Article pubs.acs.org/EF
Evolution of Hydrogen Chloride and Change in the Chlorine Functionality during Pyrolysis of Argonne Premium Coal Samples Naoto Tsubouchi,*,† Takeomi Saito,‡ Noriaki Ohtaka,‡ and Yasuo Ohtsuka‡ †
Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 060-8628, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
‡
S Supporting Information *
ABSTRACT: In order to understand chlorine chemistry in coal pyrolysis, the dynamics of HCl evolution and changes in chlorine functional forms during temperature-programmed pyrolysis of eight Argonne premium coal samples have been examined with an online HCl-monitoring technique and by the Cl 2p X-ray photoelectron spectroscopy (XPS) method. The rate profiles of HCl evolved show at least three distinct peaks at 390, 520, and 600 °C, and the presence of these peaks depends strongly on the type of coal. The HCl peak at 390 °C appears with four coals alone and becomes considerably small by water washing, whereas the high-temperature peaks above 450 °C observed with almost all of the coals do not change significantly after washing. Yields of HCl up to 1000 °C are in the range of 50−90% in many cases, and the yield tends to decrease with increasing atomic Ca/Cl ratio in coal. The chlorine XPS analyses show that the chlorine in each coal is enriched at the surface and composed of inorganic and organic functional forms. The extent of the enrichment and proportion of organic chloride species increase after pyrolysis at 450 °C, whereas they decrease at high temperatures of 800 and 1000 °C. Some model experiments followed by the chlorine XPS measurements show that the reaction of HCl with carbon active sites proceeds readily at 500 °C to produce organic C−Cl forms, which release HCl again above 500 °C upon reheating in an inert gas. On the basis of the abovementioned results, it is possible that HCl evolved below 450 °C in coal pyrolysis comes predominantly from water-soluble chlorine functional groups in coal, whereas HCl formation above 450 °C originates mainly from organic chlorides, which could be present inherently in coal and/or may be formed by secondary reactions of HCl evolved at a lower temperature with carbon active sites in the nascent char. pyrolysis up to 800 °C in many cases,20 whereas most of the fluorine in coal is retained in the solid phase even in the coexistence of a significant amount of H2O at 1000 °C.22 It has also been suggested that coal-derived HCl may react in situ with reactive components present in the char during pyrolysis and gasification to produce several types of chlorine functional groups. Some of them may be transformed again into HCl in pyrolysis at a higher temperature and at the latter stage of char gasification.20,21,23 These observations indicate that secondary reactions between HCl and char may be one of the crucial factors determining the behavior of the release and retention of chlorine in coal and char. It is of interest to discuss chlorine chemistry during coal pyrolysis in more detail. In the present paper, therefore, we first investigate the dynamics of HCl evolution and changes in the surface chlorine forms during temperature-programmed pyrolysis of coals, then examine the possibility of secondary reactions of HCl with carbon active sites in char by using activated carbon (AC) materials, and finally clarify some factors controlling the retransformation of organic C−Cl forms produced via such reactions into HCl.
1. INTRODUCTION Chlorine is a minor element in coal, but it is the primary halogen in it, and the content roughly ranges from 100 to 2000 μg/g of coal.1−3 It has been reported that chlorine (denoted as coal-Cl) is released predominantly as hydrogen chloride (HCl) upon pyrolysis, combustion, and gasification1,4−9 and that the HCl causes corrosive effects on their facilities,1,9−11 affects the speciation and vaporization of trace elements, such as antimony, mercury, and alkali metals,12−15 and may be involved in the formation of hazardous chlorinated compounds.16 There is also further concern that HCl causes corrosion problems on gas turbine materials and deterioration of the fuel cell’s performance in integrated gasification combined cycle and fuel cell systems under development.17,18 As reported previously,18 the HCl concentration for gas turbine manufacturers is limited to 1−2 ppmv. For molten carbonate fuel cell applications, on the other hand, the level may be more stringent (
