Primary operating conditions of the gasification reaction (as shown in Fig. 2) are biomass feed rate and composition, equivalence ratio or steam-to-biomass ratio, types and amounts of other oxidizing agents, reactor temperature profile, and supplemental heat in case of an indirectly heated gasifier. These operating conditions affect yield and properties of products (also shown in Fig. 2) such as syngas (or producer gas) flow rate and gas composition, and contents of tars, particulates, NH3, and H2S. Overfeeding biomass can lead to plugging whereas underfeeding can lead to underutilization of reactor volume. Contents of cellulose, hemicellulose and lignin in lignocellulosic biomass also effects the products. However, in general, thermochemical processes can utilize lignin and accept biomass with variable contents of cellulose, hemicellulose and lignin. Equivalence ratio (ER) and/or steam to biomass ratio significantly affect the products. In

Figure 2. Gasification process variables.

air gasification, increase in ER results in an increase in reactor temperature due to increased degree of biomass oxidation. However, with an increase in ER energy efficiencies and concentration of CO and H2 gases initially increase and then decrease after reaching optimum levels. Sharma et al. (2011) found the optimum ER to be 0.32 achieving the maximum hot gas efficiency of 75% for fluidized-bed gasification of switchgrass (Sharma et al.

2011) . Supplying steam into gasifier has shown to increase H2 content and reduce tar content in the syngas due to steam reforming reactions (Narvaez et al. 1996). However, gasification temperatures need to be kept at high level (above 750-800°C) to promote the steam reforming reactions (Lucas et al. 2004; Kumar et al. 2009; Gupta and Cichonski 2007).

Temperature profile of the gasifier is one of the most influential factors affecting the yield and composition of products. Temperature profile in turn depends on amount of oxidizing agents supplied and heat added, if any. Gonzales et al. showed that contents of H2 and CO increased while contents of CH4 and CO2 decreased when the temperature was increased from 700 to 900°C (Gonzalez et al. 2008). CO/CO2 increased linearly from 0.85 at 700°C to 2.7 at 900°C. The trend is possibly due to increase in Boudouard reaction, which becomes predominant at high temperature. Boateng et al. observed that with an increase in gasification temperature from 700 to 800°C, gas yield, gas heating value, energy efficiency, and H2 content increase while CH4, CO and CO content decreased (Boateng et al. 1992). The literature overwhelmingly suggests improvement in gas composition with increase in gasification temperature. However, additional energy penalty, ash agglomeration and need of strong reactor material at high temperature (>900°C) limit its applicability.