A pilot-scale fireside deposit study of co-firing Cynara with two coals in a fluidised bed

摘要

Costs of biofuel production from energy crops can be reduced by applying the crop residues in heat and power production. Perennial herbaceous crops like Cynara cardunculus L. are challenging fuels because they tend to have high ash and chlorine contents. Coals, however, are often rich in aluminium silicates and sulphur, and co-firing of these biofuels with coal could be expected to reduce operational problems. In addition, CO_2 emissions are lower than during coal firing alone. Blends of Cynara and two coals, South African bituminous and Spanish sub-bituminous coal, were combusted in a 20 kW bubbling bed pilot reactor to ascertain the ability of the coals to reduce operational problems by alkali capture. The Cynara fuel sample contained almost 2 wt% chlorine. The South African coal was rich in kaolinite capable of capturing alkalies from chlorides to produce alkali aluminium silicate and HC1. The Spanish coal was rich in sulphur (mostly present as FeS_2), and produced high concentrations of SO_2 that partially oxidised to SO_3. The SO_3 can capture alkalies from chlorides by sulphation. Up to 30% Cynara, on energy basis, could be co-fired with Spanish coal without operational problems, whereas the same percentage of Cynara with South African coal led to strong Cl deposition. Co-firing of Cynara with both coals resulted in high HC1 emissions (up to 1500 mg/Nm~3 in 6% O_2). In addition, co-firing of the Spanish coal led to very high SO_2 emissions (up to about 16,000 mg/N m~3 in 6% O_2). Thus, a power plant capable of firing such blends must be equipped with flue gas cleaning equipment for effective SO_2 and HC1 capture in the flue gas channel after the superheaters, or else the quality of the Cynara must be markedly improved by changing the harvesting technology and fertilisers, which could be major sources of high ash and chlorine content in the fuel.