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Powering business with waste residues:
How thermally dried municipal solid waste and sewage sludge can replace fossil fuels sustainably.
With the current global shift towards landfill reduction, green energy production, and utilization of methane gas, waste-to-energy solutions have proved effective in reducing the problem of waste disposal and the environmental impact of using fossil fuels. Improved waste management as well as the growing need for alternative fuels are the keys to making treatment of waste more resource-efficient. One way of achieving this is to focus not only on reducing the volume of waste disposal, but also on reuse, recycling, and recovery technologies.
Reduce, reuse, profit: how to catch three butterflies with one net
For years now, the volume of waste has kept growing with the world’s population, as has the population’s demand for energy, which is largely generated from non-renewable fossil fuels. The need to replace these energy sources is becoming more urgent as time passes, which is why more and more companies are jumping on the bandwagon of converting waste into fuel – which sounds like part of an ideal solution for both of these challenges from an ecological point of view. Municipal Solid Waste (MSW) is one of the largest waste capacities globally and requires more and more costly space for dumping on landfill sites. This is where the demand for efficiency in waste-to-energy solutions adds an economic level to this topic as well, not only by reducing the mass of waste and saving on landfill costs, but also by producing profitable alternative fuels.
Some good examples of a successful increase in the calorific value of components extracted from MSW are Refused Derived Fuel (RDF) and Solid Recovered Fuel (SRF), which can both be reused as high-calorific fuel. These alternative fuels consist largely of combustible components found in municipal waste, such as plastics, fibers and fabrics, and are produced by shredding, sorting, and drying the light fraction of solid waste to obtain a high-calorific and stable material. Drying with advanced systems can raise the net caloric value to between 16 and 20 MJ/kg, making RDF and SRF able to replace fossil fuels in many applications, such as cement kiln plants, steel furnaces, or power plants. If you use the right solution, you will be able to catch three butterflies with one net.
Converting garbage to profit? Two areas in the UK achieve this with efficient drying.
Transforming a widely diverse assortment of waste into new resources requires precise handling, as the source’s material characteristics, such as moisture content, particle sizes, and bulk density, can vary widely. Yet, RDF and SRF require a consistently low moisture content to reach a high calorific value.
Thermal drying is a key process step in achieving the high quality standards needed in renewable fuel. A well-proven and highly efficient drying process turns the wet RDF into a valuable energy source for further downstream processes, for example gasification, combustion, or densification.Among several thermal systems that are capable of converting wet RDF into a valuable product, belt and drum drying systems are the most proven ones. Both are convective drying systems. While the belt dryer can utilize low-temperature heat sources (≤90 °C possible), the drum dryer requires high-temperature heat sources of around 300 °C. Independently of the heat sources used, bothdrying systems are able to reduce the moisture content significantly down to less than 10% water content while increasing the calorific value of RDF.
RDF consists of several types of materials, such as plastics, paper rejects, textiles, rubber, as well as undesirable material like residual glass, stones, and metals. Different particle sizes and densities arise from this mix, and these are rather difficult to handle. Rough particles cause wear and tear. Thus, proper feeding is the main challenge for RDF dryers. Based on extensive pilot testing and operational field experience with different RDF and MSW materials, the international technology group ANDRITZ has developed specially designed feeding devices for drum and belt drying systems to overcome these difficulties and ensure a proper and trouble-free drying process as well as homogeneous drying of both light and heavy particles.
One of the main goals of RDF and SRF producers is not only to reduce mass and gain partial independence from fossil fuels, but also to generate a profit from the sale of these products. One local authority that achieved the goal of economic profit with renewable fuels is the town of Swindon, UK. A state-of-the-art municipal solid waste plant was established to process MSW as well as commercial and industrial waste with the goal of producing RDF and SRF for sale. With a capacity of 48,000 tons per year of diverse waste fluctuating between 20% and 50% moisture content, drying the waste to achieve a uniformly low moisture content was a key process step for the plant to successfully produce high-quality SRF. Downstream of an RDF pretreatment facility, an ANDRITZ Vandenbroek drum dryer was selected as the right drying solution to obtain a uniform, low moisture content. For the waste-to-recycling plant in Swindon, this solution has proven effective because it achieves the required SRF standards of 15-20% moisture content in the final product while creating a low volume of exhaust air, despite the diversity of the moisture content in the various waste components. Controlled product moisture content is achieved continuously through special drum internals, allowing for high-quality RDF for profitable re-sale.
