Again and again we hear the message that resources are scarce, and that we have to ensure that they are not lost in our waste. Explanations as to which resources we are talking about and how we avoid wasting them, however, are in short supply. Waste may contain many different resources depending on origin and prior use. The resources of waste may be considered a material resource, an energy resource or a nutrient resource, and typically a waste fraction will comprise a mixture of these in varying quantities. When evaluated from a materials and nutrients point of view, however, many waste fractions do not contain any significant recyclable resources without them first being subjected to thorough sorting and cleaning processes – processes which both economically and in terms of energy are very costly. At modern waste to energy facilities with combined heat and power production a highly efficient recovery of the most important resource of these waste fractions - energy - can be ensured. When material resources are recycled, an actual environmental benefit only occurs if it results in savings of virgin materials. Similarly, for it to be an environmental benefit, the recovery of energy has to supplant other energy production whereby the consumption of fuels and/or materials is spared. Despite global efforts to reduce our reliance on fossil fuels by developing alternative renewable energy production, the production of both power and district heating, will in the coming 20 years continue to be broadly based on the use of fossil fuels. This is why the utilisation of the energy resource present in waste both saves the consumption of fossil fuels and the emission of greenhouse gases (GHGs). Even though many European countries are planning a conversion of their energy production from being fossil fuel based to being biofuel based, energy recovery from waste will continue to be an environmental benefit as it will save bio resources, which are expected to become a scarce. The Energy Tower The utilisation of the energy resource of waste is the objective of a new facility, dubbed the Energy Tower, in Roskilde, Denmark. The design of the facility is focused on maximum utilisation of the energy resources in the waste, such that it may be harnessed with practically no losses. The grate and boiler for the plant have been supplied by German waste to energy technology specialist, MARTIN while the flue gas treatment system was supplied by LAB – a subsidiary of CNIM – and the turbine by and MAN Diesel & Turbo. The large amber-coloured façade is designed by Erick van Egeraat, and at night the backlighting of the perforated façade will transform the spire to an illusion of a glowing beacon, symbolizing the energy production inside the facility. By utilising the energy resources from waste, it is not only the national consumption of fuels that is saved, but the environment also benefits from lower GHG emissions and resources are preserved. Energy Optimisation In the Energy Tower waste is utilised through conversion into energy in the form of electricity and district heating. The focus is on reaching the highest possible efficiency in order to avoid any loss of resources. The plant's waste treatment process produces heat in the boiler room which is used for the production of steam in the boiler. Through this process some 85% - 90% of the energy content of the waste is transformed into steam, and the steam is subsequently converted to electricity as well as district heating. Earlier generations of waste to energy facilities stopped the energy recovery process at this point. However, increased focus on energy efficiency at the Energy Tower has resulted in the installation of flue gas condensation in order to increase the production of district heating. With flue gas condensation the temperature of the district heating water returning from the city is raised, and heat production is further increased by approximately 10%.
With flue gas condensation technology and an absorption cooler driven by district heating water, efficiency approaches 100% The facility's owner, waste management company KARA/NOVEREN, decided to go even further. In the recovery processes a certain amount of electricity is required to operate pumps and fans etc. However, at the Energy Tower the plant's consumption of electricity is reduced by establishing a component cooling system driven by district heating. The component cooling system is a necessary installation in any modern process plant, and hence also a necessity in the Energy Tower. However, instead of cooling away the heat with a traditional electrically powered cooling compressor, KARA/NOVEREN has established an absorption cooler driven by district heating water. The need for cooling is at its highest during the summer when the general need for district heating in turn is at its lowest and as such has the lowest recycling value. The total process energy consumption is thus lowered, and the utilisation of the energy resource in the waste is maximised. With the high utilisation of energy resource, the total energy efficiency rate of the Energy Tower is almost 100%. On the basis of 200,000 tonnes of waste being processed at the plant each year, KARA/NOVEREN will produce electricity corresponding to the consumption of some 44,000 households, while the production of district heating will correspond to the consumption of around 26,000 households. As such, the Energy Tower will be one of the most modern and efficient waste to energy facilities in Europe, with the energy recovery increased by 35% compared to the facility's old units. Furthermore, two old 1980s units will be taken out of operation when the new facility is operational. With future alterations of framework conditions for waste to energy plants putting even more emphasis on the utilisation of energy resources in waste, along with a higher valuation of the energy resources recovered, the waste to energy facilities of the future will most likely be equipped with heat pumps coupled to flue gas condensation. This will increase the energy efficiency even further and enable flue gas condensation in systems with relatively high flue gas temperatures. Feedstock & Finance The new €175 million Energy Tower is being built, financed, owned and operated by KARA/NOVEREN - an inter-municipal waste management company owned by nine municipalities west and south of Copenhagen. In addition to the Roskilde waste to energy facility, which includes three units from 1981, 1989 and 1999, the company operates 14 civic amenity sites, a recycling station, a landfill site for residual waste and two transfer stations.
