Biomass for H&P_Gasification

8/13/2019 Biomass for H&P_Gasification

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JEE651 Biomass for Heat and PowerSemester 1/2006

Thermal conversion technologies-Gasification

Harmen, S.T., M.T.


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Thermo-chemical conversion


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Gasification

Gasification involves devolatilisation and conversion ofbiomass in an atmosphere of steam or air, carried out at

elevated temperatures (i.e. 500-1400C) and atmospheric or

elevated pressures

Gasification produces a medium or low calorific gas, called

Synthesis Gas


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Synthesis gas consists primarily of hydrogen (H2) and

carbon monoxide (CO), with lesser amounts of carbondioxide (CO2), water (H2O), methane (CH4), higher

hydrocarbons (C2+), and nitrogen (N2)

Different oxidant used produces gases with different heating

value:

Air-based gasifiers: 4 and 6 MJ/m3(107-161 Btu/ft3)

O2 and steam-based gasifiers: 10 and 20 MJ/m3

(268-537 Btu/ft3)

Producer gas contains 70-80% of the energy originally

present in the biomass feedstock


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This gas can then be used for a number of applications:

Used as cooking gas Burned in a diesel engine

Fuelled in a combined cycle power generation cycle

involving a gas turbine topping cycle and a steam turbine

bottoming cycle

Raw materials for chemical syntheses: methanol, DME,diesel


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Syngas-to-Liquids Processes

SyngasCO + H2

Methanol

H2OWGSPurify

H2N2over Fe/FeO

(K2O, Al2O3, CaO)

NH3

Cu/ZnOIsosynthesis

ThO2or ZrO2

i-C4

Alkali-doped

ZnO/C

r2

O3

Cu/Zn

O;Cu/ZnO

/Al2O

3

CuO

/CoO/A

l2O

3

M

oS2

MixedAlcohols

Oxosynthesis

HCo(C

O)4

HCo(C

O)3P(B

u3)

Rh(C

O)(P

Ph3)

3

AldehydesAlcohols

Fischer-Tropsch

Fe, C

o,Ru

WaxesDiesel

OlefinsGasoline

Ethanol

Co

,Rh

Formaldehyde

Ag

DME

Al2

O3

zeolites

MTOMTG

OlefinsGasoline

MTBEAcetic Acid

carbon

ylatio

n

CH3

OH

+CO

Co,R

h,Ni

M100M85DMFC

Dir

ectUse

homolo

gatio

n

Co

isobutylene

acidic

ionexchange

Graphics courtesy of Richard Bain, NREL


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Principal reactions occurring during gasification stepExothermic Reactions(1) Combustion Volatiles/char + O2 CO2(2) Partial Oxidation Volatiles/char + O2 CO(3) Methanation Volatiles/char + H2 CH4(4) Water-Gas Shift CO + H2O CO2 + H2(5) CO Methanation CO + 3H2 CH4 + H2O

Endothermic Reactions

(6) Steam-Carbon reaction Volatiles/char + H2O CO + H2(7) Boudouard reaction Volatiles/char + CO2 2CO

The chemistry of gasification


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Directly heated gasification

Pyrolysis and gasification occur in single vessel

Heat produced from reaction 1&2 (about 15%) is used forall endothermic reactions (including pyrolysis)

Heating value of produced gas is lower

Indirectly heated gasification

Need two chambers: one acts as gasifier, the other aschar combustor

This approach separates reaction 1 from other gasificationreactions and reaction 2 is suppressed

Heating value of produced gas is higher


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Indirectly-heated fast fluid-bed and Indirectly-heated bubblingfluid-bed


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TypicalGas Composition Product gas composition of wood and charcoalof low to

medium moisture content (wood 20 %, charcoal 7%) operatedin co-current gasifiers

Component Wood gas (vol%) Charcoal gas(vol%)Nitrogen 50-54 55-65Carbon monoxide 17-22 28-32Carbon dioxide 9-15 1-3Hydrogen 12-20 4-10Methane 2-3 0-2Heating value (MJ/Nm3) 5-5.9 4.5-5.6


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Mechanism of tar formation- When biomass is heated molecular bonds break

produce small molecules (gas) and larger molecules (knownas primary tar)

- These primary tars, which are always fragments of theoriginal materials, can react to secondary tars by furtherreactions at the same temperature and to tertiary tars at high

temperature

- Tar formation pathway can be visualised as follows:Mixed oxygenates Phenolic ethers Alkyl phenolics

(400 C) (500C) (600 C)

Heterocyclic ethers PAH Larger PAH(700C) (800 C) (900C)


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- Example of tar formed


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Biomass gasification for the heat application has generally

been successful, but much less success has been realizedfor power applications, where gas quality is of prime

importance.

