Energy & Fuels 2008, 22, 1955–1964
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Tar Formation and Destruction in a Fixed Bed Reactor Simulating Downdraft Gasification: Optimization of Conditions S. Monteiro Nunes, N. Paterson,* A. A. Herod, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2AZ, U.K. ReceiVed NoVember 6, 2007. ReVised Manuscript ReceiVed January 21, 2008
There are concerns about the impact of low residual tar concentrations on the long-term operational reliability of downdraft, biomass/waste fired gasifiers. A two-stage laboratory scale fixed-bed reactor has been constructed for simulating the release and destruction of tars in this type of gasifier. The commissioning and preliminary results from the reactor have already been reported. The experimental program has now been extended to investigate the effects of variation in the operating conditions in the second stage of the reactor on the emitted residual tar concentrations. The effects of temperature, char type, char particle size, residence time, and effect of addition of diluted air to the throat have been studied. The tar concentration decreased with increasing temperature, decreasing bed particle size, and increasing residence time. Addition of a limited amount of air also reduced the tar content. Tests with several different feedstocks have been done and these have suggested that the final tar emission does depend on the initial feedstock. There is clearly scope to minimize the tar emissions from different feedstocks, by optimizing the conditions in the second stage. The tars from the first and second stages, operated under different conditions, have been characterized by gas chromatography/mass spectrometry (GC/MS) and size exclusion chromatography (SEC). The results show increasing polynuclear aromatic content with increasing second stage temperature. Overall, the work has shown that the tar emissions from downdraft gasifiers can be reduced to low levels with relative ease but that complete removal will require careful manipulation of reaction parameters, dependent to some degree on feedstock properties.
Introduction One way to limit fossil fuel utilization and the rising concentration of CO2 and other pollutants in the atmosphere is to use biomass and organic wastes, as a replacement fuel for coal. These biofuels are mainly comprised of carbon, oxygen, and hydrogen,1 but the difference between these and the fossil fuels is that biomass is part of natural, short-term cycle and its use as fuel may be considered as carbon neutral. The use of biomass and waste fuels will gather momentum over the next decade, with much research and development effort directed toward the optimization of their use in a range of technologies. Using agricultural land to grow energy crops on a scale that would enable significant fossil fuel replacement is not sustainable. It would reduce the area of land available to grow food crops. The debate on this is ongoing, but food production is likely to take priority over energy crops.2 Meanwhile, there is a strong case for maximizing the use of existing biomass and waste materials as fuels, rather than deliberately growing new energy crops. This route is more economic, as well as providing significant environmental benefits. A reduction in the volume of waste for disposal is achieved by the utilization of these materials. This will reduce the waste disposal costs incurred by industry and communities, which currently amount to approximately $60 per ton for nonhazardous materials.3 These * Corresponding author. E-mail:
[email protected]. (1) Turare, C. Biomass Gasification Technology and Utilisation. www. members.tripod.com/∼cturare/bio.htm, ARTES Institute, University of Flensburg, Germany, July 1997. (2) Kandiyoti, R. Int. J. Power Energy Syst. 2004, 24, 205–214. (3) Our energy future creating a low carbon economy; energy white paper, UK Government February 2003.
charges will rise as landfill sites become increasingly scarce and expensive, as a result of more stringent environmental legislation. There is a range of technologies that can be fuelled with biomass/waste materials. In the UK, its application to pulverised fuel (PF) combustion has been proven at the large scale during cocombustion trials in several power stations. Power producers have been encouraged to cocombust biomass with coal through the availability of renewable obligation credits (ROCs). Although, there is criticism of the way in which the ROC system operates, as it does not give sufficient priority to the option of using locally produced biomass, which would provide the maximum benefit. As currently organized, the same subsidy is given for biomass imported into the UK (which incurs an extra transport C cost), compared with indigenous material and it is possible that the ROC framework will be adjusted to overcome this deficiency. The use of biomass fuels in this context is also limited to about 5% of the total charge, since biomass ashes are corrosive and tend to alter the viscosity and other properties of molten ashes (slags). There is also a growing interest in the use of fluidized bed combustion and gasification and fixed bed gasification in the production of power from biomass/waste in locally based CHP schemes.4 However, most biomass gasification systems suffer from difficulty in eliminating the tar contents of fuel gases. Tars formed during the gasification of biomass are complex mixtures usually containing thousands of compounds with wide ranges of boiling points. The compositions and amount of tars depend on factors such as type of gasifier, moisture and size of biomass, (4) Dayton D. A ReView of the Literature on Catalytic Biomass Tar Destruction; NREL milestone report (NREL/TP-510-32815), National Renewable Energy Lab: Golden, CO, December 2002.
