Syngas Compositions, Cold Gas and Carbon Conversion Efficiencies for Different Coal Gasification Processes and all Coal Ranks

This Paper presents comparison of syngas compositions, for all coal ranks, as produced from different types of coal gasification processes currently in use, namely entrained flow, fluidized bed, and fixed bed gasifiers. Cold gas and carbon conversion efficiencies were investigated. The syngas composition varies with the applied gasification process. The importance of this research arises from the fact that gasifiers produce the environmentally clean fuel required to run any Integrated Gasification Combined Cycle power system. A procedure was conducted to get estimates for bituminous coal. It is important to have knowledge of the chemical reactions which take place in each gasifier and the raw syngas produced from the specific reaction involved to develop an Integrated Coal Gasification Combined Power Generation plant with CO 2 recovery in order to increase the cycle efficiency and mitigate CO 2 emission and other pollutants. The results indicate that the entrained flow gasifier is the dominant one.


The Gasification Process
The real reactions associated with the gasification process are immensely complicated and change with the feed material properties [2].
The gasification of coal comprises three chief steps, as shown in Figure 1: (a) pyrolysis and devolatilization, (b) volatiles cracking and combustion, and (c) char gasification. These processes are explained briefly.  [2].

Pyrolysis and Devolatilization
The interaction between pyrolysis and gasification under various conditions of heating is depicted in Figure 2. If the heating is slow then the pyrolysis reactions start at about 350°C. The gasification reaction of volatile matters (VM) and char with steam is rather slow at such temperature. The concentration of volatiles outside the coal particle increases quickly, and gasification only takes place after devolatilization is accomplished. However, in case the heating rate is high, then both pyrolysis and gasification occur concurrently, so that high concentration of volatiles is never permitted to build up. This explains why a clean gas in such a short time is produced from high-temperature entrained-flow gasifiers. In Ref. [3], it is indicated that in contrast with a counter flow moving-bed process, where lump coal is employed, the rate of heating is slow and a high volatiles concentration grows up and discarded unreacted from the reactor by the syngas.
The steam-char reaction is the supreme contributor to the production of both H 2 and CO, which are the primary reactive constituents of the syngas [2].

Water-Gas Shift (WGS)Process Inside Gasifiers
The water-gas shift reaction is an equilibrium process: CO + H 2 O (g) ↔ CO 2 + H 2 . The forward reaction is exothermic, in which CO and steam are converted to H 2 and CO 2 . The forward reaction is energetic at temperatures less than 700°C. At higher temperatures, near 1000°C, the net reaction is slow and negligible. More than 1200°C, the backward reaction becomes commanding. The reaction rate of the WGS is usually slow without using catalysts; however, in the gasifier, the reaction rate is usually enhanced by the catalytic effect of metallic components in coal. In gasifiers which utilize the quench method to cool down the syngas to near 200°C, the residence time is very short to achieve any remarkable forward WGS reaction, despite that the equilibrium constant value is large at low temperatures, because the catalytic effect from metals in coal is feeble in the quenched syngas since most of the metals have become molten slag, which is extracted during the gasification process, before quenching takes place [2].

Methanation
The methanation reaction [C + 2H 2 → CH 4 ] is predominantly exothermic and pressure favourable, so it is generally inactive in high-temperature atmospheres, for instance in high-temperature entrained flow gasifiers [2].

Types of Gasifiers
Detailed descriptions of gasifier types and their operation can be found in many references. Operating data and innovative gasifiers studied prior to the 1980s are reported in some early studies. Recent approaches to gasification such as catalytic, molten Lurgi of 1600 ton/day capacity was manufactured [7]. how coal is converted into ash and the gas production across the gasifier as the temperature goes up. It is a back-mixed or wellstirred reactor where there is a consistent mixture of fresh coal particles mixed with older ones, some of which are partially gasified, and some are totally gasified. The mixing regime allows consisting temperatures throughout the bed. The gas flow into the reactor (oxidant, steam, recycled syngas) must be adjusted such that to suspend coal particles within the bed but not too high to entrain them out. As the gasified coal particles gets smaller and lighter, they will escape from the gasifier. In order to avoid particle agglomeration, it is important to ensure that the temperatures within the bed to be lower than the initial ash fusion temperature of coal [6].  [6].

