5.thermodynamics of gasification

8/13/2019 5.Thermodynamics of Gasification

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Thermodynamics of

Gasification

Prof. Dr. Javaid RabbaniKhan


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Thermodynamics & kinetics

For the theoretical background to anychemical process :

Thermodynamics(the state to which the

process will move under specific conditions ofpressure and temperature, given sufficient

time)Kinetics

(what route will it take and how fast willit get there)


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REACTIONS

During the process of gasification of solid carbon are inthe form of Coal,

Coke, Char,

The principle chemical reactions are those involving Carbon,

Carbon monoxide,

Carbon dioxide,

Hydrogen,

Water (or steam), and

Methane


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Combustion reactions

C+ O2 = CO 111 MJ/kmol

CO+ O2=

CO2283 MJ/kmol

H2 + O2 = H2O 242 MJ/kmol

Boudouard reaction

C+CO2 2 CO +172 MJ/kmol

water gas reactionC+H2O CO+H2 +131 MJ/kmol

Methanation reaction

C+2 H2 CH4 75 MJ/kmol


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CO shift reaction

CO+H2O CO2 +H2 41 MJ/kmol

Steam methane reforming reaction

CH4+H2O CO2 +3 H2 + 206 MJ/kmol


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For real fuels (including coal, which also

contains hydrogen) the overall reaction:

CnHm + n/2 O2 = n CO +m/2 H2

where

for gas, as pure methane, m= 4 and n = 1,

hence m/n = 4, and

for oil, m/n 2, hence m = 2 and n = 1,and

for coal, m/n 1, hence m = 1 and n = 1


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Thermodynam ic Equ il ibr ium

In general, the forward and the reverse reactionstake place simultaneously and at different rates

For any given temperature these reaction rates

are proportional to the quantity of reactantsavailable

For CO shift reaction, the forward reaction rate,rf, is proportional to the molar concentrations of

CO and H2O per unit volumerf = kf [CO] [H2O]

Where Constant of proportionality kf istemperature dependant


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Similarly, for the reverse reaction

rr = kr [CO2] [H2]

Over a period of time these two reaction rateswill tend to reach a common value and the gascomposition will have reached a state ofequilibrium

where Kp is the temperature dependantequilibrium constant for the CO shift reaction


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Assuming ideal gases this can also be

expressed as

where PCOis the partial pressure and vCO

is the volume fraction PCO/P of CO in the

gas


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For the Boudouard reaction

For the water gas reaction


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For the reforming reaction

where P is the total absolute pressure of the gas

The temperature dependency of these

equilibrium constants can be derived from acorrelation as

T is the absolute temperature in Kelvin


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THERMODYNAMIC MODELING OF

GASIFICATION

The designer has the task of calculating a limited

number of design cases

Throughputs of the different feedstocks,

Gas compositions,

Heat effects,

Quench requirements,

Startup and shutdown requirements,

Optimal conditions for the design feedstocks,

Process control requirements.


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Purpose of Gasification Modeling

The calculation of the gas composition.

The calculation of the relative amounts of

oxygen and/or steam and/or heat requiredper unit fuel intake.

The optimization of the energy in the form

of the heat of combustion of the productgas or, alternatively, of the synthesis gasproduction per unit fuel intake.

To provide set points for process control


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Calculations comprising the gasification

are based on

Thermodynamics,

Mass and energy balances and

Process conditions,

Temperature Pressure

The addition or subtraction of indirect heat


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For coal both must be known

Proximate analysis

fixed carbon

volatile matter

Moisture

ash

Ultimate analysis

elemental, apart from ash


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Feedstocks

Feedstocks for gasification may vary from

natural gas to heavy oil residues

coal

waste streams and biomass

For calculations following must be known

The elemental composition

The standard heat of formation of the fuels


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Coal

Composition and combustion data for coalare often very confusing as based on

as-received (ar),moisture-free (mf),

ash-free (af),

ash-and-moisture-free (maf)

The heating value can be given as

LHV

HHV


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Moderator

The most common moderator used in gasificationprocesses is steam but CO2 also used as moderator

The steam must have a minimum temperature

corresponding to that of saturated steam at the pressureprevailing in the gasifier, otherwise condensation in thelines to the gasifier will occur.

In general, steam is used that is superheated to atemperature of 300400C.

At pressures above 40 bar this superheat is mandatory,since otherwise the steam becomes wet on expansion.


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Equations

The following equations will apply in virtually allgasification processes

Carbon balance.

Hydrogen balance.

Oxygen balance.

Dalton equation, stating that the sum of the mole fractions in theproduct gas equals unity

Heat balance

Reaction constants of the relevant reactions Sulfur balance.

Nitrogen balance

Ash balance.

Argon balance


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The remaining three variables in case ofheterogeneous gasification and two in the caseof homogeneous gasification may be selected

from the following list:1. Fuel used per kmole product gas.

2. Blast (oxidant) used per kmole product gas.

3. Moderator (mostly steam) used per kmole

product gas.4. Heat loss from the gasifier reactor or heat

required for the gasification.

5. Gasification temperature


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DEDUCTIONS FROM THE

THERMODYNAMIC MODEL

Effect of Pressure

Effect of Temperature


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Effect of Pressure

practically all modern processes are operatedat pressures of at least 10 bar and up to ashigh as 100 bar.

we can compare the energy required to provide100,000Nm3/h raw synthesis gas at 45 bar byeither

1. gasifying at a relatively low pressure (5 bar)

and compressing the synthesis gas, oralternatively,

2. compressing the feedstocks to 55 bar (allowingfor pressure drop in the system) and gasifyingat the higher pressure.


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Variation of Syngas Compositions with

Temperature at 1000C


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Variations of Yields with

Temperature at 1000C


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Effect of Temperature

The temperature is generally selected on thebasis of the ash properties

For process control purposes where ratiosbetween fuel, oxygen, and/or steam are known,the temperature can be calculated

Since most modern gasification processes

operate at pressures of 30 bar or higher,temperatures of above 1300C are required inorder to produce a synthesis gas with a lowmethane content


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Variations of Syngas Compositions and

Yields at 1500C


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Variation of Yields with Pressure

at 30 bar


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Optimum Operating Point

Eff ic iencies

The two most commonly encountered are

cold gas efficiency (CGE)

carbon conversion


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Cold gas efficiency

Cold gas efficiency is defined as :


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Carbon conversion efficiency

Carbon conversion efficiency is defined as


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Cold gas efficiency


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Syngas yields for coal

5.thermodynamics of gasification

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