Environ. Sci. Technol. 2002, 36, 2193-2197
Occurrence and Destruction of PAHs, PCBs, ClPhs, ClBzs, and PCDD/Fs in Ash from Gasification of Straw A R J A H . A S I K A I N E N , * ,† MIKKO P. KUUSISTO,‡ MATTI A. HILTUNEN,‡ AND JUHANI RUUSKANEN† Department of Environmental Sciences, University of Kuopio, P.O. Box 1627, Fin-70211 Kuopio, Finland, and Karhula Research and Development Center, Foster Wheeler, P.O. Box 66, Fin-48601 Karhula, Finland
Two experiments were performed with an atmospheric circulating fluidized bed gasifier (ACFBG), the first with pelletized straw and the second with loose straw, to investigate the occurrence of polycyclic aromatic hydrocarbons (PAHs), chlorophenols (ClPhs), polychlorinated biphenyls (PCBs), polychlorinated benzenes (ClBzs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) in the bottom ash and fly ash formed during gasification. Only PAHs were present in large amounts, and only in the fly ash, ranging from 300 to 555 mg/kg ash in the tests with pelletized straw and from 73 to 118 mg/ kg ash in those with loose straw. These amounts are so high that environmentally safe disposal or reuse of the ash would be difficult, so the development of a technique to handle the problem was included in the project. The method investigated was to burn the fly ash in a circulating fluidized bed (CFB) boiler in order to destroy the PAHs. This worked surprisingly well, eliminating 99% of the PAHs, without any further formation of the other harmful organic compounds analyzed. Thus, this method could actually be useful in practice. Especially the fact that the formation of PCDD/Fs was minimal during gasification and further treatment of the ash in the CFB boiler makes the gasification technique highly competitive relative to conventional combustion methods.
Introduction The use of biomass, as a fuel in the production of energy, has become more common in recent years. Werther et al. (1) have published an extensive overview of techniques for the combustion of agricultural residues and problems associated with this. Alongside conventional techniques, gasification is one of the newest approaches and has attracted a lot of attention in recent years. Cofiring of biomasses with another fuel is the most common technique used worldwide (2), but the possibility of burning biomasses as a single fuel, especially in gasification, has attracted more attention in the past couple of years. Gas * Corresponding author phone: 358 17 162893; fax: +358 17 163191; e-mail:
[email protected]. † University of Kuopio. ‡ Foster Wheeler. 10.1021/es0157907 CCC: $22.00 Published on Web 04/10/2002
2002 American Chemical Society
can be produced from solids in several ways, of which circulating fluidized bed (CFB) gasification is especially fascinating due to the advantages that it has over other gasification systems and over combustion: • An air-blown CFB gasifier is simple to construct, and the process performs smoothly and is easy to operate. This guarantees good availability of the gasification process. • The resulting gas can be used to replace natural gas, oil, or coal. The CFB gasifier may be constructed as a separate plant, but it can also easily be fitted later to an existing highefficiency PC boiler, oil boiler, or lime kiln, for example. • The atmospheric circulating fluidized bed gasifier (ACFBG) is highly flexible in terms of the choice of fuel, which allows cheap local biomasses and wastes to be used as fuels. • The level of investment required is low, as are operating costs, and the gasifier can be operated from a control room like any other boiler. • The benefits of large unit size mean that it can be built for a wide range of fuels and outputs of 20-200 MWth. • High carbon and energy efficiencies achieve conversions of up to 95% with fuels of high volatile content, especially biomass, and up to 80-90% with fossil fuels of low volatile content. • The bed chemistry can be maintained, and the harmful components of synthesis gas, such as sulfides, HCl, and tars, can be controlled with case-specific bed makeup additions. Typical fuels for an atmospheric CFB gasifier include wood, wood waste, bark, demolition wood, production waste, and household waste, which typically include packagings made of plastics, paper, and cardboard. Other materials such as peat and shredded tires can also be gasified successfully. A project aimed at developing the ACFBG technology for use with straw was started in 1998. The gas was intended to be used as a supplementary fuel in a modern coal-fired power plant. This project included two separate parts: (1) development and testing of a straw feeding and gas cleaning system and (2) evaluation of the procedures to ensure reuse of the residues. The main objectives of the first part were to describe the conditions necessary for gasification, gas cooling, dust separation, and gas combustion and to investigate the occurrence of harmful organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), chlorophenols (ClPhs), polychlorinated biphenyls (PCBs), polychlorinated benzenes (ClBzs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs), in the residues (fly ash and bottom ash). The first part of the project consisted of two gasification experiments, and the test plans and process conditions were based on preliminary tests with straw performed by the Technical Research Centre of Finland in its process development unit (PDU) gasifier (3). The main objective in the second part of the project was to reduce the carbon content of the ash and eliminate any organic compounds, formed during gasification, from the residues.