Fluidized Bed Gasifier
A cyclone downstream the gasifier will capture the larger particles that are entrained out and return them back to the bed.
The residence time of coal particles in these gasifiers is shorter than that in a moving bed one [6]. The main characteristics of these gasifiers are extensive solids recycling, uniform and moderate temperature, and moderate oxygen and steam requirements.  gasifiers must perform at high temperatures in order to attain high carbon conversion efficiency. Therefore, shows that the majority of entrained flow gasifiers employ oxygen rather than air and operate above the slagging temperature of coal [6]. The processes that need a high throughput capacity in a single reactor generally use entrained bed type, as in IGCC, since the reactor size can be reduced by the fast residence time (typically less than 5 sec) as well as by high pressure. Although large scale operation of entrained bed gasifiers have been successfully operated commercially, however, the experience is not long enough as in the case of fixed or fluidized bed gasifiers. It is concluded that the foremost disadvantage of entrained bed gasifier is its high capital cost due to the compact configuration of parts [7].   i) The radiant syngas cooling can increase the cycle efficiency by 4 -5% over full quench types.

Concluding Remarks on Gasifiers
Gasifiers need further improvements and developments in order to increase their efficiencies and performance, for more availability of IGCC systems. C + CO 2 = 2CO + 172 MJ/kmol (4) The water gas reaction C + H 2 O = CO + H 2 + 131 MJ/kmol (5) The methanation reaction C + 2H 2 = CH 4 + 75 MJ/kmol (6) As reactions with free oxygen are all complete under gasification conditions, reactions (1), (2) and (3) do not need to be considered in determining an equilibrium syngas composition.
The three heterogeneous (i.e. gas and solid phase) reactions (4), (5) and (6) are sufficient. In general, we are concerned with situations where the carbon conversion is also essentially complete. Under these circumstances, Equations (4), (5), and (6) can be reduced to these two homogeneous gas reactions: The water gas shift reaction CO + H 2 O = CO 2 + H 2 + 41 MJ/kmol (7) The steam methane reforming reaction Note that by subtracting the moles and heat effects from reaction (4) from those in reaction (5), one obtains reaction (7), and by subtracting reaction (6) from (5), one obtains reaction (8).
But most gasification processes rely on a balance between reactions (1) (partial oxidation) and (5) (water gas reaction). For real fuels (including coal, which also contains hydrogen), the overall reaction can be written as: Where -For gas, as pure methane, m = 4 and n = 1, hence m/n = 4 -For oil, m/n = 2, hence m = 2 and n = 1 -For coal, m/n = 1, hence m = 1 and n = 1.
Gasification temperatures are so high that, thermodynamically as well as in practice, no hydrocarbons other than methane can be present in any appreciable quantity.

Data and Assumptions
To perform the calculations, numerical data and assumptions should be made. The coal ranks considered cover the high grade ones (anthracite and bituminous) and low ranks (sub-bituminous and lignite). Mass percentages of constituents of coals are depicted in Table 1 [5]. To get coal chemical compositions, we must calculate mole fractions of all species and hence their normalized values with respect to carbon, and these are presented in Table 2. Coal compositions as extracted from the normalized mole fractions, in Table 2, are shown in Table 3.
Equations for complete combustion of coals are indicated in Table 4.   The heating values used in calculations of cold gas efficiencies are listed in Table 5. For all present calculations, we assumed a flow rate of coal in gasifiers= 2500TPD. Oxygen is blown in gasifiers and the pressure= 30 atm.

