Minimising CO₂ Emissions from Coal Gasification

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 low-grade coals can be co-gasified with methane to obtain CO2-free syngas ideal for power generation.

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 coal-bed 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 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.

2. Analysis of coal-methane Co-gasification

A systematic method of obtaining the stoichiometric reactions and thermally balanced region for the gasification of a South African coal from Bosjesspruit mine is studied. A carbon-hydrogen-oxygen (CHO) ternary or bond equivalent diagram is used for this type of analysis. A material balance, thermodynamic equilibrium as well as insight on the product composition of the gasification process can be accomplished with the ternary diagram. The feed and product highways are used to determine the stoichiometric reactions, indicated by the intersections, for the gasification process. Thereafter, the thermal nature of the reactions (endothermic or exothermic) can be used to determine the thermally balanced region of operation as done in other previous studies.

2.1 Ternary bond equivalent diagram

The composition of coal, indicated by coal composition charts, can be represented by three atomic species, namely, carbon (C), hydrogen (H) and oxygen (O). Hence, a ternary (CHO) graphical illustration can be used to represent the gasification reactions [2]. The ternary diagram is also referred to as a bond equivalent (BE) phase diagram. The diagram comprises of an equilateral triangular grid with lines parallel to the three sides drawn within the triangle and C, H and O on each vertex (see Figure 1). The vertex thus represents the pure component i.e., 100% C, H or O. The edges of the triangle represent a binary mixture of the atomic species. For example, a point alone the C-H edge will represent a species composed of C and H only (without any O). Furthermore, any point within the triangle will represent a mixture of the three atomic species with different compositions [9].

Figure 1.

Representation of a CHO ternary diagram [9].

The bonding capacity of the constituent elements is used to denote the species. For a species C_xH_yO_z, the bond equivalent compositions can be calculated using bonding capacity of each element and normalised accordingly.

2.2 Feed and product highways

The feed and product components and highways are plotted in Figure 2. Methane (CH₄) is represented by the point between 100% C and 100% H such that C and H have combined to achieve their normal valencies. The point midway between 100% H and 100% O represents water (H₂O) and the point midway between 100% C and 100% O represents carbon dioxide (CO₂) [2]. In this manner, using the BE composition calculations, the components are plotted. The Ultimate Analysis of the Bosjesspruit coal determines its molecular formula to be CH_0.75O_0.16 [8]. From the Proximate Analysis % [8]: moisture is 3.9, Ash is 32.8, Volatile Matter is 21.6 and Fixed Carbon is 52.2. The calorific value of the coal, as received, is 18.88 MJ/kg rendering it a low quality sub-bituminous coal.

Figure 2.

Feed and product highways for co-gasification of Bosjesspruit coal with methane.

The following calculations are used for the BE composition of the coal:

$$C=\frac{4\left(4\right)}{4\left(4\right)+1\left(0.75\right)+2\left(0.16\right)}=0.79$$E1

$$H=\frac{1\left(0.75\right)}{4\left(4\right)+1\left(0.75\right)+2\left(0.16\right)}=0.15$$E2

$$O=\frac{2\left(0.16\right)}{4\left(4\right)+1\left(0.75\right)+2\left(0.16\right)}=0.06$$E3

Additionally, the feed and product components are joined by the lines referred to as the feed and product highways. For example, the feed highways are the lines connecting coal to oxygen and methane to oxygen. The product highways are connected from H₂ to CO/CO₂ and H₂O to CO/CO₂ points.

2.3 Representation of important reactions for Co-gasification

The intersections between these highways represent the important gasification reaction points (stoichiometric reactions) as summarised in Table 1. The triangular points, in Figure 2, indicate the exothermic reactions (r1 to r6) and the circled points are the two endothermic reactions (r7 & r8).

