## 2 Method
### Model for Simulations  
Figure 3 shows the mathematical blocks for the calculation of mass and energy balance of the conceptual process described above. CHEMCAD (v. 7.1, Chemstations Inc., Houston, TX, USA) chemical process simulator was used. Dry, ash-free, biomass feedstock is characterised by the empirical molecular form C\({}_{\alpha}\)H\({}_{\beta}\)O\({}_{\gamma}\)N\({}_{\psi}\)S\({}_{\epsilon}\). This feedstock is considered to be combustion solid with a lower heating value (LHV) of 20 MJ per kg of dry, ash-free biomass. The assumption is made that the humidity content of the biomass is 5% on a dry, ash-free  
basis and that it contains 0.5% of inert, inorganic material on a dry, ash-free basis. A flow of 1000 kg/h of ash-free, dry biomass is fed into Zone-I of the gasifier, along with its water and ash content. The biomass stream BIOM shown in Figure 3 is considered to contain dry, ash-free biomass along with humidity and some inorganic material. This inorganic material is considered inert. For the purpose of calculation, the following reactions are considered for Zone-I of the gasifier:  
\[ \begin{split}\mathrm{C_{\alpha}H_{\beta}O_{Y}N_{\psi}S_{\varepsilon }+a\,O_{2}\to b\,CO_{2}+c\,H_{2}O+d\,NO_{2}+e\,SO_{2}}\end{split}  \tag{7}\]  
\[ \begin{split}\mathrm{C_{\alpha}H_{\beta}O_{Y}N_{\psi}S_{ \varepsilon}\to f\,H_{2}+g\,CO+h\,CO_{2}+i\,H_{2}O+j\,CH_{4}+k\,C_{2}H_{4}+1 \,COS+m\,H_{2}S}\\ +n\,NH_{3}+p\,ORG+q\,CHARORG\end{split}  \tag{8}\]  
Figure 3: Block calculation diagram. Reaction (7) represents the partial oxidation of biomass to generate a gas mixture made of CO\({}_{2}\), H\({}_{2}\)O, nitrogen dioxide (NO\({}_{2}\)), and sulphur dioxide (SO\({}_{2}\)). Reaction (7) supplies the heat required for the pyrolysis process (Reaction (8)) to proceed. Reaction (8) models the pyrolysis process in which ORG represents any organic material (including tars, but excluding the char) coming out of the pyrolysis process, CHARORG represents the organic part of the solid carbonaceous material (the char) exiting the gasifier, C\({}_{2}\)H\({}_{4}\) is ethylene, COS is carbonyl sulfide, H\({}_{2}\)S is hydrogen sulfide, and NH\({}_{3}\) is ammonia. The CHAR stream exiting the gasifier contains an organic part (called CHARORG) along with the inert inorganic (called INORG) content of the biomass feed. Organics (ORG) and CHARORG are represented by the respective empirical formulas: C\({}_{\alpha}\)-H\({}_{9}\)-O\({}_{\gamma}\)-N\({}_{\Psi}\)-S\({}_{\epsilon}\)- and C\({}_{\alpha}\)-H\({}_{9}\)-O\({}_{\gamma}\)-N\({}_{\Psi}\)-S\({}_{\epsilon}\)-. Table 1 lists the values of the coefficients used in the calculations, related to the composition of ORG, CHARORG, and the dry, ash-free biomass. Tables 2 and 3 show stoichiometric coefficients for Reactions (7) and (8), respectively. In Reaction (8), for the purpose of the calculations, the assumption is made that the pyrolysis of 1 kg of dry, ash-free biomass generates 0.18 kg of organic material (ORG) and 0.8 kg of CHARORG and that 20% of the carbon from the biomass is converted into CO\({}_{2}\), 15% into CH\({}_{4}\), 7.5% into C\({}_{2}\)H\({}_{4}\), and 0% into COS.  
As shown in Figure 3, the gasifier is characterised by four mathematical blocks:  
* COMB: Adiabatic, stoichiometric calculation of Reaction (7);
* PYR: Stoichiometric calculation of Reaction (8) assuming a heat loss of 100 kW is specified (1.8% of the energy content of the biomass);
* CHARSEP: Separation of the organic part CHARORG from the inorganic components of the char that originate from the biomass;
* OXY: Calculation--using Gibbs free energy minimization algorithm--of the equilibrium composition after injection of O\({}_{2}\)/steam mixture, assuming a heat loss of 150 kW (2.7% of the biomass energy content). The first three blocks model Zone-I, as shown in Figure 1 (or standard gasifier in Figure 2), while the OXY block models Zone-II of the gasifier (tar conversion zone). The pressure is specified at 4 bars for these blocks, except for block OXY, in which pressure is specified at 3 bars.