-
Design of a 50 kWe Bio-oil Fueled Rankine Cycle Cogeneration
Power Plant
Angela Hsu
A thesis submitted in partial fulfillment of the requirements
for the degree of
BACHELOR OF APPLIED SCIENCE
Supervisor: J. S. Wallace
Department of Mechanical and Industrial Engineering University
of Toronto
March 2007
-
ABSTRACT
The objectives of this thesis are to design a 50 kWe bio-oil
fueled Rankine cycle cogeneration power plant and to determine the
feasibility of constructing and operating such a plant. A
simulation model was constructed to incorporate the properties and
characteristics of each component and the operational limits of the
entire system. The results of the simulation indicate a maximum
electrical efficiency of 3.45 percent and a maximum cogeneration
efficiency of 85 percent, both occurring at 100 percent process
load demand. An economic analysis was conducted to compare the cost
of generating heat and electricity using the system to the current
cost of purchasing energy from a utility supplier. The total annual
cost of the system is approximately US$313,000. The total annual
cost of purchasing the equivalent amount of energy generated by the
system is US$484,600 at 100 percent process load demand and
US$435,600 at 89 percent process load. The results from the
technical and economical analyses indicate the system is only
economically feasible if cogeneration is implemented. The system
does not generate enough electricity to be economical without
implementing cogeneration.
ii
-
ACKNOWLEDGEMENTS
I would like to take this opportunity to thank Professor James
S. Wallace for his guidance
and support throughout the project. His first-hand experience
and knowledge of the energy
generation sector provided clear and insightful direction for
the project. I would like to thank
my parents for their love and support throughout my life. Last
but not least, I would like to
thank my fiance for supporting me through this entire
project.
iii
-
TABLE OF CONTENTS TABLE OF CONTENTS
.....................................................................................................
IV LIST OF FIGURES
..............................................................................................................VI
LIST OF TABLES
..............................................................................................................
VII LIST OF
EQUATIONS.....................................................................................................VIII
CHAPTER 1:
INTRODUCTION.........................................................................................
1
1.1 THESIS OVERVIEW
.....................................................................................................
1 1.2 OBJECTIVES
...............................................................................................................
1
CHAPTER 2: BACKGROUND AND THEORY
............................................................... 3
2.1
BIO-OIL......................................................................................................................
3 2.2 THE RANKINE
CYCLE.................................................................................................
6 2.3 COGENERATION OR COMBINED HEAT AND POWER
(CHP)....................................... 11
CHAPTER 3: LITERATURE
REVIEW...........................................................................
12 3.1 BIO-MATTER FOR ENERGY PRODUCTION
.................................................................
12 3.2 BIO-POWERED SYSTEMS
..........................................................................................
13 3.3 A NON-BIO-POWERED RANKINE ENGINE
................................................................
17
CHAPTER 4: SYSTEM AND PARTS REQUIREMENTS
............................................ 19 4.1 OVERALL SYSTEM
REQUIREMENTS..........................................................................
19 4.2 STEAM TURBINES AND ELECTRIC GENERATORS
...................................................... 20 4.3
BOILER SPECIFICATIONS
..........................................................................................
24 4.4
PUMPS......................................................................................................................
26 4.5 CONDENSATE
TRAPS................................................................................................
29 4.6 PRESSURE REDUCING
VALVE...................................................................................
30 4.7 HEAT
EXCHANGER...................................................................................................
31 4.8 STEAM SEPARATOR AND WATER STORAGE TANKS
................................................. 32
CHAPTER 5: TECHNICAL ANALYSIS
.........................................................................
34 5.1 THERMODYNAMIC
LIMITATION................................................................................
34 5.2 MODEL ASSUMPTIONS
.............................................................................................
35 5.3 SYSTEM SET-UP
.......................................................................................................
35 5.4 SIMULATION
............................................................................................................
36 5.5 SIMULATION RESULTS
.............................................................................................
40
CHAPTER 6: ECONOMIC
ANALYSIS...........................................................................
43 6.1 COST SUMMARY
......................................................................................................
43 6.2 ELECTRICITY COST PER
KILOWATT-HOUR...............................................................
44 6.3 HEATING COST PER
KILOWATT-HOUR.....................................................................
45 6.4 COST FOR BOTH ELECTRICITY AND HEAT
................................................................ 46
6.5 ECONOMIC SENSITIVITY ANALYSIS
.........................................................................
46
iv
-
CHAPTER 7: DISCUSSION AND
RECOMMENDATIONS......................................... 49
7.1 DISCUSSION
.............................................................................................................
49 7.2
RECOMMENDATIONS................................................................................................
51
CHAPTER 8:
CONCLUSION............................................................................................
52
REFERENCES......................................................................................................................
53 APPENDIX A: FULL SCHEMATIC OF A 50 KWE BIO-OIL FUELED POWER
PLANT...................................................................................................................................
56 APPENDIX B: TURBINE EFFICIENCY
CALCULATION.......................................... 57 APPENDIX
C: SIMULINK MODEL AND
FORMULAE.............................................. 58 APPENDIX
D: SIMULATION RESULTS
.......................................................................
64 APPENDIX E: ECONOMIC ANALYSIS SUPPLEMENTAL
DATA........................... 67 APPENDIX F: EQUIPMENT COST
SOURCES.............................................................
71
v
-
LIST OF FIGURES Figure 1: Simple Rankine Cycle
..............................................................................................
6 Figure 2: Pre-heating of Feedwater Steam (Heat Exchanger)
................................................. 8 Figure 3:
Regeneration Direct Heat Transfer from Turbine
................................................. 9 Figure 4:
Regeneration Open Feedwater
Heater.................................................................
10 Figure 5: Steam
Reheat..........................................................................................................
10 Figure 6: Bio-oil Fueled Cogeneration Power Plant Schematic
(Simple) ............................. 19 Figure 7: Turbosteam BP50
Turbine Genset
[16]..................................................................
23 Figure 8: Clayton E-154 Steam Generator
[18].....................................................................
25 Figure 9: Clayton Feedwater Pump
[18]................................................................................
28 Figure 10: Federal Pump Type VRC Condensate Return Unit
[21]...................................... 28 Figure 11: Spirax
Sarco FT14 Ball Float Steam Trap
[22].................................................... 29 Figure
12: How a Spirax Sarco Ball Float Steam Trap Works [22]
...................................... 30 Figure 13: Spirax Sarco
Pilot Operated Self-Actuated Pressure Reducing Valve [24].........
31 Figure 14: Shell and Tube Heat Exchanger
...........................................................................
31 Figure 15: Armstrong Heat Exchanger [25]
..........................................................................
32 Figure 16: Steam
Separator....................................................................................................
33 Figure 17: Feedwater Tank Inlet Enthalpy
Results................................................................
41 Figure 18: Electrical Efficiency Results
................................................................................
41 Figure 19: Cogeneration Efficiency Results
..........................................................................
42 Figure 20: Pump 1 Inlet
Enthalpy..........................................................................................
50 Figure A-1: Full Schematic of a 50 kWe Bio-oil Fueled Power
Plant................................... 56 Figure C-1: Simulink
Model Part 1
.......................................................................................
58 Figure C-2: Simulink Model Part 2
.......................................................................................
59 Figure D-1: Electrical Efficiency Results, All Process Load
Demand Levels ...................... 64 Figure D-2: Cogeneration
Efficiency Results, All Process Load Demand Levels ................
64 Figure D-3: Pump 1 Inlet Enthalpy Results, All Process Load
Demand Levels ................... 65 Figure D-4: Feedwater Tank
Inlet Enthalpy Results, All Process Load Demand Levels...... 65
Figure D-5: Fuel Rate Results, All Process Load Demand
Levels........................................ 66 Figure D-6:
Additional Mass Flow Rate Results, All Process Load Demand Levels
........... 66
vi
-
LIST OF TABLES Table 1: Comparison between Induction and
Synchronous Generators [14] ........................ 22 Table 2:
Turbosteam BP-50 Turbine Specifications [16]
...................................................... 23 Table 3:
Clayton E-154 Steam Generator Specifications
[19]............................................... 26 Table 4:
Comparison between Positive Displacement and Roto-dynamic Pumps
[20] ........ 27 Table 5: System Component
Efficiencies..............................................................................
34 Table 6: Mathematical Approximations of Thermodynamic Values
.................................... 37 Table 7: Simulation Input
Parameters....................................................................................
38 Table 8: Simulation Output
Parameters.................................................................................
39 Table 9: Electricity Cost per Kilowatt-hour at Various Bio-oil
Costs................................... 47 Table 10: Annual
Equivalent System Cost at Various Interest
Rates.................................... 48 Table B-1: State
Values to Calculate Turbine
Efficiency...................................................... 57
Table E-1: Equipment, Operation and Maintenance
Costs.................................................... 67 Table
E-2: System Energy Output Values
.............................................................................
67 Table E-3: Electrical Cost per Kilowatt-hour
........................................................................
