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Energy & Fuels 2006, 20, 2672-2680
Validation of a Continuous Combined Heat and Power (CHP) Operation of a Two-Stage Biomass Gasifier Jesper Ahrenfeldt,*,† Ulrik Henriksen,† Torben K. Jensen,† Benny Gøbel,† Lars Wiese,‡ Alphons Kather,‡ and Helge Egsgaard§ Biomass Gasification Group, Department of Mechanical Engineering, Technical UniVersity of Denmark, Building 402, 2800 Kgs. Lyngby, Denmark, Department of Energy Systems and Marine Engineering, Hamburg UniVersity of Technology, Denickestrasse 15, 21073 Hamburg, Germany, and Biosystems Department, Risoe National Laboratory, 4000 Roskilde, Denmark ReceiVed NoVember 8, 2005. ReVised Manuscript ReceiVed August 29, 2006
The Viking gasification plant at the Technical University of Denmark was built to demonstrate a continuous combined heat and power operation of a two-stage gasifier fueled with wood chips. The nominal input of the gasifier is 75 kW thermal. To validate the continuous operation of the plant, a 9-day measurement campaign was performed. The campaign verified a stable operation of the plant, and the energy balance resulted in an overall fuel to gas efficiency of 93% and a wood to electricity efficiency of 25%. Very low tar content in the producer gas was observed: only 0.1 mg/Nm3 naphthalene could be measured in raw gas. A stable engine operation on the producer gas was observed, and very low emissions of aldehydes, N2O, and polycyclic aromatic hydrocarbons were measured.
1. Introduction Since the beginning of the 1990s, the reduction of CO2 emissions has become one of the main focus areas of research and development in the power-plant industry. One way to achieve this goal is to increase the energetic use of biomass, which, compared to other renewable energy sources, offers the advantage of no temporal dependency on wind or sunshine. In the last 15 years, a large number of plants burning biomass to operate a steam turbine were commissioned in Europe. Most of them have an output of less than 20 MWe and show electrical efficiencies of less than 30%. If the plants produce combined heat and power (CHP), electrical efficiency typically drops to 20% or less. At small scale units with electrical outputs of less than 5 MWe, gasification plants feeding the gas to an engine offer an interesting possibility for reaching higher electric efficiencies. Therefore, in the past, a large number of fixed bed gasifiers were developed and tested for supplying a gas engine with fuel. Unfortunately, most of these plants could not be operated over a long period of time, because it was either impossible to achieve a stable operation or a too high tar concentration in the produced gas made continuous engine operation impossible. At the Technical University of Denmark (DTU), the Biomass Gasification Group (BGG) has been working with the research and development of staged gasification in fixed beds since 1989. During this period, different designs have been investigated.1 The two-stage process is characterized by having pyrolysis and gasification in separate reactors with an intermediate high-
temperature tar-cracking zone.2 This allows for a very fine control of the process temperatures, resulting in extremely low tar concentrations in the produced gas.3 After success with the basic process and design, the BGG decided to build a continuous running gasifier for long-term testing. As a result of this work, a demonstration and research plant has been built. The gasification plant named “Viking” (see Figure 1) is a small-scale plant with a nominal thermal input of 75 kW. The plant is running automatically and unmanned, fueled by wood chips, and feeding its gas to a gas engine. The main target for the Viking project has been to demonstrate and test a long-term CHP operation of a gasifier coupled with a gas engine. The plant was commis-
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +45-45935761. † Technical University of Denmark. ‡ Hamburg University of Technology. § Risoe National Laboratory. (1) Henriksen, U. Termisk forgasning af biomasse. Ph.D. Thesis, Department of Mchanical Engineering, DTU, MEK-ET-PHD-2004-01, 138 pages, ISBN 87-7475-321-5, with parts in Danish and English.
(2) Gøbel, B.; Bentzen, J. D.; Hindsgaul, C.; Henriksen, U.; Ahrenfeldt, J.; Fock, F.; Houbak, N.; Qvale, B. High Performance Gasification with the Two-Stage Gasifier. In proceedings of 12th European Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Amsterdam, The Netherlands, June, 2002, pp 389-395. (3) Henriksen, U.; Ahrenfeldt, J.; Jensen, T. K.; Gøbel, B.; Bentzen, J. B.; Hindsgaul C.; Sørensen, L. H. The design, construction and operation of a 75 kW two-stage gasifier. Energy 2006, 31, 1542-1553.
