THERMODYNAMIC ANALYSIS OF PLASMA GASIFICATION SYSTEMS

INTERNATIONAL FLAME RESEARCH FOUNDATION

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IFRF Members’ Conference – Flexible and clean fuel conversion to industry

Freising, Germany, 1, 2, 3 June 2015 – Paper n. 11

THERMODYNAMIC ANALYSIS OF PLASMA GASIFICATION SYSTEMS

Aytaç Şanlısoy, [email protected], Melda Özdinç Çarpınlıoğlu,

[email protected]

GAZIANTEP UNIVERSITY

Faculty of Engineering, Department of Mechanical Engineering, 27310, Gaziantep,

TURKEY

ABSTRACT

In this paper, the available literature on the thermodynamic modelling and analysis of plasma

gasification system is criticized. The reactor and plasma generation sections have a critical

importance for production of syngas and reliability of the system. Therefore, the reactor is

taken as control volume and performance of the gasification system with respect to

thermodynamic laws is analyzed. The mass balance for the reactor and the heating values of

the reactants and products are specified. (The chemical reactions for the coal gasification by

air and steam are examined.) Furthermore, the first and second law of the thermodynamics

are applied to obtain the energy balance and required minimum power for plasma gasifier is

determined. The cold gas, hot gas and second law efficiency of the process revealed in terms

of heating value, mass flow rate and standard exergy of the substances.

1. INTRODUCTION The usage of coal, waste and organic matters needs more efficient and environmental

methods to generate desired form of energy. In conventional way, the waste and coal

incinerated to gain heat energy, but the emission released to environment. To decrease the

emission, the fuels gasified by absence or oxygen starved media. By gasification, the fuels

decomposed to its components and mainly CO, H2, CO2 and hydrocarbons are obtained.

There are several methods to gasify the matters such as depending on reactor type; updraft or

downdraft fixed bed, bubbling or circulating fluidized bed, depend on power supply; allo-

thermal and auto-thermal process, depending on reaction media; pyrolysis and gasification

[1]. As it known, the increase of gasifier temperature enhances the conversion of fuel. The

plasma gasification method has unique advantages of very high temperature, high response,

simple and compact design in comparison with other methods [2].

1.1. The Gasification Gasification is conversion of materials into a mixture of gaseous spaces that primary consist

of CO, H2, CO2, and CH4. Conversion of materials to syngas takes place by partial

oxidization of the carbon in gasification material. Gasification occurs at high temperatures by

using controlled amount of supply gases which can be air, steam or oxygen [3]. Syngas

content can change depending on the reactor temperature, gasification material, residence

time of material in reactor, supplied gas type, supplied gas rate, gasification technique,

reactor type and etc.

Gasification is a promising technology that converts simply biomass, municipal solid wastes

and hydrocarbons into syngas which includes mainly Carbon monoxide, hydrogen, carbon

dioxide and methane.

1.2. Thermal Plasma Gasification Plasma gasification; powered by an external energy source, operates at very high

temperatures in an oxygen starved environment to decompose input waste material into

elemental molecules. Products are syngas and inert vitreous (glass-like) materials. Syngas can

be used to generate electricity or liquid fuel production. Plasma technology breaks down

nearly all the materials excluding the radioactive materials to their elemental form. As a result

of high temperature toxic compounds decomposed to harmless chemical elements. In

conventional technologies the remaining ash and char should be handled as final product.


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Plasma gasification system consists of gasification reactor, feeding system, equipment to

handle the slug, ash, tar and char, syngas treatment system, and a monitoring and control

system.

The process in the reactor has a critical importance for the performance of gasification. In

literature it is possible to find thermodynamic analysis of the integrated gasification system.