No.	Reaction	Heat of reaction (kJ/mol)
r1	$C{H}_4+0.5O_2\to \mathit{CO}+2H_2$	−35.50 (exothermic)
r2	$C{H}_4+O_2\to {\mathit{CO}}_2+2H_2$	−318.70 (exothermic)
r3	$C{H}_4+1.5O_2\to \mathit{CO}+2H_{2}O$	−519.10 (exothermic)
r4	$C{H}_4+2O_2\to C{O}_2+2H_{2}O$	−802.30 (exothermic)
r5	$C{H}_{0.75}{O}_{0.16}+1.12O_2\to C{O}_2+0.375H_{2}O$	−272.53 (exothermic)
r6	$C{H}_{0.75}{O}_{0.16}+0.92O_2\to C{O}_2+0.375H_2$	−181.85 (exothermic)
r7	$C{H}_{0.75}{O}_{0.16}+0.61O_2\to \mathit{CO}+0.375H_{2}O$	10.68 (endothermic)
r8	$C{H}_{0.75}{O}_{0.16}+0.42O_2\to \mathit{CO}+0.375H_2$	101.35 (endothermic)

Table 1.

Balanced stoichiometric reactions for Bosjesspruit coal with the addition of methane.

The region bound by the stoichiometric reactions (r1-r8) shaded in light grey, referred to as the stoichiometric or gasification region, represents an important mass balance constraint for any gasification process that uses coal, methane and oxygen as feed. It is in this stoichiometric region that all solid carbon will convert to gas and all methane will reform to gas comprising H₂, CO, CO₂ and H₂O only. Operating out of this region (dark grey regions), by changing feed stoichiometry, will lead to excess amounts of feed not converting in the gasification process and hence be an inefficient conversion process and undesirable.

2.4 Thermally balanced reactions and region

The thermal nature of the reactions (endothermic or exothermic) indicated by the intersections can be used to determine the thermally balanced region (TBR) of operation. The TBR limits the gasification region such that there is no nett heat added or released to the gasifier; the endo- and exothermic reactions are paired thereby resulting in thermally balanced points with a heat of reaction of zero (kJ/mol). To develop a boundary around all the thermally balanced reactions, the extreme reactions, which are said to be linearly independent, are used. Operating within the thermally balanced region is desirable as it increases the thermal efficiency of the plant, eases operation and makes provision for economic savings. Generally, real and practical gasifiers will operate slightly away from the TBR on the “hot” side of the balance in order to use this excess heat to pre-heat the feed and or offset heat losses in the gasifier as explained by [2].

To determine the thermally balanced region of operation, the exothermic reactions (r1 to r6) are balanced with the endothermic reactions r7 and then r8. This results in the 12 reactions in Table 2. For example, r1 is balanced with r7 as follows:

