# The Influence of Co-Firing Coal with Biomass Syngas on the Thermodynamic Parameters of a Boiler
## 4 Results and Discussion
### Effect of Co-Firing on Thermal Efficiency  
Figure 7 reveals the results of the exhaust gas temperature, thermal efficiency, and coal consumption rate through heat balance calculations. As the biomass syngas is entrained, the alterations of the component, velocity, volume, and physical characteristics of the flue gas, which happen in the processes of convective and radiant heat transfer in the boiler, would result in the exhaust gas temperature variation according to Figure 7a. The exhaust gas temperature increases with increasing biomass syngas consumption rates. The highest variations of the exhaust gas temperature for various consumption rates under a full load are 9 \({}^{\circ}\)C for the wood syngas, 10 \({}^{\circ}\)C for the palm syngas, and 14 \({}^{\circ}\)C for the straw syngas. It can be inferred from Equations (8) and (9) and the assumption made in Section 3 that the thermal efficiency (\(\eta\), %) is mainly determined by the heat losses owing to the exhaust gas (\(q_{2}\), %), while the other heat losses remain unchanged. The growth of the exhaust gas temperature would lead to the growth of \(q_{2}\), and then the thermal efficiency would descend with increasing biomass syngas consumption rates (see Figure 7b). A combination of the available heat of the working fluid and the thermal efficiency would reduce the coal consumption rate, as the biomass syngas consumption rate increases (see Figure 7c). As  
Figure 6: Some parameters comparison related to heat transfer of a low-temperature superheater under various boiler loads (wood syngas, 450 \({}^{\circ}\)C). (**a**) Heat transfer coefficient; (**b**) temperature difference; (**c**) heat transfer quantity.  
\(3\times 10^{4}\) m\({}^{3}\) h\({}^{-1}\) of wood syngas is entrained, the thermal efficiency is reduced by 0.4%, while the coal consumption rate is declined by 5.1%. The variations of the exhaust gas temperature and thermal efficiency under various coupling percentages of the fuels between this current study and the previous studies [23,24] are quite different due to the different fuels co-firing with the coal, while it could be confirmed from these three studies [23,24] that the coal consumption rate would significantly descend with increasing coupling percentages of the fuels.  
The alterations of the exhaust gas temperature, thermal efficiency of the boiler, and coal consumption rate under various boiler loads after introducing \(3\times 10^{4}\) m\({}^{3}\) h\({}^{-1}\) of the wood syngas are shown in Figure 8. Coupling wood syngas with the coal would lead to the growth of the exhaust gas temperature (see Figure 8a), but reduce the thermal efficiency of the boiler and the coal consumption rate (see Figure 8b,c). The alterations of the exhaust gas temperature and the thermal efficiency are not monotonous as the boiler load is changed. However, the increment (for the exhaust gas temperature) or the decrement (for the thermal efficiency and the coal consumption rate) rises monotonously with declining boiler loads. When the boiler operates under 30% THA, the exhaust gas temperature is increased by 7.5%, while \(\eta\) and \(B\) are decreased by 0.5% and 13.2%, respectively.