Particulate and Aromatic Hydrocarbon Emissions from a Small-Scale

Apr 18, 2014 - In gasifier–generator systems, producer gas is used to feed an internal combustion engine connected to a generator. Little is known a...
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Particulate and Aromatic Hydrocarbon Emissions from a Small-Scale Biomass Gasifier−Generator System Jaimie E. Hamilton, John M. Adams, and William F. Northrop* Department of Mechanical Engineering, University of Minnesota Minneapolis, Minnesota 55455, United States ABSTRACT: Distributed gasification of biomass has potential as a renewable energy alternative, but the emissions from gasifier−generator systems raise potential environmental and human health concerns. Producer gas from gasifiers contains high concentrations of aromatic compounds, tars, and particulate matter. In gasifier−generator systems, producer gas is used to feed an internal combustion engine connected to a generator. Little is known about the extent to which spark-ignited engine combustion eliminates these harmful components. This study measured emissions at three locations of a 10 kW electric, fixedbed, downdraft gasifier−generator system to determine the effectiveness of the packed-bed filter and the spark-ignited engine to reduce pollutant concentrations in the producer gas. Emissions were compared to regulated levels. Particulate matter concentrations were approximately 75 mg/Nm3 in the pre-filtered producer gas and were reduced by about 99% by the packedbed filter to about 1 mg/Nm3. Particulate matter concentrations were below regulated levels and did not change significantly because of the combustion in the engine for the conditions tested. Combustible compounds were 99% consumed in the engine, and carbon monoxide concentrations in the engine exhaust were below regulated levels. While the concentrations of benzene and toluene in the engine exhaust were each approximately 10 ppm, it is expected that they would not exceed permissible exposure limits for ambient air if the system was installed in a properly ventilated area. Leak-free and properly maintained gasifier− generator systems could be implemented in rural communities to efficiently use woody biomass without significant risk of human exposure to carbon monoxide, particulate matter, or light aromatic hydrocarbons.

1. INTRODUCTION Worldwide, 1.4 billion people lack access to electricity. These populations predominately live in rural communities in the developing world. Increased access to affordable, clean, and safe combined heat and power has been shown to contribute to social and economic development through increased productivity and job creation.1 Locally grown biomass-derived energy can be produced sustainably, unlike fossil-fuel-derived energy, because the carbon emitted during the conversion and combustion of biomass is for the most part offset by carbon taken up during the growth of the plant.2 Unfortunately, biomass gasification is not a completely clean energy source because it produces gaseous combustiongenerated compounds, such as oxides of nitrogen (NOx) and particulate matter (PM).3 Incomplete conversion of biomass during gasification is also known to produce heavier tar compounds and high levels of aromatic hydrocarbons.4 PM is generated and emitted during the conversion and combustion of biomass from integrated gasifiers.3 Although little is known about PM formed from biomass gasification, PM produced by complete biomass combustion is well-understood. Fine particles (1 μm) are generally formed from the fragmentation of inorganic and organic non-volatilized material.5 While biomass combustion and gasification differ in the air/fuel ratio during the thermochemical conversion, it is reasonable to assume that the PM is formed in similar ways. Additionally, during gasification, tars are formed from devolatilzation of the biomass and soot is formed from hydrocarbons at high temperatures. Hydrocarbon emissions, © 2014 American Chemical Society

particularly volatile organic compounds and polycyclic aromatic hydrocarbons, are dangerous to respiratory health. Benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds and naphthalene generated during gasification are linked to cancer, liver problems, and neurological damage, while PM has detrimental effects on the respiratory health of humans and adds to environmental pollution. A comprehensive review of these pollutants and other pollutants and their relationship to cancer causation is found in a review by the International Agency for Research on Cancer.6 Directive 2008/50/EC7 set forth from the European Parliament regulates many environmental pollutants from stationary generator systems, including NOx, PM10, PM2.5, and benzene. While this directive gives target ambient air limits for systems in the European Union, systems used in developing countries for rural electrification may not be monitored or regulated as strictly. For these applications, guidance for appropriate emissions may be set by understanding the level of contamination in the systems and determining exposure limits based on various international standards. Tars are generally accepted to be organic compounds produced during thermal or partial oxidation of biomass that condense under the operating conditions of the system with molecular mass greater than benzene.8 Efficient removal of tars is a major barrier to wide-scale commercialization of biomass gasification systems.9,10 The presence of tars is an indication of wasted energy in the conversion of the biomass to energy, and Received: February 20, 2014 Revised: April 17, 2014 Published: April 18, 2014 3255

