1.6.4.1.3 Fast pyrolysis
Fast pyrolysis (Figure 5) is a high temperature process, in which the biomass feedstock is heated rapidly (~ 300 °C/min) to 450-600 °C in the absence of air. Under these conditions, organic vapours, pyrolysis gases and charcoal are produced. The vapours are condensed to bio-oil. Typically, 60-75 wt. % of the feedstock is converted into oil. The products of fast pyrolysis can be found in all gas, liquid and solid phases, see table 1.
Gaseous products include H2, CH4, CO, CO2 and other gases depending on the organic nature of the biomass for pyrolysis. Liquid products include tar and oils that remain in liquid form at room temperature like acetone, acetic acid, etc. Solid products are mainly composed of char and almost pure carbon plus other inert materials.
The goal of fast pyrolysis is to produce liquid fuel from lignocellulosic biomass that can substitute for fuel oil in any application. The liquid can also be used to produce a range of specialty and commodity chemicals. The essential features of a fast pyrolysis process are very high heating and heat transfer rates, which often require a finely ground biomass feed. The main product of fast pyrolysis is bio-oil, which is obtained in yields of up to 80 wt% of dry feed basis, together with by-product char and gas, which are used within the process to provide the process heat requirements, so there are no waste streams other than flue gas and ash. Fast pyrolysis for liquids production is currently of particular interest because liquids can be stored and transported more easily and at lower cost than solid biomass.
Figure 5. Schematic of the fluidized bed fast pyrolysis process [4]
Bio-oil is a dense complex mixture of oxygenated organic compounds. It has a fuel value that is generally 50–70% that of petroleum bases fuels and can be used as boiler fuel or upgraded to renewable transportation fuels. Its density is > 1 kg L-1, much greater than that of biomass feedstocks, making it more cost effective to transport than biomass.
In fast pyrolysis, biomass rapidly decomposes to generate vapours, aerosols, gases and some charcoal. After cooling and collection, a dark brown mobile liquid is formed that has a heating value of about half that of conventional fuel oil. While it is related to the traditional pyrolysis processes for making charcoal, fast pyrolysis is an advanced process that is completed in seconds, with carefully controlled parameters, to give high yields of liquid.
The essential features of a fast pyrolysis process for producing liquids are:
Very high heating and heat transfer rates at the reaction interface, which usually requires a finely ground biomass feed.
Carefully controlled pyrolysis reaction temperature of around 500 °C and vapour phase temperature of 400-450 °C. The effect of temperature on yields and product spectrum is discussed in the section on pyrolysis liquid below.
Short hot vapour residence times of typically less than two seconds.
Rapid cooling of the pyrolysis vapours to give the bio-oil product.
Table 3. Composition and properties of typical wood-derived pyrolysis bio-oil [10]
-
Property
Unit
Value
C
H
N
O (balance)
wt%
46
7
< 0.01
47
Water content
Ash content
Solids content
wt%
25
0.02
0.04
LHV
LHV
MJ/kg
MJ/Ltr
16
19
Density
kg/m3
1 170
pH
–
2.9
Kinematic viscosity (40 °C)
cSt
13
A fast pyrolysis process includes drying the feedstock to typically less than 10% water in order to minimise the water in the product liquid oil, grinding the feed (to around 2 mm particle size in the case of fluid bed reactors) to give sufficiently small particles to ensure rapid reaction, fast pyrolysis, separation of solids (char), and quenching and collection of the liquid product (bio-oil).
Virtually any form of biomass can be considered for fast pyrolysis. While most work has been carried out on wood because of its consistency and comparability between tests, nearly 100 different biomass types have been tested by many laboratories, ranging from agricultural wastes such as straw, olive pits and nut shells to energy crops such Miscanthus and Sorghum, forestry wastes such as bark, and solid wastes such as sewage sludge and leather wastes.
Due to large amounts of oxygenated components present, the oil does not mix readily with hydrocarbons. The degradation products from the biomass constituents include organic acids (like formic and acetic acid), giving the oil its low pH. Water is an integral part of the single-phase chemical solution. The (hydrophilic) bio-oils have water contents of typically 15 - 35 wt.%. Typically, phase separation does occur when the water content is higher than about 30 to 45%. Table 3 shows properties of typical wood-derived pyrolysis oil.