The amount of compounds that occur in biomass

gasification tars can be as high as several hundreds oreven several thousands for low temperature tars. The

amount and composition depend on:

- Type and properties of the biomass (moisture,

particle size)

- Gasification conditions (P, T, residence time)- Type of gasifier (reactor configuration)


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Fixed bed gasifiers: updraft and downdraft

Biomass Gasifiers


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A typical design for a fixed-bed gasification plant for the

generation of electricity will include Biomass receiving and storage

Drying

Gasification

Particulate removal

Generator system

Drying Carried out in a rotary flue gas dryer

Hot flue gases from the engine exhaust will be used toevaporate water from the biomass feed.

The final moisture content of biomass fed to the gasifiershould be


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In gasifier, the product gases flow through the hot part of the

bed and heavy tars produced in pyrolysis crack to form morecombustible gas components.

For gas cleaning, fuel gas is led through a cyclone to the airpre-heater, where gasification air is heated to 300C cooled to approximately 40C, and part of water vapourpresent in gas will condense finally filtered through afabric filter to remove the remaining solid particulates.

In dual fuel (engine) operation carried out by VTT Energyin 1995, approximately 15% of the energy fed into the diesel

engine is supplied with diesel oil while the rest of therequirement was provided by fuel gas from a fixed-bedgasifier.


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Fluidized-bed gasifier: Bubbling and Circulating


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Atmospheric-pressure fluidised bed gasification is

commercially proven technology for coal, peat and woodwastes.

A fluidised-bed gasification system for power generation

consists of the following major units:

biomass receiving and storage

milling and drying gasification and tar cracking

water wash of the raw gas

generator system


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Biomass CFB GasifierDemonstration Project(K Y M I J R V I P O W E R S TAT I O N, L A H T I , F I N L A N D)


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Gasifier types: Advantages & Disadvantages


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Integrated gasification combine cycle(IGCC)

This system uses simple Brayton gas cycle (generally

associated with the gas turbine) and is essentially acombined cycle


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The biomass-based IGCC electric generating plants normally

consist of the following process sections: Fuel receiving, sizing, preparation, and drying

Gasification and gas cleaning (Gasification Island): Wood

feeding unit, Gasifier, Char combustion and air heating,

Primary cyclone, Tar cracker, Gas quench, Particulate

removal Power Island

Gas turbine and generator

Heat Recovery Steam Generator (HRSG)

Steam turbine and generator

Condenser, cooling tower, feed water and blowdown

treating unit

General plant utilities and facilities


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The products of gasification are burnt in a combustor to drive agas turbine while the heat in the exhaust flue gases is recoveredin an HRSG that in turn drives a steam turbine

This results to high overall efficiencies with the prospect of evenhigher efficiencies if high temperature turbines and hot gascleanup systems were developed


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1000 MW Fuel capacity1 MW 10 MW 100 MW100 kW1 kW

Pressurised Fluidised Bed

Circulating Fluidised Bed

Downdraft

Updraft

Bubbling Fluidised Bed

Preferred Gasification Technologies at Different Scales


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100 MWe

IGCC

5 10 15 20 25

Dual fuel diesel engine + steam cycle

Dual fuel diesel engine

Gas engine

95

Fuel cell (not yet commercial)

(more robust than gas engine)

Preferred Gasification-Based Electricity Technologies vs. Scale


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Contaminant Potential problem Treatments

Particulates Erosion Cyclones, filters

Alkali vapors Corrosion Cool/condense/remove

NH3, HCN Emissions Capture, scrubbing

H2S, HCl Corrosion, emissions Capture, scrubbing

Tars and oils Deposition, equipment

clogging; waste watertreatment

Cracking, scrubbing,

filtering, combusting(including catalytic tar

converter

Product gas cleanup


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Product gas cleanup requirement for different applications