The Energy Tower will offer a total operating efficiency some 35% greater than the two 1980s units it will be replacing In Denmark, the municipalities are responsible for, and own, all waste (municipal and commercial) generated within their boundaries, except for pre-sorted recyclables. KARA/NOVEREN manages the waste generated in the municipalities that own it which comprise of around 400,000 citizens and 20,500 businesses. In 2011, 67% of the waste was recycled, 28% was converted to heat and electricity at its waste to energy facility, 4% was landfilled, and 1% was sent to special hazardous waste treatment. The company is a non-profit based on a cost coverage principle. The gate fee at the waste to energy facility is one of the lowest in Europe and amounts to only DKK200 (€27) per tonne of waste (excluding taxes and VAT). The low gate fee is attributable to the efficiently operated facility on the one hand and to the extensive energy recovery on the other. The electricity produced at the facility is sold to the national grid, and the heat is sold to the district heating network owned by the Copenhagen district heating transmission company, VEKS, which supplies heat and hot water to the equivalent of 150,000 families. In the summer, the entire consumption of heat and hot water in VEKS's transmission area is covered by KARA/NOVEREN and one other waste to energy facility in Copenhagen. The annual treatment capacity of the facility is around 250,000 tonnes. Once commissioned in the summer of 2013, the Energy Tower will replace the two 1980s units, giving the facility a total annual capacity of around 350,000 tonnes. Energy recovery and recycling in Parallel It is beyond any doubt that part of the European resource efficiency ambition is to maximise recycling. We should do all we can to further increase recycling percentages, without compromising on economic and environmental sustainability.
Work on the facility is almost complete and commissioning is expected to commence in summer this year At the same time we have to increase the effort to maximise the recovery of energy from residual waste, which cannot be recycled in a sustainable manner. Denmark has for many decades demonstrated that high recycling rates, even among the highest in the world, are compatible with waste to energy, and that with innovative thinking and dedicated effort both recycling and energy efficiency can be further increased in parallel. Many other European countries are following suit after having dealt with the stigma of waste incineration and unsuccessfully tried out alternative ways of utilising the resources in their waste. They now know that the Energy Tower in Roskilde is among the places to go if they want to study waste management excellence. Kim Brinck is chief consultant and Susanne Wellington Hansen is communications & marketing manager at Ramboll Energy This article is on-line. The energy recovery process The high energy recovery efficiency in the Energy Tower is achieved by combining and integrating the design of the boiler with the flue gas treatment. During the incineration process, energy is extracted from the combustion by production of steam in the boiler while the temperature of the flue gas is decreased. At the end of the boiler system the temperature is reduced to approximately 170°C. At this point, the flue gas is contaminated with dust and acidic components so further reduction of the temperature would cause operational problems with condensation of liquid acid. This would not only increase the corrosion of steel components, but also result in fouling on tubes etc. Therefore further energy recovery must be done after de-dusting of the flue gas and by equipment made of corrosion-proof materials. The first step in the flue gas treatment is de-dusting by bag house filtration. By keeping the temperature above 170°C, clogging of bags and formation of sticky fly ash product are prevented. After this process the flue gas is almost totally free of dust, and additional energy is subsequently recovered from the flue gas by further cooling to approximately 120°C in a low-temperature economizer (LT Eco). Energy from the LT Eco is returned to the water steam circuit as air preheating for the combustion processes (thermal input in boiler system is increased), which increases the production of steam in the boiler. From the LT Eco the flue gas is cleaned by a two-stage wet scrubbing system. As part of the scrubbing process, the temperature of the flue gas is lowered to approximately 55°C, and the humidity increases (the flue gas becomes saturated with water). The relatively low temperature of the flue gas after the scrubbing makes it unsuitable for further thermal input to the boiler system, but it can be used as an additional energy input for district heating. Consequently the two-stage scrubbing system is followed by a condensing scrubbing system where district heating water returning from the city is preheated by the water loop in the condensing scrubber, and the condensation process recovers the latent energy hidden in the humid flue gas. The flue gas is cooled to a temperature of approximately 2-3°C above the temperature of the district heating water returning from the city so the magnitude of this energy recovery process is highly dependent on this temperature. From the condensing scrubber the flue gas is emitted to the atmosphere. Almost all of the energy in the waste is used in the process by production of either electricity or district heating, and the thermal loss to the surroundings is minimized, which results in an energy recovery efficiency rate of almost 100%. With respect to material recovery, inert materials from the waste (ashes and metals) are transformed to bottom ash during the combustion processes, and after incineration metals are extracted and recovered from the ashes for further reuse and thus replace the use of virgin materials. The metal-free bottom ash itself is also reused for the foundations of roads and car parks etc.., and replaces the use of aggregates. The Energy Tower will offer a total operating efficiency some 35% greater than the two 1980s units it will be replacing