BIOGAS UPGRADING ON DAIRY DIGESTERS

Transcription

1 BIOGAS UPGRADING ON DAIRY DIGESTERS Anaerobic Digestion Systems Series By Nicholas Kennedy, Associate in Research, Center for Sustaining Agriculture and Natural Resources, Washington State University, Georgine Yorgey, Associate in Research, Center for Sustaining Agriculture and Natural Resources, Washington State University, Craig Frear, Assistant Professor, Center for Sustaining Agriculture and Natural Resources, Washington State University, Dan Evans, President, Promus Energy, Jim Jensen, Senior Bioenergy and Alternative Fuels Specialist, WSU Extension Energy Program, Washington State University, Chad Kruger, Director, Center for Sustaining Agriculture and Natural Resources, Washington State University FS180E Page 1 ext.wsu.edu FS180E

2 Biogas Upgrading on Dairy Digesters Introduction Biogas that is generated from the anaerobic digestion (AD) of dairy manure is a reliable source of renewable energy that can be utilized in a variety of different ways (EPA et al. 2014; Persson et al. 2006). It can be combusted in boilers to produce thermal energy, burned in engines and generator sets to generate electricity and heat also known as combined heat and power (CHP) or upgraded into hydrogen fuel, gasoline, methanol, or renewable natural gas (RNG), sometimes also called biomethane (Krich et al. 2005). In the United States (US), the vast majority of animal manure digesters produce CHP, which requires little purification of the biogas after it is generated during anaerobic digestion. As of July 2015, only a small handful of digesters in the US were producing RNG, and none of these were in the Pacific Northwest. However, recently the prices for electricity production have fallen in many parts of the US, reducing the profitability of digesters (Costa and Voell 2012). Low prices have been a particular problem in the Pacific Northwest (PNW), where abundant hydropower also keeps electrical prices low (Coppedge et al. 2012). As received electrical rates have continued to decrease, interest in other higher value end-uses for biogas has grown. Using data from a dairy in Washington State, a companion factsheet in this series, Anaerobic Digester Project and System Modifications: An Economic Analysis (Galinato et al. 2015), illustrates how the economics of RNG can compare favorably to those of CHP once environmental credits are factored in. Among upgraded products (those refined beyond the standard needed for CHP), hydrogen, gasoline, and methanol all have high conversion costs at present. Renewable natural gas, less expensive to produce, is utilized in a variety of different forms including pipeline quality gas for use as a thermal energy, compressed natural gas (CNG) fuel, and liquefied natural gas (LNG) (Figure 1). Figure 1: Biogas generation and end-use at dairy digesters (modified from Jensen 2011 and Ryckebosch et al. 2011) (graphic by Nicholas Kennedy). FS180E Page 2 ext.wsu.edu

3 CNG is currently the most popular end-use of upgraded dairy biogas due to economics and will therefore be highlighted here. Information on other end-uses can be found in a report by Krich et al. (2005). Compressed natural gas (CNG) The economics of CNG are linked to the decoupling of diesel and fossil CNG pricing, government incentives such as renewable identification numbers (RINs), and nationwide installation of fossil-based CNG infrastructure (Gorrie 2014). As inexpensive fossil-derived natural gas has come into the market in the US in recent years, the price of CNG (both fossil and renewable) has dropped compared to gasoline and diesel. This has been a major driver in the still nascent shift from gasoline and diesel to CNG in the US. While this shift creates opportunities for renewable RNG, RNG also needs to be able to meet or beat the falling fossil fuel-derived CNG prices in order to remain price competitive. The costs of producing renewable RNG with current technologies are slightly higher than the costs to produce fossilderived CNG. However, if produced under the right conditions, scale, and business plan and given current government incentives (e.g., RINs), it is possible for biogasderived CNG to compete with fossil-derived CNG (Evans, personal communication). Upgrading biogas to CNG requires that non-methane impurities are removed and that the gas is compressed