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BIOMASS GASIFICATION SYSTEM

Thermo-chemical principles: Effect of pressure, temperature and introducing steam and oxygen. - Design of Fixed and Fluidized Bed Gasifiers and their operation - Safety aspects.

CASE STUDY: BIOMASS GASIFIER (MNES PROJECT): TUMKUR DISTRICT,

KARNATAKA. STAND ALONE GASIFIER: POWER 60 TO 100 KW FOR 15

VILLAGES.

REF: BUISNESS STANDARD: 25, FEB. 2000

NAME OF PRODUCT GAS IS ‘PRODUCER GAS’

GASIFIER, GAS CLEANING TRAIN, DUAL FUEL INTERNAL COMBUSTION

ENGINE OR GAS TURBINE

CASE STUDY: PERFORMANCE OF SMALL GASIFIER DUAL FUEL ENGINE

SYSTEM Ref: Biomass 19, (1989), 75-97

Till July 2000, 1704 biomass gasifiers with an aggregate capacity of 34.36

MW have been installed. Forty biomass combustion based power projects

are aggregating 222 MW

ENGINE—ALTERNATER COMBINATION FOR ELECTRIC POWER

ENGINE—PUMP COMBINATION FOR WATER

******

Gasification Technologies, By John Rezaiyan and Nicholas P.

Cheremisinoff, Taylor and Francis, Florida, USA. 2005

[665.7P5, 100419] Jan, 2006, NITT Library.

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THERMOCHEMICAL GASIFICATION OF BIOMASS

TO GENERATE PRODUCER GAS

AS MEDIUM CALOIFIC VALUE FUEL

Desirable characteristics of biomass for gasification

1. Average moisture content of the biomass should be well within 50%.

2. Average sizes of the feedstock should be between 1.25 and 7.62 cm.

3. Average higher heating value should not be less than 10 MJ/kg.

4. Ignition temperature of the feedstock should be low. 5. Chemical composition of the feedstock should be

uniform. 6. Ash content should be less than 10%. 7. Fusion temperature of the ash should be over 1150

o C.

8. It should be relatively simple and economical to collect, store and handle the biomass.

9. It should be constantly and adequately available to meet the gasifier load demand.

Some parameters to be considered for proper gasification process:

1. FUEL FOR

GASIFICATION

2. GASIFIER/

DEVICE

3. PRODUCT

GAS

4. ACCESSORY

FOR CLEANING

5. END USE OF

PRODUCER GAS

a) Collection,

handling,

storage,

upgrading

systems; their

costs

a) Type, Size,

Cost, Reliability

a) Gas chemical

composition,

content of tar,

volatile matter,

moisture,

particulates

a) System,

b) Efficiency,

a)Design of direct

combustion burner or

I.C. engine for dual

fuel operation

b) Composition:

moisture, ash

content, heating

value

b) Environmental

impact, Pollution

control

b) Calorific

value of the gas.

c) Costs b) Efficiency, costs,

pollution control,

reliability.

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Biomass Gasification

Overview:

When biomass is gasified by thermo-chemical reactions the product gas is called as

producer gas or low BTU gas. The objective of Biomass Gasification is to recover the

stored chemical energy of the solid in a product that is a gaseous fuel that can be broadly

usable in variety of applications. Agro residues are the commonly used feedstock and

Gasification can be called a renewable energy technology.

Design Program for Biomass Gasification Power Generation

To make basic design calculations to determine operating condition and

equipment specifications, we have to make chemical equilibrium calculation,

stoichiometric calculation, enthalpy calculation and chemical engineering calculation.

Design and studies can be made for:

(1) Wood Gas Cart

(2) Comparison of Wood vs. Charcoal

(3) Hardwood Power Generation

According to a design manual published by Forestry Department of FAO (Food

Agricultural Organization) of UN, there are four types of reactor design.

(1) Up draught or counter current gasifier

(2) Cross-draught gasifier

(3) Downdraught or co-current gasifier

(4) Fluidized bed gasifier

Downdraught or co-current gasifier is most suited for engine. There are 3 different types

among Downdraught or co-current gasifier.

