19.11.08Since the introduction of an emission limit of 200 mg/Nm3 in Germany, flue gas in incineration plants must be treated with the aid of a system that enables the reduction of nitrous gases.
Both non-catalyst (SNCR) and catalyst (SCR) procedures are currently employed for the reduction of nitrogen oxides in these types of incineration plants, however, in the near future the SCR (Selective Catalytic Reduction) technology will increasingly gain in significance. The highest NOx separation efficiency is achieved with this technology. Furthermore, catalysts for NOx reduction and degradation of dioxin and furan can be combined in SCR reactors especially cost effectively. Argillon GmbH presented exemplary solutions at the Berlin Conference on Waste Management and Energy.
The 37th Federal Ordinance on Waste Incineration and Co-Incineration (BImSchV) specifies for German incineration plants the reduction of the emission limit for nitrogen oxide from previously 200 mg /Nm3 to 100 mg/Nm3. As the levels currently prescribed in the Netherlands, in Austria and in Switzerland permit a limit of only 70 to 100 mg/Nm3, according to national regulations, SCR technology is employed in these countries, as a rule. Numerous plants in France and Belgium have also been retrofitted with SCR catalysts in recent years, in part with combined dioxin reduction measures. Likewise, SCR plants are in operation or in the planning stage in Italy, Spain, Sweden and Norway.
Dr. Eng. Wolfgang Schüttenhelm, Head of the of the Business Segment Catalyst Systems and Power Plants at Argillon in Redwitz, Upper Franconia said: "SCR technology is state of the art technology in many countries now due to the benefits as opposed to the non-catalyst process." The recognised expert introduced exemplary solutions for waste incineration and flue gas treatment and the newest developments in the sector of the SCR concepts and the application of SCR catalysts at the Berlin Conference on Waste Management and Energy.
For Schüttenhelm the benefits of the SCR technology are quite obvious: "As a result of the optimum incorporation of the SCR technology in a total plant concept a cost effective reduction of nitrogen oxide succeeds with extremely high separation efficiency, at the same time achieving a minimum escape of ammonia. In particular raw gas circuits with or without electrical precipitators as well as low temperature SCR plants that are arranged in accordance to the modern flue gas purification process are the trend", he explained to the conference participants. The operational experience has provided the evidence that the range of application of the SCR in waste and refuse derived fuel and biomass combustion plants has been considerably extended. A further benefit: the energetic efficiency of a plant can even be increased due to the downstream heat recovery system. The SINOx® plate catalyst is especially suitable for raw gas circuits due to its resistance to contamination and dust accumulation. The essential benefits of the SCR technology can be defined by the key statements expressed below:
- High NOx precipitation rate of up to 95% in waste incineration plants
- Combination with dioxin and furan precipitation is possible
- No residual materials in comparison with other procedures for dioxin precipitation
- Low escape of ammonia
- Contamination of residual materials deriving from the flue gas purification is prevented
- Can be employed in low-dust and high-dust zones
- SINOx® plate catalysts permit raw gas circuits at a low pressure loss, high resistance to catalyst toxins and a low SO2 conversion rate
- Combination with semi-dry and dry processes enables low temperature SCR
- Easy operation and low maintenance
- No residual materials in comparison with other procedures for dioxin precipitation
- Long holding time of the catalysts, in particular in the clean gas circuit
So far the majority of German waste, biomass and refuse derived fuel combustion plants have been planned with SNCR technology because lower investment costs are incurred with non-catalyst NOx reduction and also because it is easier to achieve the relevant defined emission limits. However, when the scheduled increasingly stringent limits are disregarded, a whole range of drawbacks should be considered in relation to the forgoing of the SCR technology:
- The formation of thermal NOx is favoured as higher temperatures develop in refuse derived fuel combustion plants. This increases the demands to be met by the NOx-separation system.
- The reactant costs for a SNCR are approximately twice as high as for the SCR technology. This also included higher costs for storage and transport, apart from the rising environmental pollution due to SNCR technology.
- The escaping of ammonia from a SNCR rises exponentially to the removal capacity. In general, it is possible to maintain a moderate and acceptable ammonia escape of up to 10 mg/Nm3 with a NOx removal of approximately 60 percent. Only a multi-stage arrangement of injection and extensive measuring and control technology can restrict the escaping of ammonia appropriately during a high degree of NOx removal. Whereas with the SCR technology the catalyst is correspondingly designed to accommodate the authorised degree of escaping ammonia. A typical design specification amounts to 5 mg/Nm3 at full load, which is first reached towards the end of the service life of the catalyst after several years. As ammonia gas is one of the climate gases, the SCR technology also presents a more environmentally friendly solution in this case.
- Part of the ammonia escaping proceeds to the bottom ash and into the residual material from the flue gas purification - which could lead to odours in event of humidification. This unfavourable effect can have a negative impact on the costs of waste tip management. In countries like Switzerland and other European countries that specify a stabilisation of residual materials from flue gas purification, the prevention of ammonia content in residual materials is essential in order to avoid supplementary investment and operating costs for downstream adsorption plants, the so-called NH3 strippers.
