Since its start-up in 2006, the Torsvik CHP plant in Sweden has been demonstrating that energy from waste can be deployed with very low impact on the environment, while safely replacing fossil fuels. Alstom Power’s Bo Herrlander describes the plant and its clean-up technologies.
The use of biofuel in combined municipal district heating and electricity generation in Sweden is gaining attention as the country moves to reduce its use of oil and its carbon dioxide emissions. The biofuel may be virgin wood or recycled, e.g. demolition wood, municipal solid waste and certain industrial and business wastes.
Supported by the Euopean Union’s Waste Frame Directive, which defines energy recovery status for efficient waste-to-energy plants, Sweden started up a waste-to-energy plant outside Jönköping. The plant is notable for an environmental control system supplied by Alstom to optimize emissions performance and heat recovery.
THE TORSVIK CHP PLANT
The Torsvik combined heat and power (CHP) plant, developed by Jönköping Energi AB, was put into operation in the summer of 2006. The new plant uses a grate type boiler to combust 162,000 tonnes per year of waste, replacing old oil-fired boilers to reduce carbon dioxide emissions by 30%.
Torsvik is designed to produce 350 GWh of heat and 100 GWh of electricity annually, with an overall plant efficiency of 92% and in conformity with the Waste Frame Directive energy efficiency criteria. The heat is delivered to the district heating network in Jönköping and Huskvarna, which supplies around 30,000 households and municipal localities in these communities. The Torsvik plant covers half of this demand. Meanwhile, the electricity produced is fed to the grid.
The municipal waste burned at the plant can be sorted or mixed combustible waste depending on the community where it originates. In Jönköping, source sorting is gradually being implemented and food waste is separated from other combustible waste. The food waste is converted to biogas and bio-fertilizer. The biogas is used in vehicles while the bio-fertilizer replaces synthetic fertilizer.
Waste fuel delivered to the Torsvik plant is dumped in a 17,000 m3 bunker. The fuel is a mix of 40% municipal solid waste and 60% industrial waste, with an average heating value of 11.7 MJ/kg. Two 10.5-tonne cranes feed the 20-tonnes/hour waste-fired boiler.
The fuel is combusted on a 64 m2 water-cooled grate, releasing a fuel effect of 61 MW at the design point. The combustion requires 90,000 Nm3/h combustion air at this point. The boiler produces steam at a temperature of 380°C and a pressure of 41 bar at the design flow of 22 kg/s.
|The Torsvik CHP plant in Sweden|
Steam raised by the boiler drives a steam turbine that is connected to a gearbox to reduce the shaft speed to 1500 rpm to suit the 14 MW generator. The generator delivers a voltage of 10.5 kV.
Steam leaving the turbine is condensed and the latent heat recovered to the district heating water. This latent heat amounts to 42 MWth and is fed to the district heating network. Without electricity production the condenser recovers 56 MWth of latent heat.
Flue gas from the boiler goes through a two-stage cleaning process before the latent heat in the flue gas is recovered in the condenser. The condenser is integrated with a heat pump, which in total delivers a net effect of 10 MWth to the district heating network. The cleaned flue gas is released to the atmosphere via a 120-metre concrete stack at a maximum rate of 127,000 Nm3/h.
The heat demand varies according to the time of day and season. In order to optimize the energy conversion into heat and electric power – and to enable a steady operation that smoothly handles demand alterations – a 34-metre high district heating net accumulator with a capacity of 6000 m3 has been installed. This has a capacity of 75 MWth and is corrosion protected by nitrogen.
FLUE GAS CLEANING
The plant is designed to fulfil air and water emission requirements set by the Swedish authorities, which are more stringent than EU emission regulations. Jököping Energi AB has given great importance to the overall performance of the flue gas cleaning system in terms of low emissions to air and water, low utility consumptions, and high energy recovery rate. The firm selected a solution comprising an Alstom flue gas desulphurization system (trademarked NID) followed by a wet scrubber and a flue gas condenser.
The NID system is a highly efficient gas cleaning system, developed using experience from Alstom’s continuous development of various dry and semi-dry systems since the 1970s.
|Table 1. Emissions to air|
The NID process is based on the reaction between SO2 and Ca(OH)2 in humid conditions. The humidified mixture of hydrated lime and reaction product is injected into the NID system absorber and cools the inlet flue gas by evaporation. The cooled flue gas then flows to the dust collector, preferably a fabric filter or an electrostatic precipitator (ESP), where the particles in the flue gas are removed and recycled back through the NID system.
The NID concept has demonstrated very low emissions combined with low consumption of lime. The Alstom flue gas condensing system combined with a heat pump optimizes the heat recovery rate.
The gas cleaning system is designed to smoothly follow the gas flow variations from the boiler depending on load and fuel heat value. The operating range extends from 60,000 Nm3/h up to 127,000 Nm3/h.
The NID system at Torsvik consists of a special hydrator/mixer/humidifier, a NID reactor and a fabric filter. The active absorbents are lime, for the acidic gas components, and activated carbon, which adsorbs dioxins/furans and mercury in the flue gas. Quick lime is fed to the hydrator integrated in the mixer/humidifier. This hydrator has a special design that converts quick lime to slaked lime. The freshly slaked lime has a larger specific surface than commercially available pre-slaked lime, which improves the absorption of acid gas compounds.