68 Table E-4: Household Energy Consumption [32]
.................................................................
68 Table E-5: Cost of Purchasing Electricity from Utility Supplier
[33] ................................... 69 Table E-6: Household
Annual Heating [32]
..........................................................................
69 Table E-7: Residential Gas-fired Water Heater Information [35]
......................................... 69 Table E-8: Cost of
Purchasing Natural Gas for Heating from Utility Supplier [36]
............. 70
vii
-
LIST OF EQUATIONS Equation 1: Rankine Cycle
Efficiency.....................................................................................
8 Equation 2: Cogeneration Power Plant System
Efficiency.................................................... 11
Equation 3: Turbine
Efficiency..............................................................................................
23 Equation 4: Carnot Efficiency
...............................................................................................
34 Equation 5: Cost per Kilowatt-hour Formula
........................................................................
44 Equation 6: Annualized Equivalent Cost Formula
[31]......................................................... 45
Equation B-1: Turbine Exhaust Steam Quality
(Isentropic).................................................. 57
Equation B-2: Turbine Exhaust Steam Enthalpy (Isentropic)
............................................... 57 Equation B-3:
Turbine
Efficiency..........................................................................................
57 Equation C-1: Turbine Outlet Formulae
................................................................................
60 Equation C-2: Steam Separator Formulae
.............................................................................
60 Equation C-3: Mass Flow Rate
Formulae..............................................................................
60 Equation C-4: Condensate Tank
Formulae............................................................................
61 Equation C-5: Pump 1 Formulae
...........................................................................................
61 Equation C-6: Feedwater Heater Formulae
...........................................................................
61 Equation C-7: Feedwater Tank Heat Loss
Formulae.............................................................
62 Equation C-8: Feedwater Tank
Formulae..............................................................................
62 Equation C-9: Pump 2 Formulae
...........................................................................................
62 Equation C-10: Power Formulae
...........................................................................................
63 Equation C-11: Heat Formulae
..............................................................................................
63 Equation C-12: Efficiency Formulae
.....................................................................................
63
viii
-
CHAPTER 1: INTRODUCTION
1.1 Thesis Overview
This thesis aims to analyze the efficiency and economics of a
bio-oil fueled Rankine cycle
cogeneration power plant with a gross power output of 50 kWe.
The service area of the
power plant is located within the northern regions of the
province of Ontario.
The power plant utilizes water as the working fluid. Water is
readily available and relatively
inexpensive to acquire. The system components required are those
found in standard steam
cycle-based power plants. These include a steam turbine and
generator set, a steam generator
and pumps. This thesis outlines the criteria used in the
component selection process to
achieve the desired design.
Cost is almost always the primary consideration for any design
project. As part of this thesis,
an economic analysis is performed to determine the feasibility
of developing the small-scale
cogeneration power plant in Northern Ontario.
Steam turbines with power outputs less than several hundred
kilowatts have relatively low
efficiencies compared to the several hundred megawatt output
units used in traditional power
generation. However, the combination of a growing need for
alternative energy sources, the
relatively small environmental impact of small-scale
cogeneration systems and the growing
interest in distributed power generation systems requires that
studies, such as the one
presented in this thesis, be conducted.
1.2 Objectives
The thesis consists of two main objectives: design a
cogeneration power plant to meet the
design objectives and conduct a technical and economic analysis
of the power plant to
determine the feasibility of constructing and operating the
power plant.
1
-
The design objectives include the following. The power plant
must generate 50 kWe, utilize
water/steam as the working fluid, the chosen components must be
commercially available
and non-customized, and the steam generator must be capable of
burning bio-oil.
As part of the technical analysis Simulink, a sub-program of
Matlab, is used to simulate the
plant performance. Current energy rates are used for the
economic analysis.
2
-
CHAPTER 2: BACKGROUND AND THEORY This chapter provides theory
and background information regarding bio-oil, the Rankine
cycle, and cogeneration.
2.1 Bio-oil
Bio-matter Sources
Bio-oil is derived from bio-matter. The two main sources of
bio-matter used in the
production of bio-oil are energy crops and wastes [1]. Energy
crops are plants that are
specifically grown for fuel production, and wastes include
plant, animal, and human activity
wastes, such as landfill garbage [1]. This project will utilize
bio-oil generated from plant and
animal waste.
Energy Crops
According to Boyle [1], the growing interest in energy crops is
due to several reasons:
1. The need for alternatives to fossil fuels to reduce net CO2
emissions
2. The search for indigenous alternatives to imported oil
3. The problem of surplus agricultural land
Depending on the location and the availability of land,
different energy crops can be grown
to satisfy local energy needs [1].
Boyle [1] discusses two categories of energy crops: woody plants
and others. Woody plants
are grown using the short rotation forestry (SRF) method, also
known as the short rotation
coppice (SRC) method [1]. The SRF/SRC method is as follows [1].
Fast-growing trees are
planted 10,000 to 15,000 per hectare and are cut down close to
the ground after a year of
growth. The trees re-grow and continue to grow for 2 to 4 years
before they are cut again.
This cycle can be repeated for up to 30 years.
Agricultural crops such as sugar cane, maize and miscanthus (a
grassy plant) are widely
grown for use as bio-fuel [1]. The advantages of agricultural
crops include high yields, the
3
-
use of conventional farming techniques, and flexible land use as
a result of the annual cycle
of the crop [1].
The specific types of energy crops grown vary widely and are
dependent on the availability
of land, sunlight exposure, climate, and soil conditions.
Wastes
Boyle [1] lists four categories of waste suitable for use as
bio-fuel.
1. Wood residues
2. Temperate crop wastes
3. Tropical crop wastes
4. Animal wastes
The background information presented in the sections below was
extracted from Boyle [1].
Wood Residues
Currently, the majority of wood residues from tree-trimming and
plantation thinning are
usually left at the site to ensure nutrients are returned to the
soil. Due to the large amounts of
space required to transport the residues it was traditionally
more economical to leave the
residues at the site. With the development of new harvesting
techniques, a fraction of the
residues can be transported to power plants as fuel for heat
and/or power generation.
It is recognized that the removal of these residues from the
site will decrease the amount of
nutrients returning to the soil. However, the long term affects
of residue removal on soil
nutrient levels have not been determined and are beyond the
scope of this thesis.
Temperate Crop Wastes
Each year more than one billion tons of wheat and corn residues
are generated. In the past,
the excess crops were burned in the field. This practice was
later banned due to high levels
of pollution. Now the residues are used to generate bio-gas for
the local area.
4
-
Straw is another plentiful crop waste. However, straw is
considered a relatively expensive
fuel due to its low mass density. Transportation and storage is
expensive since a lot of space
is required. A solution to this problem is the production of
high-density (1 ton per cubic
meter) pelletted straw. The pelletted straw requires less space
to transport and store.
Tropical Crop Wastes
The residues from sugar and rice, the two main tropical crops,
are already being used as fuels
around the world. The fibrous residue of sugar cane, bagasse, is
used in sugar factories to
generate steam and electricity. Even though transportation of
bagasse is costly, the sale of
the generated electricity often produces enough profit to cover
the costs. The bagasse could
also be used to produce ethanol, which further increases its
appeal as a fuel source.
Animal Wastes
Animal manure accounts for 10 percent of the methane emissions
in the United States. This
methane can be harnessed for use as bio-gas through anaerobic
digestion. Anaerobic
digestion utilizes bacteria to break down the organic material
to produce bio-gas. The
remaining residue from the process can be used as
fertilizer.
Poultry litter, and other wastes similar in water content, can
be used in combination with
wood shavings and straw in direct combustion power generation.
This is suitable in rural
areas where farming is prevalent.
To summarize, the type of organic waste used to produce bio-oil
will differ depending on
what is available. In temperate climates, wood residues and
temperate crop wastes are
plentiful. Animal waste may also be available if the climate is
suitable for animal farms. In
tropical climates, all four types of wastes may be
plentiful.
Pyrolysis: The Production of Bio-oil
Bio-oil is produced through the process of pyrolysis. As
explained by Boyle [1], pyrolysis is
a process where the volatile compounds of bio-matter are
collected and condensed to produce
bio-oil. This is achieved by heating bio-matter with low levels
of oxygen. A variation on the
5
-
above mentioned process is fast pyrolysis. Fast pyrolysis, as
described by Blackaby [2],
exposes the bio-matter to temperatures as high as 500C with
almost no oxygen to produce
bio-oil. The resulting bio-oil is typically acidic [1] and very
viscous [2]. This makes bio-oil
a difficult fuel to work with. However, the energy content of
bio-oil is approximately half
that of crude oil [1]. Taking into consideration the
availability and energy content, bio-oil is
an attractive fuel source.
Bio-oil Summary
Bio-oil is produced using one or more types of organic waste
through the process of
pyrolysis. The location of the power plant will determine which
category of organic waste is
used to produce bio-oil. The cogeneration power plant for this
thesis is intended for Northern
Ontario and other similar locations. This eliminates tropical
crop wastes since such crops
cannot be grown in such northerly climates.