to CNG standards (Table 1). All natural gas engine manufacturers have their own natural gas standards to ensure engine performance and reduce malfunction and corrosion. To date, there has been no consistent standard set for pipeline quality natural gas or CNG in the PNW, though standards implemented in other or by specific pipeline providers can be used as a guideline (Rutledge 2005; Table 1). Standards for compression also vary but are typically 3,000 to 3,600 pounds per square inch (PSI) at 70 F (Jensen 2011). These varying standards are one (though not the only) barrier to adoption of biogas upgrading technologies. Thus, the creation of consistent expectations across various states and pipeline providers would likely help facilitate increased adoption of biogas upgrading by dairy AD digesters. Composition of raw biogas Problematic constituents in raw biogas include water, hydrogen sulfide, carbon dioxide, siloxanes, oxygen or air, and ammonia (Table 1). Each of these is described more fully here. Water vapor Raw biogas is generally saturated with water vapor since it is collected from the headspace of the digester (Ryckebosch et al. 2011). The amount of water saturation is highly dependent on temperature and pressure. Water vapor is detrimental because it can react with hydrogen sulfide, ammonia, and carbon dioxide to form acids causing corrosion in compressors, gas storage tanks, and engines (Ryckebosch et al. 2011). Removal techniques for water vapor are discussed in further detail in the section Impurity removal technologies for biogas used for CHP below. Hydrogen sulfide Hydrogen sulfide is one of the most detrimental impurities in dairy biogas. Hydrogen sulfide is corrosive to internal combustion engines (Fulton 1991), and it can be an environmental and human health hazard due to its odor and toxicity (Speece 1996). In addition, in the exhaust of CHP engines, hydrogen sulfide is almost completely converted to sulfur oxides (SOx), a precursor to fine particulate matter and a major concern to air quality regulators (Ecology 2012). Table 1. Composition of raw biogas, pipeline quality gas, and CNG (adopted from GTI 2009, Rutledge 2005, and Northwest Pipeline LLC) FS180E Page 3 ext.wsu.edu

4 SOx is often strictly regulated, requiring potentially expensive add-on hydrogen sulfide reduction units prior to introduction of the biogas to CHP engine sets. Removal techniques for hydrogen sulfide are discussed in further detail in the sections Impurity removal technologies for biogas used for CHP and Additional technologies used for upgrading biogas to RNG below. Carbon dioxide Carbon dioxide is not detrimental to equipment or human health. However, it does decrease the energy potential of biogas compared to natural gas (Persson et al. 2006). Raw biogas has a British thermal unit (BTU) value of around 612 BTU per square cubic feet (scf), which is much lower than pipeline quality natural gas at 1031 BTU/scf (Bothi 2007). Removal techniques for carbon dioxide are discussed in further detail in the section Additional technologies used for upgrading biogas to RNG below. Siloxanes Siloxanes are compounds that contain Si-O and organic radicals (methyl, ethyl, and other organic groups) bound to the silicon atom (Ryckebosch et al. 2011). Siloxanes accumulate on rotor blades, spark plugs, and other engine parts, reducing engine performance and causing damage (Ajhar et al. 2010; Dewil et al. 2006). Fortunately, siloxanes in dairy-based biogas are generally at concentrations below those deemed detrimental, though they can be problematic in biogas derived from other sources (e.g., sewage sludge, landfill) (Ryckebosch et al. 2011). Therefore, dairy digesters typically do not need additional infrastructure for removal when biogas is used in CHP engines, boilers, or when upgraded to RNG, though options do exist (Ryckebosch et al. 2011). However, co-digestion with organic materials that include cosmetics, pharmaceuticals, or anti-foaming agents could increase the presence of siloxanes in biogas, warranting removal techniques. Oxygen/air High concentrations of oxygen or air in biogas can create an explosive mixture. In dairy-based digesters, oxygen concentrations are typically well under the threshold that requires removal (Ryckebosch et al. 2011). The presence of oxygen can be an indicator of leaks within