|Superheater pipes corroded by chlorine|
There are 3 groups of biomass used as powerplant fuel around the world based on its quality such as calorific value, ash content and ash chemistry. First, woody biomass includes all woody trees such as eucalyptus, calliandra, gliricidia and so on. The second group is agro-industry wastes such as ricehusk, baggase, coconut shell, palm shell and so on. While in the third group is grasses.
Although in general the biomass has a low concentration of chlorine (except straw) compared with coal, heavy corrosion occurs in a powerplant that use biomass fuel and cause leaks in the pipes of heat exchanger with less than 10,000 hours of operation. Corrosion of heat transfer apparatus is strongly associated with ash deposits in the pipes. The mechanism of the complex reactions of chlorine corrosion in boiler pipes like this chart:
|The mechanism of corrosion chemical reaction of chlorine|
Superheater steam pipes are generally not designed to accommodate the chlorine in biomass fuels. Corrosion mechanisms above are quite well understood by designers and users of biomass boilers in Europe, indications are superheater pipes installed in the low temperature gas zone. Another technology that is able to accommodate biomass fuels with high chlorine levels such as agro-industry wastes and grasses are Circulating Fluidised BedCombustion (CFBC). Temperature pulverized coal (PC) boilers which operate at temperatures up to 1500 C 1400 causing severe corrosion in the superheater pipes, while the CFBC operating at lower temperatures ranging from 850C to 900C.