**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:

*CH*0*:*75*O*0*:*<sup>16</sup> þ 0*:*738*O*<sup>2</sup> þ 0*:*318*CH*<sup>4</sup> þ 1*:*17*H*2O ! 0*:*318*CO*<sup>2</sup> þ *CO* þ 1*:*011*H*<sup>2</sup> þ 1*:*17*H*2O (5)

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.
