The Thermochemical Conversion of Biomass into High-Value Products: Microwave Pyrolysis - The Economic Utilisation of Food Co-Products - page 49

from 30-32 kJ/g in conventional heating to 34 kJ/g in MW. For the gases, it was
observed that at 500 1C the gas obtained from MW pyrolysis exhibited a
calorific value twice that of gas from conventional pyrolysis.
It has been shown that MW pyrolysis of the coffee hulls produces more gases
but fewer oils than conventional pyrolysis; the char yields being virtually
identical for both pyrolysis methods. This suggests that in microwave treatment
secondary cracking reactions of the oil components occur to a greater extent
than in conventional heating.
Use of microwave irradiation in low-temperature pyrolysis is relatively
unexploited. Ruan and coworkers have investigated microwave biomass
pyrolysis at temperatures between 200-600 1C, using a variety of biomass sources
including cellulose, pine, corn stover, canola, municipal waste and aspen. 20,54
They also investigated temperature optimisation to minimise side reactions and
gain a greater control of the product composition. Analysis of pyrolysis oil in
terms of water content, density, pH, viscosity, elemental analysis and calorific
value were reported. 55 The properties reported, including calorific values, were in
the range of similar bio-oils (19 kJ/g) produced by conventional pyrolysis
process. The influence of pyrolysis conditions such as irradiation power,
temperature and time on elemental and chemical composition of gas and chars
were also presented. The authors reported that at elevated temperatures they
observed increased yields of hydrogen gas and decreased yields of char.
More recently, microwave-induced pyrolysis at temperatures between 250
and 560 1C has been investigated by Huang et al. 21 They looked at pyrolysis of
rice straw and identified solid, liquid and gaseous products. The microwave
power and particle size of feedstock were both key parameters affecting the
performance of microwave-induced pyrolysis. After the pyrolysis of rice straw,
three main products were generated and collected separately. About half of rice
straw sample was transformed into H 2 -rich fuel gas (H 2 ,CO 2 , CO, CH 4 ,
respective percentages of 55, 17, 13, 10 vol.%).

d n 9 r 3 n g | 8

3 .

3.5.2 Low-Temperature Microwave Processing
Microwave treatment of biomass has been focused on high-temperature
pyrolysis, gasification and liquefaction of the starting materials. Pyrolysis of
biomass is usually conducted at temperature above 350 1C. Examples of
substrates employed in this type of pyrolysis are numerous and include plant
biomass such as wood or agricultural residues, plastics and municipal waste. It
has been shown that there are a number of advantages of microwave pyrolysis
above conventional due to high rate of pyrolysis, better controllability, energy
eciency and selectivity. This selectivity may be in the ratio of solid-liquid-gas,
or it may be in composition of the bio-oil.

3.5.2.1 Cellulose Pyrolysis
Lignocellulosic biomass consists of three structural polymers (hemicellulose,
cellulose and lignin) along with various extraneous components. 56 These