CAPTURE OF SO_2/NO_x EMISSIONS FROM COMBUSTION OF COAL TREATED WITH TALL OIL PITCH AND CALCIUM 3-HYDROXY-PROPIONATE

摘要

This paper reports on the sulfur dioxide emissions (SO_2) from the combustion of coal treated with tall oil pitch and calcium 3-hydroxy-propionate. Crude tall oil (CTO) is a byproduct of the wood-based pulping process in pulp mills. Tall oil pitch (TOP) is the non-volatile residue obtained in the fractional distillation process of crude tall oil. The yield of TOP in the refining process amounts to 15-40%, depending primarily on the quality of the crude tall oil. The utilization of this by-product is still an unsolved problem, even if a part of it may be used in products, such as biofuel, asphalt emulsions and printing inks. In this study, tall oil pitch (TOP) is used to prepare coal-based synthetic fuels, treated with 3-hydroxy-propionic calcium salt. The chemical 3-hydroxy-propionic acid (3-HP) is prepared by chemical processes involving the hydration of acrylic acid or by fermentation of sugars from biomass. A 3-HP calcium salt has been produced and it was found that its solubility in water is very high (in the order of 100 g salt / 100 ml water). When emulsions of TOP and 3-HP in water are applied to coal they exhibit the property of sealing the coal from excess moisture. In this study, formulations were produced to maximize the amount of calcium complex incorporated within the pitch emulsions. Emulsions were then applied to the coal at nominal amounts of 1, 3 and 5 wt%. In addition to the emulsions, solutions of the 3-HP salt in water were also applied to coal also at nominal amounts of 1, 3 and 5 wt%. During combustion of treated coals the aforementioned calcium 3-hydroxy propionate acts as a sorbent, upon transformation to calcium oxide/hydroxide, and chemically reacts with the released SO_2. Combustion of pre-dried coals (treated both with emulsions and solutions) was conducted in a laboratory-scale facility, at 1200 ℃ and globally fuel-lean conditions, and SO_2 as well as NO_x capturing efficiencies of the embedded sorbent complexes were assessed. Results showed that TOP-Ca-3-HP emulsion-treated coals emitted 28 % less SO_2 (for the 5% formulation) than the untreated coal, and 15-20% less NO_x. For the same amount of Ca-3-HP uptake, the emulsion-treated coals performed better than the solution-treated coals in SO_2 emission reduction.