Investigation of Performance for Entrained Flow Gasifier Through Simulations

Pakistan has proven huge coal reserves but still unable to harvest the energy due to unavailability of ingenious technology. Coal gasification is robust, efficient and environmental friendly technology but it is highly sensitive to the coal characteristics. Scant literature is available on the development of the coal gasification technology which suits the characteristics of indigenous coal reserves. This papers presents the simulation of entrained flow gasifier for using indigenous coal through process modeling software namely Aspen Plus® to evaluate various system flow steps within an entrained flow gasifier. General techniques have also been discussed for creating the equilibrium-bases simulations of gasification systems. Peng-Robinson equation was used to correlate the volume of species with respect to state variables (temperature and pressure). The effects of composition of different indigenous coals like Thar, Lakhra and Sonda were investigated through simulations along with O/C (Oxygen-to-Carbon) ratio. Parametric study revealed that the O/C ratio along with other related system parameters have great influence on the performance. Sulfur could be available in different forms in coal like pyrite, sulfate or organic sulfur so appropriate form of sulfur in feedstocks should be corrected for better accuracy of model results. The highest percentages of CO i.e. 44.2, 37.8, and 46.6% were obtained from Thar coal (air dried form), Lakhra coal and Sonda coal respectively at the 0.3 O/C ratio. The decrease in LHV and HHV (Lower and Higher Heating Values) of syngas was observed on increase of O/C ratio for all coal types. The composition of as received Thar coal gave maximum LHV (1.5x10 KJ/Kg) and HHV (1.78x10 KJ/Kg) at the 0.3 O/C ratio. The future work could be extended by simulating biomass composition in the developed model of gasifier in transient simulations.


INTRODUCTION
S ustainable development requires the efficient and economic conversion of coal due to its major utilization for electricity production across the globe [1]. Gasification is considered to be one of the most efficient technologies to convert raw, low-cost coal into clean and highly priced chemicals, fuels, and power.

Investigation of Performance for Entrained Flow Gasifier Through Simulations
coal reserves which is a huge amount [2] and could be a breakthrough for country through exploitation of gasification technology for producing energy and other chemicals from abundant ingenious resource [3].
Fundamentally there are three basic configurations for gasifiers, namely fixed bed gasifier, fluidized bed gasifier and entrained flow gasifier. The fundamental difference in these configurations is the fuel and oxidizing agent feeding style and form of fuel. Entrained flow gasifiers are found most efficient but complex in operations.
In entrained-flow gasifier, the gasifying agents such as oxygen/air and steam including the mixture of gas products flows counter currently with solid feedstock like coal [4]. The whole process in entrained flow gasifier carried out at the temperatures below than the ash fusion point for avoiding the clinking issues. The coals with none clinking properties are usually preferred for entrained flow gasifiers. The size of coal being used that ranges from 6.35-38.1mm with oxygen enter from the top of the reactor and from the reactor bottom steams are blown. The coal particles travel from different zones within the reaction chamber of entrained flow gasifier usually known as devitalization zone, drying zone, combustion zone, and gasification zone. Drying zone is at start where the moisture of coal gets evaporated. Generally, drying process occurs its reaction under 300°C but it also depends on moisture content available in the coal [5]. The drying process ends up then devolatilization process begins. The coal is transferred into elevated temperature so that it produce the gases char and tar. The product gases formation, char and tar composition depend on the category of coal, heating rate, pressure, and temperature. The size of particles largely affects the overall reaction rate, quantity and quality of product syngas due to slow heat transfer rate and slow reaction time [6].
After the devolatization, combustion of char and volatiles starts which is controlled by limited oxygen supply. The general combustion reaction is described by reaction Equation (1) [7].
Where 'z' is the stoichiometric coefficient and defines the complete or partial combustion of carbon in the fuel.
After the limited combustion the temperature of the chamber reaches above 800°C and filled with enough combustion product gases like CO 2 Reaction of Hydrogen combustion: The principal advantages of entrained flow gasifier is its ability to produce tar-free gas from the coal used as feedstock along with production of ash in frit or inert slag form. Consumption of high amount of oxygen as compared conversion efficiency with high carbon content fuels.
Likewise, the quality of syngas produced from entrained flow gasifier is of high quality because it contains less methane and higher CO and H 2 mole fractions. The feed systems, chamber of vessels and its internals, entrance of fuel and oxidant are varying in different designs of entrained-flow gasifiers [8].

DEVELOPMENT OF MODEL
This section provides the development of kinetic model for downdraft entrained flow gasifier in Aspen Plus® software. The model approach was proposed by Chaung and Wen [14].
The model features are enlist below:  The model of steady state.

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The occurring physical and chemical processes in gasifier are accounted for in this modeled, char gasification, coal pyrolysis and volatile combustion.

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The consideration of char in the gasification is defined by the kinetics reaction.

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The time of residence solids is calculated by the hydrodynamics is taken into account.

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The solid segment and gas segment is mixed with each other promptly and flawlessly.

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The pressure drop is negligible in the gasifier.

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The coal particles size is supposed to be spherical as well as uniform.

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The coating formed of ash leftover on the particle at the time of reaction depends upon the model shrinking core unreacted.

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The coal particle temperature inside is assumed to be unchanged.
The process containing the species of chemicals are presented in Table 1.

Simulation Method
The model is developed in Aspen Plus®V10 for whole process of gasification is shown in Fig. 2 where P is pressure, R is general gas constant, T is temperature, Z is compressibility factor of real gas and v is specific volume.

Measuring Performance of Gasifier
There are two ways in which the performance of gasifier is measured that is efficiency and heating value of syngas.
Commonly efficiencies of gasifier are of three types welldefined by Silaen [22]: The produced syngas quality is analyze by the heating value which is prime factor of gasifier. The calculation of

Cases of Simulation for Various Inputs
The simulation conditions input are given in Table 8

RESULTS AND DISCUSSION
The Aspen Plus®8.4 software, a standard flowsheeting, is used in current research work for development of entrained-flow gasifier using coal as

Effects of O/C Ratio and Type of Feedstock on Syngas Composition
The Maximum 26% CO 2 was produced from Thar (AD) and Lakhra coals at 0.6 O/C ratio. For Thar (AR) and Sonda coal the maximum CO 2 was achieved at O/C ratio of 0.7 i.e. 21 and 26% respectively. The results are in agreement with previous findings [9].

Effects of O/C ratio and type of feedstock on Temperature of Syngas
The outlet temperature of product syngas from different feedstocks was also recorded during the simulations of gasification at varying O/C ratio and is shown in Fig.7. research [11]. The main cause of this performance is due to the converting gasification into the combustion at greater O/C ratios.

Fuel Conversion Efficiency
The efficiency of conversion of all fuels was calculated using Equation (8)

Quality of Syngas
The heating value would be the term for measuring the quality of syngas. The syngas composition, pressure, and temperature was utilized in each case to calculate the LHV and the HHV using material stream convergence method in Aspen HYSYS. Fig. 9(a-b)

Investigation of Performance for Entrained Flow Gasifier Through Simulations
was achieved from Thar as received composition. Thar coal (as received) produces a syngas with 19% CH 4 at 0.3 O/C ratio, which is highest amount and for that factor, the unexpected higher HHV and LHV values are obtained. In other type of coals there is less moisture than the Thar (AR) and for that reason the highest amount of methane is obtained because moisture work in the gasifier as the steam and producing more methane. The Sonda coal produced second best quality of syngas, which is 1.