Entrained Flow Gasification
Reactions in gasifier:

Syngas Compositions Produced from Gasifiers
Partial combustion of coal inside the gasifier produces raw syngas consisting of different gases. Some of these are re-burned again in the turbine, such as CO, H 2 , and CH 4 . Some are disposed off, including H 2 S. Some species will withdraw heat from the system, such as N2 and water. Some gases such as CO 2 are harmful to the environment. Tables 6, 7, and 8 give the raw syngas compositions, as obtained from the above calculations, for all coal ranks, for the three studied gasifiers.
The numbers in Tables 6, 7, and 8 indicate the mole percentage of each gas component produced in the syngas by the partial combustion of coal. The mass ratio of each gas was not calculated, and it was sufficient to adopt the mole percentage values because they are more precise in expressing the gas composition since the mole mass for each gas species is different.    The raw syngas produced from gasifiers needs a cleaning system in case used as fuel for a gas turbine in an IGCC plant.
Cleaning syngas means the removal of particulate matter, metallic compounds, and other undesirable pollutants such as sulphur. The cleaning system may be pre-combustion or post-combustion. A water gas shift system may be included to enhance the H 2 content in the gas. A CO 2 capture system can be employed if desired. Therefore, IGCC plants are more environmentally cleaner than current ones especially if CO 2 capture system is adopted.
It should be realized that there are many technological problems which should be encountered for a successful IGCC technology.
The most important issue is the gas turbine which is the core of a combined cycle system. The differences in the properties between syngas and NG dictate new considerations in the design of the gas turbine in an IGCC system. For instance, the lower calorific value of syngas fuels requires a significant increase in the mass flow rate of fuel supply to the gas turbine as compared with burning NG. As a result the output power will increase and this needs a different design of the turbine to allow for the increased flow rate. Also the higher content of H 2 in syngas which has a higher flame velocity may lead to difficulties in controlling the combustion mechanism. Also, syngas combustion is different from NG because of the presence of H 2 and CO which have higher adiabatic flame temperatures than CH 4 . So, new issues should be looked after for a proper reliable gas turbine performance. Table 9 exhibits the CGη and CCη for the different gasifiers and coals. Figure 6 lists some cold gas efficiencies for various gasification processes for all coal ranks. The cold gas efficiency is the ratio of fuel heat content to the syngas heat content at ambient conditions and is a measure of how efficiently fuel energy is converted into syngas energy. For the cases reported, the not fixed/moving bed is most efficient but has the disadvantage that some of the syngas energy is produced in tars. The listed efficiencies (~86%) support the rule of thumb that approximately 15% of the feedstock heating value is used to convert the feedstock to syngas.  The results depict that the entrained flow gasifier, for all coal ranks, is the predominant one because of its low cold gas efficiency which means low loses in coal fired because of the high temperature in the gasifier, higher carbon conversion efficiency (100%), low tar, and the low methane produced which means lower emission. The second good, for all coal types, is the fluidized bed gasifier.

Conclusions
Irrespective of new discoveries of natural gas reserves and new techniques (such as hydraulic fracturing or fracking) being developed to increase cheaper natural gas production, coal will continue to be a major energy source to produce electricity in the world, either for economic reasons or as a strategy to safeguard national energy security and independence. The conventional way of burning coal is environmentally unfriendly; therefore, it is essential that cleaner methods of utilizing coal be developed. IGCC is one of the most promising methods to generate electricity in a more efficient, environmentally friendly manner than conventional fossil fuel plants. Example step by step manual calculation procedure was conducted for one coal (bituminous).
Syngas composition depends incredibly on the gasification process and coal type. Each gasification technology has its own specific chemical reaction and operating conditions. The entrained flow gasifier satisfies most of the requirements needed for the appropriate syngas for IGCC plants, for all coals. This gasifier produces raw syngas with zero CH 4 , high H 2 and CO, and low CO 2 , H 2 O, N 2 , and H 2 S.
The entrained flow gasifier is the most viable of the three because of its high carbon conversion efficiency which means low loses in coal fired and low cold gas efficiency, high temperature in the gasifier, low tar, and low methane produced resulting in low emissions, and it produces a clean gas in such a short time because of the employed high temperature. Compare this with a countercurrent fixed-bed process, which uses lump coal, the heating up rate is slow with a built up of high volatiles concentration that are removed unreacted from the reactor by the syngas.
For the technology of IGCC plants to be competitive and economically feasible, their availability should be increased and the capital cost to be reduced. Achieving these, IGCC systems will be fully commercial and of increased widespread use (table 10).