Anaerobic Digestion Fall 2011 Final...

Cornell University Sustainable Design | Anaerobic Digestion Subteam | Fall 2011 Final Deliverable

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Anaerobic Digestion

Fall 2011 Final Deliverable

Cornell University Sustainable Design

Sustainability Research Facility

Team Members:

Ryan Ashley

Vardahn Chaudhry

Ann Lu

Maddy Messer

Adam Pranda

Alexander Rojas

Marina Shumakovich

Jason Wright

Faculty Advisor:

Dr. Lars Angenent


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Table of Contents

Introduction ..................................................................................................................................... 1

Design Considerations ..................................................................................................................... 2

Reactor ......................................................................................................................................... 2

Plug-flow reactor (PFR) ............................................................................................................ 2

Continuously stirred tank reactor (CSTR) ................................................................................ 4

Biogas Processing ......................................................................................................................... 5

Water scrubbing ...................................................................................................................... 6

Pressure swing absorption ...................................................................................................... 7

Iron oxide bed .......................................................................................................................... 8

Liquid phase oxidation ............................................................................................................. 8

Air/oxygen injection ................................................................................................................ 8

Biological filter ......................................................................................................................... 8

Storage ..................................................................................................................................... 9

Structural Integration ................................................................................................................. 10

Pre-processing ....................................................................................................................... 10

Placement .............................................................................................................................. 11

Temperature .......................................................................................................................... 12

Odor ....................................................................................................................................... 13

Flare ....................................................................................................................................... 14

Education ............................................................................................................................... 15

Off-site ................................................................................................................................... 15

Feedstocks ..................................................................................................................................... 16

Food waste at Cornell ................................................................................................................ 16

Types & quantities of waste ....................................................................................................... 16

Transportation ........................................................................................................................... 17

Limiting parameters ................................................................................................................... 17

Seasonal Factors......................................................................................................................... 19

Contacts ..................................................................................................................................... 19

System Analysis .............................................................................................................................. 19

Energy Estimates ........................................................................................................................ 19


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Cost Estimates ............................................................................................................................ 20

Conclusions .................................................................................................................................... 21

Recommendations ..................................................................................................................... 21

Timeline ...................................................................................................................................... 22

Appendix ........................................................................................................................................ 23


1

Introduction

We investigated the possibility of incorporating an anaerobic digestion system within the Sustainability Research Facility, a proposed “living laboratory” that will act as a hotspot for energy initiatives and projects on Cornell University’s campus. The system would be capable of handling pre-consumer food waste produced at both the facility and other dining halls on Cornell’s campus to generate renewable biogas.

Anaerobic digestion takes in organic matter and produces output biogas primarily composed of methane. At the same time, it reduces the overall volume of food waste by producing digestate, which can be used as fertilizer in Cornell’s numerous composting locations. The system will be energy positive and will offset the need to burn propane or natural gas for heating the new facility. Using biogas as a substitute results in fewer greenhouse gas emissions and shifts the resource cost from fossil fuels to a purely renewable source. Without anaerobic digestion, food waste composting emits nitrogen, phosphorous, and ammonia, which each have 30-70 times the greenhouse gas content of CO2.1 We also intend for our design to serve as an educational and academic resource that will help Cornell utilize similar efforts in other university facilities and teach the value of sustainable usage of food waste.

1 Holm-Nielsen, J.B., T. Al Seadi, and P. Oleskowicz-Popiel. 2009. "The future of anaerobic digestion and biogas

utilization". Bioresource Technology. 100 (22): 5478-5484.


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Design Considerations

Figure 1. Flow diagram for the low-solids anaerobic-digestion process.

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Reactor

Plug-flow reactor (PFR)

In a plug-flow anaerobic digester, the incoming materials are dispersed evenly through a vessel and are mixed. Hence, when the materials move through the actual reactor there has to be no additional mixing and the feed can move uniformly in a single direction. It is important that the feedstock is homogeneously mixed as it moves through the reactor because it has to allow the microorganisms in the digester to all be digesting the same materials and producing the same amount and type of gasses.