If we leave Swindon for a 350-mile drive north, we reach Levenseat Limited in Scotland, which also operates a waste recycling plant, including gasification to convert MSW into energy. The goals in Levenseat were even more ambitious – saving 1.4 million tons of waste from going into landfill and delivering a reduction of around 1.3 million tons in greenhouse gas emissions. The electricity to be supplied by this plant will meet the needs of 18,000 households.
As in Swindon, the international technology group ANDRITZ was approached for a solution. While the waste recycling plant will convert municipal solid waste into energy using a gasification process, the ANDRITZ belt dryer technology was chosen to prepare the RDF material for the downstream gasification process. 72,000 tons of wet MSW per year will be converted into energy by applying pre-treatment, drying and gasification. The gasification process itself requires a constant moisture content and a steady heating value. Like in Swindon, the moisture content of the MSW to be converted varies strongly, therefore a belt dryer is an essential part of this process to make sure the RDF produced and converted achieves constant standards of 20% moisture content and a constant high calorific value. Since the MSW to be dried contains a large amount of food waste, an acidic and caustic scrubber package controls emission thresholds for ammonia and H2S.
And the common denominator? State-of-the-art drying systems that help reduce the landfill needed, cutting dumping costs, powering households, and making profits with alternative fuels by using what citizens throw into the trash day by day.
Let’s visit the sewers, there might be money in there as well!
Pecunia non olet – this Latin saying translating into “Money doesn’t stink” is a phrase that is ascribed to the Roman emperor Vespasian (ruled AD 69–79) about taxing the distribution of urine from public urinals in Rome’s sewer system. Let us take this opportunity and see if there is “money in the sewers” in modern times as well, as renewable energy from waste is not found on the landfill sites alone. The use of municipal sewage sludge in waste-to-energy plants as well as co-combustion of sewage sludge in municipal solid waste plants is attracting more and more interest because it also has a high potential and calorific value. As in the conversion of MSW, sewage sludge conversion helps solve the disposal problem while also providing an alternative source of energy. The experience of ANDRITZ with its drying technologies (belt, drum, paddle and fluid bed dryers) shows constant results with a calorific value of 10 to 17 MJ/kg after being dried to approx. 90% dry solids, making sewage sludge an attractive renewable energy source.
However, this solution becomes even more efficient when combining a sewage sludge drying installation and a municipal solid waste plant using the steam generated by the municipal solid waste combustion process and waste heat to operate the sewage sludge drying line. Without the need for external heating sources for the sewage sludge drying line, thermal energy costs are reduced to a minimum although they account for the largest share of the operating costs in the drying lines. For example, when using a paddle or fluid bed dryer for the sludge drying line, off-gas volumes are so small that they can easily be treated thermally in the combustion process itself, eliminating the need for any biological or other multi-stage filter for treating the off-gas.
With the many solutions that have already proved effective, the move towards using renewable energy sources is not being prevented by a lack of know-how. The future challenges to handle ever-growing garbage heaps and the need for more energy may be overcome simply by making existing waste-to-energy solutions more acceptable and even more efficient than the state-of-the-art technologies already demonstrate.
Complete wastewater technologies, services, and automation solutions from a single source
In developing countries, achieving a reliable base infrastructure for drinking water, wastewater treatment, and municipal solid waste disposal is challenging. In industrialized countries, water quality, new energy sources, and environmental regulations can play a driving role. Whatever the case, the right mechanical and thermal separation solutions as well as pumping systems are vital to providing safe and affordable living conditions for people around the world.
ANDRITZ is one of a select few global suppliers to combine a complete range of multiple thickening, dewatering, and drying technologies and services, together with screening, and ancillary solutions. To complete this holistic solution-oriented portfolio, ANDRITZ offers highly efficient pumps specialized for various tasks in waste water applications such as single-stage centrifugal pumps as well as dry and wet installed sewage pumps. This affords plant operators the flexibility to choose the best individual equipment or service from a range of options or a complete system with all components fully integrated and fine-tuned. From initial consulting and feasibility studies to pilot tests, on-site optimization, and full-service contracts, ANDRITZ can draw on a wealth of references and hands-on experience to support wastewater treatment plants with decades of high operational efficiency. And when it comes to process control, the Metris addIQ control systems from ANDRITZ combine all its extensive operation, troubleshooting, and start-up experience in one tailored automation solution. With the broad portfolio of scalable automation solutions, applicable for a wide range of purposes (e.g. predictive maintenance, process monitoring, troubleshooting, long-term trending, etc.), ANDRITZ is a true full-service provider helping to reduce risks, increase the availability and through this improve the ecological footprint as well as the long-term business case for its clients.
Author: Doris Thamer, Sales Director Thermal Systems MSW & Biomass, ANDRITZ
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