Energy & Fuels 1989,3, 735-740
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Heterogeneous Cracking of Wood Pyrolysis Tars over Fresh Wood Char Surfaces Michael L. Boroson,t Jack B. Howard, John P. Longwell, and William A. Peters* Department of Chemical Engineering and Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received May 8, 1989. Revised Manuscript Received August 14, 1989 Char-induced conversion of newly formed wood pyrolysis tars was measured for independent variations in temperature and tar-char space time. Tar vapors were generated by controlled devolatilization of shallow (- 2 cm deep) packed beds of 45-250-pm particles of sweet gum hardwood. The tar vapors were extensively diluted in a helium carrier gas and then rapidly conveyed to an adjacent reactor for controlled thermal treatment with (or without), 0.2-12 cm deep beds of fresh char from pyrolysis of the same type of wood. Heterogeneous conversion is defined as the net tar loss from exposure to the wood char (Le. after subtraction from the total tar lost during thermal treatment the amount of tar destroyed by vapor-phase cracking upstream and downstream of the char bed). Heterogeneous conversion was significant but essentially constant at 14 f 7 wt 9% of tar (to 2 standard deviations), for temperatures from 400 to 600 "C and space times from 2.5 to 100 ms. In contrast, vapor-phase tar conversion upstream of the char bed ranged from 0% at 400 "C to 30% (wt % of tar) at 600 "C. The implication is that an otherwise thermally stable fraction of newly formed wood pyrolysis tars is very reactive in the presence of wood char. Literature data and the present results, including measurements of char yields from devolatilization of wood beds of different depths, imply that this fraction contains oxygen, is more aromatic t h m the whole tar,and may amount to as much as 35 w t % of the total quantity of tar released at the surface of pyrolyzing wood. Lignin is indicated as a major source, but not the only source, of this char-reactive tar fraction. Carbon dioxide and CO are definite products of this char-induced tar conversion, and additional char (coke) is a highly probable product.
Introduction Various forms of biomass, including wood, have been used as heating and cooking fuels for millenia. In modern technology, biomass combustion arises in electric power generation, municipal wastes incineration, propellant burning, and fires. Being renewable, biomass is also of interest as a feedstock for manufacturing synthetic fuels and chemicals. Wood may offer special advantages as a fuel or feedstock because of its low sulfur and nitrogen content. Thermal decomposition (pyrolysis)of the starting material is usually a key step in the thermal chemical reactions of solid organic fuels. Pyrolysis typically transforms the parent solid into two major products, an immobile solid (possibly containing solvent-extractable liquids and often denoted as char) and transportable volatiles, usually consisting of gases and liquids. Typically liquids are further divided into low-boiling-point compounds and "tars". The latter are often complex mixtures containing many organic compounds representing wide ranges of boiling point and chemical functionality. Consequently, tars are often defined operationally, by using parameters specific to a given experiment, for example organic material condensing in a reaction chamber at ambient conditions. Although somewhat arbitrary, these definitions prove very useful in developing quantitative understanding of the thermal reaction behavior of complex organic materials.'Y2 In many research apparatuses, and most practical processes, some volatiles can undergo additional (secondary or postpyrolysis) thermal reactions before quenching, with *Towhom correspondence can be addressed at Room E40-483B, Energy Laboratory, MIT,Cambridge, MA 02139. Present address: Rogers Corp., 1Technology Drive, Rogers, CT 06263.