2.2 The Rankine Cycle
Steam power plants utilize the Rankine cycle. A simple Rankine
cycle consists of four main
stages: work input, heat input, work output, and heat
output.
Figure 1: Simple Rankine Cycle
6
-
Work Input
Work input into the system is accomplished using one or several
water pumps. Water
returning from the process heater is usually at a saturated
liquid state at a low pressure
(atmospheric or sub-atmospheric). The water entering the boiler
is required to be at a much
higher pressure, thus one or several pumps are required to
increase the pressure of the
saturated liquid water.
Heat Input
A boiler (or steam generator) introduces heat into the system.
The boiler heats the liquid
water converting it into steam. The steam is usually heated
until it reaches the superheated
steam region to maximize the efficiency of the system. If the
steam does not reach the
superheated region, at the very least, it must be at the
saturated vapour state. This ensures
high quality steam passes through the steam turbine. A high
quality steam consists largely of
steam and a small fraction of condensate (as suspended water
droplets). Steam containing a
large fraction of water droplets can damage a turbine as the
mixture passes through.
Work Output
To harness the energy contained within the steam, a steam
turbine is used to convert the
thermal energy into mechanical energy. Steam passing through the
turbine blades expands
and rotates the turbine shaft. The turbine shaft is connected to
an electrical generator which
harnesses the mechanical energy and converts it into useable
electrical energy.
Heat Output
The steam exiting the turbine is a low pressure, high quality
two-phase mixture. Water
pumps cannot handle high-quality two-phase mixtures, thus the
steam must be condensed
back into a liquid state. This is accomplished through a process
heater. The process heater
extracts the latent heat contained within the mixture,
condensing the mixture to the saturated
liquid state.
Efficiency
The cycle efficiency is conventionally calculated as the net
work over the net heat input.
7
-
boiler
pumpsturbine
in
inout
in
net
QWW
QWW
QW ===
Equation 1: Rankine Cycle Efficiency
Cycle Efficiency Improvements
There are several variations of the Rankine cycle in which the
overall efficiency of the
system can be increased. These variations include pre-heating
the feedwater, regeneration,
superheating, and re-heating.
Pre-heating
Pre-heating the feedwater increases the temperature of the water
entering the boiler. This
reduces the amount of heat input from the boiler and increases
the cycle efficiency. This can
be accomplished through regeneration or the use of a heat
exchanger.
Figure 2: Pre-heating of Feedwater Steam (Heat Exchanger)
Regeneration
There are two methods of regeneration. Both methods utilize the
high temperature, high
quality steam passing through the turbine. The first method
involves passing the saturated
liquid from the condenser through the turbine for pre-heating.
This is similar to what occurs
within a heat exchanger. The heat in the steam passing through
the turbine is transferred to
8
-
the condensate which causes the steam to condense inside the
turbine. However, this method
has two major problems. The heat transfer area in the turbine is
minimal, and the turbine exit
steam is of very high moisture content. Exit steam with high
moisture content indicates a
large region within the turbine where a two-phase mixture
existed. As stated above, this is
undesirable.
Figure 3: Regeneration Direct Heat Transfer from Turbine
The second method involves the extraction of high temperature
and high quality steam. This
extracted steam is used in either an open or closed feedwater
heater to increase the
temperature of the feedwater. An open feedwater heater allows
direct mixing of the high and
low temperature fluids. A closed feedwater heater is essentially
a shell and tube heat
exchanger. The high temperature and high quality steam can be
extracted either from the
turbine or the boiler. Extracting the steam from the turbine
does not reduce the quality of the
steam at the turbine exit. This method is advantageous since the
heat transfer rate between
the two streams of hot and cold fluid is much higher than in the
previous method, and the
turbine exhaust steam is unaffected.
9
-
Figure 4: Regeneration Open Feedwater Heater
Superheating
As stated earlier, superheating the steam involves heating the
steam until it reaches the
superheated region. Superheating increases the cycle efficiency
by increasing the work
output.
Re-heating
Re-heating takes the exhaust steam from a high pressure turbine
and re-heating the steam
prior to passing the steam through a low pressure turbine. This
increases the amount of work
extracted, increasing the cycle efficiency.
Figure 5: Steam Reheat
10
-
2.3 Cogeneration or Combined Heat and Power (CHP)
Traditional power generation only utilizes the electrical energy
produced by the turbine-
generator set. The turbine exhaust steam usually contains
insufficient energy for further
electrical energy generation. The heat in the exhaust steam is
typically rejected to large
bodies of water or the atmosphere as waste heat [3].
In a cogeneration power plant, the thermal energy removed at the
condenser is used either to
pre-heat the working fluid within the system [3] or used as
process heat to satisfy other
heating loads, such as space or water heating [4].
The cogeneration system efficiency is calculated as the sum of
the net work and the useable
heat retrieved over the heat input.
( ) ( )boiler
retrievedpumpsturb
in
outnetoncogenerati Q
QWWQ
QW =+=
Equation 2: Cogeneration Power Plant System Efficiency
The increase in system efficiency is realized by reducing or
eliminating the need to generate
the equivalent amount and quantity of steam using a separate
boiler.
11
-
CHAPTER 3: LITERATURE REVIEW
This chapter reviews and summarizes research relevant to
bio-powered systems. The
systems discussed include bio-mass gasifier gas turbines and
bio-oil turbines. A solar-
powered/fuel-assisted Rankine engine is also presented as a
point of comparison for this
project.
3.1 Bio-matter for Energy Production
Fossil fuels became the primary energy source when higher
temperatures were required by
industrial processes. These higher temperatures could not be
achieved by burning bio-matter.
Despite that, bio-matter still plays a significant role in
energy production. According to Bain
and Overend [5], bio-power is the single largest source of
non-hydro renewable energy in the
United States. The average size of existing bio-power plants is
20 MW with an average bio-
mass-to-electricity efficiency of 20 percent [5]. A power plant
as small as 20 MW has
several disadvantages. The small size leads to higher capital
costs per kilowatt-hour of
power produced [5]. Low efficiency ratings result in increased
sensitivity to fluctuations in
the price of fuel [5]. The result is an increase in the cost of
electricity to 8 to 12 cents per
kilowatt-hour [5]. In Ontario, the average cost of electricity
is 6 Canadian cents per kilowatt-
hour [6], not including other charges. Bio-power is currently
too expensive to compete with
traditional power generation in densely populated areas.
However, in rural areas where the
cost of electricity is higher than in cities, bio-power may be
more economical.
Bain and Overend [5] suggest three methods to lower the cost of
utilizing bio-matter. These
methods are applicable both in rural and suburban areas.
1. Co-firing
2. Gasification
3. Direct-fired combustion
12
-
Co-firing
The first cost reduction method suggested by Bain and Overend
[5] is to co-fire bio-mass
with coal in existing systems. The authors state this would
lower the capital investment
since only slight modifications to existing systems are
required.
Gasification
Gasification is the process in which the volatile compounds in
the bio-matter are extracted in
either air or steam to produce a medium to low energy bio-gas
[5]. Bio-gas is used as fuel
in a gasification combined cycle (GCC) which consists of a gas
turbine topping cycle and a
steam turbine bottoming cycle [5]. According to the authors, the
first generation of bio-
matter GCC systems could have efficiencies double that of
current coal-based GCC systems.
In CHP applications, bio-matter GCC systems have the potential
to achieve 80 percent
efficiency [5]. Higher cycle efficiency results in a lower cost
per kilowatt-hour.
Direct-fired combustion
Direct-fired combustion involves burning bio-matter to produce
heat. The resulting
combustion gases are used to produce steam in a Rankine cycle
[5]. The efficiency of the
steam system can be increased by 10 percent by incorporating
re-heat, regeneration and other
efficiency boosting methods [5].
The three cost reduction methods described above lower the costs
of utilizing bio-energy
either through raising the efficiency of the system or by
reducing the investment costs
required. The implementation of these solutions leads to
promising future prospects for bio-
energy as a source of power.
3.2 Bio-powered Systems
Bio-mass Gasifier Gas Turbine Power Generation
Bio-mass gasifier gas turbine technology combines advanced
Brayton cycle power
generation with bio-mass gasifiers [7]. Larson and Williams [7]
states the unit capital costs
of gas-turbine systems are relatively low and insensitive to
size. Larson and Williams [7]
13
-
also believe the advantage of gas turbines lies in the ability
to achieve higher peak cycle
temperatures than steam turbines, thus achieving higher cycle
efficiencies. Although
commercial bio-matter gas turbines are currently unavailable,
Larson and Williams [7] have
listed three cycles that could potentially be converted into
biomass-integrated gasifier/gas
turbine (BIG/GT) systems.
