The gasification process requires the addition of an oxidizing or gasifying agent, such as air, oxygen, steam, carbon dioxide or combination of these. Depending on gasifying agent used, different calorific values of the product gas will be obtained (Wang, 2008). For example, if steam is used as the gasifying agent, the heating values of the product gas are typically 10-15 MJ/m3N and in case of air typically 3-6 MJ/m3N (Wang, 2008) for gasification of biomass. Typical gas compositions using air and oxygen/steam as gasifying agents in fluidized bed gasification are shown in Table 6.1.

Two parameters used to describe the measures of the air or oxygen flowrate is the equivalence ratio (ER) and the superficial velocity (SV). ER and SV are defined as the ratio of airflow to the airflow required for stoichiometric combustion of the biomass, which indicates the extent of partial combustion, and as the ratio of air flow to the cross-sectional area of the gasifier, which removes the influence of gasifier dimension by normalization, respectively (Yamazaki, 2005). Therefore, both the ER and the SV are directly proportional to the airflow. In case of using air as the gasifying agent, the air supplies with oxygen for the combustion process and by varying the air flow, in for example fluidized bed gasification; it will influence the degree of combustion, which also affects the gasification temperature. A higher airflow rate, results in a higher temperature, resulting in a higher biomass conversion. This is true as long as oxygen is not supplied in excess causing a too high degree of combustion of the biomass, resulting in decreased energy content of the product gas. If a too high air flow is used, the residence time will be shorter, which may reduce the extent of biomass conversion. Other factors of importance for the effect of ER on the product gas are e. g. temperature and steam to biomass ratio.

Steam as a gasifying agent increases the partial pressure of H2O in the gasifier. This favors the water gas reaction (eq. 6.4) as well as water gas shift (eq. 6.6) and methane reforming reactions (eq. 6.5), where the latter two occur at gasification temperatures above 750-800°C (Kumar, 2009; Turn, 2010; Lucas, 2004). The addition of steam results in an increased amount of H2 in the product gas. The temperature of steam is generally lower than the gasification temperature and therefore a large amount of heat is needed to elevate the steam temperature and thereby maybe lowering the temperature in the gasifier. This need more pronounced above a certain steam-to — biomass ratio where the product gas will be affected negatively. It is therefore recommended to preheat the steam or any gasifying agent before introducing it into the gasifier, to induce a higher gasification temperature.