Figure 1. Viking demonstration and research plant.
10.1021/ef0503616 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
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Energy & Fuels, Vol. 20, No. 6, 2006 2673
Figure 2. Flow chart of the Viking gasification plant.
sioned in the autumn of 2002 and has been operated for more than 3337 h (3017 h with engine operation). To determine the energetic efficiency and validate stable operation at a high load over a longer period of time, in April 2003, a 9 day measurement campaign was performed, involving the Danish Technological Institute (DTI), Hamburg Harburg University of Technology (TUHH), University of Stuttgart (US), and Risø National Laboratory (RNL), as well as the BGG itself. The measurements included characterization of the wood fuel, verification of a stable gasifier and engine operation, characterization of the producer gas, determination of the mass and energy balance for the plant, and characterization of engine emissions and waste products. 2. Experimental Section A description of the experimental setup and measurements conducted during the campaign is given in the following sections. 2.1. Experimental Setup. In this section, the build up of the Viking demonstration plant and the gasification process is described. The Feeding System. The wood chips are feed to the gasifier through a feeding system. The storage container with a conveyer in the bottom feeds the wood chips to a system of screw conveyers. The first two screw conveyers transport the wood chips to a lock hopper system, which consist of two slide valves separated by a third screw conveyer. This system secures that no gas leaks from the pyrolysis unit. After the second slide valve, the wood chips are fed into a cache. From the cache, a forth screw conveyer feeds the fuel to the pyrolysis reactor. When the rotational speed of this screw is controlled, the feeding rate of the wood chips is controlled. The cache is purged with a small flow of nitrogen to maintain slight overpressure. This is done to avoid a back flush of gas from the pyrolysis reactor and thus tar condensation in the feeding system. The Pyrolysis Reactor. The pyrolysis reactor consists of a screw conveyer that transports the biomass through the reactor. The pyrolysis reactor is externally heated by the exhaust gas from the gas engine. In the first part of the pyrolysis reactor, the biomass is dried, and in the second part of the pyrolysis reactor, the pyrolysis
takes place. Pyrolysis demands higher temperatures than drying, and to satisfy this, the exhaust gas from the engine is split into two streams (see Figure 2). One stream is heated further by heat exchanging with the hot producer gas, and the other is led directly to the pyrolysis unit, where it joins the other stream. During the pyrolysis process, volatiles are released from the biomass because of heating. At the entrance of the reactor, the temperature is 280300 °C and the pyrolysis end temperature is 550-600 °C. The solid fraction produced during slow pyrolysis is char. The char has approximately half the size (volume) of wood chips, while the shape is the same; no agglomeration problems are seen.4 The Gasification Reactor. The products of the pyrolysis process, char and gas, are lead to the gasification reactor. Prior to the gasification process, the pyrolysis gas is partly oxidized to produce the heat necessary for the endothermic char gasification (see Figure 2). Temperatures of 1100-1300 °C are reached in this oxidation zone. Because of the high temperature and the partial oxidation of the volatile pyrolysis products, the tar content in the volatiles is reduced by about a factor of 100.5 Char gasification in the char bed is controlled by a complex set of reactions, which can be described by Langmuir-Hishelwood kinetics;6 the reacting gasification agents in the process are H2O and CO2.7 The temperature in the bed ranges from 1100 to 800 °C. Throughout the bed, the composition of the gas phase will be in equilibrium with H2O, H2, CO2, and CO.8-10 This equilibrium can (4) Hindsgaul, C.; Henriksen, U. Particle Distribution in a Fixed Bed Down Draft Wood Gasifier. Paper V2AII.10 in proceedings of 14th European Conference and Exhibition on Biomass for Energy, Industry and Climate Protection; Paris, France, October, 2005. (5) Brandt, P.; Larsen, E.; Henriksen, U. High tar reduction in a twostage gasifier. Energy Fuels 2000, 14, 816-819. (6) Laurendeau, N. M. Heterogeneous kinetics of coal char gasification and combustion. Prog. Energy Combust. Sci. 1978, 4, 221-270. (7) Gøbel, B.; Henriksen, U.; Qvale, B.; Houbak, N. Dynamic modelling of char gasification in a fixed-bed. In proceedings of the conference “Progress in Thermochemical Biomass Conversion”; Tyrol, Austria, September 17-22, 2000; 15 pages. (8) Biba, V.; Maca´k, J.; Klose, E.; Malecha. J. Mathematical model for the gasification of coal under pressure. Ind. Eng. Chem. Process. DeV. 1978, 17, 92-98.
