biomass characterization and...

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Biomass Characterization and Gasification

January 24, 2013

CHEN 4470: Process Design Practice

Sushil Adhikari, Ph.D.Biosystems Engineering Department

Biomass Properties

• Physical Properties – Density, size, shape, area

• Chemical Properties – Heating value, proximate analysis, ultimate

analysis

• Biomass Constituents – Hemicellulose, cellulose and lignin

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Proximate Analysis

• Proximate Analysis (weight percentage)

– Moisture Content (wet basis/dry basis)–ASTM E871

– Ash Content—ASTM D1102

– Volatile Matters-- ASTM E872

– Fixed Carbon

• Ultimate Analysis (ASTM D 5373-02)

– Carbon (E 777)

– Hydrogen (E 777)

– Nitrogen (E 778)

– Oxygen

– Other elements-S, Cl..

• Carbon, hydrogen and nitrogen areconverted into carbon dioxide, water vapor,nitrogen, respectively for quantification.

• Usually, oxygen is calculated from thedifference (100-C-H-N).

Ultimate Analysis (contd.)

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Heating Value

• Heating value represents the heat releasedwhen the chemical compound isstoichiometrically combusted.

• Heating value is expressed in terms of higher(gross) heating value (HHV) or lower (net)heating value (LHV).

• While measuring HHV, the products ofcombustion are cooled to the initialtemperature of the compound. In LHV, thewater produced during combustion is notcondensed.

Table: Proximate, ultimate and heating value analyses (dry weight basis) of selected biomass feedstocks

Switchgrass Hybrid

poplar

Pine

woodchipsbSugar cane

bagasse

Wyoming Elkol

coal

Proximate Analysis

Fixed Carbon

Volatile Matter

Ash

14.34

76.69

8.97

12.49

84.81

2.70

18.01

81.71

0.28

11.95

85.61

2.44

51.4

44.4

4.2

Ultimate Analysis

Carbon

Hydrogen

Nitrogen

Oxygen‡

Sulfur

Chlorine

46.68

5.82

0.77

37.38

0.19

0.19

50.18

6.06

0.60

40.43

0.02

0.01

49.33

5.03

0.53

44.70

0.13

0.003

48.64

5.87

0.16

42.82

0.04

0.03

71.5

5.3

1.2

16.9

0.9

n/a

HHV, MJ/kg 18.06 19.02 19.40 18.99 29.50

‡calculated from difference. n/a= not available.

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Enthalpies of Formation

• Enthalpies of formation is quite useful forthermodynamic calculations such as Gibbsfree energy of minimization.

• The standard enthalpy of formation of aparticular biomass sample is equal to thesum of heats of formation of the products oncombustion minus the HHV. If you use the minus sign, then you

should use “-” for the HHV because of exothermicity. Otherwise, you can use “plus” sign withoutworrying any sign for the HHV.

• It is assumed that ash is inert.

• Standard enthalpies of formation at 298 K ofthe combustion products are as follows:

CO2 = -94.05; H2O =-68.37; NO2 =8.09; SO2

=70.95 in kcal/g-mol

Gasification:Biomass:

CH1.44O0.66

High Temperature(800-900oC)

Insufficient Oxidizing agent

(Air, O2, H2O and CO2)

Products:

H2

CO2

Syngas:

CO

CH4

Small solid or liquid fractions

Biomass Gasification

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Biomass Gasification

• Partial oxidation of biomass to produce a lowcalorific-value fuel called syngas or producer gas.

• Main components of the producer gas are CO, H2,CO2, CH4, N2, and H2O.

• Chemical transformation can take place in fixed,moving, or fluidized bed or entrained flow gasifiersat temperatures of 1400 to 1800°F with pressuresfrom 1 to 30 atmospheres.

Syngas Potential

Source: Jenny B. Tennant. NETL Overview of DOE’s Gasification Program

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Conversion of Syngas to Fuels

Power

Gasification Steps

1. Drying (>150 oC)

2. Pyrolysis or Devolatilization (150-700 oC)

3. Combustion (700-1500 oC)

4. Reduction (800-1100 oC)

Processes 1, 2, and 4 absorb heat whereasstep 3 releases heat.

Source: Prabir Basu, 2006. Combustion and Gasification in Fluidized Beds

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Drying

• Every kg of moisture in the biomass takesaway a minimum of 2260 kJ to vaporizewater (Basu, 2010).

• Typical moisture content of freshly rangesfrom 30 to 60% and for some biomass itcan exceed 90%.

