Escape 24 biomass power plant with integrated drying-effective utilization of waste heat

Biomass Power Plant with Integrated Drying: Effective Utilization of Waste HeatTesfaldet Gebreegziabher, A. O. Oyedun, Y. Zhang , M. J. Wang , Y. Zhu , J. Liu , C. W. Hui

June 16-2014

Contents1. Biomass fired power plants2. Energy efficiency improvement options3. Drying of biomass 4. Process description of base case5. Dryer Integration with power plant

I. Integration study with hot air dryer(HAD)II. Integration study with Hot air dryer (HAD) and

steam dryer(SSD)III. Integration study with flue gas dryer (FGD)

6. Results7. Conclusions

1.Biomass fired power plants

Use direct combustion to convert stored biomass energy to heat and power with the help of a steam cycle.

Have low efficiency than the conventional coal power plants. Low efficiency is due to relatively high moisture content, low

heating value of feedstock and flue gas heat loss (Stack losses).

Figure 1 Schematic drawing of biomass fired power plant

1.1 Moisture content

Much of the heating value is used to evaporate the water and results in lower efficiency.

Using dry fuel in combustion systems improves efficiency, increases steam production, reduces fuel use, lowers emissions and improves boiler operation.

Although drying biomass prior to combustion process have an impact on efficiency, it is an energy and capital intensive process.

Using waste heat from other sources can make drying process more economical.

1.2 Stack losses In power plants , air and fuel are mixed and burned to

generate heat, some of which is transferred to boiler to produce steam.

When the heat transfer reaches its practical limit, the combustion gases are usually removed from the boiler through a stack.

At this exit condition, in reference to the ambient conditions, the exhaust gases still hold significant heat energy and leads to reduction in system efficiency.

Recovering part of heat from this flue gases can lead to an increase in steam cycle efficiency.

2. Energy efficiency improvement options

The energy efficiency of a power plant can be improved either by using dry feedstock in combustion process ; or

by using waste heat recovery system to capture and use some of the energy in the flue gas;

Figure 2 Power plant with heat recovery options

3. Drying of biomass In our study drying of biomass was considered as efficiency

improvement option via LHV enhancement. Biomass considered is Empty fruit bunches(EFB).

Figure 3 LHV of EFB as function of moisture content

3.1 Methodology A base case and three drying options for generating 12.5MW

power from empty fruit bunches containing 70% moisture content are considered.

Hot air dryer (HAD) Hot air dryer (HAD) and superheated steam dryer (SSD) Flue gas dryer (FGD).

Mathematical models of the steam power plant and the drying processes are developed.

Water97_v13.xla is used for calculating stream properties of the units.

Pinch analysis is used to show the effectiveness of the different integration options.

4. Base case Mathematical modeling

Mass Balance

Energy balance

Optimize the over all efficiency defined by:

Figure 4 Process flow diagram of base case

By simultaneously changing the flow rates, pressures and temperatures of steam and boiler feed water (BFW) in the power plant and the temperature, flow rate and drying level of the biomass in the drying process.

In the base case MP and LP steam pressures for BFW preheat are selected arbitrarily.

Feed moisture content

4.1 Base case constraints

Process constraints applied in the base case

ηturb=80%, ηboiler=90% and ηpump=100%

P12=100bar, P14=18bar, P1=3bar, P2=0.1bar, P16=3bar, P13=18bar, P5=3bar,

P7=17bar, P9=100 bar

Moisture content of EFB, X(%)=70%

LHVEFB at 0wt% moisture = 15820 kj/kg

= =100kg/h

T6=330K ,T8=400K,T12=900K

Pout=12.5MW

In the base case MP and LP steam pressures are selected arbitrarily for preheating Boiler feed water(BFW).

4.2 Base case solution

Figure 5 Base case composite curves

Excel 2010 Standard GRG Non-linear Solver was used to solve the optimization problem.

While solving the optimization problem, the stream properties of the units are determined automatically by Water97.

The over all efficiency is calculated to be 20.08% with 45,589 kg/h EFB feed requirement.

Stream data are extracted and composite curves are plotted as in Figure 5.

4.3 Optimum BFW Preheat temperature and pressure of base case In this study LP and MP steam are allowed to vary in the following

ranges: (2 bar ≤ LP ≤ 5 bar and 15 bar ≤ MP ≤ 40 bar) The cycle efficiency was improved to 20.80% with 42,934 kg/h EFB

feed requirement. LP = 2.4 bar and MP = 19.8 bar (3 bar and 18 bar-base case). T of Stream 6 and 8 are 411K and 491K respectively. (330 and

400K-base case)

Figure 5 Base case composite curves

5. Dryer Integration with power plant

During thermal drying, EFB is dehydrated through heat transfer with air, steam or Flue gas .

The optimum temperature and pressure values of LP and MP ,and the optimum temperature of BFW are used in all the integration study.