A sample plug flow reactor was examined from a case study. This plug flow reactor has high efficiency and is fairly simple to operate and maintain. It contains an anaerobic inclined reactor, a grinding system, a mixer, a loading unit, a biogas discharging system, a gas analyzer, and a data-acquisition device. The anaerobic reactor is cylindrical-shaped and has a large volume because of increased surface area due to expanding the length. In order to prevent losses of energy the reactor can be isolated with wool which should be placed between two galvanized

It appears that the reactor, mixer, biogas discharging system, and potentially the gas analyzer are definitely needed in the SRF digester since those are the parts that keep the plug flow

2 A. Hilkiah Igoni, M.J. Ayotamuno, C.L. Eze, S.O.T. Ogaji, S.D. Probert, Designs of anaerobic digesters for producing

biogas from municipal solid-waste, Applied Energy, Volume 85, Issue 6, June 2008, Pages 430-438, ISSN 0306-2619.


3 reactor functioning. The digester needs to be at a certain pH and temperature in order for the thermophilic organisms to digest at their fastest rates.

Figure 2. Pilot-scale plug flow anaerobic digester.

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Gas production of a plug flow reactor is also very high compared to other types of reactors. Case studies also show that plug flow reactors are frequently used for the processing of semi-solid and solid waste, which is the type of waste that a dining facility would be producing. The table below shows some comparisons of the gas productions of different reactors. This table supports the idea that the plug flow reactor will be best suited for SRF needs. It indicates that the plug flow reactor produces more biogas than completely mixed models (similar to the batch reactor). Since it is possible that there will not be very much feedstock (small cafe), it is essential that the reactor produces maximum amount of gas for the amount of feedstock that we have.

Completely Mixed Plug Flow

3 Lastella, G., C. Testa, G. Cornacchia, M. Notornicola, F. Voltasio, and V. K. Sharma. 2002. "Anaerobic digestion of

semi-solid organic waste: biogas production and its purification". Energy Conversion and Management. 43 (1): 63-75.


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Hydraulic retention time (days) 15 30 15 30

Specific volume (m3 gas/m3 reactor/days) 2.13 1.13 2.32 1.26

Specific gas production (m3 /kg VS Added) 0.281 0.310 0.337 0.364

Gas composition (%CH4) 55 58 55 57

Volatile solids reduction (%) 27.8 31.7 34.1 40.6

Table 1. Relative properties of different reactor designs.4

Another design consideration of the anaerobic digester is biogas collection. In a plug flow reactor, the biogas exists the digester at the top of the long cylindrical reactor. One needs to build a channel going from the tube to some kind of container for the biogas. One can also store the biogas in a very large storage tank covering the top of the reactor.

In summary of the previous arguments, the plug flow reactor appears to be well suited for the SRF’s needs. This is because chances are that the feedstock will not be very large since the eatery in the SRF is not going to be very substantial, constant stirring of a batch reactor would be unnecessary and would result in more energy used. Additionally, case studies have indicated that plug-flow reactors are better suited for small volumes of non-liquid feedstock.5

Continuously stirred tank reactor (CSTR)

A continuously stirred tank reactor (CSTR) receives a continuous inflow of reactants and thus continuously produces biogas and sludge. The CSTR is assumed to contain matter of a uniform composition: the exit stream has the same composition as the material the tank. While the stirring process significantly increases biogas yield6, there is an energy cost for the stirring itself.

Both PFR and CSTR designs are capable of handling solid food wastes at high organic loading rates, but the CSTR is easier to operate and more economically viable as it requires fewer pumps than a PFR. In a CSTR, there is low accumulation of the liquefied waste in the reactor because it is completely stirred. Meanwhile, a substantial amount of energy is spent pumping waste in a PFR to prevent accumulation and potential digester clogging which is offset by an agitator in the CSTR. Additional advantages of continuous operation, temperature control, easy adaptation to two-phase runs, high overall system control, simplicity of construction, low operating costs of labor, and the ease of cleaning make the CSTR an attractive reactor design.

