# 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 Radiant Heat Transfer
The \(k_{\mathrm{C,q}}\) for a coal-fired boiler can be disregarded, while \(k_{\mathrm{K}}\mu_{\mathrm{K}}\) would not change owing to the unchanged coal characteristics [28]. The \(k_{\mathrm{q}}\) rises as a result of the rising volume fraction of CO\({}_{2}\), while \(k_{\mathrm{h}}\mu_{\mathrm{h}}\) decreases due to the decreasing fly ash concentration after the introduction of biomass syngas. Consequently, the \(k\) descends as the biomass syngas consumption rate rises according to Figure 3d. The heat absorption of flue gas (\(Q_{\mathrm{H}}\), kW) is determined by the average temperature and radiative characteristics of the flame, and the size and shape of the furnace [24]. The average temperature of the flame decreases due to the falling theoretical combustion temperature. Compared with the solely coal-fired condition, the heat absorption ascends after introducing the biomass syngas. For the introduction of wood syngas, the heat absorption descends with rising biomass syngas consumption rates. However, the heat absorption for introducing palm and straw syngases does not present a monotonous trend (see Figure 3e), since the complex combustion products produced by burning mixed fuels can contribute to radiant heat transfer in numerous ways [28]. Since the difference between the available heat and heat absorption of flue gas ascends, the enthalpy of flue gas, which reflects the variations of the volume, specific heat, and temperature in flue gas, rises as the biomass syngas consumption rate increases. It can be seen that \(T_{1}^{\prime\prime}\) declines with ascending biomass syngas consumption rates in Figure 3f. \(T_{1}^{\prime\prime}\) decreases by 2.1% for palm syngas and straw syngas, while that for wood syngas can reach 2.3%, as 5 \(\times\) 10\({}^{4}\) m\({}^{3}\) h\({}^{-1}\) of biomass syngas is entrained. To evaluate the influence of boiler load variation on the thermodynamic parameters after the introduction of biomass syngas, the consumption rate of the biomass syngas would not change (3 \(\times\) 10\({}^{4}\) m\({}^{3}\) h\({}^{-1}\) in this study) because of the difficulty of adjusting the load of the gasifier. The comparisons of the primary thermodynamic parameters between burning coal alone and co-firing coal with biomass syngas (3 \(\times\) 10\({}^{4}\) m\({}^{3}\) h\({}^{-1}\) of wood syngas) under different boiler loads are revealed in Figure 4. The results demonstrate that the available heat and water vapor volume fraction ascend after coupling the wood syngas, while the radiant attenuation factor and furnace exit flue-gas temperature descend after the introduction of syngas. Meanwhile, the influence of wood syngas introduction on the theoretical combustion temperature and heat absorption is weakened. For the solely coal-fired condition, the available heat declines with falling boiler loads, while that for the wood syngas introduction shows a reverse trend. It could be confirmed that the introduction of wood syngas affects the available heat more significantly under a low load (see Figure 4a). The syngas from the gasification of the woody biomass contains low levels of nitrogen, a high volume fraction of combustible materials, and a relatively high heating value, which would result in a higher water vapor volume fraction after the introduction of wood syngas (see Figure 4c). As the consumption rate of biomass syngas remains unchanged, operating the boiler under lower loads means a higher consumption rate percentage of biomass syngas than that of coal. Thus, the water vapor volume fraction increases more dramatically under low boiler loads. The variation in the water vapor volume fraction would affect that of the radiant attenuation factor, which can be seen in Figure 4d. The difference ratios are 4.6% for the water vapor volume fraction and 12.1% for the radiant attenuation factor under 30% THA. The other three thermodynamic parameters decrease when the boiler load descends (see Figure 4b,e,f).