Wang et al. [4], studied the energy and exergy analyses of an integrated system and they

found that the highest exergy destruction occurs in gasification system. Similarly, Bang-

Moller et al. [5], performed Exergy analysis and optimization of a biomass gasification, solid

oxide fuel cell and micro gas turbine hybrid system and the gasification component of the

integrated system has the largest exergy destruction. Ozturk and Dincer [6], performed the

thermodynamic analysis of multigenerational system by using solar power tower and coal

gasification system. They found that irreversible chemical reactions in the gasification

process is the main reason of the large exergy destruction. Another result is that, increasing

the reference temperature causes better performance. Sandeep and Dasappa [7], performed

the thermodynamic analysis of oxy-steam biomass gasification and they compared the

performance of oxi-steam gasification with air gasification. Kalinci et al. [8], performed the

exergoeconomic evaluation of sewage sludge plasma gasification systems. It is obvious from

the literature survey, the high temperature properties and better conversion properties of

plasma gasification system has advantages compared with conventional methods. If the

performance of the plasma gasification method evaluated by thermodynamic analysis and

obtain high performance syngas production, it will be possible to replace conventional

gasification systems of integrated systems. The aim of this study perform the thermodynamic

analysis of plasma gasification process.

2. DESIGN OF THE PROBLEM The system considered here is the plasma gasification system which used to dispose the waste

materials, low grade coals, plastics and etc. This study includes the parametric analyses of the

efficiency of the system. For this purpose, the first law of thermodynamic will apply to the

main sections of the system, heating value of the fuels (waste materials, low grade coals,

plastics and etc.) will be determined and the efficiency of the system will be obtained.

The model system consists of plasma system, fuel and plasma gases supply system, reactor,

filtration and combustion sections as shown in figure1. Also system includes gas analyze and

temperature measurements sensors in reactor. For the analysis of the system, gasification

reactor was taken as control volume. In gasification reactor while the inputs are steam, air,

and fuel, as a result of chemical reaction, the outputs were syngas, slugs and ash. The

produced syngas sent to the combustion chamber to obtain heat in desired form.

Figure 1: Block diagram of the microwave gasification system


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3. THERMODYNAMIC MODELLING

In this section, the mass balance, chemical reactions, first law of thermodynamics, second law

analysis, lower and higher heating values will be described for this model system.

3.1. Mass balance In gasification reactor while the inputs are steam, air, and fuel, outputs were syngas, slugs

and ash. The produced syngas sent to the combustion chamber to obtain heat in desired form.

The mass balance of the reactor can be written as in equation 1.

�̇�𝑠𝑡𝑒𝑎𝑚 + �̇�𝑎𝑖𝑟 + �̇�𝑓𝑢𝑒𝑙 = �̇�𝑠𝑦𝑛𝑔𝑎𝑠 + �̇�𝑎𝑠ℎ (1)

3.2. The chemical reactions in the reactor The typical chemical reactions occurs in reactor explained below when the coal considered as

fuel. When the air supplied to the reactor, the governor reactions were oxidization. However

in the case of steam supplying, the water gas reaction, water decomposition and Boudourad

reaction occurs [9].

Reactions for oxygen supply;

Oxidization

𝐶 + 𝑂2 → 𝐶𝑂2

𝐶 +1

2𝑂2 → 𝐶𝑂

Coal cracking

𝐶𝑋𝐻𝑌 +1

2𝑥𝑂2 → 𝑥𝐶𝑂 +

1

2𝑦𝐻2

Reactions for water supply;

Water-gas reaction

𝐶 + 𝐻2𝑂 → 𝐶𝑂

𝐶 + 2𝐻2𝑂 → 𝐶𝑂2 + 2𝐻2,

Water decomposition

𝐻2𝑂 → 𝐻2 +1

2𝑂2

Boudourad reaction

𝐶 + 𝐶2𝑂 → 2𝐶𝑂

Finally chemical reaction can be written as below;

(𝐶 + 𝐻2𝑂 + 𝐻2 + 𝑂2 + 𝑎𝑠ℎ) + (𝑂2 + 3.76𝑁2) + 𝐻2𝑂 → 𝐶𝑂 + 𝐻2 + 𝐶𝑂2 + 𝑁2 + 𝑎𝑠ℎ

3.3. First law analysis of thermodynamics In this section, the first law analysis can be done by choosing the reactor as control volume

and writing the energy balance for the input and outputs. The Process assumed as steady

process. The microwave plasma gasification system is an Allo-thermal gasification process,

which means that the process occurs with the external power supply. In the case of steam

input, the first law equation can be written as in equation 2 [10].