No.	Reaction
A	$C{H}_{0.75}{O}_{0.16}+0.7579O_2+0.3007C{H}_4\to 1.3007\mathit{CO}+0.375H_{2}O+0.6014H_2$
B	$C{H}_{0.75}{O}_{0.16}+0.641O_2+0.0335C{H}_4\to 0.0335C{O}_2+\mathit{CO}+0.375H_{2}O+0.067H_2$
C	$\mathit{C}{\mathit{H}}_{\mathbf{0.75}}{\mathit{O}}_{\mathbf{0.16}}\mathbf{+}\mathbf{0.6383}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathbf{0.0206}\mathit{C}{\mathit{H}}_{\mathbf{4}}\mathbf{\to }\mathbf{1.0206}\mathit{CO}\mathbf{+}\mathbf{0.4161}{\mathit{H}}_{\mathbf{2}}\mathit{O}$
D	$C{H}_{0.75}{O}_{0.16}+0.6341O_2+0.0133C{H}_4\to 0.0133C{O}_2+\mathit{CO}+0.4016H_{2}O$
E	$\mathit{C}{\mathit{H}}_{\mathbf{0.75}}{\mathit{O}}_{\mathbf{0.16}}\mathbf{+}\mathbf{0.6263}{\mathit{O}}_{\mathbf{2}}\mathbf{\to }\mathbf{0.0377}\mathit{C}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathbf{0.9623}\mathit{CO}\mathbf{+}\mathbf{0.375}{\mathit{H}}_{\mathbf{2}}\mathit{O}$
F	$C{H}_{0.75}{O}_{0.16}+0.6248O_2\to 0.0554C{O}_2+0.9446\mathit{CO}+0.3542H_{2}O+0.0208H_2$
G	$\mathit{C}{\mathit{H}}_{\mathbf{0.75}}{\mathit{O}}_{\mathbf{0.16}}\mathbf{+}\mathbf{1.8475}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathbf{2.8549}\mathit{C}{\mathit{H}}_{\mathbf{4}}\mathbf{\to }\mathbf{3.8549}\mathit{CO}\mathbf{+}\mathbf{6.0849}{\mathit{H}}_{\mathbf{2}}$
H	$\mathit{C}{\mathit{H}}_{\mathbf{0.75}}{\mathit{O}}_{\mathbf{0.16}}\mathbf{+}\mathbf{0.738}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathbf{0.318}\mathit{C}{\mathit{H}}_{\mathbf{4}}\mathbf{\to }\mathbf{0.318}\mathit{C}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathit{CO}\mathbf{+}\mathbf{1.011}{\mathit{H}}_{\mathbf{2}}$
I	$C{H}_{0.75}{O}_{0.16}+0.7129O_2+0.1952C{H}_4\to 1.1952\mathit{CO}+0.3905H_{2}O+0.375H_2$
J	$C{H}_{0.75}{O}_{0.16}+0.6726O_2+0.1263C{H}_4\to 0.1263C{O}_2+\mathit{CO}+0.2526H_{2}O+0.375H_2$
K	$C{H}_{0.75}{O}_{0.16}+0.6064O_2\to 0.2711C{O}_2+0.7289\mathit{CO}+0.1017H_{2}O+0.2733H_2$
L	$\mathit{C}{\mathit{H}}_{\mathbf{0.75}}{\mathit{O}}_{\mathbf{0.16}}\mathbf{+}\mathbf{0.5989}{\mathit{O}}_{\mathbf{2}}\mathbf{\to }\mathbf{0.3579}\mathit{C}{\mathit{O}}_{\mathbf{2}}\mathbf{+}\mathbf{0.6421}\mathit{CO}\mathbf{+}\mathbf{0.375}{\mathit{H}}_{\mathbf{2}}$

Table 2.

Thermally balanced reactions for Bosjesspruit coal with the addition of methane.

$$r_1+r_7=\left(\frac{10.68}{35.50}\right)C{H}_4+\left(\frac{10.68}{35.50}\right)0.5O_2\to \left(\frac{10.68}{35.50}\right)\mathit{CO}+\left(\frac{10.68}{35.50}\right)2H_2$$E4

$$+C{H}_{0.75}{O}_{0.16}+0.61O_2\to \mathit{CO}+0.375H_{2}O$$

$$=C{H}_{0.75}{O}_{0.16}+0.7579O_2+0.3007C{H}_4\to 1.3007\mathit{CO}+0.375H_{2}O+0.6014H_2$$

$\therefore$ The heat of reaction for $r_1+r_7=\left(\frac{10.68}{35.50}\right)\left(-35.50\right)+10.68=0\mathit{kJ}/\mathit{mol}.$

The BE compositions for the thermally balanced equations are calculated and represented graphically by the black region in Figure 3. The enclosed area represented by the 5 reactions shows the thermally balanced region of operation – all thermally balanced reaction operate in this area. The area of operation to the right of the region represents exothermic reactions where the gasification syngas product comes out “hotter” than the feed. Also, the area to the left of the region represents endothermic reactions where products come out “colder” than the feed temperature.

Figure 3.

Thermally balanced region for gasification of Bosjesspruit coal with methane.

Table 2 summarises the 5 thermally balanced reactions in bold (C,E,G,H,L) that form the extreme boundary points and any other thermally balanced reaction (A,B,D,F,I,J,K) can be obtained by linear combinations of these 5 reactions. The MATLAB^(C)CONVHULL function was used to determine the extreme thermally balanced points.

3. Implications of thermally balanced regions

A systematic method of obtaining the stoichiometric reactions and thermally balanced region was developed above from which some very important results are noted. Firstly, the co-gasification of coal and methane is theoretically possible to produce syngas comprising H₂, CO, CO₂ and H₂O at varying compositions. Secondly, these results are consistent and independent of the type of gasifier chosen: fixed bed, fluidized bed or entrained flow, allowing to assess a number of gasification types within a single diagram.

Hence, this allows for the targeted approach to designing co-gasification based systems for either IGCC processes or gas to liquids processes and are discussed below.