Extent of Gas Cleanup Required

Little Modest Higher Highest

Direct burning

IC engine

Gas turbine

Fuel cells

Fuel synthesis


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Characteristics:

- China produces > 700 million t/year of agriculturalresidues, most of which is used for cooking and heating bydirect combustion in rural areas

- Several hundred small biomass gasifiers are currentlyoperating in China to provide cooking gas in rural villages,which helps reduce terrible indoor air pollution problems

- But cooking and heating demands alone are too smalland unsteady to result in good economics. Adding powergeneration allows increased scale and greater capacityutilizationmore favorable economics.

China Small-Scale Biopower Case Study


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If used in combined heat and power production, half of total

residues produced in China could provide clean cooking gasfor 230 million people (27% of rural population) and generate

270 TWh of electricity

Case study project: Hechengli Village, Jilin Province

- A 200 kWe Gasifier/Engine + Cooking Fuel

- Construction and commissioning completed in Aug

2004, but due to institutional problems - no commercialoperation yet

Clean Gas


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200 kWe

Gasifier/Engine + Cooking Fuel in Hechengli Village

EngineGenerator

Gas

Clean Up

Gasifier

BiomassStorage

GasStorage

Households& Factories

Gas Distribution for

Cooking, Heating &

Process heat

Electricity to

Households & Factories Electricityto Grid

Engine

Exhaust

Auxiliary

Power for

CHP plant

Gross

Output

Utility Grid

Blower

Diesel engines preferred over spark-ignition: more efficient, durable, reliable,

simpler maintenance. But requires dual fueling: typically 70% diesel replacement.

Clean Gas

CO 20

H2 15

CH4 2

CO2 10

N2 53

Btu/ft3

(hhv)133


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Hechengli Plant Design

Planned

1800 Nm3/hr gasifier capacity

200 kW engine-generator

224 cooking and heating

customers

500 Nm3gas storage

60 yuan/ton biomass cost

0.2 yuan/Nm3gas price

0.5 yuan/kWh electricity price

Actual

1800 Nm3/hr gasifier capacity

200 kW engine-generator

125 cooking customers with 0

heating customers

300 Nm3gas storage

90 yuan/ton biomass cost

0.2 yuan/Nm3gas price

0.58 yuan/kWh electricity price

* Producer gas cooking-only projects in Jilin have not beeneconomically viable without subsidy Electricity generation isessential for commercial viability


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Impact of Capacity Factor on Cost

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Capacity Factor

PowerGeneratingCost(1

997UScents/kWh)

Total Lifecycle Capital

Investment, 1997 US$/kW

1200

800

Calculated Power Generating Costfor 100 kWe BiG/ICE System

as Function of Capacity FactorBiomass @ $1/GJ; diesel @ $0.26/liter;25-year investment; 12% internal rate of retun

Source: E.D. Larson, Small-scale gasification-based biomass power generation,Proceedings of the Workshop on

Small-Scale Biomass Power Generation, Changchun, Jilin Province, China, 12-13 January 1998.


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Issues of tar: Small Gasifier-ICEs

Tar content must be < 30-50 mg/Nm3in gas to IC engine

Wood chip gasifiers:

Ankur gasifier (Imbert type) cracks tar with high

temperatures at throat and catalytic action of charcoal in

reduction zone to give ~5 mg/Nm3at gasifier exit withsubsequent water scrubbing and filtration aimed primarily at

particulate removal

IISc gasifier (stratified, open-top type) cracks tars thermally

with long residence times to give ~100 mg/Nm3tar in raw gas.

Tar levels are further reduced to 10-30 mg/Nm3with waterscrubber and sand-bed filter.


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Commercialization

Economic viability requires reasonably high capacity factors. Unit capital costs must be reduced to insure competitiveness

(especially for smaller systems):

Standardize system design to lower cost of manufacture,installation, servicing, etc.

Aggregate the market to lower transaction, maintenance,and others costs for suppliers.

R&D and commercialization of advanced technologiesthat reduce costs (e.g., microturbine), especially asbiomass costs rise.

