Study on Deposits on the Surface, Upstream, and Downstream of Bag Filters in a 12 MW Biomass-Fired Boiler

Deposits on the surface, upstream, and downstream of bag filters in a 12 MW biomass-fired grate furnace in China were collected, sampled, and analyzed by X-ray fluorescence (XRF) and X-ray powder diffractometry (XRD). Major elements in the surface and upstream deposits were Si, Ca, K, Cl, S, and Na; the share of Al, P, Mg, and Fe was relatively poor. In comparison to the upstream deposits, the high concentrations of K, Na, Cl, and S in the surface deposits were indicative of the formation of sub-micrometer particles in the boiler, which were subsequently captured on the bag filters. However, high concentrations of Si, Al, and Ca in the upstream deposits could trap chloride via the formation of aluminosilicate and calcium chloride. In addition, the relatively lower contents of K, Na, Cl, and S in the upstream deposits went against the formation of sub-micrometer particles and led to upstream deposits that were more like an aggregate of fly ash and unlike the surface deposits, which were as hard as stone and adhered to the surface of bag filters when flue gas passed through the bag filters. XRD analysis further proved the above results. The intensity of sylvine and halite phases in surface deposits were significantly higher than those in the upstream deposits, and the intensitise of quartz and berlinite phases in the surface deposits were obviously lower than those in the upstream deposits. Higher contents of sylvine and halite as well as lower contents of quartz and berlinite in the surface were indicative of the formation of sintered ash in the boiler. Then, it adhered to the surface of the bag filters. At the exit of bag filters, a mysterious substance evinced as NH4Cl by XRD was generated by the recombination reaction of NH3 (g) and HCl(g) at the outlet temperature of bag filters. The purity of NH4Cl was almost 100%.