• For the production of a fuel gas, mostgasification system use dry biomass with amoisture content of 10 to 20%.

Pyrolysis

• Complex physical and chemical processesoccur during the pyrolysis process.

• It starts slowly at 350 oC, accelerating to analmost instantaneous rate above 700 oC.

• During pyrolysis process, large compoundsare broken down and evaporate with othervolatile components.

Biomass + Heat Char + Gases+Vapors/liquid (tar or PAHs)

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Combustion

• Oxidation or combustion is one of themost important reactions in thegasification.

• All the thermal energy needed forendothermic reactions are provided duringthis step.

• Oxygen supplied to the gasifier reacts withcombustible products, resulting theformation of CO2 and H2O.

Gasification Chemistry

Biomass

Oxygen

Steam

Syngas

Source: Jenny B. Tennant. NETL Overview of DOE’s Gasification Program

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Reactions

• Combustion Reactions

• Boudouard Reaction

• Water-Gas Reaction

• Methanation Reaction

• CO shift Reaction (Water-Gas Shift Reaction)

• Methane Steam Reforming Reaction


Reactions (cont.)

• Combustion Reactions

C+1/2 O2 CO (H = -111 MJ/kmol)

CO+1/2 O2 CO2 ( H = -283 MJ/kmol)

H2 + ½ O2 H2O ( H = -242 MJ/kmol)

• Boudouard ReactionC+CO2 2CO ( H = +172 MJ/kmol)

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Reactions (contd.)

• Water-gas Reaction

C+H2O CO+H2 ( H = +131 MJ/kmol)

• Methanation Reaction

C+2H2 CH4 ( H = -75 MJ/kmol)

• Methane Steam Reforming Reaction

CH4+H2O CO + 3H2 ( H = +206 MJ/kmol)

Reactions (contd.)

• Water-gas Shift Reaction

CO+H2O CO2 + H2 ( H = -41 MJ/kmol)

• For real fuel, the overall reaction can bewritten as:

CnHmOp + ??O2 CO +CO2+H2+CH4+H2O+tar

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Heating Value of Syngas

The higher heating value of the syngas can be calculated by thevolumetric fraction and the higher heating values of gascomponents, which is given by

Types of Gasifier

Updraft Gasifier

Downdraft Gasifier

Crossdraft Gasifier

Source: Olofsson et al., 2005.

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Mobile BIOMAX

• Features• Field deployable.• Self contained and doesn’t

need grid connection.• 25 kWe generating capacity.• 50 lbs biomass consumed

per hour.

Mobile BIOMAX (contd.)

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Biomax control system 64 control points (temps, pressures,

flows, motors, engine, generator, etc.) 30 auto alarms with text messaging or

email. Auto remote start up and shut down. Full data logging – downloadable. Remote trouble diagnosis / software

upgrades. Manual on-site push button start-stop.

Fluidization Regimes

Source: Introduction to Fluidization Technology by Dr. Karl V. Jacob and Dr. Ray Cocco on April 13, 2011 at ChemE on Demand

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Types of Gasifier (cont.)

Bubbling Fluidized Bed Gasifier

Entrained Flow Gasifier


Fig. Auburn University’s bubbling fluidized bed gasifier and biomass feeder

Fig. Gas conditioning system

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Advantages/ Disadvantages

• Updraft Gasifier– Size, shape and moisture content of biomass

particles are less critical than with a downdraftgasifier.

– Design is simple and results in a fairly highheating value of the gas.

– The quality of the syngas is generally quite low.

– High temperature near the reactor grate cancause blocking due to ash fusion


• Downdraft Gasifier– Produced gas is generally of relatively good

quality and has low level of tars.

– Up to 99.9% of the formed tar is consumedminimizing tar cleanup.

– Syngas contains relatively high levels of CO2

since a large portion of the biomass isoxidised.

– Heating value is low.

– Size and shape and low moisture content ofbiomass particles must be controlled withinclose limits.

Advantages… (cont.)


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Advantages/ Disadvantages (cont.)

• Crossdraft Gasifier

– Design is simple.

– Quality of syngas is generally poor.

– Heating value of the syngas is low and the tarcontent is high.



• Bubbling Fluidized Bed Gasifier– Reactor allows high rates of throughput, higher

than fixed beds.– Results in good mixing, optimized kinetics,

particle/gas contact and heat transfer as well aslong residence time.

– High carbon conversion rates and, consequently,high yields.

– Sand bed makes it possible to use in-bed catalyticprocessing.

