# The Influence of Co-Firing Coal with Biomass Syngas on the Thermodynamic Parameters of a Boiler
## 1 Introduction  
CO\({}_{2}\) emissions, which contribute to 76% of entire greenhouse gas emissions, can lead to a number of problems driven by global warming [1]. Moreover, power generation industries in China are facing tremendous pressure to mitigate CO\({}_{2}\) emissions, which should be lower than 550 g kW\({}^{-1}\) h\({}^{-1}\)[2]. Therefore, reducing CO\({}_{2}\) emissions has become a primary issue, especially emissions from the combustion of fossil fuels. Biomass can be adopted as an alternative energy resource to partially substitute coal in a boiler, which is a promising solution to reduce the emissions of CO\({}_{2}\) and NO\({}_{x}\), as well as improve the combustion characteristics of fuel owing to its renewability, CO\({}_{2}\) neutrality, and relatively high volatile-matter content [3; 4; 5]. Co-firing technologies include direct, indirect, and separate co-firing [6]. For direct co-firing, fuels are milled before entering the furnace. Although this method is a straightforward and economical option, it can create some challenges, such as the pre-treatment and feeding of biomass, fouling, and corrosion, which prevents it from being widely applied [7]. Separate co-firing can be adopted to increase the steam capacity of an existing boiler as a separate biomass-fired boiler is added to the entire system, which is costly and redundant [6]. For indirect co-firing, the biomass must be converted into syngas by a gasifier at first, and then the biomass syngas enters the furnace and is co-fired with the coal [8]. The interaction between alkali species from the biomass and the sulfur from the coal during co-firing might alleviate the ash deposition created by biomass combustion [9]. A highnumber of problems related to corrosion and deactivation can also be mitigated by indirect co-firing [7; 10]. The distinct benefit of the indirect co-firing method is the preservation of some substances (containing K, Na, S, and Cl) that ordinarily contribute to the high-temperature corrosion of the water-cooled wall in the furnace. In addition, serious fouling of the heat exchangers produced by the inorganic components that contain K, Na, S, and Cl can be prevented [7]. Therefore, the indirect co-firing of coal with biomass syngas is an attractive scheme. A great number of experimental [9; 11; 12; 13; 14; 15; 16; 17] and numerical studies [8; 18; 19; 20; 21; 22] have been performed on co-firing in a boiler. The researchers mainly focus on the impacts on NO\({}_{x}\) emissions [12; 19], slagging/ash deposition [9; 13; 14], combustion characteristics [15; 16], and temperature distribution [8; 18; 19]. The variations in the main thermodynamic parameters, such as the radiant attenuation factor, furnace exit flue-gas temperature, thermal efficiency, and coal combustion rate, are not fully evaluated. Additionally, the thermal calculations of the principles of mass conservation, heat conservation, and heat transfer are precise and helpful in understanding the design, operation, and retrofit of boilers through the variations in the thermodynamic parameters [2]. However, only a few studies [23; 24] (our previous studies) have been conducted to investigate the influence of co-firing on the thermodynamic parameters through thermal calculations, to our knowledge, but not for co-firing with the biomass syngas, which is beneficial to the reduction in CO\({}_{2}\) emissions. Moreover, it is significant to study the indirect co-firing method under different loads, especially a low load, in conjunction with the current deep-peaking issue. Biomass syngas, especially syngas from the gasification of woody biomass, contain low levels of nitrogen, virtually no sulfur, and have a relatively high heating value, which can lead to an apparent reduction in NO\({}_{x}\) emissions [12]. Thus, it is beneficial to study the effect of co-firing as the boiler load alters.