the digester itself or gas collector infrastructure. Higher than normal levels of oxygen within biogas can also lead to downstream concerns for particular technologies, so it is advantageous to produce a biogas with as low oxygen levels as possible. FS180E Page 4 ext.wsu.edu Ammonia Gaseous-free ammonia can also be present in biogas as organic nitrogen (N) is consumed during the AD process and converted to ammonia and other forms of inorganic N. After reacting with water, ammonia in biogas can be corrosive to mechanical parts (Ryckebosch et al. 2011). In dairy manure biogas, gaseous ammonia is generally present at concentrations that are too low to merit a separate ammonia removal process. However, when co-digestion is carried out, ammonia levels can increase to levels necessitating removal. If required, ammonia can be removed using activated carbon and other adsorption processes used to remove carbon dioxide from biogas (Hagen 2001). Nitrogen can be an air quality issue downstream in the form of nitrogen oxides (NOx) that are emitted when biogas is combusted in engine/generator sets (EPA 2008). NOx emissions can be reduced with a number of technologies, including burning biogas lean, selective catalytic reduction, selective non-catalytic reduction, NSCR/3-way catalysts, and exhaust gas recirculation (Ecology 2012). One solution is to upgrade biogas to RNG. When RNG is utilized in transportation vehicles, NOx is reduced when it is burned via catalytic converters. Impurity removal technologies for biogas used for CHP Some impurities in biogas are removed even if the biogas is going to be used for CHP and not upgraded to RNG. Other impurities are only removed if RNG is produced. Because upgrading technologies for RNG are most commonly layered on top of primary technologies that focus on impurities problematic for CHP, these are discussed first. Water vapor removal Water vapor can be removed with physical separation of condensed water, conducted via refrigeration (i.e., chillers) through the use of demisters, cyclone separators, moisture traps, or water traps (Ryckebosch et al. 2011; Schomaker et al. 2000). Chemical adsorption and absorption (e.g., silica, aluminum, glycol, and hygroscopic salts) can also be used but these methods are generally more expensive than refrigeration (Ryckebosch et al. 2011).

5 Hydrogen sulfide removal In a dairy digester context, the four most common methods used for removing hydrogen sulfide when needed are the addition of iron salts, oxygen injection to the digester, physicalchemical adsorption after digestion, or a biological filter. Adding iron salts is relatively inexpensive and effective. Salts are added either to the digester itself or to the influent before it is pumped into the digester. The reaction between the iron salts and sulfur forms iron sulfide salt particles, reducing the production of hydrogen sulfide (Wellinger and Lindberg 2005). AD project developers have also found that codigesting dairy manure with blood or dissolved air flotation (DAF) waste can supply iron and drastically reduce hydrogen sulfide production during AD (Kennedy et al. forthcoming). A second common technique for removing a considerable fraction of hydrogen sulfide from biogas is injecting oxygen into the digester. Oxygen injection promotes the formation of sulfur-consuming bacteria (mainly Thiobacillus) that convert hydrogen sulfide to elemental sulfur in the process of cellular respiration (Díaz et al. 2011; Ryckebosch et al. 2011). An addition of 2 6% oxygen to the headspace of the digester is typical (Ryckebosch et al. 2011). Support media for the bacteria is also often provided at or near the surface, supplying additional surface area as well as a wetting action that helps the bacteria to propagate and thrive. It is important, though, that proper control of oxygen dosing rates is maintained as residual unreacted oxygen within the biogas stream can interfere with RNG purification. Also, overdosing the reactor can inhibit anaerobic bacteria and create an explosive mixture in the absence of careful control (Ryckebosch et al. 2011). Physical-chemical adsorption techniques implemented after digestion are quite common as well. In particular, these include treatment with iron oxide or materials dosed with iron oxides (also known as iron sponge), activated carbon, and water