1026
Energy & Fuels 1997, 11, 1026-1032
Release of Chlorine from Biomass at Pyrolysis and Gasification Conditions1 E. Bjo¨rkman* and B. Stro¨mberg TPS Termiska Processer AB, Studsvik, S-611 82 Nyko¨ ping, Sweden Received February 24, 1997X
The working hypothesis for the study was that the main part of the chlorine in biomass is in an inorganic form and therefore should not vaporize appreciably below the melting point of the corresponding salt (around 700 °C) because the vapor pressure over solid salt is negligible. In the study, biomass fuels (sugarcane trash, switch grass, lucerne, straw rape) were subjected to pyrolysis in a flow of nitrogen, and the weight of the residue and its chlorine content were measured and compared to the original fuel. Contrary to the hypothesis, the results showed that during pyrolysis of biomass 20-50% of the total chlorine evaporated already at 400 °C, although the majority of the chlorine was water soluble (in grass 93%) and therefore most probably ionic species. At 900 °C, 30-60% of the chlorine was still left in the char. At 200 °C less than 10% of chlorine had evaporated from the fuel, indicating that the chlorine is not associated with water. Another result was that there was no significant difference in the chlorine release between biomass and synthetic waste, i.e., a mixture of organic and inorganic chlorides. These results are contradictory with the starting hypothesis and can therefore have new implications for the use of these fuels in combustion and gasification processes.
Introduction Emission of hydrogen chloride is the third most important contribution to the global acidification from human activities. The two first are SO2 and NOx. The HCl is a local pollutant, contrary to the other two, since it is easily dissolved in rain droplets and, therefore, usually falls down near the emission source. The hydrogen chloride emission from combustion and gasification processes has been calculated to 3.5 Mt/year. The major part of the estimated global contribution of HCl to the atmosphere is evaporation from seas. Even with a redeposition of 90% HCl to the seas, the estimated emission will reach approximately 120 Mt/year. Another HCl source in the atmosphere is methyl chloride (CH3Cl), which reacts with the OH radical and forms HCl. The main emission sources for CH3Cl are microorganisms, marine plants on the sea, and the burning of vegetation on land. The total amount of HCl produced from CH3Cl in the atmosphere has been estimated to 4 Mt/year.2 Chlorine, together with alkali metals, has been shown to play an important role in deposit formation during biomass combustion. The chlorine facilitates the mobility of many inorganic compounds, especially potassium, and can stabilize high-temperature gas phase alkalicontaining species.3 X Abstract published in Advance ACS Abstracts, September 1, 1997. (1) This work was in part presented at the Conference on Development in Thermochemical Biomass Conversion held in Banff, 1996. Due to misunderstandings, the article was not printed in the proceedings, and the text presented here is based both on that contribution and on additional results. (2) Sloss, L. L. IEA Coal Res. London 1992, IEACR/45. (3) Baxter, L. L.; Miles, T. R.; Miles Jr., T. R.; Jenkins, B. M.; Milne, T. A.; Dayton, D.; Bryers, R. W.; Oden, L. L. Developments in Thermochemical Biomass Conversion; Blackie Academic and Professional: London, 1997; pp 1424-1444.
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The emitted chloro compounds from combustion processes are formed from chlorine in the fuel, and the amount is dependent on the concentration and the use of secondary gas treatment. Both the amount and the origin of the chlorine vary between different types of fuels. In coal, the concentration varies normally between 50 and 2000 mg/kg, and the origin is mainly groundwater which has been incorporated into the coal after its formation. A smaller amount is from prehistoric vegetation. In the coal, the main part of the chlorine can be found as chloride ions in the moisture within the pores of coal particles, which upon drying forms NaCl. Other forms of chlorine in coal are inorganic and organic chlorides.2 The concentration of chlorine in peat is 200-500 mg/kg2 mainly from marine sources, minerals in the soil, and depositions from human activities. In biomass the chlorine content can vary between
