This report discusses potential future systems for waste-to-energy production in the Baltic Sea Region, and especially for the project REMOWE partner regions, the County of Västmanland in Sweden, Northern Savo in Finland, Lower Silesia in Poland, western part of Lithuania and Estonia.
The waste-to-energy systems planned for in the partner regions are combustion of municipal solid waste (MSW) and solid recovered fuels from household and industry as well as anaerobic digestion of sewage sludge and agriculture waste.
The potential future waste-to-energy systems in the partner regions include increased utilization of available waste resources. Examples of resources possible to use are straw that could be used for ethanol production and biowaste from households and manure that could be used for biogas production. If the utilization in all partner regions would reach the same level as already exists in the County of Västmanland it would correspond to an increased energy supply of 3 TWh/year which corresponds to about 2.5 % of the total energy use in the partner regions year 2008.
An important aspect of future anaerobic processes for biogas production is the possibility to use the residue. West Lithuanian biogas production residue is planned to be dewatered up to 90 % of dry matter to make future utilization options possible.
Pre-processing is necessary to be able to use the residue from digestion of solid waste as fertilizer. The pre-processing should include crushing, removal of metals, wood and plastics, and pulping. Without pre-processing it is possible to combust the residues with energy utilisation. Results from an investigation of the residues from biogas production tests using substrates from the project partner regions show a remaining energy potential of the digestate corresponding to 17 to 50% of the biogas energy. A combination of digestate combustion and fertilizer use could be a possibility.
Hydrothermal carbonization, HTC, is a process that could be of interest to use for treating digestate in order both to utilize the energy left after biogas production but also for sanitation of the digestate. In this process heat is released and coal is produced. This process could also be of interest for waste-to-energy conversion of waste which is usually not usable for other biological process like e.g. biogas production, for example sharp leaved rush, straw or leaves from gardening etc. Initial tests on pulp and paper waste show promising results.
Among the possible processes for increasing the output of biogas from anaerobic digestion using ultrasound technology for thickening of surplus waste water sludge can be mentioned. It allows increasing the biogas production up to 30 % and reducing the amount of organic substances in the digested sewage sludge by up to 25 %. Another area of possible improvement is the mixing in the digester. The mixing is important for distribution of microorganisms and nutrition, inoculation of fresh feed, homogenizing of the material and for the removal of end products of the metabolism. Studies of the digester for biowaste in the County of Västmanland indicate that about 30 % of digester volume can have dead and stagnant zones.
Waste-to-energy utilisation could also be possible to realise by further development and introduction of new processes and concepts. An option for solving the problem of old sewage sludge could be to use it in a gasifier to convert it to energy rich gases. Microbiological conversion of waste can be further developed to produce several different products, such as heat, power, fuels and chemicals, the development of so-called biorefineries. Also the biorefinery‘s water management can be renewed in order to remove toxic substances, minimize environmental impacts and produce pure, clean water. Finnoflag Oy has developed a technology that converts waste materials into e.g. fuels, chemicals, plastic and rubber via low-energy routes. The Finnoflag technology is based on the PMEU (Portable Microbe Enrichment Unit) which is a new innovative instrument for use in the microbe detection process and that is designed to create an optimal growth environment for microbes.
For fibrous and well-structured biowaste dry digestion could be a good option for biogas production. Several different configurations have been tested in Germany. The garage digestion method has the advantage that an extensive pre- treatment of substrate is not necessary and no pumps or stirrers, which can be destroyed by disturbing materials, are involved. However, the efficiency of garage digesters is low compared to other digestion methods due to lack of effective substrate turbation. More research work is needed to improve the efficiency. Tests of five existing dry digestion processes show that the Tower-digester is the most suitable dry digestion method for household waste. Among the reviewed plants the Dranco-tower digester showed the best efficiency in reference to biogas potential. The plant design is robust enough to handle substrates like household waste with fractions of disturbing materials. The mixing in the reactor is based on the force of gravity and the used pumps are powerful and very resistant.
Pyrolysis is a process of interest for converting wood based waste into energy products such as gas, bio-oil and/or solid fuel/carbon. This has been identified as a process suitable for a new business model with a franchise based model, offering an earning opportunity for small size entrepreneurs.
Possible improvement of existing and new waste-to-energy systems also includes increasing the overall efficiency of the utilization of waste resources by integration of several processes. A study on integrating pellets production from the residues from straw-based ethanol production with an existing combined heat and power plant shows that the total production cost can be reduced by the integration.
Possible development of waste-to-energy systems for the partner regions could be the following:
Estonian - biogas production using the organic waste, use of the digestate as fertilizer on demand or combustion for power and heat production, recycling plants for paper, plastics and other recyclable wastes and combustion for power and heat production after recycling
North Savo, Finland- the same options as for Estonia is of interest. Added to this is the potential for power and heat production from large amount of wood waste. Also the possibility for pellet production from wood waste could be of interest.
Western Lithuania- the same system as mentioned for Finland is also of interest for western Lithuania.
Lower Silesia, Poland- also for Lower Silesia high amounts of organic wastes is suitable to use for biogas production in anaerobic digestion. Recovered derived fuel (RDF) is already used as fuel for power and heat production. There are also some attempts to involve combustion of residual mixed waste in 1-2 of the most densely populated areas.
County of Västmanland, Sweden- Here a system for separate collection of the biowaste from households, digestion of the fraction together with ley crop silage from regional farmers to produce biogas and use of the digestate from the digestion process as fertilizer at farmland already exists and a new power and heat plant using recovered derived fuels is under construction. Further waste- to-energy plants for production of bioethanol from straw and biogas from agricultural waste could be possible.
The potential future waste-to-energy systems are not only dependent on available technologies for waste-to-energy conversion but also on the development within the waste and energy areas including also economic and political aspects. There is a growing interest for waste prevention in waste management within the EU, and growing concern about food losses and food waste at global and national levels. During past decades the waste amounts have steadily increased with economic growth but due to waste prevention actions a decoupling of the waste amount and economic growth is foreseen. This has to be considered in development of future waste-to-energy systems. Further, policies and goals concerning fossil fuel free transportation systems and low carbon energy systems is of importance.
Analysis of combined previous proposed scenarios for energy demand and use development and waste amount development for waste-to-energy in Sweden 2010 to 2050 shows that the contribution of waste-to-energy to the total energy supply in 2050 varies from 6 to 47 % depending on the scenarios combined. The lowest contribution occur for scenarios with low waste amounts combined with energy scenarios with low changes in energy demand while the highest contribution occurs for scenarios with high amount of waste combined with energy scenarios with large decrease in energy demand.
Västerås: Mälardalen University , 2012. , 39 p.