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Figure 3. Diagram of the engine setup.
be described by a single, temperature-dependent reaction, the socalled water-gas shift reaction.11 When the producer gas leaves the gasifier, it has a temperature of about 750 °C and is in equilibrium at this temperature. As the gas is further cooled, the water-gas shift reactions become so slow that the gas composition in practice is frozen at the 750 °C equilibrium.12 When the partially oxidized pyrolysis products pass through the char bed in the char gasification reactor, the tar content is further reduced by nearly another factor of 100.5 This results in an extremely low tar content in the produced gas. The gasification reactor is built with a metal shell. In the top part of the reactor, where the partial oxidation occurs, the temperatures are at the highest (up to about 1300 °C) and only gas is present. An alkali-resistant refractory lining is used as the reactor wall in this part of the reactor. In the char bed, stainless steel is in direct contact with the char. This design provides a gastight reactor wall, keeping the gas from bypassing the char bed, because this would result in an increased amount of tar in the produced gas.3 Geometrically, the reactor is constructed as concentric cylinders and cones. At the bottom, the cylinder meets a square grate. The grate is a moving grate, allowing ash and char to pass through to decrease the pressure drop over the char bed. The movement of the grate is triggered when the pressure drop across the char bed in the gasification reactor exceeds a certain threshold [e.g., 300 mm water gauge (WG)]. Two sets of screw conveyers remove the ash. The Gas Conditioning System. When the producer gas leaves the gasification reactor, the temperature of the gas is about 800 °C. To cool the gas, it is passed through a system of heat exchangers (see Figure 2). In the first heat exchanger, the gas heats a part of the stream of the exhaust gas, and in the second heat exchanger, the gas is heat-exchanged to preheat the air for the gasifier. The gas is then cooled to just above the water dew point (about 90 °C) in a gas/water cooler and cleaned in a bag house filter. After the bag house filter, a paper cartridge filter acts as a security filter. After this, the gas is cooled further to about 50 °C and the condensate is removed. To ensure that droplets produced during the condensation are removed, the gas is then passed through (9) Yoon, H.; Wei, J.; Denn, M. M. A model for moving bed coal gasification reactors. AIChE J. 1978, 24, 885-903. (10) Groeneveld, M. J. The Co-Current Moving Bed Gasifier. Ph.D. Thesis, TU Delft, 1980. (11) Turns, S. R. An Introduction to Combustion; McGraw-Hill: New York, 2000. (12) Knoef, H. A. M., Ed. Handbook, Biomass Gasification; BTG Biomass Technology Group: Enschede, The Netherlands, 2005, ISBN 90810068-1-9.