Two major activity areas with particular relevance forThailand:

Small-scale systems for crop residue utilization, e.g. inChina

Larger-scale systems for sugarcane residue utilization,e.g. in Brazil


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Fischer-Tropsch Fuels

(straight-chain CnH2n , CnH2n+2)

F-T fuels of interest include high-cetane,low-aromatic, no-sulfur diesel substitute andnaphtha as chemical feedstock upgradableto gasoline blendstock.

F-T fuels production is commerciallyestablished, and growing rapidly.

From coal:

Since 1950s in South Africa, 175kbbl/day (bpd) total capacity

20k bpd, Inner Mongolia (2007)

120k bpd, China letter of intent signed

5k bpd demo, Gilberton, Pa (2008)

33k bpd, Wyoming (in planning)

57k bpd, Wyoming (proposed)

From stranded natural gas:

From 1990s in Malaysia: 13k bpd

Planned:

Qatar, 2005: 34k bpd

Nigeria, 2006: 34k bpd



Dimethyl Ether

(CH3OCH3)

Ozone-safe aerosol propellant, chemicalfeedstock.

Current global production ~150,000tons/year by drying methanol (CH3OH).

Similar to LPGmild pressure needed tokeep as liquid.

Good diesel-engine fuel: high cetane #, no

sulfur, lower NOx, near-zero soot. Rapidly expanding production worldwide to

supply (initially) markets for cooking andheating fuel (LPG substitute).

110,000 tpy (from NG) facility to startin China, 2005

800,000 tpy (from NG) facility to start

in Iran, 2006 At least two 800,000 tpy (from coal)facilities in planning in China.

Sweden bio-DME activities at Varnamogasification pilot-plant facilityaiming atheavy-vehicle applications.

Two synthetic liquid fuels of interest


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Catalytic synthesis of fuels from CO+H2

Three reactor designs: Fixed-bed (gas phase): low one-pass

conversion, difficult heat removal

Fluidized-bed (gas phase): betterconversion, more complex operation

Slurry-bed (liquid phase): much higher

single-pass conversion (e.g., 80% vs.40% for F-T)Once-through designsfavored when electricity can be sold

Liquid phase FT reactors are commercial

LP-MeOH commercially demonstrated

LP-DME near commercial

Focus here on OT process designs with

LP synthesis.

Basic overall reactions:

Methanol

Dimethyl ether

Fischer-Tropsch liquids

322 OHCHHCO +

233233 COOCHCHHCO ++

222 H O- C2HCO ++ H -

TYPICAL CONDITIONSP = 50-100 atm.T = 200-300oC

Liquid Phase Reactor

Synthesis gas

(CO + H2)

Cooling water

SteamCatalyst

powder

slurried

in oil

Disengagement

zone

Fuel product (vapor)

+ unreacted syngas

Synthesis gas

(CO + H2)

Cooling water

SteamCatalyst

powder

slurried

in oil

Disengagement

zone

Fuel product (vapor)

+ unreacted syngas

catalyst

CO

H2

CH3OCH3CH3OH

CnH2n+2

(depending

on catalyst)

catalystCO

H2

CH3OCH3CH3OH

CnH2n+2

(depending

on catalyst)


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Summary of Key Characteristics

Gasification

Only fully commercial for direct gas combustion

applications

Gasifiers need uniform size and composition of feed with

low moisture

Gasifiers are relatively simple technologies, but

considerable investment is needed in gas cleanup for high-

value uses of the gas

Clean gas can meet stringent air emissions limits

Comparatively favorable economics at smaller scales (e.g.,< 10 MWe)


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Advantages of gasification(compared to conventional combustion technologies)

The combined heat and power generation via biomassgasification techniques connected to gas-fired engines or gasturbines can achieve significantly higher electrical efficienciesbetween 22 % and 37 % compared to biomass combustiontechnologies with steam generation and steam turbine (15 % to18 %).

Due to the improved electrical efficiency of the energy conversionvia gasification,

1. the potential reduction in CO2is greater than with combustion.

2. The formation of NOx compounds can also be largely

prevented, although the NOx advantage may be partly lost ifthe gas is subsequently used in gas-fired engines or gasturbines.

3. Significantly lower emissions of NOx, CO and hydrocarbonscan be expected when the produced gas is used in fuel cells.

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Biomass for H&P_Gasification