Investigation of Ash Sintering during Combustion of Agricultural Residues and the Effect of Additives

Abstract

In the process of creating sustainable heat and power production systems it is important to find alternative, renewable fuels that are carbon dioxide neutral. Preferably these fuels should be domestic, thus diminishing the need for transportation. One option could be to use existing residues from local agriculture and food production. Development of combustion methods suitable for such residues is presently being pursued by a number of companies. Because many biomass fuels have compositions that makes them inclined to cause ash sintering problems and emissions of acid gases, there may be a need for the use of additives to decrease such problems. The aim of this work was to examine the ash characteristics of some agricultural crops and residues and to find mixtures of fuels and additives that can form the basis for production of fuel pellets with minimal problems. The work is focused on biomass fuel pellets for small-scale grate-fired combustors. Three additives (limestone powder, kaolin, and sodium bicarbonate) were investigated regarding their effects on the ash melting behavior. The results show that calcium carbonate and kaolin both serve as good additives to prevent the formation of slag. The best antislagging effect was achieved when both additives were used. Sodium bicarbonate can be used as a sulfur binding additive, but this cannot be recommended since it increases the slag formation considerably. The conclusion is that combustion of agricultural crops and residues may be hampered by problems such as slag formation and ash fouling. However, through the use of suitable additives, the ash sintering characteristics can be improved significantly. This means that agricultural residues can be competitive fuels on the energy market in the future.

Ash Deposition in Biomass Combustion or Co-Firing for Power/Heat Generation

“…Various technologies for reducing ash deposition as well as corrosion have been studied. These include: (1) [10,20,41,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76]; (2) co-firing bio-fuels with low fouling-tendency fuels [8,55,[77][78][79][80][81]; (3) pretreatment of the feedstock to reduce the alkali metals [82,83], and (4) modification of the boiler (e.g., modification of the re-heater and super-heater in order to allow for larger spacing, more soot-blowing and a decrease in the live steam temperature to less than 500 °C, etc.) [60].…”

“…Al-Si-type additives are mostly the first choice in the literatures [61,62,[67][68][69]72] due to their strong ability for converting vapor/liquid-phase KCl and KSiO 3 to potassium aluminum silicates (e.g., KAlSiO 4 , KAlSi 2 O 6 , KAlSi 3 O 8 , etc.) with high melting-points (>1100 °C) by the reactions indicated in Equation (3).…”

“…The calcination process usually occurs at a high temperature and prolonged time [34,68]. However, calcium additives containing CaO, CaCO 3 and Ca(OH) 2 are more active in biomass combustion, in particluar for P&K-rich biomass fuels, to help convert vapor K species into high-melting-point potassium silicates/phosphates [63,65,[69][70][71]. Correspondingly, when biomass fuels containing high contents of K, Si and Ca, P-based additives could be used to reduce ash sintering and bed agglomeration [64,65].…”

Lab-Scale Investigations During Combustion of Agricultural Residues and Selected Polish Coals

“…The sintering degree (SD) for the collected slag samples was assessed applying the four-degree scale through visual inspection and manual strength test (Fernandez Llorente et al, 2008;Steenari et al, 2009):…”

“…Potassium chloride sulphuration in flue gas by injecting some sulphur compounds before the superheater section is an effective method of chlorine release from ash (Kassman et al, 2013). Aluminosilicates of the kaolinite group appeared to be effective in potassium retention (Hardy et al, 2013;Steenari et al, 2009). A promising approach to prevention of intensive slagging is fuel-mixing (Thesis et al, 2006).…”