**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].

The bonding capacity of the constituent elements is used to denote the species. For a species *CxHyOz*, 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

**Figure 1.** *Representation of a CHO ternary diagram [9].*

**Figure 2.** *Feed and product highways for co-gasification of Bosjesspruit coal with methane.*

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 *CH0.75O0.16* [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.

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

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

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

$$O = \frac{2(0.16)}{4(4) + 1(0.75) + 2(0.16)} = 0.06\tag{3}$$

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).


#### **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:

$$r\_1 + r\_7 = \left(\frac{10.68}{35.50}\right) \text{CH}\_4 + \left(\frac{10.68}{35.50}\right) 0.5 \text{O}\_2 \to \left(\frac{10.68}{35.50}\right) \text{CO} + \left(\frac{10.68}{35.50}\right) 2 \text{H}\_2 \tag{4}$$

$$+ \text{CH}\_{0.75}\text{O}\_{0.16} + 0.61 \text{O}\_2 \to \text{CO} + 0.375 \text{H}\_2\text{O}$$

$$= \text{CH}\_{0.75}\text{O}\_{0.16} + 0.7579 \text{O}\_2 + 0.3007 \text{CH}\_4 \to 1.3007 \text{CO} + 0.375 \text{H}\_2\text{O} + 0.6014 \text{H}\_2$$

<sup>∴</sup> The heat of reaction for *<sup>r</sup>*<sup>1</sup> <sup>þ</sup> *<sup>r</sup>*<sup>7</sup> <sup>¼</sup> <sup>10</sup>*:*<sup>68</sup> <sup>35</sup>*:*<sup>50</sup> ð Þþ �35*:*<sup>50</sup> <sup>10</sup>*:*<sup>68</sup> <sup>¼</sup> <sup>0</sup> *kJ=mol:*

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


#### **Table 2.**

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

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.

**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,

**Figure 3.** *Thermally balanced region for gasification of Bosjesspruit coal with methane.*

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.