– Syngas is rich in particulates


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• Entrained Flow Gasifier– Almost tar free syngas

– Leach-resistant molten slag

– A high percentage of energy is converted intosensible heat.

– Production of biomass powder is an extracost.


Composition of Gas Yield

• Fuel Composition

• Gasifying Medium

• Operating Pressure

• Temperature

• Moisture Content of the Fuel

• Mode of Bringing the Reactants into Contact


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Gas Composition (cont.)

Component Composition, %

Nitrogen 50-54

CO 17-22

CO2 9-15

H2 12-20

CH4 2-3

Heating Value, kJ/m3

5000-5900

Source: Wood gas as engine fuel. FAO 1986. pp.19

Gas composition presentedhere is from downdraftgasifier operated at 20% MC.

Effect of Operating Parameters

• Temperature

• Pressure

• Feed Characteristics– Fuel Reactivity

– Volatile Matters

– Ash

– Moisture Content


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Volatile Matter

• Fuels with high volatile matter content areeasier to gasify.

• Also, char produced from gasificationprocess is more porous and easier togasify.

• Biomass has high volatile matters andproduces high tar content.

• High tar content makes gas clean-upprocess difficult.


Ash Content• Ash content does not have direct influence on

the gas composition.

• However, it affects the practical operation ofgasifier.

• Ash can be removed either in solid or liquidform.

• In fixed and fluidizing beds, ash is removed insolid form.

• If the ash is removed in the solid form,feedstocks should have high ash-melting/softening temperatures and the gasifiershould be operated at well below meltingtemperature.

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Ash Content (cont.)

• The relationship between ash meltingtemperature and composition is a complicated.

• It mainly depends on SiO2-Al2O3-Cao-FeO.

• High in silica and alumina will result high inash-melting temperature. But, the ratio ofsilica/alumina is also equally important.

• It is reduced by the presence of CaO and FeO.

• Ash-melting temperature of coal is more than1200 oC but biomass can have significantly lowerthan 950 oC.

Ash Content (cont.)

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Constituents Fraction

(N2 balance) Higher Heating Value

Syngas Composition from Different Feedstocks

0.0

5.0

10.0

15.0

20.0

25.0

O2 CO CO2 CH4 H2

%v

ol.

Peanut hullsSaw dustPoultry LitterWood chips

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Peanuthulls

Saw dust PoultryLitter

Woodchips

MJ

/m3

Gautam et al. (2009), ASABE Annual International Meeting. June 21-June 24, 2009, Reno, NV

Gasification Processes and Methanol Production

Process Condition Gasifier Type

Circulating fluidized bed

Bubbling fluidized bed

Entrained

Feedstock (wood), t/d 1650 1650 1650

Steam, t/t dry feed 0.31 0.3 0.03

Oxygen, t/t dry feed 0 0.3 0.5

Air, t/t dry feed 1.46 0 0

Gas. Temp., oC 927 982 1045

Gas. Press., psi 14.8 507 357

Exit gas (dry)

H2 (vol.%) 21.1 30.7 33.9

CO (vol.%) 46.8 22.2 50.7

CH4 (vol.%) 14.9 12.0 0.2

CO2 (vol.%) 11.3 35.2 14.9

H2/CO 0.45 1.38 0.66

Source: Klass (1998).

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Design Consideration

• Gasifier Efficiency– Cold gas efficiency – Hot gas efficiency

• Carbon Conversion• Equivalence Ratio

Cold gas efficiency = (Heating value of productgas/Heating value of feedstocks)x100 %

It is important to specify whether the heating values are onhigher heating value or lower heating value basis.

Design Consideration (contd.)

The gas is not cooled before combustion and thesensible heat is also useful. Therefore, sometimes,hot gas efficiency is also used for such applications.

Hot gas efficiency = (Heating value ofproduct gas + Hsensible /Heating valueof feedstocks) x 100 %

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Carbon conversion = {1 –Carbon in gasificationresidue/Carbon in feedstocks} x 100 %

or{Carbon in gas composition/Carbon in

feedstocks} x 100 %

• Care is required to interpret the data. Highermethane concentration could result in highercold efficiency and good for powerapplication but it is not the optimum choicefor a synthesis gas applications to producefuels and chemicals.


• Equivalence Ratio (ER):

= (A/F)actual/(A/F)stoichiometric

• The quality of syngas depends upon the value ofER.

• A low value of ER (<0.2) results in severalproblems including excessive char formation.• A high value of ER (>0.4) results in excessiveformation of CO2 and H2O.• Typical range of ER is ~0.2 -0.4.

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