Applicability of Fuel Indexes for Small-Scale Biomass Combustion Technologies, Part 1: Slag Formation

Several
methods are available to predict the combustion behavior
of fuels. Fuel indexes have been developed either for specific fuel
types (e.g., coal, biomass) or their utilization in combustion technology
(fluidized bed, grate systems). This study deals with the validation
of fuel indexes for biomass fuels utilized in small-scale appliances
for residential heating. Laboratory analysis data of 33 biomass-derived
fuels were used for determining indexes for predicting slag formation
tendencies. Indexes were selected that have been reported and previously
applied in the literature. They vary in terms of their derivation:
ratio or concentration of specific components that are relevant for
ash chemistry, temperature-based indexes, and empirical correlations.
Combustion tests with 9 different small-scale appliances were conducted
to gain experimental data. The appliances had a nominal load between
6 kWth and 140 kWth. After each experiment,
the fraction of fuel ash that formed slag was quantified. Because
of several boiler–fuel combinations in total, data from 90
combustion experiments were available for evaluation. The comparison
of the quantified slag with the calculated slagging indexes showed
that the applicability was strongly dependent on the (chemical) background
of the respective index. Also, the fuel composition (e.g., fuels rich
in calcium, silicon or phosphorus) plays an important role. Thus,
available indexes are not applicable without restrictions and require
a closer look on fuel properties and possible ash transformation mechanisms.
Overall, the comparison of the fuel indexes with practical data (slag
formation) also indicated an influence of the combustion technologies
and operation conditions. The comparison of indexes that predict particulate
matter and nitrogen oxide emissions with data measured during combustion
experiments was evaluated as well. These results will be described
in the second part of the present work.