another paper cartridge filter (similar to the security filter), which acts as a demister. A roots blower controls the gas flow.3 The filter in the bag house filter consists of ordinary polyethylene bags, which are back-flushed by nitrogen. When the pressure drop in the filter exceeds 75 mm WG, the back flush is initiated, reducing the pressure drop to about 25 mm WG. To ensure a stable gas composition for the engine, the gas is led through a mixing tank. In the tank, the gas is cooled further and the condensate is removed. When the gas leaves the mixing tank, the gas has a temperature of 10-30 °C depending upon the ambient temperature. The main constituents of the produced gas are CO, H2, CO2, CH4, N2, and H2O. The gas is a low-calorific gas, meaning that the energy density is low. On dry basis, the lower heating value is 5.5-6.5 MJ/Nm3 (see Figure 6). In comparison to natural gas with a lower heating value of 39 MJ/Nm3, this is very low. On the other hand, the producer gas has a stoichiometric air/fuel ratio of about 1.2 N m3air/N m3fuel compared to 10.6 for natural gas. Because of this relationship between the heating value and the stoichiometric air/fuel ratio, the amount of energy in the fuel and air mixture supplied to the engine is only 10-12% lower for this producer gas than for natural gas. The Gas Engine. The produced gas is fed to a naturally aspirated three-cylinder 3.1 L DEUTZ spark-ignition gas engine operating at a variable load. The engine is connected to a producer gas and natural gas line. Natural gas is used during the start up. During the start up of the plant, the engine is operated on natural gas to produce the necessary heat for the drying and pyrolysis reactor. When the gasifier is running stable, the gas-cleaning system is activated. For a short period, the gas is burned in a flare. This is done to secure a stabilized gas cleaning. When the plant is producing a good quality of gas, the engine is switched to a producer gas operation. The engine runs an electrical generator, and the produced electricity is supplied to the electricity grid. The produced heat from cooling the producer and exhaust gases as well as from the cooling of the engine is supplied to a heating system to simulate a district heating operation. The return temperature from the central heating system is kept constant at 35 °C. The engine is operated with a full open throttle at all loads during operation on the producer gas. A roots blower supplies the gas to the engine, and the combustion air is naturally aspirated by the engine. Gas and air are mixed by a mixing device located in the supply pipe prior to the intake manifold. The local gas grid supplies the natural gas. Figure 3 shows a diagram of the engine setup. Plant Control. The gasifiers control and security systems are automatically handled by a programmable logic controller (PLC), and the plant can be operated unattended. Different control approaches for the plant can be applied. During the measurement
Continuous CHP Operation of a Two-Stage Biomass Gasifier
Figure 4. Sample of the wood chips applied during the test. Table 1. Measurements of the Composition of Wood Chips Used as Fuel for the Viking Plant component
method
measure 1
ash (wt %, dry) HHV (MJ/kg, dry) LHV (MJ/kg, dry) C (wt %, dry) H (wt %, dry) N (wt %, dry) S (wt %, dry) Cl (wt %, dry) O (wt %, dry) humidity (%)
550 °C, app. 20 h ISO 1928 ISO 1928 ASTM 5373 ASTM 5373 ASTM 5373 ASTM 4239C ASTM 4208, IC
0.91 19.60 18.28 48.90 6.20 0.17 0.022 0.063
measure 2
49.00 6 0.4 0.07 44.00 32.20
campaign described here, the following control approach on the gasifier was applied: the producer gas flow to the engine is kept constant. To obtain this, the roots blower, which is a volumetric machine, is operated at constant speed. The air supply to the gasifier is adjusted to give atmospheric pressure at the biomass fuel inlet in the pyrolysis unit, and the char bed is kept at a constant level. This control strategy results in a very stable operation of the gasifier because of the constant gas flow from the gasification reactor. On the other hand, possible variations in gas composition will result in minor variations in engine performance and emissions. 2.2. Measurements and Methods. The following gives a description of the measurements and methods applied during the campaign. 