1. Steam-injected gas turbine
2. Intercooled steam-injected gas turbine
3. Combined cycle
According to Larson and Williams [7], both BIG/GT and
double-extraction/condensing
steam turbine (CEST) cogeneration systems convert 60 percent of
the energy in bio-mass
fuel into steam and electricity. However, BIG/GT systems produce
three or more times the
amount of electricity of CEST systems, making them a prime
candidate for power
generation. Larson and Williams [7] discuss several BIG/GT
projects that were underway
during the time the article was written. Two of these projects
will be briefly discussed below
according to the information extracted from Larson and Williams
[7].
Combined cycle district-heating cogeneration in Varnamo,
Sweden
This was the first BIG/GT Combined Cycle plant. The plant has a
nominal electrical output
of 6 MW and a nominal thermal output of 9 MW. To generate the
stated outputs, 20 MW of
bio-mass input is required. The plant was modeled around a
modified European Gas Turbine
Typhoon gas turbine and became fully operation in October
1995.
IVOSDIG cycle in Finland
The IVOSDIG cycle utilizes wet feedstocks. The wet fuel is dried
in a pressurized dryer to
produce high pressure steam. This high pressure steam is
recovered and injected into the gas
turbine. The system has a nominal output of 92 MW and an
efficiency of 35 percent when
utilizing 70 percent moisture content peat.
With relatively low capital costs, insensitivities to sizing,
and reasonably high cycle
efficiency, BIG/GT systems are a very promising technology as
illustrated by the two
14
-
projects discussed above. However, BIG/GT systems may be
unsuitable for rural areas.
Transporting the bio-matter from rural areas to gasification
plants to produce bio-gas may be
more costly than locally preparing the bio-matter for direct
firing. This is due to the lower
population density of rural areas. Lower population density
means less power demand. This
results in a higher cost of power on a per kilowatt basis. This
is also a reason why small-
scale power generation is more cost-effective than traditional
centralized power generation.
Centralized power generation requires expensive power lines to
be installed to transport
power. Small distributed power plants, such as the one developed
for this thesis, will
eliminate the need for expensive electrical towers.
Bio-oil Gas Turbine
Blackaby [2] discusses modifications made to a GT2500 gas
turbine which will be
summarized in the following paragraphs. A Canadian-based
company, Orenda Aerospace
Corporation, in collaboration with a Ukrainian manufacturer,
Zorya-Mashproekt, modified
the GT2500 system at a DynaMotive power plant to utilize
bio-oil. Due to the viscous nature
of bio-oil, the oil is pre-heated prior to entering the high
pressure pumps.
The West Lorne bio-oil plant located 50 kilometers southwest of
London, Ontario produces
70 tons of bio-oil per day. The oil is used to power the GT2500
gas turbine, which produces
2.5 MW of electricity. This is enough electricity to meet the
demands of the Erie Flooring
plant. The remaining electricity is exported to the local grid.
The surplus heat generated by
the turbine is used to generate 12,000 pounds of steam per hour
for the Erie Floorings
industrial operation. The plant has entered into a 3 year
contract with a third party for the
sale of excess bio-oil.
Blackaby [2] also mentions a new project between DynaMotive and
an Ojibway community
in Northwestern Ontario. Alex Peters, President of the
Whitefeather Forest Management
Corporation, is quoted by Blackaby [2] as saying: Our community
runs on fuel oil
generators, so we have to fly in fuel, which is very expensive.
But we have all these trees we
can get the bio oil out of.
15
-
Many rural communities are surrounded by an abundance of
bio-matter that could be
converted into bio-oil for energy production. Utilizing bio-oil
CHP systems, bio-oil may be
the most cost-effective energy production method.
Other Bio-powered Systems
BIG/GT systems are not the only bio-powered systems currently
under development. Bain
and Overend [5] provides four examples of other bio-powered
technology. The examples
discussed below were extracted from Bain and Overend [5].
Gasifier spark ignition engine in Littleton, Colorado
The CPC project consists of a fixed-bed downdraft gasifier. The
gasifier feeds gas to a spark
ignition engine which is coupled to a generator. The system
capacity ranges from 12 kW to
25 kW. Two units have been installed: one in the Philippines and
the other in California
(both in 2001).
Stirling engine in Indianapolis, Indiana
The heat used to drive the Stirling engine comes from the
combustion gases of a modified
pellet stove (burner). A large portion of the heat from the
exhaust gases of the engine is
recovered and transferred to the incoming combustion gases, thus
improving the overall
efficiency. The system is designed for 3 kW to 18 kW and is
being targeted at residential
and small industrial markets.
Microturbine in Mission Viejo, California
Flex Energies has designed and fabricated a 30 kW proof of
concept Flex-MicroturbineTM.
The unit is designed to utilize very low heating value gases
(3.7 MJ/Nm3) with very low
emission levels. Once the proof of concept design is
successfully tested, three additional
prototypes will be designed. The prototypes will be tested using
landfill gas, anaerobic
digester gas, and gasification producer gas.
16
-
Fluid Bed Gasifier CHP plant in Orinda, California
The Carbona Corporation plans to design, fabricate and prototype
a CHP system that will
utilize a fluid bed gasifier to fuel internal combustion
engines. The 5 MW electric and 9 MW
hot water system will be located in Denmark for residential
heating. Wood chips will be the
fuel source.
As the above projects illustrate, there are many technologies
that can utilize bio-matter for
power generation. Further research and development into each
technology will reveal which
is most suitable for each application (industrial, residential,
etc.).
3.3 A Non-Bio-Powered Rankine Engine
As a point of comparison for the work developed in this thesis,
the work conducted by Gari,
Khalifa and Radhwan [8] regarding a solar-powered/fuel-assisted
Rankine engine for power
generation in Jeddah, Saudi Arabia will be discussed
briefly.
The solar-powered Rankine engine steadily generates 36 kW of
electrical power using steam
at 230C. The steam is produced using a boiler which is connected
to a heated oil loop. The
oil loops are heated using 400m2 of single-axis tracking
concentrating parabolic-trough
collectors and an auxiliary gas-fired heater. With a two-stage
turbine, the systems
theoretical efficiency is 23.2 percent.
Gari, Khalifa and Radhwan [8] discuss another study where solar
energy was used to
generate steam at 100C. This steam was then superheated to 600C,
doubling the system
efficiency as compared to Organic Rankine cycles operating at
similar solar collector
temperatures. The same study showed a seasonal thermal
efficiency of 14.6 percent can be
achieved in Phoenix, Arizona utilizing a 200m2 evacuated-tube
collector, a 10-stage turbine
and an air-cooled condenser.
The two examples illustrate why alternative fuel power
generation is only beginning to be
considered. The efficiencies achieved are quite low and are not
acceptable for centralized
17
-
power generation stations. However, these efficiencies may be
acceptable in rural areas
where the cost of electricity is much higher than in cities.
18
-
CHAPTER 4: SYSTEM AND PARTS REQUIREMENTS This chapter presents
the overall system requirements for this project. The requirements
for
the individual components of the system are also discussed.
4.1 Overall System Requirements
The power plant for this project is a cogeneration power plant
with a gross output of 50 kWe.
The system utilizes a Rankine cycle with the inclusion of a
pre-heat stage. Water is the
working fluid chosen for the system. The steam is heated using
the combustion gases from a
bio-oil fired steam generator. The thermal energy from the
turbine exhaust steam is used by
a heating process, such as building radiators or hot water
heaters, instead of being passed
through a condenser.
To achieve the desired outputs, several key pieces of equipment
are required. A steam
turbine coupled with an electric generator, an oil-fired steam
generator, pumps, condensate
traps, pressure reducing valves, a heat exchanger, a steam
separator and water storage tanks.
A detailed schematic of the power plant is available in Appendix
A.
Figure 6: Bio-oil Fueled Cogeneration Power Plant Schematic
(Simple)
19
-
4.2 Steam Turbines and Electric Generators
Steam Turbines
The most important component of the system is the steam turbine.
The turbine converts the
thermal energy in the steam into mechanical energy. As high
pressure steam passes through
the turbine, the steam expands and hits the turbine blades,
causing the turbine shaft to rotate.
This rotation generates mechanical energy that can be harnessed
and converted into
electricity.
Steam turbines typically have power outputs in the megawatt or
gigawatt range. This ensures
high efficiency and high power output required by large power
generation facilities. A
kilowatt sized steam turbine is rare since the system efficiency
would be comparatively low
and the amount of power produced would not satisfy the needs
required by large cities. In
distributed power generation systems, multiple kilowatt sized
steam turbine would provide
the power necessary for the surrounding area.
There are several types of steam turbines commonly used in power
generation. These
include condensing, extraction, re-heat, and non-condensing
turbines. Each turbine type will
be briefly discussed below.
Condensing Turbines
In a condensing turbine, the steam is expanded well below
atmospheric pressure to extract
the maximum amount of energy from the steam [10]. The
sub-atmospheric exhaust steam
does not contain enough energy for use in any other application.
This type of turbine is
suitable for use in centralized power generation facilities,
where maximum energy extraction
from the steam is a priority.