10.1021/ef700662g CCC: $40.75 2008 American Chemical Society Published on Web 03/18/2008
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and operating conditions (pressure, temperature, and residence time). Operational problems due to the presence of tars include aerosol formation, soot formation due to repolymerization, and dehydration of tars in the gas phase and interaction with other contaminants (e.g., by adsorption) on fine particles. Heavier tar components may condense on cooler surfaces downstream. These problems can lead to blockage of particulate filters and clogging of fuel lines and injectors in internal combustion engines. One attractive feature of downdraft fixed bed gasification technology is the lower tar contents found in the fuel gases compared to tar levels found with updraft fixed and fluidizedbed systems used for biomass gasification.5 In the present study, a two-stage “hot-rod” fixed bed reactor has been constructed to simulate some aspects of downdraft gasifier operation. Experiments have been conducted to study tar destruction and how the process operating conditions affect the residual tar yield from different feedstocks. The main objective is the identification of conditions for the production of a tar free gas from downdraft gasifiers. In a previous paper, we have described the development of the equipment and some initial results. The effect of equipment configuration on tar emission was described. The fraction of original sample emitted from the first stage was high and dependent on the nature of the feedstock. The value measured with silver birch wood was 47% of original biomass, by weight (daf basis). Passage of evolved volatiles through an empty tubular second stage, heated to between 700 and 1000 °C, caused a substantial reduction in the tar product exiting the reactor. The silver birch wood tar recovered after heating at 800 °C was approximately 5%, by weight, daf basis. Clearly, the thermal cracking of the primary tars is a key factor in their destruction. Packing the heated second stage with char caused a further reduction to about 3% (by weight, daf). A limited number of tests were also done with diluted air addition to the throat (between first and second stages) and a further reduction in tar recovery was observed. In this stage of the work, the effect of operating conditions in the second stage has been studied, to identify conditions needed for complete tar destruction. The tars collected from several configurations of the reactor and sets of reactor conditions have been characterized by gas chromatography/mass spectrometry (GC/MS) and size exclusion chromatography (SEC) to identify how the nature of residual tar is affected by secondary reactions in the second stage. Experimental Details Reactor. Early developments of the hot-rod reactor configuration have been reviewed in a recent publication,6 and the evolution of the hot-rod reactor design at Imperial College has been outlined in several papers.7–11 The design of the two-stage reactor that is (5) Reed, T. B.; Das, A. Handbook of biomass downdraft gasifier engine systems; The Biomass Energy Foundation Press: Golden, CO, 1998; p 80401. (6) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Pub.: Amsterdam Oxford London New York, 2006. (7) O’Brien, R. J. Tar production in coal Pyrolysissthe effect of catalysts, pressure and extraction. PhD Thesis, University of London, 1986. (8) Bolton, C.; Snape, C. E.; O’Brien, R. J.; Kandiyoti, R. Fuel 1987, 66, 1413–1417. (9) Pindoria, R. V.; Chatzakis, I. N.; Lim, J.-Y.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 55–63. (10) Gonenc, Z. S.; Bartle, K. D.; Gaines, A. F.; Kandiyoti, R. Erdol Erdgas Kohle 1990, 2, 82–85. (11) Gonenc, Z. S.; Gibbins, J. R.; Katheklakis, I. E.; Kandiyoti, R. Fuel 1990, 69, 383–390.
