SNCR technology for large combustion plants - Operational experiences with a commercial installation in a 225 MW_(el) coal-fired boiler

摘要

In the framework of harmonising environmental emission legislation, the EU issued the current Large Combustion Plant Directive. According to this directive, the member states had to incorporate these new emission requirements into national law. National emission ceiling levels as well as emission limits were introduced. Existing coal-fired power plants are to implement measures by 2016 in order to comply (Table 1). New plants already have to meet the new standards since 2008. In order to meet this low emission limit, primary measures such as low-NO_x burners (LNB) and overfire air systems (OFA) for staged combustion are not sufficient anymore. Secondary measures being SNCR (selective non-catalytic NO_x reduction) and SCR (selective catalytic NO_x reduction) will be required. SNCR technology was not applied for large combustion plants until some years ago, because the required NO_x reduction efficiencies could not be met due to the limitations of the injection technology as well as measurements and control. Maximum achievable reduction efficiencies of about 20% were reported from former installations. However, SNCR would be the preferred option due to its significant cost benefits especially taking into account that the remaining lifetime of many older power plants is rather low compared to new power station units. Today, primary measures could be further developed to reduce NO_x emission levels to 300 to 350 mg/Nm~3 or even below. Advanced LNB technology in combination with optimised mill operation, modern overfire air systems and improved firing control systems have been introduced. CFD modelling is supporting these developments. On the other hand, ERC could improve its SNCR technology by developing special nozzles and lances having much higher injection depth and providing increased distribution of the reduction agent (Figure la and lb). In combination with modern measurement systems of boiler cross sections temperatures, more sophisticated controls and the urea-based reduction agent carbamin 5722 NO_x reduction rates of more than 40% are achievable with low ammonia slip. Aqueous ammonia cannot be applied for large boiler cross sections as it is evaporating more instantly due to its high vapour pressure whereas the water of an aqueous urea solutions needs to be evaporated in a first step before the urea molecule can be thermally decomposed. This process is requires more time and grants deeper penetration as well as an increased reaction residence time.