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Power from Waste: Recycling the waste

Fabrizio Pollastro, Foster Wheeler Italiana

Concern over global warming and the effect of pollution caused by the burning of brown fuels has become a major issue for governments. Recent encouragement from the European Union Directives to the Member States for the recovery of waste by means of recycling and the use of waste as a source of energy has produced waste-to-energy (WTE) plants across Europe.

Lomellina began operating in July 2000
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The Lomellina Energia Recycling plant in Parona, near Milan, is an integrated WTE facility for the recovery of recyclable materials, the production of refuse derived fuel (RDF), composting and electricity generation. Foster Wheeler Italiana (FWI), owns 98 per cent of the plant and was responsible for the development, construction, ownership and operation of the facility.

The plant, which began operation in July 2000, required an investment of a130 million ($121 million) and was financed by 24 per cent of equity and 76 per cent of debt. The plant is capable of handling RDF with a LHV range of 2500-4000 kcal/kg.

Modern WTE plants are very different from the old incinerators thanks to technological progress made over the last few decades. They have two priorities: respect for the environment and the efficient generation of electricity. An important improvement has been achieved by converting municipal solid waste (MSW) into a real fuel that can be easily stored, transported and efficiently burned. From the combustible fraction of MSW it is easy to obtain a product that is much more homogeneous and stable than MSW. This material is known as RDF and is a mix of particles of paper, paperboard, rubber, plastic, textile, leather and wood. Around 60 per cent of the MSW is converted into RDF and the balance yields reusable aluminium, ferrous materials, glass and compost.

RDF is an ideal fuel as it has a good heating value, a controlled chemical composition, no smell and can be used by one of the most efficient combustion technologies is available today – circulating fluidized bed (CFB) boilers. A CBF boiler is at the heart of the Lomellina plant and was selected as the technology to generate electricity because of its inherently strong environmental performance.

The Lomellina Energia recycling plant is designed to recover material and energy from 200 000 t/year of MSW. The sorting process is attractive from the point of view both of recycling and for the production of RDF, a fuel that can be easily burned and produces very low quantities of bottom ash. The net power output of the facility, which commenced operation earlier this year, is about 17 MW. Electricity produced at Lomellina is sold to Enel under a 15-year Power Purchase Agreement (PPA). For the first eight years of operation the electricity tariff includes an incentive since energy from waste is considered a renewable source of energy.

In addition to electricity sales, the MSW is also a source of revenue for the plant owners. A framework agreement has been signed with the Province of Pavia, which has appointed Lomellina as the “designated plant” for a total of 96 municipalities in the Province. This framework agreement ensures that the municipalities have to deliver all their waste to the Lomellina plant at an inflation adjustable gate-fee of a28/t of MSW. Separate waste delivery agreements have been signed with municipalities in order to detail the specific terms and conditions for waste delivery.

The contracts have a put-or-pay character, which means that even if the municipalities deliver less than the agreed committed quantity, they will have to pay an amount based on the agreed gate fee and committed quantity. The system was started in December 1999, six months prior to the scheduled power plant start up. This was necessary in order to guarantee the disposal of MSW in a district which is suffering from a serious emergency due to the closure of landfill facilities.

From waste to energy

At Lomellina the MSW waste is separated as shown in Figure 3. After the first shredding MSW is sent for separation into three streams: an organic rich fraction (60 mm); a metals rich fraction (60-120 mm) and a combustible rich fraction (120 mm). The combustible rich fraction, after the removal of ferrous material through magnetic separators, is conveyed to magnetic separators for recovery of ferrous metals and to eddy current separators for recovery of aluminium cans. The remaining material is mixed with the combustible rich fraction and the waste material is shredded to a particle size of 90 mm or less.

The RDF then passes through the last magnetic separator and is conveyed to a storage building which can store enough material for three days of boiler operation at full capacity. By compressing the RDF this capacity can be increased to six days.

The Lomellina recycling plant converts waste to RDF
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The organic rich fraction is stabilized using an anaerobic fermentation process. During this process, air for composting is partially taken from the MSW receiving and sorting building. This causes a slight negative pressure that results in a steady flow of fresh outside air through these areas. A biofilter treats the air from composting and from the MSW processing buildings. The stabilized product is sent to the secondary trommel where materials over 20 mm in size are recovered and conveyed to the RDF stream.

The remaining organic fraction can be refined using an air classifier for the separation of glass and other solid inerts. The low quality compost derived from stabilization of the MSW organic fraction can be directed to compost storage or to the RDF stream.

The RDF is delivered to three parallel fuel feed systems sized for 50 per cent capacity at full load. From here the RDF is transported to the furnace where air is injected to transport the heavier bed particles towards the drain connected to the stripper-coolers and allows for fluidification of the remaining lighter particles. Secondary air is then injected in the furnace at two elevations above the bed to achieve a staged uniform combustion. Inside the furnace RDF is burnt in this hot, fluid suspension bed, entrained in a substantial upward flow of gas. Coarse solids are separated from the gas stream, which exits the top of a cyclone and is completely cooled with saturated steam from a drum.

