Energy Conversion from biomass PII · 2012-03-27 · engine exhaust gas and ambient air; 2. Rotary cascade dryer with integral burner, using engine exhaust gas, burner exhaust gas

Energy Production from Biomass

Conversion technologies

Different paths

• Virgin biomass production– Woody crops– Agricultural crops

• Waste derived biomass– Wood residues– Temperate crop wastes– Tropical crop wastes– Animal wastes– Municipal Solid Waste (MSW)– Commercial and industrial waste

Raccolta e trattamento

Bio‐energy conversion: process options

• Biomass can be converted into useful forms of energy using a number of different processes. 

• Factors that influence the choice of conversion process are: – the type and quantity of biomass feedstock; – the desired form of the energy, i.e. end‐use requirements;– Environmental standards; – economic conditions; – project specific factors

Bio‐energy conversion: process options

• Biomass can be converted into three main products:– two related to energy :

• power/heat generation• transportation fuels 

– as a chemical feedstock

From multiple biomass resources to a variety of fuels and energy products

H.L. Chum, R.P. OÕerendrFuel Processing Technology 71(2001)187–195

Conversion of Biomass Energy into useful energy

By the method how it is utilized. • Direct

–heat energy obtained by burning wood,agricultural waste or Dung cake etc. as in stove

• Indirect– first converted into a convenient or suitable fuelin the form of solid, liquid or gases

Major energy conversion technologies of biomass‐fuelled CHP systems. 

Primary technology Secondary technology

Combustion producing steam, hot water Steam engine; steam turbine; stirling engine; Organic Rankine Cycle (ORC)

Gasification producing gaseous fuels Internal combustion engine; micro‐turbine; gas turbine; fuel cell

Pyrolysis producing gaseous, liquid fuels Internal combustion engine

Biochemical/biological processes producing ethanol, biogas Internal combustion engine

Chemical/mechanical processes producingbiodiesel Internal combustion engine

Main processes, intermediate energy carriers and final energy products

Biofuels

Biomass Energy Technology

WoodAgricultural wasteOrganic waste

Thermo-chemical

Conversion process

Direct combustion

Biomass feedstock

Gasification

Pyrolysis

Methanol Production

HeatSteam Electricity

Producer Gas (Low or medium Btu)

Synthetic fuel oil, Charcoal

Methanol

TECHNOLOGIES

END 

USES

ENERGY 

or

PRODUCT

THERMO CHEMICAL CONVERSION PROCESS

Animal manure Agricultural

waste Landfill

Biochemical

Conversion process

Aerobic

Anaerobic

Methane gas

Ethanol

F U E L

P R O D U C E D

B I O M A S S

F E E D S T O C K

Sugar or starch crop

Wood waste Pulp sludge

T E C H N O L O G I E S

Biochemical Conversion processes

Palm Sunflower Coconut Ground nut Soy beans Pulp sludge Rapeseed Cotton seed

Chemical

Conversion process

Chemical Solvent

Biodiesel

F U E L

P R O D U C E D

B I O M A S S

F E E D S T O C K

T E C H N O L O G I E S

Mechanical Extraction

Chemical Conversion Processes

Biomass Preparation

• The acceptability of fuel depend on– Its performance as a fuel, which dependsupon its combustion characteristics

– Its ability to harvested, transported andstored economically

Biomass Preparation (Contd.)

Characteristics of agricultural residues:• High moisture content (Reduce combustionefficiency, producing ignition difficulties)

• Contamination of foreign particles (Stone, Dust etc.)

• Large in size (Difficult to facilitate automatichandling)

• Often have fluffy (Low bulk and low densities).

Biomass Preparation (Contd.)

Pretreatment to make it suitable to use.– Size reduction (Shredder, chipper, grinder)

– Densification (Briquettes)

– Drying (removal / reducing of moisture)

Drying Processes

• Free drying – Material as‐is– On site drying diretta in loco, previa riduzione in tronchetti: la U si può 

ridurre fino al 18‐25% dopo due stagioni• Free‐Drying – Transformed material

– Mainly part of SRF biomass material: M  may go from 50 to 30%.– Natural degradation goes in parallel

• Forced– Exposure to heat sources (hot air).  INTEGRATION !!!

Dryer

1. Rotary cascade dryer, using engine exhaust gas and ambient air;

2. Rotary cascade dryer with integral burner, using engine exhaust gas, burner exhaust gas and ambient air,

3. Deep‐bed band conveyor dryer, using warm air heated from the engine coolant system.

Performance is modelled using a straightforward mass–energy balance, where biomass inlet and target outlet moisture content are input values, along with biomass feed rate

Dryer evaluation

• From the conservation of the mass and energy around each element, a set of equations can be derived. 