𝑄 − (𝑊𝑝𝑙𝑎𝑠𝑚𝑎 + 𝑊𝑠𝑡𝑒𝑎𝑚) = �̇�𝑓𝑢𝑒𝑙 × (𝐻𝐻𝑉𝑓𝑢𝑒𝑙) − �̇�𝑠𝑦𝑛𝑔𝑎𝑠 × (𝐻𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠) − �̇�𝑎𝑠ℎ × (𝐻𝐻𝑉𝑎𝑠ℎ) (2)


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In equation; 𝐻𝐻𝑉, 𝑊𝑝𝑙𝑎𝑠𝑚𝑎, 𝑊𝑠𝑡𝑒𝑎𝑚 and �̇� are the higher heating value, the supplied power

to reactor by plasma system, the required power to obtain steam flow rate and the mass flow

rate respectively. The supplied power from microwave system can be obtained in equation 3,

by measuring the difference between the input power and the reflected power.

𝑊𝑃𝑙𝑎𝑠𝑚𝑎 = 𝑃𝑝𝑙𝑎𝑠𝑚𝑎 − 𝑃𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑝𝑜𝑤𝑒𝑟. (3)

Cold gas efficiency:

The cold gas efficiency (ηCE) is the efficiency of the gasification process which denotes the

ratio of the output energy of syngas to input energy. The cold gas efficiency can be written as

equation 4.

𝜂𝐶𝐺 =�̇�𝑠𝑦𝑛𝑔𝑎𝑠×(𝐻𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠)

�̇�𝑓𝑢𝑒𝑙×𝐻𝐻𝑉𝑓𝑢𝑒𝑙+𝑃𝑝𝑙𝑎𝑠𝑚𝑎×𝜂𝑡ℎ(𝑝𝑙𝑎𝑠𝑚𝑎 𝑡𝑜𝑟𝑐ℎ) (4)

Also, the thermal efficiency of the microwave generator can be written as in equation 5.

𝜂𝑇 =𝑃𝑝−𝑃𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝑝𝑜𝑤𝑒𝑟

𝑃𝑝𝑙𝑎𝑠𝑚𝑎 (5)

It is also possible to define the efficiency in terms of gas exit state that is hot gas efficiency. It

is written in equation 6.

𝜂𝐻𝐺 =�̇�𝑠𝑦𝑛𝑔𝑎𝑠×(𝐿𝐻𝑉𝑠𝑦𝑛𝑔𝑎𝑠+𝑐𝑝(𝑠𝑦𝑛𝑔𝑎𝑠)(𝑇𝑠𝑦𝑛𝑔𝑎𝑠−𝑇𝑜))

�̇�𝑓𝑢𝑒𝑙×𝐻𝐻𝑉𝑓𝑢𝑒𝑙+𝑃𝑝𝑙𝑎𝑠𝑚𝑎×𝜂𝑡ℎ(𝑝𝑙𝑎𝑠𝑚𝑎 𝑡𝑜𝑟𝑐ℎ) (6)

The high heating values of coal can be find by the equation 7 [11].

𝐻𝐻𝑉 = 0.3491𝐶 + 1.1783𝐻 + 0.1005𝑆 − 0.1034𝑂 − 0.0151𝑁 − 0.0211𝑎𝑠ℎ (𝑀𝑗

𝑘𝑔) (7)

In here, the C,H, S, O, N and ash are the weight fractions of carbon, hydrogen, sulfur,

oxygen, and nitrogen, , in the coal respectively. This values were obtained from the ultimate

analysis of the coal.

The heating value is called the higher heating value (HHV) when the H2O in the products is

in the liquid form, and it is called the lower heating value (LHV) when the H2O in the

products is in the vapor form [12]. The substance enter and leave the system as higher

temperature than evaporation temperature, so the heating value should be accounted by

considering lower heating value. The lower heating value of the substance can be found by

using the equation 8 [13].