This design effectively handles the thermal variations and gives reliable operation and shortens the start-up time. The cyclone separates the entrained solids including unburned carbon from the flue gas and returns them to the furnace providing an excellent carbon burn-out. From the cyclone the flue gas flows through the idle pass for lowering the temperature before entering the convection section of the boiler at 650à‚°C and then through the vestibule over the primary and intermediate superheater sections. From the vestibule, flue gas flows down through the heat recovery area over the evaporator and the economizer and finally flows out through the lower gas exit.

CFB combustion

The use of CFB combustion technology provides several advantages over conventional grate-type boilers. RDF is burned in a hot, fluid suspension of material, entrained in an upward flow of gas. Each particle of RDF is completely surrounded by air so that the contact between the fuel and oxygen is very easy and the combustion reaction is very efficient and stable. The CFB’s turbulent mixing of hot inert bed particles and air provides an excellent combination of the ‘triple Ts’ of combustion: time, temperature and turbulence.

Temperature, typically in the range 850-900à‚°C, is uniform and stable throughout the furnace due to highly turbulent mixing and bed thermal capacity. The mixing of fuel and air due to turbulence reduces the potential for hot spots or for localized reducing atmosphere.

As a consequence of the easiness of the combustion process, excess air requirements (50 per cent) are substantially less than for conventional grate boilers. Despite the low excess air needed by the CFB, carbon monoxide (CO) emissions are very low. The low and homogeneous furnace temperature provides lower nitrogen oxide (NOx) emissions than conventional high temperature combustion and the introduction of air in stages suppresses the generation of NOx even further. Another advantage that is related to relatively low combustion temperature is the low potential for ash slagging and tube fouling. A very high boiler efficiency exceeding 86 per cent is achieved because of low air excess (50 per cent) and low unburned carbon (one per cent). This results in increased steam and electrical output per ton of waste burned.

Auxiliary burners and in-bed lances firing natural gas are used to heat the boiler and the bed during the start-up phases. Boiler feedwater enters the unit at the economizer and the water rises to the steam drum where it is distributed to the lower inlet headers of the furnace walls, the idle pass walls, the heat recovery area enclosure and the evaporator bank.

Inside the water walls and the evaporator coil water flows upward and boils. A steam-water mixture is collected at the upper outlet headers and sent to the steam drum. The steam drum then separates the water from the steam-water mixture and directs it back to the drum reservoir for circulation to the water walls and evaporator coils. The steam is dried and sent to the superheaters where heat is recovered from recirculated ash in the Intrex heat exchanger.

The design steam production is 83 t/h at 443à‚°C and 62 bar[g]. The superheated steam drives a steam turbine, single casing, condensing type, with three extractions feeding the deaerator, the two condensate pre-heaters and the combustion air pre-heater. The electric power production at generator terminals is 19 MW. The exhaust steam from the turbine is sent to an air-cooled condenser where the condensate flow is pumped from a condenser hotwell through a preheater into the deareator and then to the boiler.

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The spent bed material is in the bottom of the furnace in the lower portion of the bed. From the rear wall at the bottom of the furnace the coarser, heavier ash is discharged to the two stripper-coolers. Air is used to facilitate the discharge of the bottom ash. Each stripper-cooler is utilized to strip the fines from the ash, channelling them back to the furnace and to cool the remaining coarser ash. Bed material is admitted into the stripper-cooler when the furnace bed level reaches a set value. The bottom ashes, which are about three per cent by weight of the fired RDF, are disposed in a landfill for non-hazardous waste.

Non-hazardous waste

The flue gas cleaning system is very simple and reflects the philosophy which inspired the design of the unit: an intrinsically clean power plant firing a real fuel instead of an incinerator followed by a complicated chemical plant to clean the flue gases. The flue gases are cooled to 130à‚°C by finely dispersed water droplets which vaporize to yield the optimum moisture content for pollutants removal. The flue gases are then sent to the Venturi dry reactor where hydrated lime and activated carbon are pneumatically injected in the flue gases for the removal of acid components and pollutants.

The fly ash, the reaction products, the activated carbon and the unreacted lime are then retained by the bags of the fabric filter and periodically removed from the bag by air jet pulses and collected in the filter hoppers. The cleaned flue gases are taken by the induced draught fan and then exhausted to the atmosphere. The fly ash that is collected is partially recycled to recover unreacted lime and carbon and partially sent to the storage silos for further processing.

Without further treatment fly ash is classified as a hazardous substance so it is treated in a cold process to meet the requirements for non-hazardous waste landfilling. Fly ash is mixed with cement and water and poured in bags.

From tests on samples of fly ash, it is expected that fly ash from the combustion of RDF can pass the leaching test without any inertization treatment. Thanks to the quality of the combustion process there is no need for a DeNOx system to meet the NOx emission limit. The expected values for NOx is less than 140 mg/Nm3.