scrubbing (described in the section Water scrubbing) (Ryckebosch et al. 2011). Iron sponge techniques use a composite of hydrated iron oxide on a carrier of wood shavings or chips (Figures 2a and b; Anerousis and Whitman 1984). When gas is passed over the media, hydrogen sulfide reacts with the iron oxide to produce iron sulfides and a small amount of water. Systems generally include engineering considerations that maximize the life of the media, include online or offline regeneration, and eliminate any risk of spontaneous ignition. FS180E Page 5 ext.wsu.edu Figure 2: (a) Fresh iron sponge media and (b) an iron sponge scrubber installed in 2014 at a facility in Pixley, CA utilizing dairy manure digester gas in place of natural gas to power boilers (photos courtesy of MV Technologies). A biological filter is a fourth common strategy for removing hydrogen sulfide (Syed et al. 2006). In a biological filter, sulfur-consuming bacteria are used to convert hydrogen sulfide into elemental sulfur (Persson et al. 2006). The underlying process is similar to oxygen dosing though it occurs after AD is complete. Air is added to the biogas, which is then passed through a filter bed at a temperature of approximately 35 C. Figure 3 shows a commercial-scale biological filter installed at a dairy digester. Biological filters are gaining in popularity because they are cheaper to operate than chemical cleaning (Ryckebosch et al. 2011). Advantages include the low parasitic load and low quantity of byproducts (e.g., elemental sulfur) (Jensen 2011). On the other hand, performance is highly dependent on the activity of the bacteria, and temperature and nutrient requirements need to be met (Ryckebosch et al. 2011).

6 For a more complete review of the many removal technologies and techniques available for use at dairy digesters, please refer to Krich et al. (2005). Water scrubbing Water scrubbing is a physical separation process that uses the difference in solubility of various biogas constituents to separate impurities from the desired methane. Carbon dioxide and hydrogen sulfide have lower solubility than methane in water. Therefore, when biogas is passed through water at elevated temperature and pressure, carbon dioxide and hydrogen sulfide can be removed, leaving high purity methane gas. Two types of water scrubbers have been used in practice: single pass and regenerative scrubbers (Krich et al. 2005). Regenerative scrubbers are currently more attractive due to higher stability and fewer operational problems (Bauer et al. 2013). Figure 4 shows a regenerative water scrubbing unit at a dairy in Indiana; one tower is scrubbing hydrogen sulfide and carbon dioxide while the other tower is regenerating water for reuse. Figure 3: Biological scrubber used to remove hydrogen sulfide from biogas (photo courtesy of Energy Cube LLC). This can lead to varying or low overall removal efficiencies, necessitating secondary treatment in some cases. Lastly, the presence of oxygen and nitrogen gas in biogas can sometimes make the biological filter process difficult, requiring additional purification steps (Ryckebosch et al. 2011). Additional technologies used for upgrading biogas to RNG To upgrade dairy AD biogas to RNG, previously described technologies may be augmented or replaced by additional units for removal of carbon dioxide and other trace impurities (Krich et al. 2005). In some cases, simultaneous removal of many of the previously discussed impurities, including hydrogen sulfide, can be accomplished with these RNG-specific units as described below. The following sections provide an overview of three of the most common approaches used for upgrading biogas to RNG at dairy digesters: water scrubbing, pressure swing adsorption (PSA), and membrane separation. FS180E Page 6 ext.wsu.edu Figure 4: Water scrubbing unit located on Fair Oaks Dairy in Fair Oaks, IN (photo courtesy of Greenlane Biogas). The advantages of water scrubbing are its ability to achieve high methane purity (>97%) with little loss of methane during operation (less than 2%) (Krich et al. 2005; Ryckebosch et al. 2011). In addition, it is a reliable, relatively simple, and technologically mature process. Because of this, studies have shown that water scrubbing generally has comparatively low investment and operation costs (Bauer et al. 2013; de Hullu et al. 2008).

