A series of biomass co-combustion tests were conducted at the Masnedø combined heat and power (CHP) production grate-fired boiler, Denmark, in the period Nov. 1998 – March 1999. The fuel feedstock mixtures applied were: Danish wheat straw (100 % on a thermal base), wheat straw + wood chips (75 + 25 % on a thermal base), wheat straw + olive stones (75 + 25 %), and wheat straw + shea nuts (70 + 30 %).
During these combustion tests, air-cooled probes were exposed to the flue gas in the tertiary superheater. The metal surface temperature of the probes was kept constant at 500 ºC. Two probes were applied in each test, one was exposed for app. 1 h, the other was exposed for app. 18 hs.
Probe deposits formed have been mounted in epoxy, cross sectioned, polished and studied in a Scanning Electron Microscope (SEM). The 1 h probes are in general covered with a thin oxide scale and a thin ash layer on top of this, while the 18 h probes in addition to a multi-layer oxide scale also contain a thick ash layer. The ash layer consists of residual ash particles glued together by KCl (sometimes with a minor content of S also being present in this ’glue’). There seems to be a tendency to be more ash particles in the outer ash layers formed when co-firing straw and other biomasses, compared to ’pure’ straw-firing.
This paper provides an outline of fuel characteristics, mapping of the partitioning of K, S and Cl in the boiler, and differences in ash and deposit formation when co-firing biomasses in grate-fired boilers.