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their presence can damage downstream gasifier components.8 Tar compounds in the vapor stage condense during gas cooling on downstream components of the system, such as transfer lines, boilers, or engine inlets.8,11 Tars are typically removed via filtration before producer gas is sent to an engine to generate electricity.10 However, removal of tar compounds is a source of efficiency loss in gasifier−generator systems because tars have a considerable heating value. In some reactor designs, tar concentrations account for about 0.5% of the resulting gaseous fuel mixture but account for 7% of the energy value.12 Although many filtration techniques have been proposed, simple packedbed filters have proven efficient and cost-effective for developing world applications.13 Many previous studies have examined PM and aromatic emissions from gasifiers and how they are filtered;10 however, little has been done to show how effectively engines combust and, therefore, eliminate pollutants found in post-filtration producer gas before they are emitted into the ambient air. The goal of the study was to quantify and characterize products generated from a small-scale biomass gasifier generator system at three sampling points and under different generator electrical loads. Key pollutants present in producer gas from a small-scale downdraft gasifier operating on woodchips were quantified and characterized to determine the effectiveness of a packed-bed filter and engine combustion to destroy them before being emitted into the environment. Major pollutants measured included gases, such as CO and CH4, aromatic hydrocarbons, the foundation of tar generated by biomass gasification, and PM, which includes solid components, such as soot, ash, and fragmented materials, and organic components, such as heavier tar species.

2. EXPERIMENTAL SECTION 2.1. Gasifier−Generator System. Table 1 provides the specifications for the All Power Laboratories, Inc. (Berkeley, CA) Figure 1. Schematic diagram of the biomass gasifier−generator system, including sampling locations.

Table 1. Specifications of the Gasifier−Generator System Used in the Experimental Study parameter

specification

power output engine generator

1−10 kWe Kubota, 962 cm3, spark fired, three cylinder, natural gas Mecc Alte, 12 wire genhead, 50 or 60 Hz in single, split, or three phase hard and soft woodchips, nut shells, coffee grounds, sawdust, corn cobs, manure, coconut shell, and poultry litter 1.3−3.8 cm (0.5−1.5 in.) 0.25

feedstock feedstock size ash content fixed/volatile ratio moisture content (dry) feedstock consumption gas flow rate

skid-mounted 10 kW electric (kWe) system was equipped with a threecylinder, 962 cm3 Kubota natural gas engine, Woodward L-series governor, and Mecc Alte generator. The gasifier was instrumented with thermocouples, flow meters, and pressure transducers, as indicated in Figure 1. A process control unit (PCU) controlled the producer gas and air mixture entering the engine using a wide-band Bosch oxygen sensor. The producer gas was filtered before being sent to the engine with a rotational particle cyclone and an inert packed-bed filter filled with woodchips, sawdust, and foam. Dried pine woodchips with approximate moisture content ranging between 10 and 30% were used as the feedstock. The woodchips were gasified at approximately 900 °C, and the electrical of the load generator was varied with electrical resistance heaters, which consumed 1.5 kWe each. Gas and PM samples were taken from three colocated locations shown in Figure 1. The locations were the raw producer gas after the particle cyclone but before the packed-bed filter (A), after the packed-bed filter (B), and from the engine exhaust (C). Sampling times varied from 5 to 60 min depending upon the sample type and duration. 2.2. Emission Measurement. The ability of the reactor to produce consistent fuel composition over long test cycles with different batches of biomass feedstock and the efficiency of the engine were determined by comparing the producer gas and engine exhaust gas composition. Multiple analyzers and sampling instruments were used to collect gas samples. An Atmospheric Recovery, Inc. online, Ramen laser gas analyzer (RLGA) was used to determine the volumetric concentrations of O2, N2, H2O, H2, NO2, CO, CO2, and

10−30% (ideally 0.8 μm) at various generator loading conditions. Total PM concentrations were stable over the range of generator loading conditions, and PM production and conversion did not change significantly with generator load. The packed-bed filter showed very high efficiency at removing PM contaminants in the producer gas, as shown in Figure 5, because the concentrations of both small and large PM were reduced by over 95% for all generator loads. The engine did not significantly reduce total PM concentrations. This is interesting considering that thermal destruction of tar species is known to occur at temperatures greater than 1000 °C.8 The adiabatic flame temperature of a stoichiometric mixture of producer gas with a concentration shown in Table 2 and air was calculated to be approximately 1750 °C using Cantera, an open-source thermochemical software code. This value is similar to that found by Kim et al. in a study of engine 3259

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Table 3. Comparison of Gasifier Emissions to Established Federal and Global Regulations pollutant CO (g/kWeh) HC + NOx (g/kWeh) NMHC + NOx (g/kWeh) PM (g/kWeh) PM (mg/Nm3) SO2 (g/Nm3) NOx (g/Nm3) a

10 kWe Power Pallet

Tier 4 compression ignition non-road engine22

Phase 3 spark-ignited non-road engine (Class II)23,24

45−124 a a

6.6

610 8

0.0054−0.0114 0.8 a a

0.4

World Bank guidelines for stationary engines25

7.5

50 2 2

Not measured in the experimental study.