3.1 Endothermicity of partial oxidation of Bosjesspruit coal

The reactions representing partial combustion of Bosjesspruit coal r7 and r8 (where syngas produced has significant calorific value) are naturally endothermic. This has a major implication to IGCC where only oxygen is used to obtain syngas with high heating value (HHV). Generally, for IGCC and power applications as seen for good quality coals (northern hemisphere), the partial oxidation leads to syngas rich in H₂ and CO only (no/little CO₂) which is desirable as it represents a clean (no CO₂) gas with high HHV. However, for Bosjesspruit coal, it is not sensible to obtain a H₂-CO only gas as that reaction is endothermic (r8). To obtain the onset of exothermic reactions the addition of oxygen will be required as shown in thermally balanced reaction r8, after which further oxygen will lead to more exothermicity (for higher gasification temperatures) of the system. However, this also means that a lower HHV value gas is obtained which not ideal for IGCC operation – this implies that South African coals of low quality will not be used for IGCC purposes if only oxygen is used. From the analysis in Section 2.4 above it is concluded that the Bosjesspruit coal (low quality) can only be used in IGCC application if methane is available and needs to operate between the thermally balanced reaction G and the partial oxidation of methane (r1) (Figure 3). This operation also lies on the H₂-CO line that connects point G and r1which produces clean syngas (no CO₂), is exothermic and high HHV suitable for IGCC operations, regardless of gasifier type.

3.2 Equilibrium considerations & excess steam addition

The thermally balanced reactions (C,E,G,H,L) on the edges of the balanced region (Figure 3) are targets for a co-gasification process and are obtained by the precise ratios of coal, methane and oxygen in the feed as well as at high operating temperature (>1700 K) and pressures (>20 bar). However, to obtain these precise outcomes also requires the consideration of thermodynamic equilibrium affecting the distribution of the products in the syngas. It was determined that this can be circumvented by the addition and removal of steam in the system as shown by the following example for thermally balanced reaction H chosen arbitrarily:

$$C{H}_{0.75}{O}_{0.16}+0.738O_2+0.318C{H}_4+1.17H_2\mathrm{O}\to 0.318C{O}_2+\mathit{CO}+1.011H_2+1.17H_2\mathrm{O}$$E5

At equilibrium (without) steam addition the product distribution would have been 1.22CO, 0.79H₂, 0.096CO₂, 0.22H₂O (1700 K & 50 bar)– the addition of steam (1.17) and the subsequent condensation of the same amount leads to the reactions as written by H. It is in this context that excess steam is implied for this work.

4. Application to coal-to-liquids

A sensible polygeneration facility, as described earlier [1] where both power generation and liquid fuels/chemicals are made, requires that the co-gasification process produce a syngas that is typically rich in H₂ and CO only. A portion of this syngas would be directed to an IGCC unit for power generation and the rest to a gas-to-liquids process. This was shown above that possible operational regions would be on the line connecting thermally balanced point G and the exothermic partial oxidation of methane (r1) – refer to Figure 3. Along this line of operation, the gasification reactions are inherently exothermic and it is preferred to operate closer to point G as the reactions are not highly exothermic (for temperature control in the gasifier) and less CH₄ (more coal) is used overall. However, this also means the syngas composition does not have a high H₂-content for the downstream gas-to-liquids conversion. This section covers some of the strategies employed to obtain a syngas feed composition suitable for methanol production as given by Higman & van der Burgt [10] as an example and the details are provided in Table 3 below. It is noted that this syngas requires the presence of up to 3.5% CO₂ for the catalyst to operate optimally. While traditional methods would deploy an additional Water-Gas-Shift reactor to correct the H₂:CO ratio, in this work it is not implemented due to the additional equipment cost, raising of additional steam and the CO₂ byproduct created directly by the WGS reaction and indirectly by raising steam.

The thermally balanced point at G represents the maximum H₂ that can be produced from the co-gasification process where the syngas product temperature equals the feed temperature. Generally, a real gasification process will operate just off of this point and into the “hot” exothermic side [2]. The methanol feed, point S, is shown in the ternary diagram, Figure 4. The task for the designer is to obtain this feed point starting from point G and requires either the addition of excess hydrogen and or the removal of CO₂ to obtain the final methanol feed composition. Several scenarios are provided and discussed where the choice of additional equipment for WGS was avoided.

