Study of the Layered Structure of Deposit in a Biomass-Fired Boiler (Case Study)

摘要

Biomass-fired boilers generally experience severe deposit problems, in particular on the intermediate temperature superheaters (SH_2). In order to investigate the formation mechanisms of the deposit, mature deposits collected from the superheaters of a 30 MW biomass-fired power plant were analyzed using SEM-EDX, XRD, and XRF techniques. The results show that the deposits formed on the SH_2 have an obvious layered structure, and temperature-dependent layers exist close to the tube wall, where the crystallization process is mainly affected by the steam temperature. Referenced to the three phase equilibrium diagram data in the silicate cement field, the eutectic salt KC1-K_2SO_4-Na_2SO_4 may be formed in a secondary step after KC1 and K2SO4 condense on the tube surface. The chemical subsequence processes are also capable of analyzing the deposits' formation process and their composition along the thickness of the deposits of any of the three superheaters in this case. As the surface temperature of the deposits increases along the thickness of the deposits, Nad and Na_2S_2O_3 can be deposited together with the components KC1, K_2SO_4, K_2Ca(SO4)_2 • H_2O, K_3Na (SCM)_2, CaSO_4, and SiO_2. K_2Ca(SO_4)_2• H_2O and K_3Na(SO_4)_2 are produced through the secondary reaction in all layers after they have deposited. In addition, monocrystal KC1 (melting point 770 ℃) was found in the top layer of the deposits. The cone shape of the layered structure of the deposits also indicates that the formation of the deposits may also be affected by the pattern of the gas flow.