The steadily increasing amount of waste is one of the pressing problems of our time, yet waste to energy plants must meet increasingly stringent environmental standards. Spain’s Sogama CFB-based waste to energy plant shows the way forward.
Managing the increasing amount of commercial and industrial waste is a pressing problem. In Europe, reuse, recycling and combustion are methods being promoted in favour of landfilling. Yet the physical and chemical composition of waste is changing, making combustion a challenging option.
In addition, the environmental legislation concerning waste incineration has become more and more stringent, stimulating the development of advanced combustion technologies that can meet these challenges, both now and in the future.
One company to choose the combustion option is Sociedade Galega do Medio Ambiente (Sogama) of Spain. With the help of Kvaerner’s circulating fluidized bed (CFB) boiler technology, it is developing a waste to energy plant in Galicia, northwest Spain, that will process 650 000 t of municipal solid waste (MSW) per year, generating power for local use.
The Sogama plant, near the town of Cerceda, will be commissioned in early 2000, when it will become the largest energy from waste plant using CFB technology in the world. It will meet not only current EU environmental limits, but also the anticipated emission limits to be set under legislation currently under preparation. Sogama will recover useful materials, generate 49 MW of electricity for export to the grid, and reduce the volume of waste to be landfilled in the region.
An issue of waste
Environmental concerns and the steadily increasing amount of domestic and commercial waste being produced is leading to tight restrictions being placed on landfill disposal of MSW. However, the daily per capita generation of MSW has risen to as much as 1.5 kg in some industrialised countries, and while reuse, recycling and composting are increasing, landfill continues to dominate as a disposal method.
Combustion of MSW is considered to be an efficient and environmentally friendly method of waste disposal, and is rising in popularity, especially where energy is recovered. In Switzerland, Japan, Denmark and Sweden, more than 50 per cent of MSW generated is combusted to generate energy.
However, over the last 50 years, the physical and chemical composition of MSW has changed considerably due to an accelerated proliferation of waste organic matter, paper and plastics. This, together with increasingly stringent environmental legislation, has presented new challenges, and has stimulated the development of more advanced combustion techniques.
CFB technology has been established as an efficient method of MSW combustion for more than a decade. Fluidized bed boilers are well known for their fuel flexibility, being able to burn fuels with a wide range of calorific values, ash and moisture content. This makes them highly suited to waste combustion, where fuel characteristics can vary on a day-to day basis.
Fluidized bed boilers are characterised by good fuel distribution and vigorous fluidization, which minimises local temperature variations and ensures complete combustion of the fuel with low excess air. Excellent combustion can therefore be achieved with high boiler efficiency and low emissions of carbon monoxide (CO) and nitrogen oxides (NOx). In addition, CFB boilers can achieve very high heat transfer rates as superheaters can be located in the loop seal.
Waste fuels are often characterised by high levels of chlorine and ash which can lead to corrosion and boiler fouling. CFB boilers in waste to energy applications have design characteristics to overcome such problems:
- Chlorine present in MSW is a possible corrosion source. Low velocities through the furnace and a radiation pass after the furnace results in a relatively low gas temperature on the first heating surfaces in the back pass. This minimises the risk of corrosion and fouling.
- Fouling in the upper part of the furnace and in the back pass and high CO emissions are avoided by ensuring good mixing between the fuel and air in the furnace and an oxidising atmosphere.
- The accumulation of coarse ash in the bottom zone can be avoided by the presence of a patented water-cooled hydro beam bottom grate where more than 30 per cent of the floor area can be reserved for the removal of coarse material.
In addition, lime or dolomite can be injected into the fluidized bed for sulphur removal.
Kvaerner introduced its first CFB for MSW combustion in 1984, and has since delivered more than 20 fluidized bed boilers for this type of application. Based on its experience with these plants, Kvaerner has developed its CFB technology in line with demands on environmental performance and unit size, increasing unit scale to 75 MWth. Earlier this year, it delivered its two largest waste-fired CFB units to Sogama.
Sogama is jointly owned by Spanish utility Union Fenosa and the Galician government. The company, which was formed to manage solid waste handling and combustion in the Galician province, awarded Kvaerner Pulping the contract to supply its CFB boilers to the project in April 1998. Worth $45 million, the order includes twin CFB boilers, all auxiliary equipment and a flue gas cleaning plant. It also includes the option for doubling the plant’s capacity in the future. The project won funding from the European Union.
A competitive solution
In planning the design of its waste to energy plant, Sogama found Kvaerner’s CFB technology to offer a competitive solution to the problems of handling waste in an efficient and environmentally acceptable way. The CFB combustion, together with flue gas emission control, a dry scrubber and a bag filter dust cleaning system meets all current and proposed EC emission legislation and regional requirements.
The Sogama plant will process approximately 650 00 t of MSW into 400 000 t of refuse derived fuel (RDF) each year. The plant consists of four major parts:
- A materials recovery facility designed by Sogama, including waste reception and fuel preparation areas, to which MSW is delivered by rail from transfer stations around Galicia. MSW is fed into a trommel, which separates small and large particles. The waste is dried by hot gases from the cogeneration plant and valuable material is recovered for recycling. The remaining material is shredded and conveyed to the boilers. Recycled material will include glass, paper, iron, aluminium, plastics and cardboard.
