ON-Line alkali measurement for fuel quality control in biomass-Operated boilers

Today's power plants are shifting their combustion toward a more complex fuel mix on the grounds of environmental impact, cost, availability and regulations. Certain new types of fuel can be classified into the following groups: herbaceous material (straw and grass), agricultural by-products (pits, shells and hulls), wood, and waste fuels. These fuels contain various amounts of alkali metals, mainly potassium and sodium, as well as chlorine and sulphur, which are easily vaporised in the combustion process and are involved in processes that cause severe slagging, fouling and high-temperature corrosion problems in the furnace and further downstream in the boiler. In this study, combustion of three different biomass fuel mixes was investigated in a circulating fluidized bed boiler. An on-line alkali-chloride monitoring instrument was used to gather valuable information for evaluating the fuel quality in terms of harmful alkali chlorides, measuring the sum of alkali chlorides (potassium chloride and sodium chloride) based on ultraviolet absorption. In addition, two batches of wood biomass fuel were compared during combustion in a full-scale powder fuel boiler. In all cases the impact of changes in fuel composition on alkali-chloride formation levels were monitored quantitatively with a time resolution in the order of seconds, allowing for analysis and countermeasures. The study concluded that the employment of an on-line alkali monitoring device to prevent alkali-chloride problems is a cost-efficient, sustainable solution that extends the operational time of the boiler and reduces its maintenance costs