A controlled amount of slaked lime, water and re-circulated filter ash is carefully mixed in the humidifier/mixer. Humidification is achieved by spraying the dust with process water, which in this case is condensate from the condenser downstream of the NID system. The spray creates a water film on the surface of the dust particles.
The humidified dust together with activated carbon is then fed into the NID reactor, where it is mixed with hot, untreated flue gases from the boiler. As the water evaporates in the flue gas, the gas is cooled down to the optimal temperature for the acid gas absorption. Filter ash and fresh lime will carry all the water needed in the reactor. Finally activated carbon is added. The acid components in the flue gas react with the lime in the dust through the intense and turbulent mixing in the reactor.
From the NID reactor the treated flue gas enters the fabric filter, which collects dust carried in the flue gas in bags. The layer of dust that forms on the bag surface helps to filter out fine particulate and further absorb gaseous compounds. To benefit from this enhanced absorption, adsorption and micro-particle filtration, this cake of dust is allowed to build up until the pressure drop reaches a set point that triggers the removal of the dust layer from the fabric.
The fabric filter controller activates the pulse cleaning system, which cleans down the captured particulate to the filter hopper. The fabric filter has a flat bottom with a large discharge opening from where the dust returns back into the mixer/humidifier. Most of the solid particles that have been collected are then recycled back to the NID reactor via the humidifier.
Filter ash is re-circulated to the NID reactor to improve the lime usage and ensure that the ratio of water and solids is controlled so that the dust is free-flowing, thus avoiding any paste or slurry handling. The dust discharged from the fabric filter hopper is then conveyed to a residue silo. A level control in the filter hopper controls the quantity of solids discharged from the process to the end-product silo for disposal.
POLISHING AND CONDENSATE TREATMENT
In the wet system downstream of the NID system, the flue gas is saturated in a quencher before the emissions are further polished in a wet scrubber. This scrubber has an acid and a pH-neutral stage. In the acid stage, residual HCl and ammonia are removed from the flue gas. The neutral stage captures residual SO2.
The flue gas then passes through a tubular condenser where most of its latent heat is recovered. The amount of condensate produced in the flue gas condenser is directly related to the cooling water temperature, which is the district heating water return temperature. The cooling water temperature varies during the day and so therefore does the amount of condensate.
|Table 2. Emissions to water|
At a cooling temperature of about 50°C almost all the condensate is re-used for the saturation of the flue gases. If the cooling requires additional water this is added from another source. At lower return temperatures there is a surplus of condensate, which needs to be treated in a condensate water treatment plant.
The condensate treatment is mainly designed for polishing of heavy metals. It includes pH-adjustment, precipitation and separation of solids in a lamella separator and finally a sand filter. The clean flue gas is then let to the stack via the induced draft (ID)-fan.
The plant operates continuously – i.e. 8200 hours per year – firing 162,000 tonnes of waste. It stops for three weeks during summer for the entire plant to be overhauled and for maintenance work on the gas cleaning plant to be carried out. During the 2009 summer overhaul the bags in the fabric filter were changed after three years, a typical bag life for a waste-to-energy plant.
The waste, having an average heating value of 11.7 MJ/kg, is a mixture of municipal (40%) and industrial (60%). To create the average fuel mix the normal practice is to mix the fractions when feeding the chute to the grate.
This mode of operation, to some extent, evens out the differences in heating value and can also mitigate changes in gas component concentrations. Although the gas cleaning system is designed to handle fast variations in gas concentration, more steady-state operation reduces wear and tear.
The gas cleaning equipment is, nonetheless, designed to handle a set of maximum incoming gas concentration values.
In the autumn of 2009, the plant experienced a period when incoming HCl concentration levels were higher than the design value. This resulted in difficult-to-handle dust. To rectify this problem the plant was operated on fuels containing little chloride until there was normal dust to discharge. Even though the NID gas cleaning system can handle gas concentration peaks well, due to the high particle concentration in the reactor, this period demonstrated the importance of correct design and operation.
|Simplified flow diagram showing the flue gas cleaning|
An external company measures emission values to air and water every quarter, and the results are reported to the authorities. All reports have shown values well below permit limits. The annual average emissions to air since start are in Table 1.
The annual average emission values to water are shown in Table 2. The emission values are measured at the same time as the emissions to air. The pH is always within the permitted range and all values have been well below the permitted values since the startup of the plant.
OPTIMAL ENERGY RECOVERY
Since the start up in 2006 of the Torsvik plant, the emission values to both air and water clearly demonstrate that energy from waste can be deployed with a very low impact on the environment and safely replace fossil fuel in the district heating net.
The combination of electricity and heat production increases the overall fuel energy deployment. Efficient and optimized heat recovery takes care of all available heat energy and optimizes the exergy conservation in the energy streams delivered from the plant.
Optimal energy recovery considers both primary and latent heat. The latent heat recovery benefits from a well designed flue gas cleaning system, thus minimizing the need for wastewater treatment before the condensate water is let to a recipient. The efficient gas cleaning demonstrated in the Torsvik plant is a prerequisite for cost-effective recovery of latent heat in the condenser.
Bo Herrlander is with Alstom Power Sweden AB, Sweden. Email: firstname.lastname@example.org