- A boiler island incorporating two 75 MWth CFB boilers, a dry flue gas cleaning system and a baghouse filter. This section accounts for one third of the total cost of the plant.
- A turbine island consisting of two steam turbines driven by steam from the two boilers. A total of 49 MW of electricity is generated for export to the local grid. Cooling is provided by a water-cooled condenser and a cooling tower.
- A cogeneration plant based on four gas engines, generating 21.6 MW of electricity and hot flue gas at 385°C used for drying the MSW.
Fuel and fuel handling system: Each boiler has three parallel fuel feeding lines leading to three separate fuel inlet chutes through the furnace front wall. In each line, the RDF is received in a 50 m3 boiler silo and is conveyed further by a collecting screw conveyor to the dosing bin feeder, which secures an even volume flow of RDF. The fuel is then sent to an eddy current separator for final removal of aluminium, and via a rotary valve down to the fuel chute.
The fuel chute is equipped with high velocity air nozzles that will spread the fuel evenly over the cross-section of the furnace in order to achieve a homogenous and complete combustion, leading to low emissions of NOx, CO and organic compounds.
The CFB boilers: Each of the two CFB boilers is designed to burn 23.7 t/h of RDF with a moisture content of 28 per cent and a net calorific value of 12 500 kJ/kg. The boilers can be operated within a load range of 70-100 per cent and a with a variation in the net calorific value of the fuel of 9200-16 700 kJ/kg.
The main parts of the boiler system are a water-cooled furnace with two integrated water-cooled cyclones and loop seals containing the final superheaters, and an external ash-classifier. The cyclones are followed by a single pass radiation cavity and a convection pass with superheater banks, boiler banks and economiser banks. The boiler is equipped with a conventional steam soot blowing system cleaning the banks in the convection pass.
The boiler is top-supported and designed for natural circulation. Saturated water from the drum is distributed through a number of downcomers to the bottom part of the boiler, the wall tubes in the furnace, the cyclone loop seal and the radiation cavity/back pass enclosure. The water/steam mixture is transferred back to the steam drum by a number of steam-separating connecting pipes. The circulating system is integrated between all components.
The furnace front and rear wall are bent into a ‘U’ shape to form a water-cooled windbox for primary air, below the fluidized bed furnace. The primary windbox also serves as a combustion chamber for the start-up burner, located at the furnace front wall.
All four walls in the lower and middle part of the furnace are refractory lined for erosion protection and to sustain furnace temperature above 850°C for two seconds after the last injection of air. This is a European requirement for firing waste. Auxiliary burners are also required to secure the furnace temperature at 850°C before adding the RDF during start-up, and in case of a drop in temperature during boiler operation.
Each boiler features two hot gas cyclones for separation of the bed material entrained by the flue gas and leaving the furnace at the furnace top. The separated material is returned to the lower part of the furnace via a loop seal. The loop seal contains a bubbling fluidized bed and is equipped with a number of air nozzles to ensure material transport. It is also designed to prevent flue gas from the furnace entering the cyclones through the bed material return leg.
The loop seal is a unique feature of the CFB process, and is an excellent location for the final superheater because the heat transfer coefficient in the bubbling bed is is five to ten times that in the back pass, so the superheater area required is reduced by 50-90 per cent. In addition, the gaseous atmosphere in the loop seal contains much less chlorine, minimizing the risk of corrosion to the high temperature superheater material.
The cyclones are constructed from water-cooled membrane walls, which form part of the water circulation system. This unique design feature means that the cyclones expand in the same way as the furnace and back pass enclosure. This allows them to be gas-tight welded to the furnace and the back pass, thus avoiding all expansion
bellows of large dimensions, which can result in costly maintenance. The cyclone interior is fully refractory lined with a thin layer for erosion protection, reducing maintenance costs and shortening start-up time.
The furnace is followed by a single pass radiation cavity for lowering the flue gas temperature to a temperature that makes the ash dry and non-sticky. This minimizes deposits and corrosion attacks. The bottom of the radiation cavity is equipped with an ash extraction conveyor system.
Coarse fuel ash entering the furnace is transported through the bottom bed by the directed primary air nozzles, to discharge openings in the bottom plate from where it is fed to the ash classifier. This acts as a high velocity fluidized bed, which elutriates the small bed particles from the coarse particles in the discharged ash and sends them back to the furnace. In addition, the coarse ash is cooled by the excess air before it leaves the classifier, which minimizes the loss of sensible heat. This bed ash, as well as the boiler ash and filter ash, will be transported to a local landfill.
The flue gases are cleaned by an ABB New Integrated Desulphurisation (NID) system using a reactor and a baghouse filter. The system is a dry flue gas cleaning process based on the reaction between SO2/HCl and Ca(OH)2 under humid conditions. No pre-collector is required.
Emissions from the plant will meet EU requirements, with guaranteed values of particulates at 10 mg/Nm3, CO at 50 mg/Nm3, NOx at 300 mg/Nm3, and SO2 at 50 mg/Nm3.
According to Kvaerner, site work on Sogama is well advanced ready for commissioning to start early in 2000. The major components of Kvaerner’s supply contract, including the boiler island, are nearing completion, and insulation work is going on. Commissioning is due to be completed in mid-2000.