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tars especially prone to further conversion. Tar secondary reactions may occur homogeneously or at interfaces (heterogeneously), within (intraparticle) or outside (extraparticle) the pyrolyzing solid. The extent of these reactions depends upon their kinetics and on the time available for reaction. The latter is determined by the tar transport rates within and away from the substrate. In wood pyrolysis, extraparticle secondary reactions of tar can include vapor-phase cracking and heterogeneous conversion on wood-derived chars. Depending on the reaction conditions, these extraparticle secondary reactions of the tars can exert modest to major influence on products yields, compositions, and release rates in pyrolysis and related thermal chemical transformations of wood. Extraparticle, vapor-phase secondary reactions of newly evolved wood pyrolysis tars have been studied quantitatively3-I and qualitatively.8 However, there is need for improved understanding of the htereogeneous conversion of wood pyrolysis tars in the presence of wood char. The present paper provides quantitative data on the conversion of newly formed wood pyrolysis tars, induced by extraparticle contacting with fresh char obtained by pyrolysis of the same type of wood. The tar reaction conditions are pertinent to wood gasification, liquefaction, and combus(1)Suuberg,E. M. Sc.D. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1977. (2)Boroson, M. L. PbD. Thesis, Department of Chemical Engineering, MIT, Cambridge, MA, 1987. (3)Mattocks, T. W. M.S. Thesis, Dept. of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 1981. (4)And, M. J., Jr. Znd. Eng. Chem. Prod. Res. Dev. 1983,22,366-375. (5)Diebold, J. P. M.S. Thesis, Department of Chemical and Petroleum-Refining Engineering, Colorado School of Mines, Golden, CO, 1985. (6) Liden, A. G. M.A.Sc. Thesis,Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, 1985. (7) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. AZChE J. 1989,35,120-128. (8)Evans, R. J.; Milne, T. A. Energy Fuels 1987,1,123-137.
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736 Energy & Fuels, Vol. 3, No. 6, 1989 Table I. Chemical Composition of Sweet Gum Hardwood (wt % of Extractive Free Wood)"vb cellulose 43.2 hemicellulose (xylan + glucomannan) 31.1 lignin 27.3
Boroson et al.
n
"From N ~ n n Source .~ of data: Chang.14 bThese three percentages total more than 100, because the analytical methods somewhat overdetermine the percentages of the individual components (Andrew@). Table 11. Elemental Composition of Sweet Gum Hardwood (wt % of Wood)" oxygen 44.64 carbon 49.46 hydrogen 6.13 From Nunn! ratory.
Source of data: analysis by a commercial labo-
tion. The tar and char were both derived by pyrolysis of sweet gum hardwood, of interest for this study because (1) this type of wood has received attention as a candidate energy crop in the southern United States and (2) data on yields of char, tar,and several other volatiles are available for pyrolysis of this wood under conditions where extraparticle secondary reactions between tar and char are virtually eliminated:JO thus providing a base case for comparison with the present work. Furthermore, our recently published7study of vapor-phase secondary reactions of tars from this wood provides yield determinations and kinetics parameters for correcting raw data on char-induced tar conversion, for tar lost by vapor-phase cracking upstream and downstream of the char bed.
Experimental Section Samples of sweet gum hardwood were obtained from Professor H.-M. Chang a t the Department of Wood and Paper Science, North Carolina State University. Chemical and elemental analyses of this type of wood are shown in Tables I and 11, respectively. Wood samples were prepared for the experiments by sieving to give a particle size range of 45-250 pm. The wood was stored over silica gel desiccant. Effects of wood particle size, wood moisture content, and char mineral matter on tar conversion were not determined. The experimental approach was to subject newly formed wood pyrolysis tars to controlled extents of postdevolatilization thermal treatment in the presence (or absence, see work reported in ref 2 and 7) of wood char beds, and to measure the effects of that thermal treatment on tar conversion. To these ends, a twochamber tubular reactor system (Figure l),developed by Serio"J2 for systematic studies of extraparticle secondary reactions of coal pyrolysis volatiles, was adapated for the present measurements. In the present work, wood tar vapors were generated in an upstream reactor (no. 1)of this apparatus, by controlled devolatilization of shallow (-2 cm deep) packed beds of 45-250-pm particles of sweet gum hardwood. The tar vapors were extensively diluted in helium carrier gas (to typical concentrations estimated a t Pd-Ru > Pd > Ru >> Au. The presence of O2 in the feed tends to decrease the NO, conversion. The control of reaction temperature is crucial to the catalyst selectivity due to the competitive reaction between H2 and Oz. Optimum reaction temperature and space velocity lead to the minimum hydrogen requirement.
Introduction The control of NO, emission from stationary sources is required to protect the environment. Selective catalytic 0887-0624/89/2503-0740$01.50/0
reduction is one approach that has been studied extensively. A number of gases, H2, CO, CHI, and NH,, can reduce the NO, to elemental N2.'-' However, only NH3 0 1989 American Chemical Society
