Future conditional: The role of MBT in recovering energy from waste

Mechanical-biological treatment (MBT) produces energy-rich refuse-derived fuels which can be combusted for their energy value. Yet despite its potential role in helping to solve today’s energy needs, its future rests in the balance. Within IEA’s Bioenergy Task 36, the MBT project - which concludes this year - investigates the challenges ahead. by Patrick Wheeler Waste has huge potential for energy generation. For example, it has been estimated that waste could supply up to 15% of the UK’s electricity demand.1It is likely that this figure will be similar in other developed countries. While conventional incineration technologies are seen as the main option for generating energy from waste, these may not always be appropriate. Balance analyses of the inherent energy released by the combustion of waste and the embedded energy released through recycling demonstrate the rationale for optimizing recycling of certain waste materials. Figure 1. Total number of MBT plants in operation or under construction. source: task 36, 2005 Click here to enlarge image Consequently, mechanical-biological treatment (MBT) can be one option for improving the conservation of resources and energy in waste management systems. Mechanical-biological treatment enhances the conservation of embedded energy through recycling and allows potentially more efficient combustion or conversion of refuse-derived fuel (RDF). In order to assess the potential role for MBT in waste management, International Energy Agency (IEA) Bioenergy Task 36 (Energy Recovery from Municipal Solid Waste) decided to evaluate MBT systems around the world and compile a database of facilities. This article includes findings from this research and reviews some of the key issues for the technology. The current status of MBT MBT encompasses a wide range of technologies aiming to process solid waste by a mixture of mechanical and biological separation. It also enables metals and other dry recyclables to be recovered. There are five main types of MBT process: incorporating anaerobic digestion to generate biogas for electricity production. Anaerobic digestion also generates a digestate to be discharged or to be dewatered, producing a compost product producing an RDF product producing a compost product and/or a stabilized material for landfilling as well as a RDF product producing a compost product stabilizing waste prior to landfill. Figure 2. Capacity of plants in each country. source: task 36, 2005 Click here to enlarge image MBT is not a new technology; mechanical sorting and biological treatment processes have been applied for many years in municipal waste management. The number of MBT plants in operation or under construction has risen from 13 in 1980 to approximately 170 in 2005.2 Worldwide, Germany has the largest number of MBT plants, followed by Italy and Spain. The total capacity of these plants is over 15 million tonnes per year, and the capacities range from under 10,000 tonnes per year to 300,000 tonnes per year.2 Figure 1 shows the increase in the number of plants in operation or under construction from 1976 to 2005. Figure 2 presents the capacity of plants by country. Juniper Consultancy Services Ltd reviewed 27 MBT technology providers, of which 15 have at least one commercial reference plant. Thus, the Juniper review concluded that there are sufficient MBT suppliers with a track record to provide a solid industry base for the development of this waste management option3. Typical examples of well established processes include the following (this list is not intended to be comprehensive): the Horstmann process, which produces RDF and compost products and/or stabilized waste for landfilling the Eco-Deco process which principally produces an RDF product. The total capacity of Eco-Deco plants installed in Italy and Spain is over 750,000 tonnes/year, and three plants planned for the UK (two are under construction) will have a total capacity of over 400,000 tonnes/year. the 3R-UR process at Eastern Creek, Sydney, Australia, which relies on energy recovery from anaerobic digestion and has a current capacity of 175,000 tonnes/year. In addition, there are other processes that deliver similar process objectives to MBT. This includes mechanical separation or autoclaves (steam treatment) which do not have a biological component but provide RDF and diversion from landfill in a way similar to MBT. (For more information see page 37.) The challenges for MBT Although a significant number of MBT processes have been developed and a large number of commercial plants have been constructed, the potential for future plants will depend on a number of commercial and technical challenges, particularly the availability of markets and/or uses for the products that MBT facilities generate. Approaches for using MBT plants to recover energy may involve anaerobic digestion, the use of the RDF product as a secondary fuel in an industrial facility, and the use in dedicated combustion facilities. Anaerobic digestion to recover energyAnaerobic digestion systems can recover substantial energy from the biodegradable fractions of the waste. The amount of energy potentially to be derived from waste is dependant on the feedstock composition and the process configuration. The export energy to be derived may vary between 0 and 100 kWh of electricity per tonne of waste input with additional heat energy available that may find use in heating schemes. Use of RDF as secondary fuel in industrial facilitiesThe use of RDF in an industrial combustion facility as a replacement for other fuels may be