Extraction Turbines
Extraction turbines allow high pressure and high temperature
steam to be extracted from an
intermediate area of the turbine [11]. The extracted steam can
be used in a re-heat process or
used in a regeneration process to pre-heat the boiler feedwater
[11] as stated in section 2.2.
20
-
Re-heat Turbines
Re-heat turbines are similar to extraction turbines, however the
steam extracted from an
intermediate section of the turbine is re-heated close to its
original temperature, and re-
introduced back into the turbine [12]. As stated in section 2.2,
this re-heat process greatly
improves the efficiency.
Non-Condensing Turbines
In a non-condensing turbine, the steam exits the turbine at a
pressure above atmospheric [10].
The turbine exhaust steam still contains enough energy to be
used in other processes, such as
heating [10].
A backpressure turbine is a type of non-condensing turbine. The
steam exits the turbine at a
specific pressure, the system back pressure. Back pressure is
the pressure applied at the
exhaust region of the turbine [13].
Electric Generators
Almost as important as the steam turbine is the electric
generator. An electric generator
converts the mechanical energy provided by the steam turbine
into useable electrical energy.
There are two types of electric generators: induction and
synchronous. A brief overview of
each generator type is presented below.
Induction Generator
An induction generator is an electric generator that receives
its excitation from the utility
[14]. This means the generator cannot produce any voltage on its
own, and that the
frequency and voltage of the power produced is governed by the
frequency and voltage of the
power from the incoming utility line [14]. An induction
generator runs at a speed that is
determined by the utility and is slightly higher than its
synchronous speed [14], the speed at
which the magnetic field in the motor is rotating [15].
21
-
Synchronous Generator
A synchronous generator is able to produce its own power and
regulate its own voltage
without being connected to the utility [14]. This means a
synchronous generator can operate
in parallel with the utility or operate independently (stand
alone) [14]. Synchronous
generators require a speed reduction gear [14]. When a
synchronous generator is used, the
turbine is designed to spin at whatever speed provides the
maximum efficiency. Thus, a
speed reduction gear is required to reduce the turbine rotation
speed to a speed suitable for
the generator [14]. Table 1 provides a comparison of the
characteristics of the two types of
generators.
Table 1: Comparison between Induction and Synchronous Generators
[14]
Induction generators Synchronous generators Parallel or
stand-alone? Can only run in parallel with
the utility. Cannot provide back-up power during utility
outage.
Can run in parallel or stand-alone. Can provide back-up
power.
Typical price comparison*
Under 700 kW, less expensive. Over 700 kW, less expensive.
Power factor issues Should not be used for more than about 1/3
of total plant electrical load.
Can be used to improve power factor. Can provide up to 100% of
plant load or more.
Complexity The common perception is that synchronous generators
are complex and difficult to operate. With modern electronics, this
is no longer an issue.
* Prices compared include turbine, generator, and complete
switchgear, including circuit breaker, utility grade electrical
protection, synchronizing equipment as required, and turbine
controls
Turbosteam BP-50 Turbine Generator Set
The turbine for this project must produce a gross electrical
output of 50 kW. The
Turbosteam BP-50 Turbine Genset meets the required electrical
output. The Turbosteam
BP-50 Turbine is a backpressure steam turbine that can be
coupled with either a synchronous
electric generator or an induction generator. A synchronous
generator is chosen to allow for
power generation independent from the provinces electrical grid.
This is particularly
important when provincial power lines are damaged.
22
-
Figure 7: Turbosteam BP50 Turbine Genset [16]
The turbine and generator set includes a turbine, generator,
pressure transmitter, generator
control panel and turbine control panel [9]. The pressure
transmitter monitors the pressure at
the turbine exit and sends a signal to the pressure controller
[9]. The generator controller
then opens or closes the turbine throttle to maintain the low
pressure set-point [9].
The turbine efficiency is calculated using a simple relation
shown in Equation 3 and the
information in Table 2. The turbine efficiency is calculated to
be 32 percent. Detailed
calculations are available in Appendix B.
Table 2: Turbosteam BP-50 Turbine Specifications [16]
Inlet Pressure 160 psig (174.696 psia) Inlet Temperature
Saturated (371F) Exhaust Pressure 12 psig (26.696 psia) Mass Flow
Rate 3880 lbs/hr Electrical Power Output 50 kW Generator Efficiency
[9] 95%
isentropic
actualturbine W
W=
Equation 3: Turbine Efficiency
23
-
4.3 Boiler Specifications
Another crucial component is the steam generator, also known as
the boiler. The basic
function of a boiler is to transform the working fluid from the
liquid phase to the gaseous
phase. To accomplish this, fuel is burned in a combustion
chamber to produce high
temperature combustion gases. The heat is transferred from the
combustion gases to the
working fluid as the working fluid passes through the boiler in
metal tubes.
The types of combustion fuel commonly used in power generation
include natural gas, light-
oils, diesel, and coal. The type of combustion fuel used
dictates the specifications of the
boiler. In addition to being able to utilize the selected fuel,
the boiler must also be able to
provide enough steam at the correct conditions to satisfy the
various system components,
such as the turbine.
Clayton E-154 Steam Generator
The 50 kWe power plant for this project utilizes bio-oil as a
fuel source. The selected boiler
must therefore be highly resilient due to the corrosive and
viscous nature of bio-oil. The
Clayton E-154 Steam Generator is chosen for this project as it
is able to burn number 6 oil.
Number 6 oil is similarly viscous but less corrosive than
bio-oil. Therefore, if the Clayton E-
154 Steam Generator can burn number 6 oil, it should be capable
of burning bio-oil, although
corrosion may be an issue in the long term.
The Clayton Steam Generator system includes a steam generator
skid, a water treatment skid
and a feedwater receiver skid [17]. The steam generator skid
includes the mounting, piping
and wiring for the steam generator and all the components for
the water treatment skid [17].
The water treatment skid includes all the mounting, piping and
wiring found on the feedwater
receiver skid, water softeners, a chemical feed system, blowdown
equipment and booster
pumps, if necessary [17]. The feedwater skid includes the
mounting, piping and wiring
necessary for the feedwater receiver [17].
24
-
The specifications for the Clayton E-154 Steam Generator are
shown in Table 3. The E-154
model is capable of providing steam at a mass flow rate of 5175
pounds per hour. The
turbine only requires 3880 pounds of steam per hour, thus the
steam generator will be
running at 75 percent of its design capacity.
A feature of the Clayton E-154 is its counter-flow design. The
feedwater is pumped up to the
top of the boiler unit and travels down the boiler in a helical
path [18]. This maximizes the
heat transfer between the combustion gases and the fluid.
Included in the package is a mechanical separator. The
mechanical separator ensures the
steam leaving the boiler has a quality of 99.5 percent (or
higher) [18]. Any moisture that is
separated out is returned to the steam generator via a steam
trap and a feedwater tank [18].
Figure 8: Clayton E-154 Steam Generator [18]
25
-
Table 3: Clayton E-154 Steam Generator Specifications [19]
Boiler Horsepower 150 BHP Heat Input 5,907,353 Btu/hr Heat
Output 5,175 lbs/hr Oil Consumption at Max. Steam Output (No. 2
oil) 42.0 gal/hr Oil-Fired Efficiency (at 75% firing rate) 85%
Electric Motor: Pump (65-300 psi) 5 HP Dimensions (Length x Width x
Height) 144 in x 88 in x 102 in
4.4 Pumps
The purpose of a pump is to add energy to a fluid. There are two
classes of pumps: positive
displacement pumps and roto-dynamic pumps. A brief description
of each pump type is
presented below.
Positive Displacement Pumps
A positive displacement pump utilizes volume change to move
fluid. A pump cavity opens
and the fluid enters the pump [20]. The pump cavity closes and
the fluid is forced out
through the pump with the aid of a piston, diaphragm, or rotor
[20]. There are two main
classifications of positive displacement pumps: reciprocating
and rotary. Reciprocating
pumps utilize a piston or diaphragm moving in a back and forth
motion to move the fluid. A
human heart is an example of a reciprocating pump [20]. Rotary
pumps utilize one or more
rotating rotors to move the fluid. A screw pump is an example of
a rotary pump [20].
Roto-dynamic Pumps
A roto-dynamic pump does not utilize a change in volume to move
the fluid. The pump adds
momentum to the fluid via fast-moving blades or vanes [20]. The
fluid increases momentum
as it moves through the open passages inside the pump [20]. The
high velocity of the fluid is
converted to a pressure increase when the fluid exits into a
diffuser section of the pump [20].
Roto-dynamic pumps can be classified into three categories
depending on the direction the
fluid takes when exiting the pump. The three categories of pumps
are: centrifugal (or radial),
axial, and mixed (between radial and axial) [20].
26
-
Table 4 compares the characteristics of the two types of
pumps.
Table 4: Comparison between Positive Displacement and
Roto-dynamic Pumps [20]
Positive Displacement Roto-dynamic Flow Rate Up to 100 gal/min.
