The relative proportions of products from fast pyrolysis processes are subject to a combination of factors including reactor configuration, vapour residence time and temperatures. Key parameters which influence the fast pyrolysis process are described later in this chapter.
2.3.1 Bio-oil
Bio-oils produced from fast pyrolysis are usually mixtures of char, water and complex organic compounds. The compounds are products formed during the decomposition of macro polymers of cellulose, lignin and hemicellulose that constitute the original biomass feedstock.
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The ratios of the components are dependent on complex relationships between the configuration of the pyrolysis process, reaction conditions, extent of char removal, condensation method and the composition of the initial feedstock. Table 2-2 shows the major physical and chemical characteristics of a typical bio-oil in comparison to heavy fuel oil
Table 2-2 Typical composition of bio-oil compared to heavy fuel oil [9].
Physical Property Bio-Oil Heavy fuel oil
Water Content, (wt %) 15-30 0.1
Bio-oils are reported to resemble the original feedstock based on elemental comparisons and because of this, usually contain highly oxygenated compounds. Table 2-3 shows the composition of bio-oil from white spruce and poplar as reported by Piskorz et al [65].
Table 2-3 Composition of bio-oil from white Spruce and Poplar [65]
Bio-oil Composition ( wt%) White Spruce Poplar
Sachharides 3.3 2.4
From Table 2-3, it can be seen that carboxylic acids can constitute up to 11% mass fraction of bio-oil. This in turn makes most bio-oils highly acidic with pH values ranging between 2- 4. Diebold [66] reports that the major organic components of bio-oil are continuously reacting in order to attain chemical equilibrium. The continuous organic reactions taking place even after the formation of the oils are responsible for the aging witnessed in
bio-41
oil during storage. These can alter the physical and chemical characteristics of the product [67]. The components of bio-oil can be classified into water soluble and water insoluble categories. The water soluble phase of most bio-oils is composed of lighter organic compounds while the water insoluble phase is composed of larger and heavy compounds commonly referred to as pyrolytic lignin [68].
Pyrolytic lignin consists primarily of tri- and tetramers of lignin sub units (hydroxyphenyl, guaiacyl and syrigyl units) as was shown in Figure 2-9. These compounds can represent up to 80 % of the original biomass and have molecular weights of between 650 and 1300 kg/kmol [69]. The high molecular weight compounds are known to be responsible for the high viscosity of bio-oil although the viscosity is also a function of the pyrolysis process configuration and the initial water content of the actual biomass feed [69].
The components of bio-oil make its properties very likely to change over time dependent on storage conditions. Polymerisation reactions within bio-oils are known to continue until heavy lignin rich fractions separate from other components into sludge like liquids [70].
An increase in bio-oil molecular weight will be observed as the product ages due to the reaction of carbohydrate based constituents. These constituents such as aldehydes and ketones can jointly account for up to 25 % of its composition [71, 72].
Bio-oil fuel properties compare differently to conventional crude because of the presence of highly oxygenated compounds. The heating values are relatively lower when compared to that of crude which is a mixture of only hydrocarbons. The high amount of water and the acidity of the product make it corrosive. The presence of water also makes it likely that bio-oil may catalyse the formation of hydrated iron (III) oxide [73]. The presence of solid fines in the oil also makes it likely to clog up fuel and injection systems.
Notwithstanding the drawbacks highlighted, bio-oil is reported to be an attractive source of high specialty chemicals when fractionated. Compounds like acetic acid, formaldehyde, hydroxyacetaldehyde and maltol can all be derived from bio-oil [74].
42 2.3.2 Char
Fast pyrolysis processes leave a residue of high carbon content char with relatively low amounts of oxygen and hydrogen. Char is usually removed as a by-product of the pyrolysis process using cyclones because it is known to reduce the yields of liquids by catalysing cracking reactions in pyrolysis vapours. Some pyrolysis configurations like circulating fluidised beds are known to utilise the char as a fuel to provide energy for the actual pyrolysis process. Many processes however collect the char product as a by-product and use it for other energy purposes as chars from fast pyrolysis can have heating values of up to 23 MJ/kg [74, 75].
2.3.3 Non condensable gases
The final product of pyrolysis processes are the non-condensable gases formed as a consequence of the thermal degradation of the biomass. The amount of non condensable gases produced from any fast pyrolysis process is dependent on numerous factors including process temperature and reactor configuration [9]. The efficiency of the vapour quenching process will also impact the amount of non condensable gases produced with very efficient quenching producing less gases. Pyrolysis gases mainly consist of carbon monoxide, carbon dioxide, methane, hydrogen, ethane and propane. The gases can be utilised for energy purposes but their use is dependent on process scale because of relatively low energy content [74]. A 7kg/h fluid bed fast pyrolysis rig at Aston University recycles the process gas through a compressor and reuses it as the fluidising gas in order to reduce costs. This is an example of effective use of pyrolysis process gases.