Böblingen avoids landfill25 March 1999
Avoidance of landfill is an increasing issue in Europe. The kind of technology used in the Böblingen Residual Waste Fired CHP plant is one solution to the problem.
The Böblingen residual waste fired (RWF) CHP plant is due to commence operation in spring 1999. The RWF plant was built by a consortium consisting primarily of ABB, Steinmüller, and Gorpfert, Reimer and Partner (GRP), a subsidiary of Electrowatt Engineering, which acted as the consulting engineers for the project. The customer is Zweckverband Restmüllheiz-kraftwerk Böblingen.
The project was initiated in 1988, with a feasibility and site study carried out by GRP. By the time the decision to build the plant was taken in 1995, in conjunction with the client, numerous waste reuse/recycling and energy recovery options had been assessed against ecological and economic criteria. Furthermore, tenders had been invited throughout the EU for eight lots covering process technology and civil work.
The decision to build a new waste-to-energy plant resulted in part from the German Technical Directive on Urban Waste (TASI). This specifies that no untreated household waste can be landfilled after 2005. However, flue gas emissions from waste incineration plants can present an environmental hazard. As a result, environmental performance was a critical design element.
During the project planning phase, the plant disposal capacity was gradually reduced in order to allow for the increase in reuse and recycling, which meant that the plant design had to be regularly adjusted. Other important project milestones were:Start of construction: Autumn 1996; Start of installation: Autumn 1997; Start of commissioning: Autumn 1998; Commencement of commercial operation: Spring 1999.
Böblingen will have a total output of 12 MWe and 28 MWt during normal operation. The plant will have a total incineration capacity of 140 000 t/a. The plant consists of two parallel process lines that are based on a grate system.
The following basic design specification was formulated:The technologies used must anticipate the state-of-the-art for the foreseeable future, and achieve a high level of environmental protection; Minimum possible waste gas emissions; All residues produced to be rendered as inert as possible; Maximum possible reuse/recycling of the residues through production of marketable end products; Disposal of unavoidable residues within the station wherever possible; Effluent-free operation and minimum consumption of potable water; Maximum possible energy recovery.
To meet the above criteria, the station was positioned on a 5.3 hectare plot on the edge of an area that had been formerly used for tank exercises.
The quantities of waste are logged on the weighbridge for incoming waste in the reception area, transported into the tipping shed and then tipped into the bunkers. Two cranes keep the tipping points clear, and are used to shift, homogenise and stack the waste, and to charge the grate firing hoppers. Bulky waste can be reduced in size with a mobile machine. A system for drying sewage sludge has been allowed for and can be retrofitted as required.
The thermal treatment involves passing the waste through the drying, degassing, incineration and post-incineration zones on the grate. The construction of the grate, together with the movement of its bars, agitate and mix the material, and then feeds it to the slag discharge point.
In the combustion chamber above the grate, and the post-combustion chamber for the main flow, the waste gases are subjected to temperatures of 1000 and 850°C respectively, for more than two seconds. This largely eliminates dioxins and halogenated hydrocarbons at an early stage. Additional burners are provided for starting the incinerator up and shutting it down. They are turned on automatically if the temperature threatens to drop below 850°C.
Because of the very strict environmental requirements that were imposed as a result of the plant location, the station has an extensive five stage waste gas cleaning system. Immediately downstream of the boiler plant is the dust collection system employing a fabric filter. Before the subsequent scrubber stage, a heat exchanger extracts the energy remaining in the waste gas. This energy will be returned to the gas after it has passed through the neutral scrubber, to achieve a higher temperature in the following cleaning stage.
The acid scrubber stage is used primarily to remove HCl and HF. If it is possible, SO2 should not be removed in this stage. This scrubber stage is therefore operated at pH<1. This leads to all of the heavy metals condensed also being removed, and going into solution, mainly in the form of metallic salts. These salts are disposed of in a salt layer near to the town of Heilbronn.
To take account of the need for optimum recycling of residues, the high chloride content of the waste gas must be converted into as useful a product as possible. In the case of Böblingen, the choice was hydrochloric acid, which is concentrated in a rectifying system into an extremely pure 30 per cent product as it passes through the flue gas cleaning system.
Like its acid counterpart, the neutral scrubber stage takes the form of a spray tower without internal fittings. In the neutral scrubber, calcium hydroxide is used mainly to remove SO2,, but HCl and HF residues are also removed from the waste gas flow. The solution is used to produce gypsum.
The activated coke reactor installed as the fourth cleaning stage is designed to achieve a high degree of purification. The heart of this part of the system is a bed of activated coke about 1 m thick. Three separate layers enhance the action and increase the efficiency.
The first layer mainly removes organic pollutants, such as dioxins, furanes and PCBs, but it also absorbs heavy metals and any residual dust. The second layer principally removes the remaining HCl, HF and SO2, as well as all other pollutants not absorbed by the first layer. The third layer acts as just a 'policing filter', rather than having a specific absorption function.
The activated coke reactor technology maximises absorption and reduces the level of pollutants – except NOx – in the clean gas to barely measurable levels.
In order to cope with the NOx content, an SCR deNOx plant with three catalyst levels is installed as the fifth and final flue gas cleaning stage. The coke filter system is able to give an extremely good quality of clean gas as the final output from the plant. This enables a very efficient denoxing to be achieved, even when it is operating at low temperatures, using an ammonia solution.
The waste gas from the denox plant is discharged to atmosphere via a chimney at 175°C. A chimney that is only 55 m high is all that is necessary.
The bottom ash is disposed of in a landfilling area near the plant.