Up to 300,000 gal/min. Fluid Viscosity
Can handle high-viscosity fluids. Can handle low-viscosity
fluids.
Pressure Very high pressure rise (300 atm). Moderate pressure
rise (a few atm).
Priming Mostly self-primng. Requires priming. Performance At
constant shaft rotation speed,
produces nearly constant flow rate and virtually unlimited
pressure rise.
Continuous constant-speed variation of performance. At zero
flow, maximum pressure rise. At maximum flow rate, zero pressure
rise.
Effects of Viscosity on Performance
Little effect on performance. Increasing viscosity sharply
degrades pump performance.
Two pumps are required for this project: a feedwater pump and a
condensate pump. Both
pumps must be capable of delivering a mass flow rate of 3880
pounds per hour to ensure
continuous re-circulation of the working fluid.
Feedwater Pump
The feedwater pump delivers water from the feedwater tank to the
boiler. As stated in
section 4.3, the feedwater pump is included in the Clayton E-154
Steam Generator system.
The Clayton feedwater pump is a positive displacement diaphragm
pump that is driven by an
electrical motor. The pump uses a flexible multi-layer rubber
membrane and hydraulic oil to
move the working fluid. The reciprocating drive pistons do not
come into contact with the
working fluid. Instead, the pistons displace the hydraulic oil
which displaces the membrane.
The Clayton pump has a maximum capacity of 5175 pounds per hour.
The system
requirement for the pump is a capacity of 3880 pounds per hour,
thus the Clayton pump
satisfies the design requirement.
27
-
Figure 9: Clayton Feedwater Pump [18]
Condensate Pump
The condensate pump delivers water from the condensate tank,
which holds the condensate
from the process load and various steam traps, to the feedwater
tank and feedwater heater.
The condensate pump chosen for the project is the Type VRC
condensate pump from Federal
Pump Corp. The unit includes a condensate receiver, a pump, and
a float switch [21]. The
receiver is made of cast iron and is designed to vent to the
atmosphere [21]. The condensate
pump itself is a bronze-fitted centrifugal pump [21]. The float
switch is mounted and wired
on the condensate return unit and allows automatic operation of
the pump [21]. The VRC-
620-2 condensate pump will have a discharge pressure of 20 psi
above the inlet pressure and
a pump capacity of 9 GPM (equivalent of 4497 pounds per hour)
[21].
Figure 10: Federal Pump Type VRC Condensate Return Unit [21]
28
-
4.5 Condensate Traps
The primary function of a condensate trap (or steam trap) is to
separate and collect the
moisture present in the steam. Condensate traps are particularly
important during system
start-up. During the start-up phase the piping throughout the
system is cold. When the hot
steam encounters the cold pipes, the steam condenses. If the
condensate is not separated out,
there is an increased potential for corrosion and damage
throughout the system, particularly
at the turbine. Condensate traps also separate air and other
impurities from the steam.
Condensate collected by the traps is re-directed back to the
condensate tank.
Figure 11: Spirax Sarco FT14 Ball Float Steam Trap [22]
Once the system reaches steady state, the pipes are warm and the
amount of condensate
throughout the system greatly reduces. Only a few of the traps
are utilized during steady
state operation. The Spirax Sarco FT14 Ball Float Steam Trap was
chosen for its durability.
The steam trap works by allowing air to by-pass the main valve
through a thermostatic air
vent during start-up [22] (Figure 12-1). As condensate is
collected the ball float is raised and
the lever mechanism opens the main valve [22] (Figure 12-2). Hot
condensate flows through
the main valve, but closes the air vent [22]. When steam enters
the trap, the ball float drops
and closes off the main valve, this prevents live steam from
passing through [22].
29
-
Figure 12: How a Spirax Sarco Ball Float Steam Trap Works
[22]
4.6 Pressure Reducing Valve
A pressure reducing valve (PRV) is used to reduce the pressure
of a working fluid. Pressure
reducing valves are often used in steam plants to reduce the
high pressure steam generated by
the boiler or from turbine exhaust to a lower pressure to be
used in another application.
There are two main types of PRVs: self-acting and pneumatic
control [23]. Self-acting
valves can operate without external power while pneumatic
control valves require a
pneumatic signal and actuator to operate [23].
The system utilizes two PRVs. One is used to relieve the
pressure in the system when the
process load is operating at less than 100 percent load. The
other is used to extract steam
from the boiler into the feedwater tank to maintain the
condensate within the tank at a certain
temperature. The Spirax Sarco Pilot Operated Self-Actuated PRV
was chosen for this
project. The pilot operated PRV is extremely accurate, easy to
adjust, and has the capability
to be turned on and off [23].
30
-
Figure 13: Spirax Sarco Pilot Operated Self-Actuated Pressure
Reducing Valve [24]
4.7 Heat Exchanger
The function of a heat exchanger is to transfer energy in the
form of heat from a high
temperature fluid stream to low temperature fluid. The heat
transfer occurs mainly through
convection. A common heat exchanger type is the shell and tube
heat exchanger. The low
temperature fluid enters from one end of the exchanger via
tubes. The high temperature fluid
enters from the other end of the exchanger in a cavity around
the tubes. Typically the two
fluids flow in opposite directions, towards then away from each
other, and around baffles to
increase heat transfer. Baffles are dividers positioned
perpendicular to the length of the tubes
which are used to direct the flow of the high pressure
fluid.
Figure 14: Shell and Tube Heat Exchanger
Fluid temperature plays an important role in heat exchanger
performance since heat transfer
is dependent on the temperature difference between the two
fluids. It is often desired to have
a large temperature difference between the two fluids.
31
-
In addition to a large temperature difference, the available
surface area also plays an
important role. In a typical shell and tube heat exchanger the
low temperature fluid travels
through many small diameter tubes. This drastically increases
the surface area available for
load. The Armstrong WS Heat Exchanger was chosen for this
project. The
rmstrong Heat Exchanger features a removable tube bundle as a
standard feature [25]. A
heat transfer.
The heat exchanger is used to pre-heat the boiler feedwater
using steam that is not required
by the process
A
removable tube bundle allows for easier maintenance operations.
The chosen unit is a 4-pass
model comprised of copper tubes, carbon steel baffles, and cast
iron head [25].
Figure 15: Armstrong Heat Exchanger [25]
4.8 Steam Separator and Water Storage Tanks
team SeparatorS
in a two-phase mixture. This is used
hen very high quality steam is required for an application. A
steam separator can be easily
a T-configuration from piping joints, as shown in Figure 16. The
two-
ondensate will be directed to the condensate tank.
A steam separator separates the steam and condensate
w
constructed using
phase steam mixture enters the T-joint and hits the back of the
joint. The steam, of lower
density, flows upward to the next part of the process and the
condensate, of higher density,
flows downwards to a condensate trap.
The steam separator will be located downstream of the turbine
exhaust. The separated steam
will proceed to the process load and the c
32
-
Figure 16: Steam Separator
Water storage tanks
Water storage tanks are used the condensate from various
oints throughout the system. The tanks are also used to hold a
small amount of condensate
operates uninterrupted during times of brief system disruption.
The
nk collects condensate from the process load and the steam traps
throughout the system.
to equalize the temperature of
p
to ensure the system
tanks are also used to help system start-up by supplying
condensate to the pumps and boiler.
The system contains two water storage tanks: a feedwater tank
and a condensate tank. The
feedwater tank pre-heats the boiler feedwater using steam from
the boiler. The condensate
ta
33
-
CHAPTER 5: TECHNICAL ANALYSIS
his chapter presents the thermodynamic limitation of the
designed system, the assumptions
input and output parameters relevant to
e simulation model, and an explanation of the system set-up.
Rankine cycle is usually designed to maximize the temperature
difference between the
sure state to maximize the system efficiency. The
aximum temperature for this system is constrained by the steam
turbine to 371F. The
T
used in the construction of the simulation model, the
th
5.1 Thermodynamic Limitation
A
highest pressure state and the lowest pres
m
turbine exhaust steam is passed through heating loads, such as a
radiator, thus the minimum
temperature for this system is the saturation temperature at
atmospheric pressure, 212F. The
Carnot efficiency (maximum theoretical efficiency) for this
system is calculated to be 19.13
percent.
( )( ) 1913.0831
672146037146021211 ==+
+==H
Lcarnot T
T
Equation 4: Carnot Efficiency
The actual system efficiency will be much lower than the Carnot
efficiency. The Carnot
efficiency assumes the components within the system are ideal
with 100 percent efficiency.
In reality, this is not the case. in the system have less
than
t Efficiency [%]
Most of the components utilized
100 percent efficiency, as listed in Table 5.
