The process of converting LB to products using pri­marily heat as the engine of conversion is thermochem­ical conversion. Thermochemical processing appears more promising than bioconversion of the lignin frac­tion of the LB in that it serves as a source of process energy and the coproducts have benefits in a life-cycle context; however, it has a detrimental effect on enzy­matic hydrolysis (Lynd et al., 1999, 2005; Lynd and Wang, 2004). This method differentiates on how much air is supplied to the conversion, as shown in Figure 27.7. If LB is heated in the presence of excessive amounts of air, specifically oxygen, then the biomass will combust. If the amount of air or elements of air is limited then gasification will occur. Lastly, if no air is allowed then pyrolysis or hydrothermal liquefaction is the outcome.

Combustion

Combustion is a result of a complicated network of exothermic chemical reactions. The reaction generates copious amounts of heat and radiation. The reaction tends to be self-perpetuating and continues spontane­ously due to the large amount of heat generated by the reaction. Specifically, combustion is when carbon, hydrogen, combustible sulfur, and nitrogen in LB react with oxygen. The process includes fusion, evaporation, pyrolysis, a gas phase, and surface reactions.

Combustion of solids can take place in many forms including evaporation combustion, decomposition com­bustion, surface combustion, and smoldering combus­tion. Evaporation combustion is when fuel, with a relatively low fusing point, fuses and evaporates by heating, and reacts with gaseous oxygen and burns. When gasses such as H2, CO, CmHn, H2O, and CO2 are produced by thermal decomposition and react with O2, it is called decomposition combustion. A common by-product of evaporation and decomposition combus­tion is char. Char burns by surface combustion. Surface combustion occurs when the material only contains car­bon and small amounts of volatile compounds such as charcoal, oxygen, carbon dioxide, or steam. When these compounds diffuse into pores inside or on the surface of the solid they burn in a reaction of the surface of the ma­terial. Lastly, smoldering combustion is a slower and lower temperature reaction. It is a form of thermal decomposition that takes place at a temperature below the ignition temperature of the volatile components of the LB. If the temperature is forced up, smoke might be produced or the reaction may ignite. If it ignites the reaction is referred to as flammable combustion. Indus­trial LB conversion processes commonly employ decom­position or surface combustion.

Gasification

Gasification is the conversion of LB from its solid form to fuel gas or syngas. Syngas is simply a chemical feedstock in its gas phase. These might be CO, H2, CH4, CO2 and N2 as well as char (Balat, 2008b; Demirbas, 2004; Li and Suzuki, 2009).

Gasification is a broad treatment method and pro­duces a variety of different results depending on how it is controlled. Manipulating pressure, temperature, heating method, and conversion agent produces specific results. Pressure is usually controlled for either normal pressure (0.1—0.12 MPa) or high-pressure conditions

(0.5—2.5 MPa). Temperatures are usually either operated under low-temperature (<700 °C) or high-temperature (>700 °C) conditions. High-temperature decomposition may occur at the ash fusion point or above. Indirect gasification occurs when heating the raw material and gasification agent using an external heat source. Direct gasification occurs when heat generated from the reaction of partial gasification of raw material and oxygen is used as the heating source. The gasification agent is another variable with significant influence on the product. An agent is any combination of air, oxygen, or steam. Additionally, carbon dioxide maybe used for a set period of time to affect the product.

There are a variety of gasifiers in use today. Fixed bed, flow bed, circulating flow bed, entrained bed, mixing bed, rotary kiln, twin tower, and molten furnace are examples of industrial gasifiers (Yokoyama, 2008).

Another method, supercritical water gasification (SCWG), is interesting because water under supercritical conditions has properties that facilitate the transport processes of compounds while remaining a benign me­dia for processing. It even acts as a catalyst for acid/ base reactions under these conditions (Calzavara et al., 2005; Nolen et al., 2003; Savage et al., 1995). Of note in SCWG is that it takes place in either high — or low — temperature conditions (Matsumura et al., 2005). How­ever, if one adds an alkali catalyst to the processing at low temperatures, it increases the conversion into oil and gas. Additionally, the catalyst lowers the tempera­ture at which the cellulose decomposes, or the onset of the gasification process (Minowa et al., 1997, 1995, 1998a, b, Murakami et al., 2012).

Pyrolysis

The conversion of LB by heating is pyrolysis (Balat, 2008a; Bridgwater, 2003; Mohan et al., 2006). Depending on the desired product, one chooses the operational con­ditions for pyrolysis. Factors such as heating rate, reactor temperature, residence time and composition of the feedstock determine the product. The goal of py­rolysis is to execute the process in the absence of oxygen and thus avoid the burning of the feedstock and instead
break down the lignocellulosic bonds and crystalline structure. By doing so under these conditions, new com­pounds are formed. Compounds such as char, bio-oil, and gasses are produced (Thuijl et al., 2003). The bio­oil formed by pyrolysis is not easily stored because it is unstable and requires additional processing prior to long-term storage (Adam, 2005).

There are three categories of pyrolysis: conventional, fast and flash. Conventional pyrolysis produces solids, liquids and gasses. Fast and flash pyrolysis produces primarily liquid and gaseous products (Demirbas, 2005).