"In response to the question, whether he employs SNCR or SCR for the reduction of nitrogen oxide, every plant owner will base his decision on the required removal capacity and the total costs calculated. A combination of the two procedures would also be conceivable, whereby the catalyst would be mainly used to degrade the escape of ammonia", according to Dr. Schüttenhelm.
The SCR technology can be integrated in the process chain of a flue gas purification plant in the form of a raw gas or a clean gas circuit. The high-dust circuit is especially familiar in the coal-fired power station sector and so far has hardly been employed in the waste incineration field due to the increased danger of poisoning from the catalyst. Thanks to the plate catalysts that have been especially developed by Argillon, the SCR-technology can now also be employed cost effectively in the boiler segment of waste incinerations plants. Raw gas SCR following electric precipitators are now frequently employed. Corresponding plants have been in operation for several years now in Switzerland, Germany and France. Plate catalysts have also proven their superiority in this case of operation.
An example of how SNCR and SCR plants can be skilfully combined was demonstrated by the successful conversion in Brescia, Italy by Argillon GmbH. Here the local waste disposal company operates two waste incineration lines and one biomass incineration line. Initially SNCR systems were installed in the boiler for NOx removal. In March 2005, Argillon was awarded the contract for the High-Dust-SCR in the first waste incineration line. The installation was successfully commissioned and went into operation in June 2006. Since this day, the SCR has been operated in combination with the previously installed SNCR. Due to the excellent performance of the plant, Argillon has this year been awarded the contract to supply components and catalysts for the second waste incineration line as well as for the biomass incineration plant. In accordance with the wishes of the customer, the SCR was designed to permit it to be operated without the upstream SNCR.
SINOx® plate catalysts have also successfully proven themselves in cases of SCR installation in hot zones following electrical precipitators. One of the reasons being the self-cleaning effect that occurs with the flexible plates, however, not with the rigid honeycomb plates. The large-scale surface area of the plate catalysts reduces accumulation of dust, even when it comes to critical dust.
A clean gas circuit honeycomb catalyst system was realised by Argillon in Hengelo, in the Netherlands. In May 2006, Argillon was awarded the contract for the installation of a SCR plant for the new biomass combustion plant. In this plant, the SCR is installed downstream from a semi-dry flue gas purification system that uses calcium carbonate as an adsorbent. Here a decision was taken for the clean gas circuit because the catalyst is subject to greater deactivation in the raw gas segment with the application of larger quantities of biomass, in particular due to alkaline elements (potassium) and phosphates. The plant, which is expected to burn approx. 140.000 tons of biomass, went into operation in spring 2008. Similar SCR plants are in the construction phase in Hameln, Germany and in Kristiansand, in Norway.
The versatility and the guaranteed future of the technology became apparent in a SCR plant, which Argillon realised in Modena, in Italy. Here the concept of a SCR downstream from a dry flue gas purification plant was implemented. In this case, sodium carbonate is employed as the adsorbent for the removal of the acidic noxious gases HCl, HF, SO2 and SO3. Argillon was awarded the contract for the retrofitting of a SINOx® low-temperature SCR for the waste incineration plant Modena in January 2008. One of the main reasons: the responsible regulatory authority tightened the approved emission value during the installation of the new fourth combustion line for an incineration capacity of 180.000 tonnes in a year. This meant that the SNCR that was originally destined no longer met the demands. The circuit consists of a fabric filter with sodium bicarbonate and activated carbon infeed, ammonia water injection and the SCR, which will be operated at a temperature of 180°C. Finally, a flue gas fan will transport the flue gases to the stack. The catalyst is designed as a honeycomb catalyst. The low operating temperature was selected in order to prevent reheating downstream from the fabric filter. The danger of sal ammoniac poisoning is very low due to the assured extremely low inlet concentration of SO2 and SO3. The plant will be commissioned and set into operation before the end of 2008.
Two similar SCR plants also supplied by Argillon have been erected in Emlichheim on the German-Netherlands border and will be commissioned soon. Here burners have been provided that will secure the minimum flue gas temperature of 180°C on the one hand as well as the regeneration of the catalysts on the other hand.
History and background
The main focus of Argillon GmbH is on environmentally friendly system solutions in the field of catalyst technology for the reduction of nitrogen oxide emissions, in particular with regard to power stations operated with fossil fuels, stationary operated diesel engines as well as diesel engines in medium size and heavy duty vehicles. At the beginning of 2008 the Johnson Matthey Group, Royston, UK purchased the company. Johnson Matthey is one of the internationally leading manufacturers of catalysts for the control of vehicle emissions and catalyst systems for the reduction of the discharge of volatile organic compounds during industrial processes.
Redwitz, November 2008
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