Figure 8. Benzene and toluene dry concentrations as a function of the sampling location and generator load.

Figure 7. Engine out (sample location C) CO and PM emissions on a specific basis as a function of the generator load compared to U.S. EPA Tier 4 off-highway emission standards.

system can be maintained by ensuring leak-free operation and complete combustion in the engine. As a general overview of the ability of the engine to reduce pollutant species found in post-filter producer gas, Table 4 gives

The system showed high efficiency at reducing the concentrations of unregulated BTEX compounds: benzene, toluene, ethylbenzene, and m-, o-, and p-xylenes. Concentrations of these hazardous compounds did not vary significantly with the generator loading condition, but their concentrations were reduced significantly through the packedbed filter and again through the engine. Benzene was reduced about 96% through the engine; toluene was reduced 75−95% in the engine; ethylbenzene was reduced almost entirely; and m- and p-xylenes were reduced 48−60%. The highest concentrations of BTEX compounds found in the pre-filtered and filtered producer gas were benzene, followed by toluene. The concentrations of these two pollutants taken at the three sample locations for the three generator loads are shown in Figure 8. NIOSH recommends a short-term exposure limit (STEL) for benzene at 5 ppm and a time-weighted average (TWA) for an 8 h work day at 1 ppm. Concentrations of benzene were well above the NIOSH recommendations for unfiltered and filtered producer gases. Although the concentrations of benzene are still higher than the TWA and STEL from the engine exhaust, which is released to the environment, NIOSH recommendations are for ambient air rather than concentrated gases. Concentrations of all BTEX compounds in the engine exhaust were less than 10 ppm. While these concentrations are still high for ambient air, if the system is operated in a well-ventilated area and the engine exhaust is highly diluted, risks to human health and the environment would be low. It is concluded from this study that low levels of hazardous compounds from the small-scale gasifier−generator

Table 4. Conversation Efficiencies of the Packed-Bed Filter and Engine removal efficiency (%) pollutant total PM EC OC combustible compounds BTEX compounds benzene toluene ethylbenzene xylenes

packed-bed filter

engine

98−99 99.5 97−99 ∼0

∼0 26−89 0−43 96−98

∼0 20 3−40 16−100

97−98 75−96 100 40−100

a summary of the conversion efficiency for measured compounds through the packed-bed filter and through the spark-ignited engine. In general, the engine was effective at reducing gaseous compounds but was less effective at eliminating PM.

4. CONCLUSION This study examined the concentration of key pollutants at three locations from a 10 kWe downdraft gasifier−generator operating with woodchips: after the reactor, at the outlet of the packed-bed tar filter, and in the engine exhaust. The data were used to calculate thermal efficiency of the processes within the 3260