Figure 4.

Target co-gasification points (P, Q) and methanol feed (S).

4.1 Excess hydrogen from electrolysis

To achieve the methanol feed point (see Figure 4) from point G, the addition of H₂ and CO₂ is necessary. Microsoft Excel’s solver tool is used to calculate the amount of H₂ and CO₂ required to achieve the correct H₂:CO ratio and CO₂ composition. The target co-gasification reaction (including additional steam for equilibrium consideration – See Section 3.2) is thus determined to be:

$$\begin{array}{l}C{H}_{0.75}{O}_{0.16}+1.4285O_2+1.7118C{H}_4+1.6543H_{2}O\\ \to 2.4065\mathit{CO}+3.7986H_2+0.3053C{O}_2+1.6543H_{2}O\end{array}$$E6

This output is represented by point P on Figure 4 and represents the syngas operating point from the exit of the co-gasification process. The final methanol feed is obtained by the addition of excess H₂ (such that H₂:CO = 2.44 as required in Table 3) obtained from a CO₂-free source such as solar-electrolysis of water. The choice of CO₂-free H₂ introduces the possibility of including renewable resources, to minimise additional CO₂ production, into existing fossil fuel based facilities and in this particular case indicates the minimum amount of renewable H₂ needed for a co-gasification process to exist. For this Bosjesspruit coal the amount of H₂ needed, to obtain the final methanol feed from point P, represents the minimum amount of renewable H₂ needed for co-gasification with methane. Point P may also be implemented for IGCC application as it has a relatively high HHV and is lowest CO₂ in the syngas.

4.2 Obtaining methanol feed by removal of CO₂ from Co-gasification process

Another possibility of obtaining the methanol feed from point G, excluding water electrolysis, is by operating the co-gasification process such that some CO₂ is allowed to be formed allowing the H₂:CO to correct itself internally (no external WGS reactor). This is represented by the point Q on Figure 4. From this point, after cleaning the syngas for contaminants (Sulphur, particulates etc) some CO₂ (1.05) is removed to obtain the final methanol feed composition as required. The balanced reaction from the co-gasification process (including additional steam for equilibrium) is thus:

$$\begin{array}{l}C{H}_{0.75}{O}_{0.16}+2.5265O_2+2.8549C{H}_4+12H_2\mathrm{O}\\ \to 2.4967\mathit{CO}+6.0848H_2+1.3582C{O}_2+12H_2\mathrm{O}\end{array}$$E7

4.3 Methanol from traditional gasification of Bosjesspruit coal

The analysis for traditional gasification of Bosjesspruit with steam and oxygen has been done elsewhere [8] and the important thermally balanced reactions are provided in Table 4 below:

Component	mol %
CO2	3.50
CO	27.59
H2	67.24

Table 3.

Methanol synthesis feed composition [2].

No.	Reaction
M	$C{H}_{0.75}{O}_{0.16}+0.599O_2\to 0.642\mathit{CO}+0.375H_2+0.358{\mathit{CO}}_2$
N	$C{H}_{0.75}{O}_{0.16}+0.544O_2+0.752H_{2}O\to C{O}_2+1.127H_2$

Table 4.

Thermally balanced reactions for traditional Bosjesspruit gasification [8].

Reaction M and N are used as basis to determine the gasification operation point that is exothermic and lies on the line that connects the final methanol feed and CO₂ point. This is required as the final step requires the removal of CO₂. Figure 4 shows the line M-N as well as the gasification point R for the coal only system. The Gibbs Free reactor was used on Aspen Plus at 1700 K and 50 bar to determine the equilibrium reaction (amount of H₂O needed for equilibrium). The resulting overall reaction, represented by R (including excess steam for equilibrium), is as follows:

$$\begin{array}{l}C{H}_{0.75}{O}_{0.16}+0.665O_2+0.2528H_2\mathrm{O}+6.1H_{2}O\\ \to 0.2576\mathit{CO}+0.6278H_2+0.7424C{O}_2+6.1H_{2}O\end{array}$$E8

5. Comparison of processes for poly-generation operation

The three methods to produce syngas for the poly-generation system was described where the H₂:CO ratio required for methanol production was obtained either by co-gasification of coal and methane then adding H₂ from electrolysis using renewable energy (4.1), or from co-gasification with CO₂ removal (4.2) or from the traditional steam-oxygen gasification (4.3) of the same Bosjesspruit coal from a South African mine.

