The various thermochemical conversion technologies that may be applied to processing of lignocellulosic biomass are presented in Fig. 7.1.

7.3.1 Combustion

The production of thermal heat and electricity from lignocellulose, as well as the production of intermediate bioenergy products such as pellets, charcoal, gases and liquid fuels derived from lignocellulose, all proceed via combustion. Combustion processes combine three elements: a feedstock as fuel, air as oxidant of the feedstock and the application of a specifically required temperature from a heat source. The carbon and hydrogen components of a feedstock are totally or partially oxidized and converted into heat. Normally, combustion precedes pyrolysis. Burning of woody material proceeds in four steps: the temperature of the starting material is increased by application of heat, which leads to the evaporation of volatile species and char formation, followed by combustion of volatiles species (primary combustion) and finally the combustion of char (Gonzalez et al. 2005).

The production of steam in a boiler, facilitated by energy derived from com­bustion, is used either directly as thermal heat or to drive steam turbines for

I Liquefaction:

Catalytic or

1 non-catalytic Sub/near/ Supercritical fluids

Fig. 7.1 Overview of thermochemical processes for conversion of woody biomass into bioenergy products

electricity production, with high boiler pressures combined with multistage turbines providing the highest overall process efficiency. Similarly, the gases produced through gasification, anaerobic digestion and pyrolysis can be combusted directly in gas turbines or gas engines, with the biomass integrated gasifier/combined cycle (BIG/CC) systems delivering significantly higher conversion efficiencies than the boiler-steam turbine process (Laser et al. 2009). The recovery of waste heat from boilers, gas engines/turbines, steam turbines and other combustion applications can provide a useful form of low quality heat (e. g. warm water), which increases conversion efficiency.

The heat energy and steam delivered by gasification systems can be used for heating purposes and electrical power generation, respectively. Known and applied for many decades, many biomass combustion technologies can be found on the market and mainly categorized in two types, fixed-bed (Water-cooled vibrating grate (VG)) and fluidised-bed systems (Bubbling fluidised bed combustion (BFBC), Circulating fluidised bed combustion (CFBC)). Fluidised-bed boilers present a series of advantages such as limited emissions and relatively complete combustion which improves overall efficiency and makes it suitable for processing of a wide range of feedstocks (Wright et al. 2010; IEA ETSAP 2010; Demirbas 2005).