**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-toliquids 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.

#### **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


**Table 3.** *Methanol synthesis feed composition [2].*

**Figure 4.** *Target co-gasification points (P, Q) and methanol feed (S).*

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:

$$\begin{aligned} &\text{2CH}\_{0.75}\text{O}\_{0.16} + \text{1.4285O}\_{2} + \text{1.7118CH}\_{4} + \text{1.6543H}\_{2}\text{O} \\ &\rightarrow 2.406\text{5CO} + \text{3.7986}H\_{2} + 0.3053\text{CO}\_{2} + \text{1.6543H}\_{2}\text{O} \end{aligned} \tag{6}$$

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:

$$\begin{aligned} &2\,\text{CH}\_{0.75}\text{O}\_{0.16} + 2.52\,\text{65O}\_{2} + 2.8549\,\text{CH}\_{4} + 12\,\text{H}\_{2}\text{O} \\ &\to 2.4967\text{CO} + 6.0848\,\text{H}\_{2} + 1.3582\,\text{CO}\_{2} + 12\,\text{H}\_{2}\text{O} \end{aligned} \tag{7}$$

#### **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:


**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:

$$\begin{aligned} &\text{CH}\_{0.75}\text{O}\_{0.16} + 0.665\text{O}\_{2} + 0.2528\text{H}\_{2}\text{O} + 6.1\text{H}\_{2}\text{O} \\ &\rightarrow 0.2576\text{CO} + 0.6278\text{H}\_{2} + 0.7424\text{CO}\_{2} + 6.1\text{H}\_{2}\text{O} \end{aligned} \tag{8}$$

### **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

$$\begin{aligned} &0.4156 \text{CH}\_{0.75} \text{O}\_{0.16} + 0.5936 \text{O}\_2 + 0.7112 \text{CH}\_4 + 0.6875 \text{H}\_2\text{O} \\ &\rightarrow \text{CO} + 2.4371 \text{H}\_2 + 0.1269 \text{CO}\_2 + 0.6875 \text{H}\_2\text{O} \end{aligned} \tag{9}$$

Co-gasification with CO2 removal:

$$\begin{aligned} &0.4005CH\_{0.75}O\_{0.16} + 1.0119O\_2 + 1.1435CH\_4 + 4.806H\_2O \\ &\rightarrow CO + 2.4371H\_2 + 0.5441CO\_2 + 4.806H\_2O \end{aligned} \tag{10}$$


#### **Table 5.**

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

Traditional CTL:

$$\begin{aligned} &3.8819CH\_{0.75}O\_{0.16} + 2.5815O\_2 + 0.9814H\_2O + 23.68H\_2O \\ &\rightarrow CO + 2.437H\_2 + 2.8819CO\_2 + 23.68H\_2O \end{aligned} \tag{11}$$

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

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 nonelectrolysis (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.