The process reactions are summarised below normalised (per mol of CO) in the syngas produced:

Co-gasification with H₂ addition from Electrolysis

$$\begin{array}{l}0.4156C{H}_{0.75}{O}_{0.16}+0.5936O_2+0.7112C{H}_4+0.6875H_{2}O\\ \to \mathit{CO}+2.4371H_2+0.1269C{O}_2+0.6875H_{2}O\end{array}$$E9

Co-gasification with CO₂ removal:

$$\begin{array}{l}0.4005C{H}_{0.75}{O}_{0.16}+1.0119O_2+1.1435C{H}_4+4.806H_{2}O\\ \to \mathit{CO}+2.4371H_2+0.5441C{O}_2+4.806H_{2}O\end{array}$$E10

Traditional CTL:

$$\begin{array}{l}3.8819C{H}_{0.75}{O}_{0.16}+2.5815O_2+0.9814H_2\mathrm{O}+23.68H_{2}O\\ \to \mathit{CO}+2.437H_2+2.8819C{O}_2+23.68H_{2}O\end{array}$$E11

Table 5 compares the various processes (per mol of CO) against the amount of excess steam required for equilibrium and the excess CO₂ produced.

Process	Excess Steam/CO	Multiple Factor Steam/CO	Excess CO2/CO	Multiple Factor CO2/CO	Comment
Co-gasification with H2 from electrolysis	0.69	1	—		no excess CO2 but requires electrolysis
Co-gasification with CO2 removal	4.81	7	0.42	1	requires CO2 removal – no WGS reactor
Traditional CTL; (simultaneous WGS)	23.68	34	2.76	7	requires CO2 removal – no WGS reactor

Table 5.

H₂O required and CO₂ produced (per Mol of CO) for various processes.

As seen in Table 5, the best case scenario which requires minimal steam and does not produce excess CO₂ is from the co-gasification of coal with methane with the addition of H₂ from renewable-electrolysis. It is noted that the small amount of CO₂ in the methanol feed (or gasification product) is a requirement for optimal catalyst performance and requires the co-gasification process to operate away from the preferred H₂-CO line where equilibrium is favoured. Hence all operations in the ternary diagram that operate away from the H₂-CO line will invariably require excess steam and hence produce excess CO₂ – albeit without a separate WGS reactor.

The traditional CTL process is by far the worst in performance as it produces the most CO₂ and requires the most excess steam. When comparing the two non-electrolysis (for high H₂) processes it is evident that the co-gasification of coal with methane is also superior to the traditional CTL process. Here, the excess steam required for co-gasification process is about 5 times less and up to 7 times less CO₂ is emitted than the traditional CTL process. This is an important result for re-looking at the way traditional CTL is done in South Africa and other developing countries intending to use low grade coal for power generation and or liquids fuel/chemicals production.

6. Conclusions

The co-gasification of coal and methane has been studied from a fundamental understanding of basic mass and energy balances for the purposes of producing syngas for polygeneration facilities where power and liquid fuels are required. Here a South African coal from Bosjesspruit mine is studied in various process routes, namely co-gasification with methane with H₂ addition from water electrolysis using renewable energy, co-gasification with CO₂ removal and the traditional gasification of coal using steam and oxygen. An important result showed that the poor quality of the Bosjesspruit coal requires the co-feeding of methane in the gasification and polygeneration process to produce a syngas rich in H₂-CO ready for IGCC purposes or further treated for liquid fuels/chemicals production. This coal would otherwise be only used for liquid fuels production resulting in high CO₂ emissions and with large requirements for water in the process.

Moreover, a technique of graphical analysis for co-gasification of coal with methane was presented forming the basis for decision making in poly-generation facilities. This allows for the quick screening of coal types as well as strategies required to design co-gasification processes. Lastly, this analysis is independent of the gasification reactor type allowing designers to narrow the options required for their purposes.