Table 5: System Component Efficiencies
System ComponenTurbine 32
Generator [9] 95 Boiler Firing Rate 85-87*
Condensate Pump 65**
Feed mp water Pu 6 **5Process Load 1 *00**
*B at partial lo wer at higher load ** y for optim ance ** all
of steam is co ensed in heating load
oiler efficiency higher Typical pump efficienc
ad and loal perform
* Assumed efficiency; nd
34
-
5.2 Model Assumption
Prior to the construction regarding the operation
r to allow for a simpler model. Refer to Figure 6 or
Appendix
for the system schematic.
oad.
to be fully condensed to a
- e pressure loss caused by the fluid traveling from the
- is considered to be negligible as the components
- exchanger) is assumed to be perfectly insulated.
F.
The heat loss
- the process load is assumed to enter the condensate tank at
122F
he system presented in Figure 6 or Appendix A, is designed to
provide the maximum
cess load demand level. To achieve maximum electrical
output,
ere must always be the maximum amount of steam passing through
the turbine. When the
process load is below 100 percent, the excess steam not utilized
by the process load must be
s
of the simulation model, some assumptions
of the system are made in orde
A
- The steam leaving the boiler is at the saturated vapour
state.
- The piping between the system components is assumed to be
perfectly insulated, with the
exception of the process l
- The steam passing through the process load is assumed
saturated liquid at atmospheric pressure.
Pump 1 will compensate for th
process load to the boiler.
The pressure drop across all other piping
are assumed to be located close to each other.
The feedwater heater (heat
- The steam (shell side) in the feedwater heater is assumed to
be fully condensed at the
shell side outlet.
- The inlet water to pump 2 is assumed to be a saturated liquid
at 26.7 psia.
- The pressure and temperature inside the feedwater tank is
assumed to remain constant at
26.7 psia and 244
- Heat loss occurs at the feedwater tank due to conduction and
convection.
remains constant.
The condensate from
and atmospheric pressure due to heat and pressure loss through
the return piping.
5.3 System Set-up
T
electrical output at any pro
th
35
-
diverted elsewhere. The excess steam is diverted through a
piping section parallel to the
process load. The steam in this section passes through a
pressure reduction valve (PRV).
The PRV allows the turbine to operate at the design condition by
maintaining the turbine
exhaust pressure at 26.7 psia. Without the PRV the pressure at
the turbine exhaust would
increase and reduce the operating efficiency of the turbine.
The diverted steam still contains useable energy. Rather than
waste the energy in the steam,
the steam is used to pre-heat the feedwater. This is
accomplished by using a heat exchanger
(feedwater heater). The diverted steam occupies the shell side
of the heat exchanger while
e feedwater occupies the tube side. The steam is assumed to be
fully condensed at the shell
nk will be at a lower temperature and pressure. To mitigate
this,
eam from the boiler outlet will be diverted to the feedwater
tank. A pressure reduction
sed to simulate the system performance. Various input and output
parameters
re necessary to perform the analysis. The parameters are either
a result of equipment
to facilitate system operations.
yed in Table 6.
th
side outlet. This condensate then travels through a steam trap
(ST6) to reduce the pressure to
atmospheric where the condensate combines with condensate
returning from the process load
and the steam separator.
As stated in section 5.2, the pressure and temperature inside
the feedwater tank is assumed to
remain constant at 26.7 psia and 244F. However, when the process
load demand is high, the
feedwater entering the ta
st
valve (PRV2) will be necessary to ensure the pressure at the
boiler exhaust is at design
conditions.
5.4 Simulation
Simulink is u
a
constraints or are chosen
To allow for flexibility in parameter adjustments, such as
pressure, saturated steam properties
were plotted in Microsoft Excel and a trendline was used to
approximate the relationships
between various properties. The approximations are displa
36
-
Table 6: Mathematical Approximations of Thermodynamic Values
Saturated Liquid State, P in ]/[0952.0ln0839.0
]/[619.81 2808.0
mf
mf
RlbBtuPslbBtuPh
+==
psia [3] 26384911614f PPPPPv ++=
103109101101103 5
]/[0161.0107 35 mlbftP ++ Saturated Vapour State, P in psia
[3]
]/[9769.1ln0815.0]/[5.1105 0152.0
RlbBtuPslbBtuPh
mg
mg
+==
Compressed Region, h in
d 5]
Btu/lbm anT in F [2
]/[016.0108102 3728 mcompressed lbfthhv ++= ]/[533.309886.0
mcompressed lbBtuTh =
Tables 7 and 8
complete Simu
present the parameters used in the Simulink model. Appendix C
contains the
link model as well as the formulae used within the m del. o
37
-
Table 7: Simulation Input Parameters
Input Parameters
Category Variable Name
in Simulink Model
Description Value
(Imperial Units)
Value (SI Units)
p_turb_in [psia] 1204.5 [kPa] Turbine inlet pressure 174.7
p_turb_out Turbine outlet pressure 26.7 [psia] 184.1 [kPa]
eta_turb Turbine efficiency 0.3 0.3191 191 Turbine
m Steamturbine 3[lb 1760 [kg/hr]_steam
flow rate to 880 m/hr]
p_pump1_in Pa] Pump 1 inlet pressure 14.7 [psia] 101.3
[kp_pump1_out Pump 1 outlet pressure 34.7 [psia] 239.2 [kPa] Pump 1
eta_pump1 Pump1 efficiency 0.65 0.65 p_pump2_in re ia] Pa] Pump 2
inlet pressu 26.7 [ps 184.1 [k
p_pump2_out ure ia] Pump 2 outlet press 174.7 [ps 1204.5 [kPa]
Pump 2
eta_pump2 Pump2 efficiency 0.65 0.65
Atank Surface area of feedwater tank 40.10 [ft3] 1.13548
[m3]
h Feedwater tank convective heat transfer coefficient
1.76 [Btu/hft2F]
] 10 [W/m2K
k Feedwater tank thermal
th tion) ]
conductivity (wifibreglass insula
0.023 [Btu/hftF
0.04 [W/mK]
Tt nternal .53 [C] Feedwater tank itemperature 243.55 [F]
117
Tamb perature
] C] Ambient tem(tank external surface temperature)
43.4 [F 23.0 [
tin Feedwater tank wall thickness (with insulation)
1.97 [in] 0.05 [m]
Feedwater Tank
Tinf Environment temperature 68 [F] 20 [C]
eta_boil Boiler cy 0.85 0.85 efficienBoiler el [27] oil 82520
q_fu Heating value of bio- [Btu/gal] 23 [MJ/L]
Net Power eta_gen fficiency 0.95 0.95 Generator e
PRV p_prvout ing valve sure a] Pressure reduc#1 outlet pres 20
[psia] 137.9 [kP
Process ss load ]
3 Load h_Lout
Enthalpy of procereturn condensate
90.08[Btu/lbm
209.5[kJ/kg]
38
-
Table 8: aramSimulation Output P eters
Output Parameters Category Variable Name
ink Modein l
Description Simulh_Tin enthalpyTurbine inlet [Btu/lbm] s_Tin
Turbine inlet entropy [Btu/lbmR] h_Tou y [Btu/lbm] t_act Turbine
outlet actual enthalpqual_Tout_act tual quality Turbine outlet
acw_turb Turbine work output [Btu/lbm] h_SSout Enthalpy of steam
branch of steam separator
[Btu/lbm] h_ST5 Enthalpy of condensate bran h ofc steam
separator
[Btu/lbm] h_FWHout Feedwater heater shell side outlet
enthalpy
[Btu/lbm] h_pump1_in Pump 1 inlet enthalpy [Btu/lbm] v_pump1_in
Pump 1 inlet specific volume [ft3/lbm] w_pump1_in Pump 1 work input
[Btu/lbm] h_pump1_out_act mPump 1 outlet actual enthalpy [Btu/lb ]
h_FWT_in Feedwater tank inlet enthalpy [Btu/lbm] h_pump2_out_act
mPump 2 outlet actual enthalpy [Btu/lb ] h_pump2_in Pump 2 inlet
enthalpy [Btu/lbm] w_pump2 Pump 2 work input [Btu/lbm] v_pump2_in
Pump 2 inlet specific volume [ft3/lb ] mq_boil Boiler heat input
[Btu/lbm]
Internal Output Variables
q_Load Load heat output [Btu/lbm] m_SSout Mass flow rate through
steam branch of steam
separator [lbm/hr] m_ST5 Mass flow rate through condensate
branch of
steam separator [lbm/hr] m_Load Mass flow rate through p or cess
load [lbm/hr] m_PRV Mass flow rate through pr essure reduction
valve
[lbm/hr]
Mass Flow Rates
m_add Additional mass flow rate into feedwater tank [lbm/hr]
QFWT Feedwater tank heat out [kW] Heat t Input/Outpu Q_boil
Boiler input power [kW]
Ppump1 Pump 1 power [kW] Ppump1 Pump 2 power [kW] Pgen Net power
output [kW]
Power Input/Output
Pturb Turbine power output [kW] Fuel Rate t Fuel_do Fuel input
rate [kW]
eta_cogen Cogeneration efficiency [%] Efficiencies eta_elec
Electrical efficiency [%]
39
-
5 tion
The results from th ion indicate ncy of 3.45 percent and
a m ogen iency of 8 0 percent process load
demand.
cess load demand varies, so do several characteristics of the
system. The
ynamics, thus the minimum process load demand for this is 89
percent. This
mitation essentially disregards all data for the system below a
process load demand level of
the boiler to feedwater tank. This efficiency is higher since
the amount of heat
put at the boiler would be lower.
