- Written by Anna Strom
Because in such thermal conversion every new process step could be a potential source of undesirable emissions, and because these need to be controlled for the purpose of safeguarding human health and the environment, Bio4Energy scientists set out to investigate the matter with a focus on toxic emissions in relation to pre-treatment technologies that are still in their infancy: Microwave-assisted pyrolysis and torrefaction. While the former is designed to produce a bio oil using microwave technology (and which oil then may be further refined into value-added specialty chemicals), the other is a form of roasting of the biomass which renders light-weight and hydrophobic solid pellets or briquettes. Both methods are performed in an oxygen free, or near oxygen-free, environment.
Unsurprisingly, then, researchers on the platform Bio4Energy Environment and Nutrient Recycling who focus on assessing the environmental and health impacts of the combustion and gasification of biomass in their daily work, have made it a priority to assess thermal process for potential formation of dioxins. In this case, on assessment of microwave-assisted pyrolysis and torrefaction with a feedstock assortment of stem wood and bark from Norway spruce and Scots pine, cassava stems, particle board and waste wood impregnated with preservatives, the results were as follows.
For both types of materials—that is, the resulting pyrolysis oil and torrefied material—the researchers concluded that the quality of the biomass feedstock and temperature during the conversion were the main factors deciding whether dioxins formed. If the process parameters (including the temperature regulation) were well controlled, and the biomass used was untreated wood or agricultural residue, toxic emissions were minimal.
However, when using waste wood, for instance from plywood board or spent telephone poles that had been treated with conservation agents, the results changed. In both cases, there were clearly detectable levels of dioxins in the resulting materials; that is in the pyrolysis oil and in the torrefied material, respectively. However, only a minimal amount of this pollution was transferred to the gas phase of the torrefaction so, in the words of the researchers, very little of it "volatilised". The scientists' recommendation then, is that the emission profile and combustion behaviour of similarly conditioned materials either be investigated before the materials are recovered as feedstock, or incinerated under controlled conditions in which process emissions are captured rather than released into the atmosphere.
"There have not been so many studies done on the impact [in terms of emissions] of these new technologies", Gao said.
In the combustion of biomass, "there is a very complex interaction between the feedstock and thermal conversion process. We wanted to contribute some input or knowledge about dioxin formation in low-temperature processes of thermal conversion. This is a first attempt", she added;
"Our message is that there is a potential risk [of formation of dioxins] if processes are not controlled".
Contributors to the work encompassed in Gao's thesis, Dioxins and dioxin-like compounds in thermochemical conversion of biomass, are acknowledged. They include researchers and technicians from Umeå University, the University of York, the Energy Research Centre of the Netherlands, the Swedish University of Agricultural Sciences at Umeå and the University of Milano-Bicocca in Italy.