The Thermochemical Conversion of Biomass into High-Value Products: Microwave Pyrolysis - The Economic Utilisation of Food Co-Products - page 54

temperature, microwave power, additives). Increasing microwave temperature
and power typically increase the yield of oil. Thus, increasing microwave power
from 500 to 1000 W increases the yield of wheat straw bio oil by 1.3; addition of
3% sulfuric acid to wheat straw halves the yield of bio-oil. It should be noted
that addition of hydrochloric acid to wheat straw causes an increase in bio-oil
yield to 22%. We believe that optimisation of microwave parameters for
specific feedstock's will significantly increase bio-oil yield.
A comparison of mass balance data for wheat straw pyrolysis form Table 3.4
(see below) and experimental data (Table 3.3) shows that in the presence of
hydrochloric acid microwave pyrolysis produces very similar quantities of bio-
oil to the conventional process. Since wheat straw is very high in potassium
content, one would expect a significant increase in char yield (and not bio-oil),
for which the microwave processing seems to compensate (Table 3.5).
Our data about temperature/microwave power influence on the mass balance
of microwave pyrolysis are in good agreement with data from the literature. 47

d n 9 r 3 n g | 8

3.5.2.5 Verification of Sample Temperature Measurement during
Microwave Experiment
The measurement of temperature is a key issue for both microwave chemistry
and pyrolysis. Bulk temperature cannot be directly measured as inside the
microwave cavity most temperature probes will be either directly heated at a
different rate from the substrate under investigation and by the nature of
temperature as a concept it cannot be measured in a nonequilibrium energy-
transfer situation as occurs under microwave irradiation (i.e. direct excitation
of rotational levels leads to non-Boltzmann distribution), but only after this
energy has been dissipated (probably relatively quickly). External temperature
measurement systems suffer from the traditional issues of lag time caused by the
heating of the material followed by transfer to the vessel by conduction.
In essence, we can see from the discussion above, one should have limited
confidence in temperature measured within a microwave cavity as a consequence
of the physical chemistry occurring. Furthermore, it is well known that within
microwave fields, due to differences in absorption of microwave energy it is
possible to have hot spots with temperatures far in excess of the bulk temperature. 7
Thorough work by Delft TU of the Netherlands investigated the issue of
temperature measurement within microwaves, the associated accuracy or
inaccuracy, probe application and spatial variation. 70 In their work, it was
observed that there was a considerable but predictable difference between an
insample fibre-optic probe and exterior infra-red detection. During microwave
pyrolysis a difference in temperature measurement between these two probes
also has been observed (see Figure 3.6).
However, below 100 1C the reproducibility between the two methods
(Figure 3.6) is good; this could be due to water vapour being evenly distributed
within the biomass bulk. At temperatures higher than the water boiling point a
difference exists between the measurement methods, due to hot spots, however,
as volatiles are evolved the hot spot is cooled by evaporation of pyrolysis

3 .