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(6) International Agency for Research on Cancer (IARC).. A review of human carcinogens. IARC Monogr. Eval. Carcinog. Risks Hum. 2012, 100, 575. (7) The European Parliament and the Council of the European. Union Directive 2008/50/EC of the European Parliament and of the Council; The European Parliament and the Council of the European: Brussels, Belgium, 2008. (8) Milne, T. A.; Abatzoglou, N.; Evans, R. J. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory (NREL): Golden, CO, 1998; NREL/TP-570-25357. (9) Maniatis, K. Progress in biomass gasification: An overview. Prog. Thermochem. Biomass Convers. 2008, 1−31. (10) Hasler, P. H.; Nussbaumer, T. H. Gas cleaning for IC engine applications from fixed bed biomass gasification. Biomass Bioenergy 1999, 16, 385−395. (11) Rabou, P. L.M.; Zwart, R. W. R.; Vreugdenhil, B. J.; Bos, L. Tar in biomass producer gas, the Energy Research Centre of The Netherlands (ECN) experience: An enduring challenge. Energy Fuels 2009, 23 (12), 6189−6198. (12) Baker, E. G.; Mudge, L. K.; Mitchell, D. H. Oxygen/steam gasification of wood in a fixed-bed gasifier. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 725−728. (13) Pathak, B. S.; Kapatel, D. V.; Bhoi, P. R.; Sharma, A. M.; Vyas, D. K. Design and development of sand bed filter for upgrading producer gas to IC engine quality. Fuel 2007, 8, 15−20. (14) Wei, L. Experimental study on the effects of operational parameters of a downdraft gasifier. Master’s Thesis, Mississippi State University, Starkville, MS, 2005; pp 1−180. (15) Hasler, P. H.; Nussbaumer, T. H. Particle size distribution of the fly ash from biomass combustion. Proceedings of the 10th European Conference and Technology Exhibition: Biomass for Energy and Industry; Würzburg, Germany, June 8−11, 1998; pp 1623−1625. (16) Cantrell, B. K.; Rubow, K. L. Development of personal diesel aerosol sampler design and performance criteria. Min. Eng. 1991, 43, 232−236. (17) Hinds, W. C. Aerosol Technology Properties, Behavior, and Measurement of Airborne Particles, 2nd ed.; John Wiley and Sons: Hoboken, NJ, 1999; pp 1−200. (18) National Institute for Occupational Safety and Health (NIOSH). Elemental carbon (diesel particulate). NIOSH Manual of Analytical Methods, 4th ed.; NIOSH, Center for Disease Control and Prevention (CDC): Atlanta, GA, 1998. (19) Birch, M. E.; Cary, R. A. Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust. Aerosol Sci. Technol. 1996, 25 (3), 221−241. (20) Zainal, Z. A.; Ali, R.; Lean, C. H.; Seetharamu, K. N. Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials. Energy Convers. Manage. 2001, 42, 1499− 1515. (21) Kim, A. M.; Smith, J. A.; Wagler, J. R.; Baldwin, D. D. Combustion characteristics of producer gas in the stationary gas engine. Proceedings of the ASME 2011 Internal Combustion Engine Division Fall Technical Conference; Morgantown, WV, Oct 2−5, 2011. (22) U.S. National Archives and Records Administration. Code of Federal Regulations, Title 40, Section 1039.101; U.S. National Archives and Records Administration: Washington, D.C., 2004. (23) U.S. National Archives and Records Administration. Code of Federal Regulations, Title 40, Section 1054.103; U.S. National Archives and Records Administration: Washington, D.C., 2008. (24) U.S. National Archives and Records Administration. Code of Federal Regulations, Title 40, Section 1054.105; U.S. National Archives and Records Administration: Washington, D.C., 2008. (25) World Bank Group. Thermal power: Guidelines for new plants. Pollution Prevention and Abatement; World Bank Group: Washington, D.C., 1998; pp 413−426.

system and to determine the ability of the spark-ignited engine to eliminate key pollutants. The results of the experimental work showed that the smallscale system resulted in higher overall system thermal efficiency at higher loads. Concentrations of gaseous compounds, PM, EC and OC, and aromatic hydrocarbons were constant as a function of the tested generator load range. The system was able to reduce the concentrations of many of the pollutants present in raw producer gas through the use of the particle cyclone, packed-bed filter, and spark-ignited engine. Concentrations of combustible compounds in the producer gas were consumed by 96−98% in the engine, and the concentrations of PM were reduced by 98% in the packed-bed filter. PM concentrations were not changed significantly through the engine, but PM emissions from the engine were still below specific federally mandated emission limits for this type of engine without the need for after-treatment. Light aromatic BTEX compounds passed through the packed-bed filter without reductions in concentration but were then mostly destroyed through combustion in the spark-ignited engine. While the concentrations of benzene and toluene were each approximately 10 ppm, it is expected that they would not exceed permissible exposure limits set forth by NIOSH for ambient air if the system was installed in a properly ventilated area. A key conclusion of this work is that leak-free and properly maintained small-scale gasifier−generator systems could be implemented in rural communities to efficiently use woody biomass without significant risk of human exposure to CO, PM, or light aromatic compounds.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-612-625-6854. Fax: +1-612-624-1578. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The All Power Laboratories gasifier−generator system used in this study was provided under Department of Energy Grant DE-EE0003239. Jaimie E. Hamilton and John M. Adams received funding from the University of Minnesota’s College of Science and Engineering. The authors also acknowledge Darrick Zarling and Winthrop Watts at University of Minnesota for their technical contributions and All Power Laboratories for their equipment support.

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REFERENCES

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dx.doi.org/10.1021/ef500437z | Energy Fuels 2014, 28, 3255−3261