7 Although biogas can be effectively treated when injected into water scrubbers without pre-treatment (Greenlane Biogas, personal communication), some reports note that hydrogen sulfide can be problematic as it can be oxidized to sulfuric acid and corrode metal surfaces inside the piping and the reactor itself (Bauer et al. 2013). This may reduce the lifespan of a water scrubbing unit (Krich et al. 2005). Furthermore, the subsequent release of separated hydrogen sulfide to the atmosphere can create serious odor concerns (Krich et al. 2005). Consequently, the bulk of hydrogen sulfide may be removed prior to water scrubbing using one of the techniques previously mentioned. However, PSA does have a high electricity requirement, resulting in higher costs compared to water scrubbing and some other purification technologies (de Hullu et al. 2008). Also, PSA involves fairly high process control and requires dry gas (water vapor must be removed prior); methane loss can occur if valves malfunction (Ryckebosch et al. 2011). If used to remove hydrogen sulfide, an additional disadvantage to using PSA is that the adsorption material adsorbs hydrogen sulfide irreversibly which decreases the operation life of the adsorbent (de Hullu et al. 2008). Thus, as with water scrubbers, additional dedicated hydrogen sulfide scrubbing systems are often utilized to remove the bulk of this impurity. Pressure swing adsorption (PSA) Membrane separation PSA is a technique used in the chemical industry to separate specific gas species from a gas mixture under pressure, according to the species molecular characteristics and affinity for an adsorbent material. For biogas purification, adsorptive materials such as activated carbon impregnated with potassium iodide, zeolites, and alumina silicates are used as a molecular sieve to preferentially target gas species at high pressure and high temperature (Krich et al. 2005). If carbon molecular sieves are used, hydrogen sulfide can be removed as well. Figure 5 shows a PSA unit used to upgrade biogas to CNG standards at a dairy in Lindsay, CA. Biogas can also be cleaned of hydrogen sulfide and carbon dioxide with semi-permeable membranes. Due to a difference in molecular affinity, carbon dioxide and hydrogen sulfide pass through a membrane, whereas methane cannot (de Hullu et al. 2008). Membrane separation is a relatively new biogas purification technique compared to the more mature water scrubbing and PSA, and it was the least frequently used biogas upgrading technology in 2012 (Bauer et al. 2013). The advantages of using a membrane separation process for the removal of carbon dioxide are compactness, low energy requirement, and relatively high methane purity (>96%) (Ryckebosch et al. 2011). However, several disadvantages also exist. Membrane separation was slightly more expensive than water scrubbing and PSA (de Hullu et al. 2008). Also, membranes are susceptible to damage by certain solvents and fine colloidal solids (e.g., graphite), which can make membranes a very fragile process for purifying biogas (de Hullu et al. 2008; Ryckebosch et al. 2011). Hydrogen sulfide is also a concern for membranes. As such, it is recommended that hydrogen sulfide is removed prior to membrane treatment. Figure 6 shows a full-scale membrane separation unit that was installed at a landfill operation. Figure 5: PSA unit used at Hilarides Dairy in Lindsay, CA; biogas is upgraded and compressed to CNG for a dedicated fleet of milk trucks (image courtesy of OWS, formerly Phase 3 Renewables). The advantages of PSA technology are high methane purity of resulting biogas (up to 98%), low emissions, tolerance to impurities, and low space requirements (Ryckebosch et al. 2011). Also, PSA does not require a lot of water or heat, making it a suitable upgrading technique for many applications (Bauer et al. 2013). Lastly, PSA is a mature technology and as such is widely used in the biogas industry, second only to water scrubbing in 2012 (Bauer et al. 2013). FS180E Page 7 ext.wsu.edu Figure 6: Membrane separation unit (photo courtesy of American Biogas Council).