constrained by waste-specific legislation (such as the Waste Incineration Directive in the EU). There are also a number of technical issues; for example, RDF has a lower carbon content than coal requiring altered air-injection patterns, and the higher levels of alkali metals can result in higher levels of fouling and corrosion. One common use for RDF is in cement kilns, but as the chlorine content can affect the quality of the cement product, the cement industry has set limits for the maximum chlorine content of the RDF product, and some plants have not been able to meet this limit. The calorific value of RDF varies with the process type. The RDF material may be similar in composition to the input waste, the only difference being that it has been dried or have some inerts removed at 10-12 MJ/kg. Or it may also consist of a highly refined plastics-rich fuel product with a calorific value up to 20 MJ/kg. Generally it is important to analyse the material characteristics of the RDF product in relation to the requirements of the co-combustion facility. The MBT process needs to be improved and refined in order to produce the quality of RDF that will be accepted by the market. Examples of German MBT plants (such as MBT Ennigerloh) indicate that a close co-operation with the industrial facility is beneficial for improving RDF quality and securing a market for the RDF material.4 Dedicated combustion facilities for RDFDedicated combustion facilities can either burn the RDF directly or further process it using advanced thermal treatment technologies such as gasification or pyrolysis. The technical issues for direct combustion of RDF will be similar to those for combustion in an industrial facility. Gasification technologies are well established for processing some biomass and waste materials such as wood, but there are currently few commercially operating plants that treat municipal wastes. RDF is more homogeneous than municipal waste as the material has gone through a mechanical sorting and pre-treatment process. The combined costs for the MBT plant and the dedicated thermal treatment facility mean that this is likely to be a much less economically attractive option than conventional waste-to-energy (WTE) systems. However, due to local political, legislative or structural circumstances, the MBT process with a dedicated combustion facility may still be more appropriate to a particular location. The efficiency of combustion and energy conversion will vary with the combustion plant, with dedicated incinerators achieving efficiencies of around 20%. However, co-combustion in power plants will give much higher efficiencies similar to those achieved with the primary fuel. The challenge to securing a market for products The RDF material produced by an MBT process will have to compete with other ‘renewable’ or ‘non-fossil’ fuels such as tyres, biomass products and energy crops. Potential users may view fuels with a high renewable content as a more attractive fuel. However, the RDF produced by many MBT processes will have a high plastics content diluting the renewable content, and thus would complicate the accounting of CO2 benefits or may not count as a renewable fuel under some regulatory systems. In contrast, the RDF produced by steam treatment (autoclave) will have a much higher biomass content (the material treated contains less plastic); thus this type of process, once it is fully developed, may be able to produce a more marketable RDF material than those that are currently being generated through MBT. Learning from German experience In Germany, MBT plants were generally able to identify markets (cement kilns and power stations) for their RDF product. However, since June 2005, landfilling of untreated waste is prohibited due to the implementation of the Landfill Directive (1999/31/EC). Household waste and commercial waste must be either pre-treated at an MBT facility or disposed through incineration. The calorific value of the commercial waste delivered to incineration usually exceeds the specification of 10-12 MJ/kg set at many incineration plants. Consequently, the throughput of the incineration plant is reduced and the commercial waste exceeding the available capacity is currently stored at the facilities. As a result, there is insufficient capacity for thermal treatment in Germany which enhances the challenge to secure markets for the RDF material. Furthermore, it is likely to increase the market price for the RDF. The German challenge clearly shows that MBT facilities must remain flexible in the RDF process and improve the RDF product as required. Furthermore, the increasing gate fee for incineration demonstrates the importance of the capacity balance between MBT treatment and incineration. Both treatment and disposal options are required for a balanced integrated waste management system. Compost The original concept for MBT was to develop processes that reduce the biodegradable content of residual waste by stabilizing it through the use of a biological process so that the material could be landfilled with lower environmental impacts. MBT processes may also produce a compost material from the mixed residual waste input which is likely to have higher levels of contamination (glass, metal, plastics) and lower nutrient levels than products generated from source-separated feedstocks. Thus, while there may be opportunities to use the compost as a soil improver, it will be very difficult for it to compete with the compost produced from source-separated materials in many of the current markets for waste-derived compost products. Most of the compost material being produced by MBT plants