The Work package Biogenic Waste to Energy of the Bioenergy Network of Excellence (NoE), a European group of eight bioenergy institutes, has published recommendations on future directions for research into management of residues from bioenergy processes. Residues from processes involving non-waste materials are less contaminated than those from the production of energy from biogenic waste. However, the report also points out that there’s a lack of knowledge regarding the quality of these residues.
An unwanted side-effect of bioenergy processes is the appearance of by-products and residues because nearly all of these products come with disposal costs. This is a big issue when considering energy recovery from waste. Bottom ashes, filter ashes and gas cleaning products from waste incineration contain toxic elements like heavy metals, salts, and organic micropollutants.
Reusing bottom ases saves disposal costs
Thanks to intense R&D efforts to improve the combustion process and the quality of the various materials used, technologies and strategies are now available to guarantee sustainable management of all residues from mwaste incineration. Bottom ashes have good potential for reuse. After pretreatment they are used as secondary building material in road construction in many EU countries. However, treated bottom ashes do not have a commercial value because they have to be pre-treated and are typically offered without charge. The main benefit is savings of disposal costs, which in many countries are higher than the gate fee at a landfill site, and will only increase in the future. From an environmental viewpoint, an additional benefit is the replacement of natural building materials like gravel.
Residues with higher quantities of pollutants, like filter ashes and gas cleaning residues can only be disposed of on special and expensive sites, preferentially underground. Reuse is mainly prohibited by the treatment cost of available processes. However, a macroeconomic evaluation might conclude that treatment, such as metal recovery from filter ashes, pays off in the long term.
Monitoring of halogens and alkali metals recommended
Residues from other bioenergy processes like biomass combustion or gasification, byproducts of pyrolysis and even the solids from anaerobic digestion are far less contaminated by heavy metals. Although for these materials, too, contamination cannot be ruled out, especially if waste materials are co-treated. In any case the concentration of halogens and alkali metals should be monitored.
In its review “Management of Solid Residues in Waste-to-Energy and Biomass Systems”, Bioenergy NoE’s Biogenic Waste to Energy work package, pointed to a lack of knowledge on the quality of residues from the above mentioned processes, at least compared to that on waste combustion residues. As a final conclusion, the review stated a need for further research on long-term reliable management strategies, especially for all types of residues from gas cleaning in all processes.
More Research needed on residues of gasification, pyrolysis and fermentation
Additionally the residues from cocombustion of waste and coal, from combustion of Solid Recovered Fuels, from gasification and pyrolysis, as well as from fermentation of biogenic matter need more detailed investigation. The challenge in all residue management scenarios – especially if these residues derive from waste or contaminated fuels – is the definition of sinks for pollutants. This task not only has a scientific and technical aspect, an environmentally sound aftercare solution must be found if biogenic waste to energy is to meet socio-economic expectations and secure public acceptance.
Bioenergy NoE, European BioenergyNews, 2007-05.