8 Concluding remarks References When CHP is the end-use of biogas, the most common biogas purification approach for dairy digesters in the US is to remove water vapor and hydrogen sulfide. Existing projects use a variety of approaches, ranging from biological processes (either via oxygen injection into the digester or via a postdigestion process) to physical-chemical adsorption processes such as iron type-sponge or activated carbon. Ajhar, M., Travesset, M., Yuce, S., and Melin, T Siloxane removal from landfill and digester gas A technology overview. Bioresource technology : biomass, bioenergy, biowastes, conversion technologies, biotransformations, production technologies 101(9): However, if RNG is the end-use, a higher degree of purity particularly in regard to carbon dioxide is required. At times, a dedicated water vapor removal unit and hydrogen sulfide scrubbing unit is still required for removal of the bulk of the hydrogen sulfide mass. Thereafter, water scrubbing or PSA is often used to remove carbon dioxide from biogas, producing an RNG fuel that can be utilized in a variety of different ways. Other technologies exist, but their application to date on dairy digesters has been rather limited due to concerns related to maturity, cost, and complexity. The best technique is also situation specific, and it is therefore critical to understand the mechanics of each purification process, its limitations, and its economics before making a decision. As electrical rates continue to drop throughout the PNW and US, current and new AD project developers are strongly considering higher value end-uses for biogas, particularly RNG. Interest is increasing due to a growing CNG industry in the US, the decoupling of CNG and diesel prices, and the potential for competitive pricing and high revenues in comparison to fossil-cng given existing government incentives. Projects are presently limited and business models must still be proven before wide-scale adoption of biogas upgrading technologies within a dairy digester platform. In addition, concerns historically plaguing CHP projects related to power purchase agreement pricing, interconnection fees, and scaling are still potentially present within a pipeline fuel model. Nonetheless, the potential exists for a new approach to AD projects on US farms. Acknowledgements This research was supported by funding from USDA National Institute of Food and Agriculture, Contract # ; National Resources Conservation Service, Conservation Innovation Grants #69-3A ; Biomass Research Funds from the WSU Agricultural Research Center; and the Washington State Department of Ecology, Waste 2 Resources Program. The authors would like to thank Energy Cube LLC, Greenlane Biogas, American Biogas Council, and OWS (formerly Phase 3 Renewables) for generously providing photos for this document. FS180E Page 8 ext.wsu.edu Anerousis, J.P. and Whitman, S.K An updated examination of gas sweetening by the iron sponge process. SPE Society of Petroleum Engineers of AIME, Dallas, TX. Bauer, F., Hulteberg, C., Persson, T., and Tamm, D Biogas upgrading-review of commercial technologies. SGC Rapport. Bothi, K.L Characterization of Biogas From Anaerobically Digested Dairy Waste for Energy Use, Vol. Master of Science, Cornell University. Ithaca. CNGInnovations Peterbilt w/ Cummings ISX Engine. Coppedge, B., Coppedge, G., Evans, D., Jensen, J., Erin Kanoa, Scanlan, K., Scanlan, B., Weisberg, P., and Frear, C Renewable natural gas and nutrient recovery feasibility for Deruyter Dairy. Washington State Department of Commerce. Olympia, Wa. Costa, A. and Voell, C Hope springs eternal: Farm digester industry in America. Biocycle 53(2): de Hullu, J., Massen, J.I.W., van Meel, P.A., Shazad, S., and Vaessen, J.M.P Comparing different biogas upgrading techniques. Eindhoven University of Technology. Deublein, D. and Steinhauser, A Biogas from waste and renewable resources : an introduction. Wiley-VCH, Weinheim. Dewil, R., Appels, L., and BaeyenS, J Energy use of biogas hampered by the presence of siloxanes. Energy Conversion and Management 47(13-14): Díaz, I., Pérez, S.I., Ferrero, E.M., and Fdz-Polanco, M Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters. Bioresource Technology 102(4):

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10 Copyright 2015 Washington State University WSU Extension bulletins contain material written and produced for public distribution. Alternate formats of our educational materials are available upon request for persons with disabilities. Please contact Washington State University Extension for more information. Issued by Washington State University Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, sex, religion, age, color, creed, and national or ethnic origin; physical, mental, or sensory disability; marital status or sexual orientation; and status as a Vietnam-era or disabled veteran. Evidence of noncompliance may be reported through your local WSU Extension office. Trade names have been used to simplify information; no endorsement is intended. Published September FS180E Page 10 ext.wsu.edu