• In the modeling, the operation of the rotary dryer is assumed to be in a steady state and thus all of the parameters and variables at a particular position within the dryer do not change with time.

The moisture content of the wood and the humidity of the air are represented by X and Y, the corresponding temperatures of the two streams are represented by Tw and Ta. The moisture evaporation rate in one element is represented by the symbol R. The heat transfer rate in the element is Q.λ  is the latent heat of water vaporization

Ref. Xu and Pang Drying Technology, 26: 1344–1350, 2008

Drying

Thermochemical conversion

Biomass thermal conversion processes

Combustion

Combustion

• Biomass for grate‐firing can be mainly grouped into waste products and dedicated energy crops . 

• Waste products include: – wood materials (e.g., saw dust, wood chips, wood logs, and bark), – crop residues (e.g., wheat straw, rice straw, corn husks);– and municipal and industrial wastes of plant origin (e.g., MSW,refuse‐

derived fuel (RDF), manure). • Dedicated energy crops are agricultural crops that are solely 

grown for use as biomass fuels:– short‐rotation woody crops like hard‐wood trees and herbaceous crops 

like switchgrass. • These crops have very fast growth rates and can therefore be 

used as a regular supply of fuel

Combustion

• The burning of biomass in air, used over a wide rangeof outputs to convert the chemical energy stored inbiomass into heat, mechanical power, or electricityusing various items of process equipment:– stoves;– furnaces;– boilers;– steam generators

• Possible use– Residential– Industrial

Combustion

• Combustion is the oxidation of the fuel for the production of heat at elevated  temperature around 800–1000 C. 

• It is possible to burn any type of biomass but in practice combustion is feasible only for biomass with a moisture content <50%,unless the biomass is pre‐dried. 

• High moisture content biomass is better suited to biological conversion processes.

• Grate firing is one of the main technologies that are currently used in biomass combustion for heat and power production.

• Grate‐fired boilers can fire a wide range of fuels of varying moisture content and show great potential in biomass combustion

Combustion

• Combustion of solids involves the simultaneous processes of heat and mass transport, progressive pyrolysis, gasification, ignition, and burning, with no intermediate steps and with an unsteady, sometimes turbulent, fluid flow. 

• Normally, combustion employs an excess of oxidizer to ensure maximum fuel conversion, but it can also occur under fuel‐rich conditions.

• The reaction of combustion can be described as 

• where e is the excess air ratio; rof =(a+b/4‐c/2) is the stoichiometric oxygen‐to‐fuel mole ratio. 

• From previous eq the concentrations of O2 and CO2 at the boiler

Furnace–boiler overall thermal energy balance 

The combustion of biomasses proceeds in different stages

The combustion of biomasses proceeds in different stages ctd.

Process of biomass combustion—principle

Fuel‐feeding system

Fuel feeding

fixed grates,  moving grates, 

A ‐ fuel feed, B ‐ .primary combustion chamber, C ‐secondary combustion chamber, D ‐ boiler, E ‐ fuel gas cleaner, F ‐ ash removal, G ‐ stack

Travelling  grates, 

Combustion: furnace

• On the grate different zones corresponding to differentcombustion regimes may be found (as a function of T and O2)

– Primary air is (PA) provide from the bottom most likely under sub‐stechiometric conditions– Secondary air is the added (SA)  to complete the gas released by the material on the grate– OverFire Air (OFA))  can be finally added to terminate the combustion process

Combustion: furnace

• Principal variable distribution is a fundamental input data for system design and control

– Modelli CFD (Computational Fluid Dynamics)

• I combustori possono avere problemi di corrosione ad alta T dovuta all’azione del Cloro (Cl)

– Si possono utilizzare additivi oppure limitare la T: è fondamentale a tale scopo la progettazione integrata con l’utilizzatore (i.e. microturbine a gas, ORC, etc)

Residential and small power production

Residential and small power production

Residential and small power production

Pellet burners

OBERNBERGER Ingwald, THEK Gerold, 2006: Recent developments concerning pellet combustion technologies – a review of Austrian developments. In: Proceedings of the 2nd World Conference on Pellets, May/June 2006, Jönköping, Seden, ISBN 91‐631‐8961‐5, pp. 31‐40, Swedish Bioenergy Association (Ed.), Stockholm, Sweden