Nunes et al. intended to simulate conditions in a downdraft gasifier (Figure 1) has been given in a recent paper.12 The reactor has two stages: the first stage simulates the pyrolysis zone of the downdraft gasifier; the second stage simulates conditions in the throat and char reduction zone. The two sections were constructed of 12 mm (i.d.) AISI 316 stainless steel and Incolloy 800HT respectively. Both sections are direct resistance heated. The heating rate in each zone is limited to a maximum value of about 10 °C s-1 to avoid excessive radial temperature variations. The temperature in each stage is controlled independently, using thermocouples. The first stage of the reactor, which represents the pyrolysis section of a downdraft gasifier, is connected to the second stage by means of a pair of flanges. The temperature of the first stage, where biomass is heated and volatile matter released is limited to 500 °C and the pressure is atmospheric. A sample weight of 1 g is used. A low, downward, flow rate of inert gas (helium) is used to sweep the products of pyrolysis into the throat. Helium or diluted air is added to the throat nozzles. The addition of air is meant to simulate, in the second stage, the flaming pyrolysis zone of commercial scale reactors. Undiluted air cannot be used in the laboratory scale simulation as this leads to the rapid combustion of the (static) packed bed and rapid overheating in the second stage. The tar trap connected to the outlet of the second stage is cooled with liquid nitrogen for capturing volatiles exiting from the reactor. The flange with a removable centerpiece contains the “V” shaped throat. It houses a set of three, equidistant, lateral nozzles, which allow injecting inert or reactive gases (e.g., diluted air) into the char-packed second-stage reactor, positioned below the base of the throat. This design aims to simulate, at laboratory scale, the behavior of the nozzles and “throat” of a downdraft gasifier. The length of the flaming pyrolysis/gasification zone, where reactive gases contact the char, is near realistic at about 20 cm. In commercial scale gasifiers, the temperature of the gas in this second zone increases sharply through partial combustion reactions, so that the tars are either thermally cracked or partially (or wholly) combusted. In this laboratory scale simulation, the throat is heated externally, using heating tape. The second reactor stage houses a static char bed where tar destruction reactions are studied; its configuration simulates the high temperature reducing zone immediately below the air nozzles of a downdraft gasifier. Experimental Procedure. A preweighed wire mesh plug formed from a strip of wire mesh was placed inside the first stage of the reactor, approximately 70 mm above the base. A known quantity of biomass or “waste” fuel was then charged into the reactor and sat on top of the plug. A preweighed quantity of char was added to the second stage reactor, again supported on a preweighed wire mesh plug. The two stages were then bolted together and the thermocouples and electrodes assembled (Figure 1). The tar trap was attached below the second stage and the lower part of the trap immersed in liquid nitrogen. The pressure was controlled using the control valve placed after the tar trap. The required gas input rates to the top of the first stage and to the throat section were set fixed. The superficial gas velocity through the first stage was 0.1 m s-1 (at 500 °C). Addition of gas through the nozzles increased the velocity in the second stage to 2 m s-1. Details of the flow system have been presented elsewhere.13The heating program, controlled by a computer, was switched on and the heating rate and maximum temperature were entered: the test was then initiated. The first stage was operated from ambient to (usually) 500 at 1 °C s-1 with 15 min holding. In contrast, the second stage was set at the intended temperature (between 700 and 1000 °C) before the start of each test. After the run, the reactor system was allowed to cool to room temperature, while helium gas flowed through the system to prevent the oxidation of the products. Once the reactor was cooled to room (12) Monteiro Nunes, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2007, 21, 3028–3035. (13) Monteiro-Nunes, S. Investigation of tar destruction reactions in a downdraft gasifier using biomass and waste feedstock. PhD Thesis, University of London, 2007.
Tar Formation/Destruction in a Fixed Bed Reactor
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Figure 1. Two-stage reactor.
temperature, the gas flow was turned off and the reactor depressurised. The tar and char recovered to determine the yields. The repeatability of the test method was established during the initial part of the test program by doing multiple tests under the same experimental conditions. The standard deviations were determined as (1% for tar yields and (0.5% for char yields. For subsequent work, each test condition was studied in duplicate and the results accepted provided the difference was within the repeatability limit. The average of the two results has been used in the data evaluation in this paper. Fuel Samples. Eucalyptus wood, an industrial sludge and silver birch wood were tested. The industrial sludge was received from P.I.R.A (the Paper Industry Research Association). Samples were dried to constant weight and then reduced to a powder using a Glen Creston grinder. The ground sample was sieved to collect the required size range of material (106–150 µm) for testing. After grinding and sieving, samples were placed again in a vacuum oven and dried at 35 °C for 16 h. The dried samples were stored under nitrogen at - 20 °C. The procedure avoided sample oxidation and changes in the moisture content of the sample. Elemental analyses and ash contents of the samples are presented in Table 1. Analytical Procedures. Tar and Char Collection Methods. Two methods have been tested; method 1 was developed first and has been used throughout this study. Initially, there were concerns that some of the tar could be lost at the relatively high temperature of the rotary evaporator used in this method. Therefore, an alternative method (method 2) was investigated, which avoided the use of rotary evaporator. The results obtained using the two methods are reviewed below. Method 1. At the end of each experiment, tars and char product were collected separately and weighed. A 1:4 methanol:chloroform
Table 1. Proximate and Elemental Analyses of Biomass Samplesa moisture content ash content volatile matter sulfur carbon hydrogenb nitrogen oxygenc
eucalyptus wood
sludge
7.5 0.5 78.5 0.11 45.6 5.5 0.30 40.4
5.9 30.4 59.3 0.80 30.8 3.6 3.32 25.2
silver birch wood 1.9-0.3
