NO_x Reduction: The Challenge for Innovative Concepts in Europe

During combustion, most of the waste’s nitrogen content is transferred to the flue gases as nitrogen oxide, NO_x. The EU Waste Incineration Directive defines a maximum emission limit value for NO_x of 200 mg/Nm³ as a daily average value referred to 11% O₂. Based on National Emission Ceilings (NEC) defined by the Gothenburg Protocol, it can be expected that the limit values for NO_x in the EU will become even more stringent. In some European countries (e.g. The Netherlands, Austria, Switzerland) a lower emission limit has already been introduced. Selective Catalytic Reduction (SCR) technologies are used in many cases to achieve the above-mentioned NO_x limits. However, there are drawbacks to SCR systems such as high investment cost. Operation cost is also quite high due to the energy consumption necessary for the reheating of the flue gas as well as the increased pressure loss. Innovative technologies are therefore required to make it possible to reconcile both requirements: reduced emissions and increased energy efficiency. Selective Non-Catalytic Reduction (SNCR) systems are based on the selective reaction of ammonia or urea injected into the upper furnace. In many cases SNCR technologies are limited by the ammonia slip which increases in case of more stringent NO_x requirements. According to the relevant BREF document, an ammonia slip limit of 10 mg/Nm³ is generally required at the stack. In order to achieve reduced NO_x values, it is necessary to implement measures to reduce ammonia slip, by means of either a wet scrubber or a High-dust catalytic converter. EfW plants in Mainz (Germany) and Brescia (Italy) are examples of operational plants combining SNCR with such a catalytic converter type. In addition R&D activities are carried out on the development of simplified reaction mechanisms to be implemented in Computational Fluid Dynamics (CFD) codes. With these tools it will be possible to describe the interaction between turbulent mixing, radiation and chemical reaction rates. Another option to achieve low NO_x values (below 100 mg/Nm³) is the reduction of NO_x by so-called primary measures, e.g. the Very Low NO_x process (VLN), which has been developed by MARTIN jointly with its cooperation partners. The VLN process is based on a grate-based combustion system. The “VLN gas” is drawn off at the rear end of the grate and is reintroduced into the upper furnace in the vicinity of the SNCR injection positions. NO_x will be reduced significantly, ensuring low NO_x emission values at the stack as required, at low values for ammonia slip. The new EfW plant in Honolulu (USA) will be equipped with the VLN process. In Coburg (Germany), the VLN process will be retrofitted in an existing installation. This paper documents the potential and the limitations of different measures for NO_x reduction as well as examples of recent innovative EfW plants in Europe using MARTIN technologies successfully.