Following table shows appropriate "Hearth Load" for each type.

Wood based Hearth Load Gas based Hearth Load

kg/cm2/h Nm3/cm2/h

No Throat 0.03 0.07

Single Throat 0.11 0.25

Double Throat 0.4 0.9

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Further recommendations by the manual by FAO are:

-nozzle air inlet velocities should be around 30-35m/sec

-throat inclination should be around 45-60deg.

-height of the reduction zone should be more than 20cm.

-height of the air inlet nozzle plane should be 10cm above narrowest constriction of the

height determined by following equation.

(nozzle height)/(throat dia.)=19.6*(throat dia,)^-0.64

-hearth diameter at inlet height should be 10cm larger than throat diameter in case of

single throat and about 20 cm larger than the throat diameter of the narrowest constriction

in case of double throat design.

1. Wood Characteristics

Carbon, Hydrogen, oxygen ratio of the dry wood was calculate from that of Cellulose

(C6H10O5)n and Lignin (C18H24O11)n. It was assumed that 70% is Cellulose and

remaining 30% is Lignin. Chemical formula for wood was taken from a reference. Water

and ash content were assumed 30 wt% and 3 wt% respectively.

2. Equipment Specifications

Design of gasifier will be made to meet available engine size.

Wood has to be charged batch wise into the container. Ashes and condensate will also be

discharged batch wise.

When water content of biomass is 30wt%, gasification air is needed. When size of reactor

is small scale, air and biomass had to be preheated.

As FAO manual shows two different figures of Hearth Load, intermediate figures of both

was taken as the basis of throat diameter.

Air nozzle controls a gasification air.

Air cooler shall be installed horizontally to ensure good natural air circulation around the

coil.

Water condenses at low load. Sufficient condensate accumulation capacity will be

provided in filter housing.

The basic flow scheme was established as follows:

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Chemical Equilibrium Calculation

There are many possible chemical reactions for the wood gassification. Chemical

equilibrium constant were calculated for following 6 equations. Eq. No.3 & 4 is called as

Water Gas Reaction. Eq. No.5 is called Methane Formation Reaction. Eq. No.6 is called

as Shift Reaction.

Chemical Equations H0 S0 Equation No.

C + O2 = CO2 -94,200 2.06 1

2C + O2 = 2CO -53,300 45.54 2

C + H2O = CO + H2 +31,230 33.41 3

C + 2H2O = CO2 + H2 +21,560 -7.89 4

CO + 3H2 = CH4 + H2O -49,300 6.51 5

CO+H2O = CO2+H2 -9,670 -10.07 6

Heat of reaction H0 was calculated from Standard Heat of Formation -H0 as listed

below. The figures were taken from Reference 4. When Standard Entropy is not

available, it was back calculated from Standard Gibbs Free Energy G0 . Eq. No.1, 2, 4

and 5 having minus H0 figures are exothermic reactions. Eq. No.3 and 4 are endothermic

reactions.

-H0 (kcal/kgmol) S0 (kcal/kgmol/K) -G0(kcal/kgmol)

O2 0 49.02 -

N2 0 45.79 -

CO2 94,200 51.08 -

CO 26,650 47.28 -

H2 0 31.23 -

H2O 57,880 45.1 -

CH4 18,070 - -12,440

Chemical equilibrium constant Kp could be calculated from Standard Heat of Reaction

H0 and Standard Entropy S0 using following Thermodynamic equations. Gas constant

R=1.987(kcal/kgmol K)

G0 = H0 - TS0

G0 = -RTlnKp

Kp = Pprod1*Pprod2/Pfeed

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When you plot G0 v.s.T, following diagram will be obtained. At 700 deg. C, Gibbs Free

Energy of Eq. No. 1 and 2 crosses. Same crossing is seen on Eq. No.3 and 6. This is

equivalent to following new equation. Over 700 deg. C, the quantity of CO exceed CO2.