Experimental Methods Gasification Experiments with Straw. The fuel used in the first gasification experiment, carried out in a pilot unit designed and built by Foster Wheeler Ltd., was pelletized straw. In the second gasification experiment a newly developed straw feeding system was tested in practice by using loose straw as the fuel. The synthesis gas was cleaned in a new baghouse before combustion. Altogether 15 test runs were made, 8 with pelletized straw and 7 with loose straw. VOL. 36, NO. 10, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic diagram of the atmospheric CFB gasifier plant. ACFBG. The gasification experiments were performed using an atmospheric circulating fluidized bed gasifier, a schematic diagram of which is presented in Figure 1. The refractory-lined gasifier reactor was inside a 10 m high gastight steel shell. The loose straw was transported pneumatically up to the screw feeder, which formed a plug of straw and delivered it through a plug loosener to a fastspeed penetration screw, which fed it into the circulating fluidized bed reactor. The pelletized straw was discharged by a screw feeder directly to the fast-speed penetration screw and fed into the reactor. The bed, which was a mixture of limestone and sand, was fluidized by preheated air (at 305351 °C) fed in at 1.4 bar by a Roots blower of capacity ) 0.25 m3/s through a distribution grid at the bottom of the reactor. The coarser solids entrained from the reactor were separated out in a cyclone and recycled back into the bottom of the reactor, while the bottom ash was carried out by a vertical pipe that transported it to a 1.5 m long water-cooled screw in which it was cooled to 100-200 °C. The hot product gas leaving the reactor at 700-850 °C was cooled to 600-700 °C in the gasifier air preheater and further to 350-400 °C in the water-tube boiler. It was then cleaned in a hot baghouse filter using a new type of hightemperature filter material (at ∼350 °C), after which it was led either to a combustion chamber, to test its combustion properties, or to a Venturi scrubber for final cleaning of dust before being burned in a flare supported by a small natural gas burner at the top of a stack of height ) 33 m. The scrubber and flare also served as a filter bypass for the total gas flow in the case of malfunction of the filter or the gasification process. The product gas composition was analyzed automatically in terms of CO, H2, CO2, CH4, C2H4, and C2H6 by a gas chromatograph. Pressure and temperature values were measured continuously at various points. Sampling. Solid samples were taken from each part of the equipment during the tests, at a typical frequency of one sample every 2-4 h. The samples from each test were collected in containers, mixed, and divided into portions for the analyses and for storing. All of the fly ash from the gasifier was stored in barrels for further testing. Combustion of Fly Ash from Straw Gasification. A third combustion experiment was carried out to examine oxidation by burning in a circulating fluidized bed (CFB) pilot plant boiler as a potential solution for the treatment and disposal 2194
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of the fly ash. Five combustion tests were performed using fly ash from the previous gasification tests as fuel. Description of the CFB Pilot Plant. Combustion tests were performed in a CFB boiler, which was separate from the ACFB gasifier. The combustor was a refractory-lined cylinder ∼10 m high with an internal diameter of 0.6 m. Primary air (up to 200 mbar) was supplied to it through a bed support plate by means of two centrifugal fans, and secondary combustion air could be introduced at three levels in the freeboard space through two nozzles located opposite each other at each level. The combustor was equipped with a gas burner for startup. Depending on its quality, the solid fuel was fed in once the combustor temperature had reached 600-800 °C. The refractory-lined cyclone used to separate the suspended solids from the combustor flue gases operated at a high temperature (up to the reactor temperature of ∼700 °C) and thus allowed additional burning time for the elutriated fuel particles. A vertical tube at the bottom of the cyclone transferred the solid material to a nonmechanical loop seal, which returned it into the combustor. The loop seal was designed to allow the ash to flow back into the combustor while blocking the flow of combustion gases and bed material into the cyclone. The gases entered a gas/water heat exchanger (boiler) with water tubes aligned perpendicularly to the gas flow. Pressurized water was circulated through the tubes to cool the gas to ∼150-220 °C. Water flows and temperatures were measured and recorded. The gas flow was directed via internal baffles to make several passes over the tubes before leaving the boiler, which was equipped with air-operated soot blowers. Fine particles were reduced below 150 mg/m3n by means of a two-compartment baghouse with air-pulse cleaning. The normal operating temperature was 140-220 °C. The fly ash fell into a hopper at the bottom of the baghouse and was removed through air-operated valves. The baghouse was bypassed at the startup, when the gas temperature was low and near the dew point. On these occasions, and also during emergencies or problems with the baghouse, dust removal was accomplished by means of a Venturi scrubber, which also removed the fine particles that escaped the baghouse stage. Process Control and Sampling. The pilot plant was equipped with a modern process control system that directed and monitored the entire combustion facility, measuring
