Minimising CO2 Emissions from Coal Gasification

*Shaakirah Cassim and Shehzaad Kauchali*

### **Abstract**

Traditional coal-to-liquid processes use gasification with excess steam to obtain hydrogen-rich syngas for downstream manufacturing of methanol or Fischer-Tropsch liquids. Such processes are shown to produce very large amounts of CO2 directly by the Water-Gas-Shift (WGS) reaction or, indirectly, by combustion in raising steam. It is shown how any coal gasifier can operate under auto-thermal conditions with methane as source of hydrogen instead of steam. This co-gasification system produces syngas for a poly-generation facility while minimising the formation of process CO2. It is shown that minimal steam is required for the process and a limit on the maximum amount of H2:CO can be obtained. Co-gasification of coal is shown to have a major advantage in that a separate WGS reactor is not required, less CO2 is formed and methane is reformed non-catalytically within the gasification unit. Furthermore, regions of thermally balanced operations were identified that enabled a targeting approach for the design of co-gasification systems. The method will guide gasification practitioners to incorporate fossil fuels and renewable-H2 into coal-to-liquids processes that require syngas with H2:CO ratio of 2. An important result shows that lowgrade coals can be co-gasified with methane to obtain CO2-free syngas ideal for power generation.

**Keywords:** *CHO*-diagram, coal-to-liquid, CO2, Co-gasification, renewable hydrogen

## **1. Introduction**

A major concern with coal-to-liquids (CTL) producing facilities is the unprecedented amount of carbon dioxide they produce. Efforts to capture and sequester this unwanted green-house gas in geological formations and, or, for enhanced oil recovery activities are encouraging albeit at a penalty cost to overall plant efficiencies and economics. For energy security in geographically stranded economies, coal and methane (either from unconventional sources such as coal-bed methane or shale gas) will play an integral role in future energy developments and what remains is the planning and execution of such activities with an environmentally conscience philosophy until carbon-free economies are the mainstay. Williams [1] suggested that a key enabling strategy leading to attractive energy-costs, without further technological developments, is "polygeneration" defined as co-production from synthesis gas of at least

electricity and one or more clean synthetic fuels such as Fischer-Tropsch (FT) liquids, methanol and dimethyl ether (DME). The advantage of polygeneration is to aid a wide range of energy needs with extremely low levels of emissions, often higher efficiencies and lower cost [1].

A key step of polygeneration facilities, using coal, is the production of syngas, comprising mainly of hydrogen, carbon monoxide, carbon dioxide, methane and steam. For these types of facilities to coexist, where the target is "clean" gas enriched with hydrogen and carbon monoxide only, a gasifier that operates as "partial combustion" of coal is required. For power generation this syngas is then fed towards an integrated combined cycle electricity power block comprising of a gas turbine and steam turbine system as in an integrated gasification combined cycle (IGCC) process. However, for liquid-fuels production such as for methanol or FT as shown by Battaerd & Evans [2], the syngas is corrected for its H2:CO ratio using an additional equilibrium-limited water-gas-shift (WGS) reactor and with excess steam to drive the reaction towards an increase in H2-content. There are thus two main, undesirable, effects of the addition of the WGS reactor: firstly, the WGS reaction itself creates CO2 and, secondly, large amounts of steam is needed for favourable equilibrium necessitating some of the original coal (or tail gases) to be combusted leading to further creation of process CO2 emitted to the atmosphere. This phenomenon is also noted in some CTL processes where traditional fixed-bed counter current operations achieve simultaneous gasification of coal (with excess steam) and the correction of the H2:CO ratio in a single piece of equipment [2]. An important strategy, in limiting the amount of CO2 produced in coal-gasification processes, is thus to avoid the phenomenon of the WGS reaction by restricting the amount of steam used.

Another strategy to correct the H2:CO ratio as used by FT processes, described in the works of Probstein & Hicks [3], is the mixing of hydrogen-rich syngas recycled from the autothermal steam reforming of tail gases comprising primarily of methane. However, this is generally acceptable practice if the initial syngas product from the gasification process has a high methane content and the FT catalyst itself produces a significant amount of methane by-product. It is an opinion that the highly inefficient autothermal steam reforming process requires a large amount of excess steam for equilibrium and also produces a large amount of CO2 due to the WGS reaction (occurring simultaneously with gasification) and indirectly from combustion in raising the steam.

