Today’s power plants are shifting their combustion toward a more complex fuel mix on grounds of environmental impact, cost, availability and regulations. 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 involved in processes that cause severe slagging, fouling and high-temperature corrosion problems in the furnace and further downstream in the boiler. In order to evaluate fuel quality in terms of generating harmful alkali chlorides, an online alkali-chloride-monitoring instrument provides valuable information. This has been achieved using an online alkali-monitoring device that measures the sum of alkali chlorides (potassium chloride and sodium chloride) based on ultraviolet absorption. Combustion of three different biomass fuel mixes has been investigated in a circulating fluidized bed boiler. 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 could be monitored quantitatively with a time resolution in the order of seconds, allowing for analysis and countermeasures. Thus, the employment of an online alkali monitoring device to prevent alkali-chloride problems is a cost-efficient, sustainable solution that extends the operational time and reduces the maintenance costs of the boiler.