2.2.1. The Wood Fuel. The plant is continuously fueled with wood chips with an average size of 25 × 35 × 6 mm (see Figure 4). During the tests presented here, the wood chips used were primarily beech with small amounts of oak. Analysis of the chemical composition, ash content, lower and higher heating value, and humidity of the wood was conducted. Measurements were made on three different batches of wood chips used during three different test periods. The applied methods for the analysis are seen in Table 1. The moisture content of the wood was measured at several times during the whole experimental campaign. This was done by weighing samples before and after drying in an oven at 100 °C. 2.2.2. The Gasifier System: Feeding System. The mass flow of biomass to the system was determined from the number of feedings within a certain period of time recorded by the data acquisition system and the mass of fuel per feeding. For a period of about 5 h, the mass and moisture contents of all fuel feedings were measured manually. Gasifier. The gasifier is equipped with continuous data acquisition that monitors the operation, temperatures, and gas flow. The airflow to the gasifier was monitored with a constant volume flowmeter. Temperatures were measured with type K thermocouples, and the gas flow was measured with a vortex flowmeter. The temperature, pressure, and flow of the cleaned gas were measured just after the gas blower (see Figure 2). The pressure of the cleaned gas was measured with a liquid-level manometer. Ashes. A total of 90% of the ash in the biomass fed to the system is extracted at the bottom of the gasifier; the remaining part is found
Energy & Fuels, Vol. 20, No. 6, 2006 2675 as fly ash in the filter. Both the amount of bottom ash and filter ash were quantified, and analyses of carbon content were carried out. 2.2.3. The Producer Gas: Gas Composition. The contents of O2, CO, CO2, CH4, and H2 in the producer gas were measured and logged continually during the campaign. The measurements were done on a dry basis, and the N2 content was determined by subtraction. O2 was measured with a paramagnetic sensor. CO and CO2 were measured with a nondispersive infrared absorbance sensor (NDIR). CH4 was measured with a flame ionization detector (FID), and H2 was measured by a thermal conductivity sensor. Nitrogen. During gasification, the biomass-bound nitrogen stays in the ash or is converted to ammonia (NH3) and hydrocyanic acid (HCN). During the campaign, ammonia was measured in the raw and cleaned gases. Tar. Tars in the producer gas have been one of the main constrictions regarding the use of producer gas as fuel for a gas engine. Tars are, according to the tar protocol,13 hydrocarbons with a molecular weight higher than benzene (>78 kg/kmol). The Viking gasifier is a low-tar gasifier, meaning that the tars are decomposed in the process and only very small quantities end up in the producer gas. To verify the low-tar designation, the tar content in the raw and clean gases was measured by three independent groups. • DTI measured tar using the Petersen column.13 The column sampling efficiency has been verified to be >99% at flow rates < 3 L/min. The absorption fluid used was isopropanol. The analyses were conducted by gas chromatography and mass spectrometry (GC/MS). The measurements are designated Tar1. • The BGG together with RNL used the solid-phase adsorption (SPA) sampling method.14 The measurements were made with different sampling tubes, one for sampling phenols and the other for sampling polycyclic aromatic hydrocarbons (PAH). For sampling phenols, SPA tubes from Supelco with an aminopropyl adsorbent were used, and for sampling PAH, tubes from Supelco with an Amberlite adsorbent (XAD2) were used. Acetone was used as a solvent for the extraction of tar from the adsorbent. All samples were spiked with deuterium-labeled phenol, guaiacol, and a PAH mixture before extraction. Sampling was made by the BGG, and the GC/MS analysis was made by RNL. The measurements are designated Tar2. • US used a newly developed FID tar measurement instrument from Ratfisch.15 The measurements are designated Tar3. Measurements were made on “raw gas”, which is the producer gas sampled before the gas cleaning train. The gas was sampled at a gas temperature of 300 °C. “Clean gas” was measured after gas cleaning and cooling at a gas temperature of 20 °C. Condensate. Finally, the mass flow of condensate removed from the gas was measured over a period of 5 days. Besides this, the PAH concentration in the condensate coming from the gas condenser (Figure 2) was analyzed. 2.2.4. The Gas Engine. The engine is equipped with continuous data acquisition that monitors the operation including the producer gas flow and the composition. Temperatures were measured with type K thermocouples. The airflow to the engine is calculated from the measured gas flow to the engine and the exhaust gas composition, in particular, from its oxygen concentration of 5.2% (by mole). Standard Emissions. The emissions of carbon CO, NOx, and unburned hydrocarbons (UHC) from the gas engine were measured (13) Neeft, J. P. A.; Knoef, H. A. M.; Zielke, U.; Sjo¨stro¨m, K.; Hasler, P.; Simell, P. A.; Dorrington, M. A.; Thomas, L.; Abatzoglou, N.; Deutch, S.; Greil, C.; Buffinga, G. J.; Brage, C.; Suomalainen M. Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases. http://www.tarweb.net/results/index.shtml. (14) Brage, C.; Yu, Q.; Chen, G.; Sjo¨stro¨m, K. Use of amino phase adsorbent for biomass tar sampling and separation. Fuel 1997, 76, 137142. (15) Staiger, B.; Wiese, L.; Hein, K. R. G. Investigation of existing gasifier and gas cleaning technologies with an online tar measuring system. Proceedings of the 2nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Rome, Italy, 2004; ETA-Florence, 2005; ISBN 88-89407-04-2.