.5 Simula Results
e simulat a maximum electrical efficie
aximum c eration effic 5 percent, both occurring at 10
As the pro
feedwater tank inlet enthalpy is of particular interest. As the
process load demand decreases
below 89 percent the feedwater tank inlet enthalpy becomes
negative. This violates the laws
of thermod
li
89 percent.
The following figures illustrate the change in the feedwater
inlet enthalpy, electrical
efficiency and cogeneration efficiency between 80 percent and
100 percent process load
demand. Figure 19 also illustrates the cogeneration efficiency
if additional steam was not
diverted from
in
Appendix D presents additional figures with the entire process
load demand range.
40
-
Feedwater Tank Inlet Enthalpy at Different Process Load
Demands
-100
-75
-50
-25
0
25
50
75
100
80 85 90 95 100
Process Load Demand [%]
Feed
wat
er T
ank
Enth
alpy
[B
tu/lb
m]
FWT Inlet Enthalpy
FWT Inlet Enthalpy,h_FWT>=0
Figure 17: Feedwater Tank Inlet Enthalpy Results
Electrical Efficiency at Different Process Load Demand
Levels
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
80 85 90 95 100
Process Load Demand [%]
Elec
tric
al E
ffici
ency
[%]
Electrical Efficiency
Electrical Efficiency withNegative h_FWT
Figure 18: Electrical Efficiency Results
41
-
Cogeneration Efficiency at Different Process Load Demand
Levels
50
55
60
65
70
75
80
85
90
95
80 85 90 95 100Process Load Demand [%]
Cog
ener
atio
n Ef
ficie
ncy
[%]
CogenerationEfficiency
CogenerationEfficiency, NoAdditional Heatat FeedwaterTank
CogenerationEfficiency withNegativeh_FWT
Figure 19: Cogeneration Efficiency Results
42
-
CHAPTER 6: ECONOMIC ANALYSIS This chapter presents an economic
analysis of the designed 50 kWe power plant. In addition
to capital costs, annual operation and maintenance costs will
also be considered. To provide
a thorough analysis of the economic nature of this project, a
comparison with current energy
rates will be conducted. Haliburton, Ontario is chosen as the
location for comparison.
Haliburton is 3 hours northeast of Toronto. The cost at which
bio-oil becomes viable will
also be analyzed.
6.1 Cost Summary
The capital cost of equipment and the annual costs of operation
and maintenance are
considered for this project. The system is relatively small
compared to centralized power
generation stations, thus it is assumed that the entire system
will be able to fit into an existing
facility. This eliminates the costs associated with constructing
a new housing facility, such
as land, construction, piping, and wiring costs.
As stated in section 5.2, the major components of the system are
assumed to be located
relatively close to each other. Therefore, the cost of piping
required between the components
is considered to be negligible when compared to the cost of the
major components (i.e.
turbine and boiler) and are excluded from the analysis.
The installation cost for a piece of equipment can vary greatly
depending on the size of the
installation and the level of expertise required. For this
project, the steam generator and the
turbine are the two components that require installation
expertise provided by the supplier.
The other components are comparatively simpler and cost
significantly less to install.
Therefore, an installation cost of 50 percent of the total
purchase price of the equipment is
estimated to be sufficient.
The cost of bio-oil is estimated to be approximately double the
cost of number 2 heating oil
since bio-oil is not as widely used. The current cost of number
2 heating oil in the United
43
-
States is approximately 245 US cents per gallon (65 US cents per
litre) [28]. Thus, the cost
of bio-oil is 490 US cents per gallon (130 US cents per
litre).
To ensure the plant is operating at optimal performance, the
major components must be
maintained. Annual maintenance costs for all the equipment are
estimated to be 3 percent of
the total capital cost (US$5700 per year). This is based on the
annual maintenance cost of
the University of Toronto Mississauga Campus microturbine
system, which is roughly 3.5
percent of the capital cost.
The system is assumed to require minimal operation supervision.
The majority of the
operational cost is due to the need to supply the steam
generator with bio-oil. This can be
accomplished at a relatively low cost either by utilizing a very
large holding tank for the fuel
which would require infrequent re-filling (i.e. every other
day), or by having an existing
operator re-fill a moderately sized fuel holding tank once or
twice a day. Either option
results in a relatively low labour cost estimated to be
US$15000.
The lifetime of the system is estimated to be 20 years. At the
end of 20 years, it is assumed
that none of the components are salvageable.
A complete listing of the relevant costs is located in Appendix
E.
6.2 Electricity Cost per Kilowatt-Hour
One of the main objectives of this thesis is to determine the
financial feasibility of
constructing and operating the system. To achieve this
objective, the cost per kilowatt-hour
of energy must be determined. The cost per kilowatt-hour can be
calculated using Equation
5.
][[$]/$kWhGenerationPowerAnnual
CostEquivalentAnnualkWh =
Equation 5: Cost per Kilowatt-hour Formula
44
-
The annual equivalent cost includes the annualized equivalent
cost of the initial capital cost.
Assuming an inflation rate of 2.1 percent [29] and a mortgage
rate of 6.35 percent [30], based
on historical data, an initial interest rate of 10 percent is
selected. Based on a lifetime of 20
years, the annualized equivalent of the initial capital cost is
calculated to be US$22,904.86
using Equation 6, which converts a present cost into an
equivalent annual cost over a certain
time period. The total annual equivalent cost of the system is
US$313,014.94.
( ) ( )
++= 111 N
N
iiiPAE
where
P = present cost
N = time period
i = interest rate
Equation 6: Annualized Equivalent Cost Formula [31]
From the simulation, the system provides a net power output of
49.3521 kW. Assuming the
system operates 24 hours a day, for 360 days a year (5 days for
equipment maintenance) the
plant produces 426,402 kilowatt-hours each year. Substituting
these values into Equation 5,
the resulting cost is US$0.7341 per kilowatt-hour. This is over
8 times the cost of purchasing
electricity from the electrical grid (US$0.0912 per
kilowatt-hour; calculations in Appendix
E).
6.3 Heating Cost per Kilowatt-Hour
In addition to producing electricity, the system also produces
heat that can be used for water
or space heating. The cost per kilowatt-hour of producing this
heat is also calculated using
Equation 5. However, the amount of power generated (as heat
transferred) varies depending
on the process load demand. Thus the cost per kilowatt-hour of
producing this heat also
varies. For all levels of demand, the cost of producing the heat
is less than the cost of
purchasing heat from a conventional supplier. The cost of
purchasing from a supplier
includes the cost of purchasing natural gas as well as the cost
of purchasing two gas-fired hot
45
-
water heaters (each assumed to have a life of 10 years). At
maximum process load demand it
costs US$0.0301 per kilowatt-hour to produce the heat whereas it
costs US$0.0429 per
kilowatt-hour to purchase it.
6.4 Cost for both Electricity and Heat
On a per kilowatt basis, it is more cost effective to purchase
electricity from a utility service
than to produce electricity. However, it is more cost effective
to produce heat through the
system than to purchase the heat from a utility service. Thus,
the results of the preliminary
economic analysis seem to be contradictory. This is explained by
the way the above analysis
was conducted. The system is a cogeneration system, thus
producing both heat and
electricity. However, a consumer purchases heat and electricity
separately and at different
rates. Thus, an analysis that separates the heat and electricity
generated by the system does
not fully encompass the systems true economic nature.
To sufficiently determine the economic nature of the bio-oil
fueled system, the annual cost of
operating the system is compared to the annual cost of
purchasing an equivalent amount of
heat and electricity from a utility service. The annual
equivalent cost of the system is
US$313,014.94, as stated in section 6.1. The annual equivalent
cost of purchasing both
electricity and heat from a utility service is US$484,572.53 at
100 percent process load
demand. At 89 percent process load demand (minimum operating
point), the annual
equivalent cost of purchasing energy is US$435,546.93. These
numbers indicate the system
becomes much more economical when cogeneration is
implemented.
6.5 Economic Sensitivity Analysis
The following analysis attempts to determine the economic
sensitivity of the system to the
price of bio-oil and the assumed interest rate. Cogeneration
will not be taken into account in
the bio-oil cost analysis, but will be included in the interest
rate analysis.
46
-
Sensitivity to the Price of Bio-oil
Electricity is currently sold at 9.12 US cents per
kilowatt-hour. This price includes all the
extraneous costs included on an electricity bill (i.e. customer
charge, delivery charge, etc.).
Table 9 shows the cost per kilowatt-hour of utilizing only the
electricity generated at various
bio-oil costs. The analysis indicates that even if the bio-oil
was free, the cost of utilizing
only electricity using this system will still be greater than
purchasing electricity from the
electric