temperatures, pressures, flows, weights, etc. at numerous points. The continuous gas analysis equipment measured O2 and CO before the boiler, O2, CO, CO2, SO2, NOx, and total hydrocarbons (THC) after the boiler, and HCl after the filter. The analyzers were calibrated on a daily basis. Solid samples of the fuel, bed material, and fly ash were taken every 2 h, and bottom ash samples were taken at the start and end of each run. The samples for each test were collected in containers, mixed, and divided into portions for the analyses and for storing. Analytical Procedure. Due to limited resources, complete analyses were performed only on the samples from five gasification tests, three in the first experiment (tests 1-3 with pelletized straw) and two in the second (tests 4 and 5 with loose straw), and two ash-burning tests (tests 6 and 7). Organic compounds, at least PAHs, were analyzed in all fly ash samples (denoted FA 1-7, the number refers to the test number) and bottom ash samples (denoted BA 1-7, the number refers to the test number) and also in the fly ash used as a fuel in the CFB combustion tests (denoted FU 6, sample from a single gasification test; and FU 7, a combined sample from all fly ash used as fuel). Composition analyses were performed on the five straw fuel samples (denoted FU 1-5, the number refers to the test number) and for the above two ash fuel samples FU 6 and FU 7. Organic Compounds. PAHs, ClPhs, PCBs, ClBzs, and PCDD/Fs were analyzed in dried, homogenized ash samples. Five to 10 g of each ash was measured into extraction thimbles washed with acetone (purchased from Merck), and 2,4,6TBrP, 2,4,6-TCB, 5 deuterated PAH compounds, and 17 13C12labeled PCDD/Fs were added as internal standards. The samples were Soxhlet-extracted with toluene (purchased from Romil) for 20 h. ClPhs were extracted from toluene with 0.1 M potassium carbonate in an extraction vessel, acetylated with acetic anhydride, and extracted with hexane (HPLC grade, purchased from Merck). The rest of the toluene was evaporated with a rotavapor and the residue dissolved in hexane. One milliliter was separated out to form the PAH sample, and the rest of the hexane was purified with concentrated sulfuric acid and evaporated to 1 mL in a nitrogen flow. The sample was then fractionated with aluminum oxide (ICN alumina B, activity class 1), the first fraction (PCBs and ClBzs) being eluted with hexane containing 2% dichloromethane (purchased from Riedel de Haen) and the second fraction (PCDD/Fs) with hexane containing 50% dichloromethane. ClPhs, ClBzs, PCBs, and PAHs were analyzed by gas chromatography/mass spectrometry (HP 6890/HP 5973) with an HP5MS column, and PCDD/Fs were analyzed with HRGC/HRMS VG 70-250 SE with a DB-Dioxin column. Compounds were identified by selective ion monitoring (SIM) and quantified by reference to external and internal standards. The analytical procedure used for PCDD/ Fs conformed to European Standard EN 1948-2:1996. Details of the analytical procedure have also been described by Ruokoja¨rvi (4, 5). Composition of Fuels. Elemental C, H, N, S, O, Na, Cl, Cd, and K analyses were performed on all of the fuels. Ash content was determined according to the DIN 51719 standard, HHV and LHV according to the DIN 51900 standard, and carbon fixation by calculation, in addition to which volatiles and moisture were also analyzed. The composition analyses employed AAS, XRF, and IR techniques and a variety of analyzers.
Results and Discussion Gasification of Straws. The straw used was mainly Danish wheat straw, of the compositions listed in Table 1. Sample FU 5 differed from the other straw samples in the amounts of Na, K, and Cl present. The amount of Cl in a fuel can affect
TABLE 1. Compositions of Straw Fuels sample
fuel C (%) H (%) N (%) S (%) O (%) ash (%) moisturea (%) HHVb (MJ/kg) LHVc (MJ/kg) fixed carbon (%) volatiles (%) Na, total (µg/g) K, total (µg/g) Cl, total (µg/g) Cd (µg/g)
FU 1
FU 2
FU 3
FU 4
FU 5
pelletized 46.2 5.7 0.6 0.2 41.7 5.5 10.5 18.5 17.2 20.1 74.5 425 17800 5890 0.1
pelletized 46.5 5.7 0.6 0.2 41.6 5.4 11.9 18.6 17.4 20.5 74.1 310 16900 5750 0.1
pelletized 45.6 5.6 0.6 0.3 41.3 6.7 9.6 18.3 17.0 20.3 73.0 310 20000 6200 0.1
loose
loose
46.4 5.8 1.1 0.2 41.0 6.4 9.8 18.6 17.4 18.3 76.2 190 14900 7210 d
47.0 5.8 0.7 0.1 41.7 4.8 13.9 18.7 17.4 16.6 78.7 100 5180 655 0.1
a As received, others in dry sample. heating value. d Not analyzed.
b
Higher heating value. c Lower
TABLE 2. Composition of the Dry Gas during the Gasification Experiments test 1 straw
pelletized CO (mol %) 13 H2 (mol %) 17 CH4 (mol %) 3 C2H6 (mol %) 0 C2H4 (mol %) 1 CO2 (mol %) 18 N2 (mol %) 50 NH3 (mg/m3n) 2870 tars (mg/m3n) 3420 benzene (mg/m3n) 3750 HCl (ppm)