C + CO2 = 2CO

6. Stoichiometric Calculation

Calculation were conducted for 400, 600, 800 and 1000 deg. C. Dry wood of 8kg/h and

oxygen in Air, A (kgmol/h) will be converted to a mixture of XO2, XCO2, XCO, XH2, XH2O,

XCH4 kgmol/h of Gas. Number of unknown variables are 6. We have 3 equations for C, O,

and H element balance as listed below.

for C element C = XCO2 + XCO + XCH4 Eq. No. 7

for O element 2A + O = 2XO2 + 2XCO2+ XCO + XH2O Eq. No. 8

for H element H = 2XH2O + 2XH2 + 4XCH4 Eq. No. 9

If we add following three equilibrium equation Kp1, Kp2, Kp3, we now have 6 algebraic

equations. Here, P is total pressure and Px is partial pressure and SUM(X) is a sum of all

procucts gas mol. including nitrogen gas. Eq. No.1 to 2 are independent of total pressure.

Chemical Equilibrium Constant for

Eq. No.1 Kp1 = PCO2/PO2 = (XCO2/XO2)

Eq. No.

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Chemical Equilibrium Constant for

Eq. No.1 Kp2 = PCO/PO2 = (XCO/XO2)

Eq. No.

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Chemical Equilibrium Constant for

Eq. No.1

Kp3 = PCOPH2/PH2O =

(XCOXH2/XH2O)(P/SUM(X))

Eq. No.

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You can obtain solutions by either solving six algebraic equations numerically or

analitically. Solver of Excel was used for numerical calculation of the following equation.

(XCOXH2/XH2O)(P/Kp3)+(2(Kp1/Kp2)+1)XCO+XH2+4XCH4+2XH2O+XO2-C-O-H-2A-N = 0

Here, N is nitrogen gas mol contained in the air feed.(kgmol/h)

7. Enthalpy Calculation

Enthalpy of the gas could be calculated from specific heat at constant pressure Cp. Cp is

a function of temperature T. Enthalpy could be calculated from an equation, having a

integral form of Cp as follows. The equation for enthalpy calculation was derived from

Reference 3. Where H* is an integral constant.

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Components Cp Enthalpy a b c

- kcal/kgmol deg.K kcal/kgmol - - -

O2 Cp = a + bT +

c/T2

H = H* + aT + bT

2/2 - c/T 8.27 0.000258 -187,700

N2 Cp = a + bT H = H* + aT + bT

2/2 6.5 0.001 -

CO2 Cp = a + bT +

c/T2

H = H* + aT + bT

2/2 - c/T 10.34 0.00274 -195,500

CO Cp = a + bT H = H* + aT + bT

2/2 6.6 0.0012 -

H2 Cp = a + bT H = H* + aT + bT

2/2 6.62 0.00081 -

H2O Cp = a + bT + cT2 H = H

* + aT + bT

2/2 +

cT3/3

8.22 0.00015 0.00000134

CH4 Cp = a + bT H = H* + aT + bT

2/2 5.34 0.0115 -

8. Heat and Material Balance Calculation

Based on calculated mol. flow of gas and enthalpy of each component, total gas enthalpy

was calculated.

Heat of reaction was calculated from heat of formation used for equilibrium calculation

according to the following equation. Reference 2.

H0reaction = H0products-H0reactants-H0vap

Where H0vap is a Latent Heat of Water in Wood and is -10,514 (kcal/kgmol).

Difference between gas enthalpy and heat of reaction plus recovered heat was defined as

Heat Loss.

Composition of the gas in mol. % at each reaction temperature are shown in the following

chart.

Chemical Engineering Calculation

Using all information made available, following items were calculated.

(1) Heating Value of the Gas ; Stoichiometric calculation of gas combustion and

subsequent heat of reaction from heat of formation. Data source of heat of formation is

Reference 4.

(2) Air Intake Volume for the Engine (air/gas) ; Stoichiometric calculation of gas

combustion.