Models of both HAD and SSD are based on gross Pout prediction with out considering combustion reaction of EFB. (Only LHV value is used for mass and energy balance).

Mathematical model of flue gas based dryer is based on combustion analysis of EFB.

5.1 HAD Integrated with power plant

Parameters ValueMoisture content of the EFB feed, X1 70%Temperature of the EFB feed, (K) 298Pressure of the air feed, P 101325PaTemperature of the air feed, (K) 298Relative Humidity of the air feed, RH1 (%) 50Specific Heat of Air, Cpda( kJ/kg-C) 1.006Specific Heat of EFB, Cpds, (kJ/kg) 1.2566Specific Heat of Water Vapor, Cpv , (kJ/kg-C 1.89Specific Heat of Water, Cpw, (kJ/kg-C) 4.186Latent Heat of Water, LHw, (kJ/kg-C) 2270

Maximum relative humidity of air (%) 95Minimum LHV of the dried EFB (kj/kg) 15,000

Modeling Air heater- Psychometric relations. HAD- Mass and energy balance.

Figure 6 HAD integrated plant

5.1.1 HAD results

LPMP

Air heaterLP

MP

The cycle efficiency is 27.36% with 34,322 kg/h EFB requirement.

Fig7: HAD mass balance

Fig 8: HAD composite curves

5.2 Multi stage dryer

Figure 9 Multi stage dryer

LP/

MP and LP steam can be extracted for Air preheat and SSD. As a huge temperature difference is observed between MP

and SSD ,LP extraction is considered in this study.

5.2.1 Detailed Multi stage dryer

Figure 10 Multi stage dryer integrated plant

Modeling Air heater- Psychometric relations. HAD- Mass and energy balance. SSD-Mass and energy balance.

5.2.3 HAD-SSD results

Figure 11 mass and Energy balance Multi stage dryer

5.2.4 HAD -SSD composite curve

LP(SSD)LP(PP)

MP

SSDAir heater

Figure 12 Composite curve of HAD-SSD

The cycle efficiency is 29.92% with 31,576 kg/h EFB requirement.

5.3 FGD integrated power plant Unlike the HAD and SSD integration cases EFB combustion process

is assumed to predict the possible amount of heat recovery from flue gases for drying biomass.

Q recoverable calculation

FGD

Flue gas

Exhaust gas

EFB Moist

Flame Temperature

FG Temperature

Exhaust Temperature

mev calculation

Figure 13 Calculations involved in FGD

TFGDE ≥ Tdw

TFGDE ≥ Tda

5.3.1 EFB properties for combustion

Ultimate Analysis wt% Molecular weight (Kg/kmol)

Wt/kg fuel

Carbon 45.53 12 0.4553 Hydrogen 5.46 1 0.0546 Nitrogen 0.45 14 0.0045 Sulphur 0.04 32 0.0004 Oxygen 43.4 16 0.434 Ash 5.12 0.0512 Total 100 1

The modeling of the combustion process is based on the ultimate analysis of EFB.

5.3.2 FGD integrated power plant

Figure 14 FGD integrated plant

Process constraints

ηturb=80%, ηpump=100%

P12=100bar, P2=0.1bar, P9=100

bar,P5=P6=3,P7=P8=15

Moisture content of EFB, X(%)=70%

LHVEFB at 0wt% moisture = 15820 kJ/kg

= =100kg/hr

T12=900K

Pout=12.5MW

TFGOB= 10K+ T11

T3= Tsat2-5K

T18=298K

5 bar < P13<40 bar

2 bar <P16<15 bar

hev =2260kj/kg

5.3.3 FGD composite curves

LP steam MP steam

LP steam MP steam

FG

Figure 15 FGD integrated plant composite curves

The calculated cycle efficiency is 24.76% with 38297 kg/h EFB containing 57.4 %moisture.

About 13% moisture removal.

6. Summary of results

System EFB F Requirementkg/hr

Air for dryingkg/hr

Final moisture(%)

Efficiency%

Base Case 45,589 - 70 20.08Base Case preheat and optimum P 42,934 - 70 22.80Integration with FGD 38297 - 57.4 24.76Integration with HAD 34322 958544 4.5 27.36Integration with HAD and SSD using LP 31576 355110 4.5 29.92

The analysis shows that integration of drying to power generation from EFB increases the overall energy efficiency.

3 multistage dryers is not considered yet.

7. Conclusion HAD and SSD dryers in multistage manner operating by

extraction of LP steam have better efficiency. In multistage drying the steam generated at SSD is a LP

steam and can be used for air preheating and this can save LP steam of power plant.

Composite curves of the overall plant are plotted for each case to indicate the effectiveness of heat integration and to provide insights for further improvement.

Water 97 is important tool and can be used for modeling and simulation of chemical process involving steam cycles in EXCEL successfully.

Future work will be analysis of multistage dryers combining air, steam and flue gas as drying medium.

Thank you