"We could be at the stage of industrial demonstration of the technology in five years. Then a certain amount of time would be needed for classification of the product. We know that it would be economically beneficial for some [existing] bioenergy operations which use fluid-bed technology to start co-firing dried sludge with [fuel wood]", according to Öhman, who is a professor in Energy Engineering at the Luleå University of Technology (LTU).
The research and development project, which is the fruit of collaboration between Bio4Energy researchers at LTU and Umeå University, has been several years in the making. Now it can go ahead thanks to a recently announced multiannual grant from the Swedish Research Council Formas.
Phosphorus is an essential nutrient for plant growth and thus for food production. It is extracted by mining in a handful of countries worldwide and its maximum production is expected to peak in the year 2030. After that predictions range from 50 to several hundred years before it runs out. Research is ongoing on a handful of methods for recycling the mineral from sludge, but which either perform inadequately (when it comes to removal of toxic heavy metals present in sludge or to phosphorus recovery rates) or are inhibitively expensive, to believe Öhman.
"With our method we could recover 80 per cent of the phosphorus that we put in our mouths [in the form of food]. The solutions which are presently out there retain too much of the toxic content of the sludge. Direct re-use of sludge is not a solution either", he said.
Clearly, the Bio4Energy team composed of experts on ash chemistry and waste treatment technologies have been mulling over this project for some time.
"One part is about building adequate, bioavailable phosphates. We have been doing that for ten years. Then there are the heavy metals. We know we can remove half of them by evaporation… and capture them in emission filters. We will optimise the method further. We will also evaluate the impact of adding silicon to the system via co-combustion with straw. The rest is engineering and for that we need to cooperate with manufacturers of biomass boilers.
"We are in discussions with one of the world's largest boiler manufacturers… and with a site in Sweden where the technology could be demonstrated", Öhman said.
For more on the method being developed, see the scientific article Combustion of Biosolids in a Bubbling Fluidised Bed: Part I: Main Ash Forming Elements and Ash Distribution with a Focus on Phosphorus, published in the scientific journal Energy & Fuels.