Biofuels produced by roasting
Roasting, the process used to roast coffee beans, could increase the energy content of Britain's main energy crops by up to 20%. Indeed, scientists from the Faculty of Engineering at the University of Leeds have studied the behavior during the combustion, after roasting, of plants cultivated especially for the production of energy.
Roasting is a mild pyrolytic process carried out under inert conditions which extracts moisture, causes partial endothermic decomposition of cell walls and alters the chemical structure of biomass polymers. This process has the merit of creating a solid product that is easier to store, transport and crush than raw biomass. It also improves the properties of biomass with respect to thermochemical treatment techniques for energy production (eg combustion, co-combustion with coal or gasification).
The Leeds researchers therefore looked at the hydrogen roasting of two energy plants (canary grass and fast-growing willow coppices) and an agricultural residue (wheat straw). Different roasting conditions were applied in order to optimize the process for the three fuels. The progress of roasting was also followed by chemical analysis (elements carbon, hydrogen, nitrogen, oxygen and ash): the researchers were able to observe that the characteristics of biofuels began to resemble those of low rank coals. In addition, the results of the analyzes indicate that the volatile compound of the biomass is both reduced and altered: the scientists therefore obtain a more thermally stable product, characterized by greater heats of reaction during combustion. The combustion behavior of raw and roasted plants was studied by differential thermal analysis and, in the case of willow, by suspending individual particles in a methane-air flame and following the combustion process via video.
The results obtained demonstrated that the treated plants required less time and energy to reach the ignition temperature, but also that they had increased energy yields during combustion. In particular, the willow has demonstrated the most interesting properties: it is the plant which retained the maximum of its mass during roasting and which presented the best energy yields. Its energy yield was up to 86%, compared to 77% for wheat straw and 78% for canary grass. Finally, when exposed to a methane-air flame, roasted willow ignites faster, probably according to the researchers because its low moisture content means it heats up faster. Roasted particles also start the combustion of carbonaceous residues more quickly than raw willow particles, although this combustion is slower for roasted particles.
Roasting is not currently used in the UK either in agriculture or in the energy sector, according to the Leeds researchers, although the method has many advantages, and not just in terms of storage. This is therefore an area they would like to explore further.
Their work has so far been supported by the Supergen Bioenergy consortium.
Source BE UK