is currently being landfilled, although some is being used in a number of countries (for example, Australia, Spain and Israel). However, the use of compost from mixed residual waste depends entirely on the legislation implemented in the various countries. Dry recyclables MBT plants can also recover dry recyclable materials including ferrous and non-ferrous metals, glass and a mixed polymer plastics product. These products will have to compete with source-separated materials in securing a market; while the glass product may well be suitable for use in aggregate substitute applications, markets for the mixed polymer product are currently limited. The future for MBT Conventional waste-to-energy will continue to have a key role to play in treating residual wastes. MBT-type processes may complement WTE systems and offer a number of potential benefits, but they face considerable challenges in realizing these. Factors that make MBT attractive include the following: null MBT plants have the potential to operate economically at lower scale and thus have the ability to process waste locally, producing a RDF product that can be burnt in more remote combustion facilities. The RDF product may be more suitable for heat recovery and electricity generation as the scale may match the smaller heat demands for individual projects in areas without district heating networks. MBT integrated with anaerobic digestion offers the potential for additional renewable energy recovery. The potential to recover additional recyclables and to conserve the embedded energy contained in these materials is beneficial in terms of resource efficiency. Although MBT is a waste treatment process, the public perception is generally better than that for an WTE incineration facility. However, a number of factors can make MBT unattractive: The overall amount of energy, as electricity, which can be recovered using a MBT process is likely to be lower than that from using conventional EfW incineration process. Lack of markets for the RDF and compost products can be a barrier for the commercial liability of the MBT facility. Processes which might produce a more marketable (higher biomass content) RDF product (such as autoclaving) are still being developed and may provide some competition for MBT in future. The poor public perception of facilities combusting waste-derived fuels can be a barrier to getting permits to operate. Greater landfill capacity will be required for process rejects (and for any products that cannot be marketed or used). Some MBT technologies are well established, and while markets have been found for the RDF products in previous years, the current market capacity is limited. A large potential market for RDF is its use in power stations, but unless co-firing solutions can be further developed successfully (both commercially and technically), the capacity for using RDF products to generate electricity may well be restricted to either MBT plants which incorporate an anaerobic digestion facility or a dedicated thermal conversion facility. There will be limited opportunities to recover heat at these plants as waste facilities are often sited away from other industry and housing due to concerns over odour and emissions. Additional development work could be conducted to produce better-quality compost products and develop suitable markets for them, and to improve the quality and range of other recyclates to enhance the conservation of embedded energy. Otherwise, the future role of MBT may well be just to treat small local arisings of waste in order to recover recyclables and stabilize the remaining waste prior to landfill. Patrick Wheeler is Deputy Task Leader of IEA Bioenergy Task 36. Research for this article has been made possible with contribution fromNicole Jaitner and Jim Poll, both of AEA Energy & Environment, UK.e-mail: patrick.wheeler@aeat.co.uk Notes Quantification of the Potential Energy from Residuals (EfR) in the UK. Report by Oakdene Hollins for The Institution of Civil Engineers and The Renewable Power Association, March 2005. Data collated by Task 36 of IEA Bioenergy, 2005. www.ieabioenergytask36.org/ Mechanical Biological Treatment: A Guide for Decision Makers, Processes, Policies and Markets. Juniper Consultancy Services Ltd., 2005. Solid replacement Fuels (SRF) for use in co-incineration plants. H. Baier, Ecowest GmbH, Ennigerloh, Germany. Zement-Kalk-Gips International, No 3-2006 (Volume 59). World energy markets and waste In 2004, world total primary energy supply was 11,059 Mtoe (million tonnes oil equivalent), of which 10.6% was supplied by combustible renewables and waste sector. While this sector has experienced slight growth since 1973, in fact the proportion of energy supplied in this way has decreased compared with other sectors. In 1973, 11.2% of our primary energy came from combustible renewables and the waste sector. Figure A. Outlook for the world total primary energy supply (‘other’ includes combustible renewables and waste, geothermal, wind and tidal energy). source: key world energy statistics - iea (2006) Click here to enlarge image World electricity generation in 2004 was 17,450 TWh, of which 2.1% was powered by ‘other sources’ - geothermal, solar, wind, combustible renewables and waste. This compares to 0.7% in 1973, indicating that we are relying increasingly on renewables and waste for electricity generation. Current estimates suggest that although the worldwide energy supply through combustible renewables and waste will increase slightly, there are no significant changes expected in future years in proportion to the world total primary energy supply - see Figure A.