2676 Energy & Fuels, Vol. 20, No. 6, 2006
Figure 5. Availability of the plant during the 400 h test run that included the 9 day measuring campaign.
and logged continually for 2 consecutive day. CO and NOx was measured with a NDIR sensor, and UHC was measured with a FID. Aldehydes and N2O. Beside standard emissions, the exhaust gas was analyzed for the content of aldehydes and laughing gas (N2O). Aldehydes were sampled by passing exhaust gas through a 2,4-dinitrophenylhydrazine (DNPH) solvent. The sample was analyzed by high-performance liquid chromatography and ultraviolet detection (HPLC-UV). For the N2O analysis, exhaust gas was sampled in Tedlar bags and the analysis was carried out by gas chromatography and electron-capture detection (GC/ECD). The measurements of aldehydes and N2O in the exhaust gas from the engine were conducted when the engine was operating on both producer and natural gases. This was done to see if there were any significant deviations with regard to these emissions when operating this particular engine on the two different fuels. The emission of PAH was likewise measured for both producer gas and natural gas operation. Sampling was done by passing exhaust gas through isopropanol, and the analysis was done by GC/ MS. Engine Lubricant. Analysis of the lubricant of the gas engine has been conducted to monitor any accelerated degrading. The accumulation of chlorine in the lubricant is especially of interest because experiences from other engine-based biomass gasification plants have shown that chlorine may be a problem concerning lubricant degradation.
3. Results and Discussion The plant was operated for a total period of 400 h, during which the measurement campaign was conducted; Figure 5 shows the availability of the plant during the whole test run.
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In the following, the results of the measurement campaign are described and discussed. 3.1. The Wood Fuel. Measurements have been made of the composition of the wood, and the results can be seen in Table 1. The table shows measurements of the composition of the wood, including ash, sulfur, and chlorine contents. The measurements of the higher and lower heating values for the wood are also given. As can be seen in Table 1, there is very little variation in the composition of the wood and the heating values. Only the ash and chlorine contents differ notably. The variation in ash content may be derived from variations in the content of bark in the wood chips. Bark contains up to 5% ash,16 while the wood will have an ash content significantly lower. An important part of the chlorine content in the wood chips comes from the production of the chips. The chips are produced from waste wood from the manufacturing of wooden floors. At the factory that makes the floors, the wood arrives as logs, which are stored outside in large piles. To maintain a high quality of the wood for manufacturing of floors, the moisture content of the wood is kept above a certain level. This means that in the summertime when it is dry it is necessary to spray the logs with water to keep them from drying up. Because the floor manufacturing factory is located at the seaside, the water used for spraying is seawater, which contains salt and hence chlorine. The chlorine content of wood chips is of special interest for engine operation. Chlorine may end up through the gasification gas in the lubricating oil of the engine and this may accelerate degradation of the engine lubricant. 3.2. The Producer Gas: The Gas Composition. The results of the measured gas composition from the Viking gasifier given as the volume percentage on a dry basis depicted as the hour average over a period of 24 h are shown in Figure 6. The gas composition was measured after the mixing tank, right before the gas enters the intake system of the engine. This means that the cyclic variation in the gas composition, caused by cyclic variations of the char flow from the pyrolysis unit, is leveled out. The figure shows that the gas composition entering the engine is very stable over time and that the largest variation is seen for the hydrogen content. Overall, variations of the hydrogen content in the producer gas are mainly caused by variations of moisture in the wood fuel. It is notable that there is a clear correlation between the hydrogen content and the lower heating value of the gas (see Figure 7), which again has a direct influence on engine load. Tar Content. Results of the Tar1 measurements are shown in Table 2. The table shows that no tars were detectable in either the clean or raw gas (not even benzene can be seen). Analyses for PAH were not performed, because it is the established
Figure 6. Gas composition given as the volume percentage on a dry basis depicted as the hour average over a period of 24 h.
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Energy & Fuels, Vol. 20, No. 6, 2006 2677
Figure 7. Correlation between the hydrogen content and the lower heating value of the producer gas. Table 2. GC/MS Analysis of Tar1 Measurements20 measuring point
raw gas
clean gas
raw gas
clean gas
raw gas
clean gas
measuring time benzene (mg/Nm3dry) toluene (mg/Nm3dry) xylenes (mg/Nm3dry) terpenes (mg/Nm3dry) phenol (mg/Nm3dry) methoxyphenols (mg/Nm3dry)
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