(3) Lower Flammability Limit of the Gas ; Data source is Reference 4.

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(4) Upper Flammability Limit of the Gas ; Data source is Reference 4.

(5)Water Dew Point of the Gas ; Data source is Reference 4.

(6) Total Energy contained in the Gas ; Calculated from Heating Value of the Gas and

Gas Volume.

(7) Engine Power Output in kW ; Normally, gasoline engine is converted to gas fuel.

Engine Analysis Program will calculate intake gas volume and thermal efficiency form

gasoline data. Int he case od power generation, normally speed is constant.

(8) Energy Conversion Efficiency of the Gasifier (gas/wood) ; Ratio of total energy

contained in the gas and that contained in the wood. Heating value of wood was

calculated from heat of formation and latent heat of water contained in the wood. Data

source is Reference 4.

(9) Main Dimensions of Gasifier ; A container having is required to house feed wood. It

was assuming Sp. Gr. of wood is 0.5 and average void is 40%.

(10) Insulation thickness ; Insulation thickness was calculated from thermal conductivity

of rock wool and allowable heat loss. For low load and for startup operation, burning rate

was calculated from heat loss calculated from given insulation thickness.

(11)Gasificaion air preheating was calculated from assumed heat transfer coefficient of

10kcal/m2 h C.

(12)Biomass preheating was calculated from Kunii-Smith Equation. Calculated heat

transfer coefficient reaches 18kcal/m2 h C. This is sufficient to vaporise all water content

in wood. For design purpose, heat equivalent to water evaporation was included in heat

loss calculation.

(13) Diameter of the throat ; Velocity was selected to ensure uniform contact of gas and

charcoal based on FAO manual.

(14) Upward gas velocity of 0.18m/sec in the annular space shall be lower than the

terminal velocity of 160 micron ash particulate calculated by Stokes' Law taken from

Reference 6.

(15)Annular space outlet temperature was calculated from air and biomass preheating and

heat loss.

(16) Diameter and total length of gas cooler ; Overall heat transfer coefficient was

calculated from gas side and water side film coefficient. Goal seek type calculation was

applied for obtaining outlet gas temperature.

(17) Pressure drop of the Nozzle was calculated from orifice calculation equation

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(18) Pressure drop of the throat was calculated by Carman's equation

(19) Capacity of Blower: Air rate at startup operation was taken as a design capacity.

(20) Diameter and Flow Velocity of Connecting Pipes; Ideal gas was assumed for

calculation actual volume of flow.

10.Performance Calculation at Startup and Reduced Load

After design type calculation, performance type calculation were conducted for reduced

operation. Turndown ratio was calculated for the case of reaction temperature of 600 and

400 deg.C respectively. Wood feed rate were determined to match heat loss by goal seek

calculation as air feed is zero.

For startup operation, reaction temperature of 900 deg. C was taken as operating

conditions. In the case, two variables, namely, wood consumption and oxygen were

calculated by Solver function of Excel.

11. Docking with Engine Analysis Program

Normally, reactor is designed for the engine. Therefore, Engine Analysis Program and

this program were combined to find best matching. Macro program was developed to do

many Solver calculations automatically.

Acknowledgements

References

1. Science Dictionary Japan 1953

2. Chemical Process Principles by Hougen and Watson, 1954

3. Mathematics I, 1956

4. Chemical Hand Book, Japan 1960

5. John H. Perry's Chemical Engineers' Handbook 1963

6. Ernest E. Ludwig, Applied Process Design for Chemical and Petrochemical Plants

Vol.1

7. Thermodynamics by Kazuo Kijima 1968

8. Data Book on Hydrocarbons by J.B. Maxwell, Sixth Edition, 1950

9. Chemical Engineering Hand Book, 1968.

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10. Chemical Engineering Lecture Note by Tesuo Maejima for 2003 Faculty of Science,

Tokyo University of Science.

11. Design Manual by Forestry Department of FAO (Food Agricultural Organization)