𝐿𝐻𝑉 = 𝐻𝐻𝑉(1 − 𝐻2𝑂) − 2440(𝐻2𝑂 + 9𝐻) (8)

Here, the 𝐻2𝑂 is the weight fraction of moisture in fuel.

HHV is used for cold gas efficiency. Because the fuel and cold gas heating values considered

at ambient temperature. However, the LHV should be used for hot syngas that its temperature

is above the evaporation temperature of the water.

3.4. Second law analysis of the thermodynamics The exergy of the stream may have chemical or physical exergy [8]. For this reason, the

exergy of stream can be defined as in equation 9.

�̇�𝑥𝑡𝑜𝑡 = �̇�𝑥𝑝ℎ + �̇�𝑥𝑐ℎ (9)


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Physical exergy occurs as a result of pressure and temperature difference between the

operation and reference conditions. The physical exergy of the elements of the fuel can

calculated as equation 10,

�̇�𝑥𝑝ℎ = �̇�[(ℎ − ℎ0) − 𝑇0(𝑠 − 𝑠0)] (10)

In equation, �̇� denotes the mass flow rate, ℎ enthalpy of the material at operation condition,

ℎ0 enthalpy of the material at reference state, 𝑠 entropy at working condition and 𝑠0 entropy

at reference state.

The physical exergy of each components of gases spaces can define as equation 11,

�̇�𝑥𝑝ℎ = ∑ 𝑥𝑖�̇�𝑥𝑝ℎ(𝑖)𝑖 (11)

Çengel and Boles [12] noticed that as a result of chemical reactions, the chemical bonds

which bind the atoms together are broken and new ones are formed. The chemical energy

formed as a result of the fission or fusion of the chemical bonds. Kotas [14], defined the

chemical exergy as “Chemical exergy is equal to the maximum amount of work obtainable

when the substance under consideration is brought from the environmental state to the dead

state by processes involving heat transfer and exchange of substances only with the

environment.” Chemical exergy expresses the exergy of a substance at ambient temperature

and pressure. The chemical exergy of the gas mixture can defined as in equation 10.

�̇�𝑥𝑐ℎ = ∑ 𝑥𝑖𝑒0(𝑖) + 𝑅𝑇0 ∑ 𝑥𝑖𝑙𝑛𝑥𝑖𝑖𝑖 (12)

Here 𝑥𝑖 denotes mole fraction of the compound in gas mixture and 𝑒0 denotes standard

chemical exergy. The standard exergy of the product gases can be found in the study of Kotas

[14]. There is uncertainties about the composition of the fuel. Therefore, empirical relation

[15] that calculate the standard chemical exergy can be used.

𝑒0 = 𝛽. 𝐿𝐻𝑉𝑓𝑢𝑒𝑙 (13)

The 𝛽 can be calculated as below, if 𝑂/𝐶 is less than 2.67.

𝛽 =1.0412+0.2160(𝐻/𝐶)−0.2499(𝑂/𝐶)[1+0.7884(𝐻/𝐶)]+0.045(𝑁/𝐶)

1−0.3035(𝑂/𝐶) (14)

As a result, the fuel exergy and the product gases exergy can be calculated and the exergetic

efficiency can calculated as equation 15.

𝜂𝑒𝑥 =(�̇�𝑥𝑝ℎ+�̇�𝑥𝑐ℎ)

𝑠𝑦𝑛𝑔𝑎𝑠

(�̇�𝑥𝑐ℎ)𝑓𝑢𝑒𝑙+𝑃𝑝𝑙𝑎𝑠𝑚𝑎 (15)

4. CONCLUSION

Thermodynamic analysis of plasma gasification systems includes the first law and second law

treatments. Since, theoretical studies are verified by a variety of experimental studies the

necessity for further research on the manner is apparent. The great range of experimental

studies in terms of their characteristics result in a different approaches in thermodynamic

modelling. In order to reveal plasma gasification there is need for both theoretical research on

efficiency and performance parameters and also experimental research.


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THERMODYNAMIC ANALYSIS OF PLASMA GASIFICATION SYSTEMS

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