Steam reforming of methane is the predominant method for the production of syngas at industrial scale. Cao et al. [4] note that natural gas based syngas are capital intensive due to expensive catalysts used and often are associated with higher energy consumption. There is thus a drive for the development of alternative technology for cost-effective production of syngas gas using geographically abundant and cheap feedstock such as coal. It is noted that there are challenges in decreasing capital investment and operational cost of coal based syngas process with flexible H2/CO ratios [4]. Firstly, coal gasification leads to low H2:CO ratios and secondly the process economics is strongly affected by the coal reactivity as this determines the carbon conversion and gas yields. Furthermore Wu & Wang [5] identified that methane could be an ideal source for H2, for syngas conversion requiring high H2, and that coalbed gas is a good methane source for co-gasification purposes. Lastly, a co-gasification experiment in a fluidized bed was performed to study the effects of adjusting the methane amount on the H2/CO ratio. Wu & Wang [5] performed similar experiments with bituminous coal and anthracite to demonstrate the combined coal gasification

#### *Minimising CO2 Emissions from Coal Gasification DOI: http://dx.doi.org/10.5772/intechopen.105587*

and methane reforming process in a single reactor. One of the objectives in the works of Wu & Wang [5] was to elucidate the catalytic effect of the unreacted coal char and ash on the partial oxidation or steam reforming of natural gas in the fluidized bed operating at 1000°C. Syngas comprising of H2:CO ratio of 1 was achieved with significant amounts of CO2 in the product. Song & Guo [6] suggest a co-gasification experiment in a moving-bed configuration using a modified large-volume blast furnace with lime-containing liquid-flux for absorption of sulphur compounds. They noted that the theoretical H2/CO ratio could vary between 0.4 (coal gasification) and 2 (partial oxidation of methane) within their system. It was experimentally observed that the H2:CO ratio was dependent on the O2/CH4 ratio in the feed. For O2/CH4 ratio in the feed below 1, the H2/CO in the product syngas is greater than 1 with over 90% gas being H2 and CO. Ouyang et al. [7] validate the need to achieve endothermic and exothermic reactions in a single reactor stating the advantages of the co-gasification of coal with methane as follows: low production cost of syngas, adjustable H2/CO ratio in range 1–2, high steam and methane conversion, energy savings and flexibility in using various carbon containing feedstock.

The work by Kauchali [8] presents an interesting theoretical basis for the analysis of coal gasification process using bond equivalent diagrams developed by Battaerd & Evans [2] for elemental carbon. Here, it was shown that theoretical gasification thermally-balanced regions could be obtained purely by analysis of the basic stoichiometry of the coal-oxidants system. The results in [8] showed that real coal gasification systems operated in, or close to the regions, predicted theoretically, and that the method proved to be an indispensable tool in understanding underground coal gasification processes.

The co-gasification of coal with methane developed above, in principle, to produce syngas with a high H2:CO ratios rely on the fact that the partial combustion of coal is highly exothermic driving the endothermic steam reforming of methane in an autothermal and balanced manner. Unfortunately, this is only true for high grade coal with high calorific values (CV in MJ/kg). In this paper it will be shown, for a typical South African coals with low CV (bituminous and sub-bituminous), that certain critical co-feed conditions (amounts of CH4 and coal) are required to be met, to achieve a CO2-free syngas, that can be used in a polygeneration facility irrespective of the gasifier type and flow configurations. In addition, the minimum H2:CO ratio achievable for the thermally balanced co-gasification of SA coal will be determined – this limit will ultimately determine the minimum amount of renewable hydrogen needed for supplementation of the syngas for liquids production. Fundamentally, the need for steam in the gasification process, as a source of hydrogen, is obviated and the endothermic partial oxidation of coal is a practical way for temperature control on the limit of flame temperature in the combustion zone.
