Behavior of Ashes and Inorganic Trace Elements During Combustion of Fuel Mixtures with Up to 100% Biomass

The cofiring of coals with biomasses is already a common practice, largely due to the German government’s decision to phase out coal use. For example, biomasses in the form of wood pellets are being employed to replace coal firing and cocombustion. In order to understand the mechanisms and subsequently counteract the increased deposit formation and corrosion caused by the increased concentrations of alkali, S, and HCl, two different coals, two woody biomasses, one sludge, and their mixtures have been comprehensively investigated regarding their release and ash behavior under simulated combustion conditions. The substances released during combustion at temperatures of 800 and 1200 °C were quantified in situ using molecular beam mass spectrometry (MBMS). Additionally, solid- and gas-phase interactions were calculated using FactSage with the GTKT and SGPS databases. The results of the calculations and the XRD data of the fuels and ashes were employed to facilitate the analysis of the complex matrix and the released compounds. The main released fraction in all experiments was SO_x and the hydrogen chloride (HCl) compounds. The distinction between the two temperatures is illustrated by the alkali elements released. These were predominantly released at the higher temperature from the biomasses. Additionally, MCl⁺ [M = Na, K] were identified, but M₂SO₄⁺ [M = Na, K] could not be detected. The effect of sewage sludge on the mixtures is dependent upon the distribution and ash content of the fuel as this influences the slagging temperature.