Waste collage Pic cred MarEdoAre mixed combustion fuels, based on different types of waste and designed for specific purposes, a thing of the future? Photos by courtesy of Mar Edo.In Sweden, toxic emissions to air from incineration of domestically-sourced municipal solid waste are generally well controlled. Moreover, in accordance with the waste hierarchy adopted by the European Union in its 2008 Waste Framework Directive, re-use and recycling are favoured above recovery. Sweden thus manages to do away with about half of the total 4.4 million tonnes of waste generated annually by its households, institutions and commercial actors before the incineration option is put to use.

However, heat recovery and electricity generation following waste incineration has become a business and the country has the capacity to burn more household waste than the 2.3 million tonnes that its citizens supply. In 2015 alone, 1.3 million tonnes of waste were imported, mainly from other European countries, and used for such waste-to-energy recovery. And when waste becomes an industry in itself, there are bound to be actors out there thinking about how to make it cleaner and finding new uses for the refuse by integrating different technologies.

For instance, staff at Vafab Miljö, a Swedish regional waste utility, have been working with Bio4Energy researchers to find ways to blend household waste and recovered wood, learning about the mixtures behaviour as a feedstock by studying its properties and testing various mechanical pre-treatments and turned the mixed waste into fuel. In the project, carried out in collaboration with Bio4Energy partner Umeå University's Industrial Doctoral School, PhD student This email address is being protected from spambots. You need JavaScript enabled to view it. has evaluated a range of fuel blends.

Edo's thesis work is based on four scientific articles, which she is careful to point are case studies, that identify a number of challenges as well as avenues to pursue that merit further research and development work:

1) Mixed fuels based on household waste and spent wood present advantages in terms of the possible increase of the energy content of the fuel and the obtention of a less toxic combustion ash. If the feedstock pre-treatment method extrusion is added, it may be possible to obtain a side stream mainly composed of food waste which may be used in the production of biogas for use as car fuel;

2) Operators ideally should increase the number of regular controls of the refuse they plan to incinerate, notably they are advised to screen batches of imported waste containing demolition and construction wood, which as a feedstock may be a source of toxic emissions in cases where the wood has been treated with conservation agents, flame retardants, paint or the like;

3) Other Bio4Energy researchers have developed a pre-treatment method in which woody biomass is compacted by roasting and demonstrated this torrefaction technology together with industrial partners. Edo and her main supervisor This email address is being protected from spambots. You need JavaScript enabled to view it., associate professor at Umeå University, have applied a simple form of torrefaction to the fuel blends in Edo's study and the results are promising in terms of minimising emissions of toxic chemicals such as dioxins and metals that spur their formation. 

Mar Edo Photo by MattiasPetterssonPhD student Mar Edo Giménez. Photo by Mattias Pettersson.

"Sweden is a leader in waste incineration. Even though most of the waste which is imported is from other European countries, mainly from other Scandinavian countries, or the UK or the Netherlands, countries are at different stages [when it comes to enforcing emission controls]. On the top of that, waste is a heterogeneous material. It becomes hard to do a forecast of the composition of the fuel", according to Edo. Therefore she would like to encourage waste incineration operators to run more frequent checks on their import batches than what is the case today.

"The results of the torrefaction tests are good. We have improved the properties of the char compared to the feedstock. We have also done the separation of the chlorine and some metals… that are catalysts for the formation of dioxins. This will not prevent the formation of these toxic pollutants when the fuel is burnt, but it could be a firt step to decrease the potential for such formation", Edo said. 

Burning issue

Edo points out that the issue of how to manage the waste generated by the world's growing population is not a minor one. According to an estimate by Eurostat, the EU's statistical arm, in 2014 people in the 28 member states brought about 239 million tonnes of municipal solid waste in the EU alone. This corresponds approximately to 475 kilogrammes of waste per capita.

"If you tell people that you are producing 500 kilogrammes of household waste per year [then] maybe you should really consider the dimension of the problem and try to do something about it", Edo said.

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Mar Edo Giménez defended her thesis, Mixed fuels composed of household waste and waste wood – Characterisation, combustion behaviour and potential emissions, 2 December 2017, from 9 a.m. at the Large Lecture Hall of Chemical-biological Building of Umeå University, Umeå, Sweden.

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