However, discontinuous supply of food waste hinders the effectiveness of a CSTR design, which works best when there is no risk of an interruption of input feedstock. To address this issue, the

4 Hayes, T. D., Jewell, W. J., Dell’Orto, S., Fanfoni, K. J. Leuschner, A. and Sherman, D. F. (1979) First

Internat. Symp. on Anaerobic Digestion, University College, Cardiff. 5 Sharma, V.K., C. Testa, G. Lastella, G. Cornacchia, and M.P. Comparato. 2000. "Inclined-plug-flow type reactor for

anaerobic digestion of semi-solid waste". Applied Energy. 65 (1-4): 173-185. 6 Rojas, C., Fang, S., Uhlenhut, F., Borchert, A., Stein, I. and Schlaak, M. (2010), Stirring and biomass starter

influences the anaerobic digestion of different substrates for biogas production. Engineering in Life Sciences, 10: 339–347.


5 subteam developed a model called an Intermittently Stirred Anaerobic Digester (ISAD), which is markedly similar to a CSTR. However, the main feature that distinguishes the ISAD from a CSTR is the stirring frequency. A pump is used to intermittently stir reactants in the digester, as opposed to an agitator that is powered by a motor connected directly to the digester. Alternatively, a pump is incorporated into the digester design to create the ISAD in order to address the discontinuous feeding of food waste into the reactor as well as to ensure that the waste is entirely mixed. The ISAD reactor would be fed at 1-hour intervals, 24 hours a day and 7 days a week. The only foreseen disadvantage of the ISAD is the requirement of electricity to power the pump located at the bottom of the reactor. However, this drawback could not be avoided as it is imperative that the reactants in the digester are completely stirred in order to optimize the generation of methane.

An intermittently stirred anaerobic digester with a variable stirring schedule would reduce the energy costs sunk into the stirring process while capturing many of the main benefits of stirred reactors, namely reduced cost and energy put into maintenance. In some cases, intermittent mixing schedules can actually increase biogas yield in comparison to continuous mixing.7

The team has not yet made a firm decision about which reactor type will be selected. The decision will be made as more information about food waste supply and usable space in the facility becomes available.

Biogas Processing

During the anaerobic digestion process, one of the byproducts that are produced during the breakdown of organic material is biogas. One of the components of the resultant biogas is methane, which can be incorporated into the daily operation of the SRF. The options that were explored for the use of the biogas in the SRF were to use it for electricity production or to use it for heating the building. From the advice of Dr. Lars Angenent, it was determined that for the given volume of waste that was going to be input into the digester and the resulting amount of biogas that was to be produced, the optimal application of the biogas was determined to be the heating of the building.

One of the foremost considerations that had to be made with biogas regarded its usable composition. From case studies that were examined regarding the production of biogas, the percentage of usable methane within biogas is around 60%. The rest of the biogas is composed of around 30% carbon dioxide, 3% hydrogen sulfide, and 7% of other gases such as nitrogen and hydrogen.8

Using the assumption that approximately 60% of the biogas would consist of methane, case studies were consulted to determine the output of biogas that would be produced given an input of dry food waste. Dry food waste was chosen for this particular application of anaerobic digestion because it was determined that wet waste could contain bacteria and parasites that

7 Prasad Kaparaju, Inmaculada Buendia, Lars Ellegaard, Irini Angelidakia, Effects of mixing on methane production

during thermophilic anaerobic digestion of manure: Lab-scale and pilot-scale studies, Bioresource Technology, Volume 99, Issue 11, July 2008, Pages 4919-4928, ISSN 0960-8524. 8 Kapdi, S. S., V. K. Vijay, S. K. Rajesh, and R. Prasad. 2005. "Biogas scrubbing, compression and storage: perspective

and prospectus in Indian context". Renewable Energy. 30 (8): 1195-1202.


6 had the potential to inhibit the anaerobic digestion process. Furthermore, given the limitation of dry waste, the source of material for the digester was fixed to be food waste from the campus dining hall. With these constraints, estimates for the volume of biogas produced could be determined. Among the case studies that contained biogas production estimates, the most relevant one for the dry waste application of the digester was the CURBI report. According to the CURBI report, the amount of biogas that would be produced from food waste by campus dining halls would be around 2600 cubic feet per day.9

Given this amount of biogas that would be produced, in order for methane to be extracted from it, the biogas would need to undergo a scrubbing process where the methane would be separated from the other constituents. The most important component that would need to be filtered out from the biogas is hydrogen sulfide. Hydrogen sulfide is corrosive to metals and would eventually cause failure of the piping and storage tanks that would transport and hold the methane.10 Therefore, the first priority of a scrubbing system would be to remove hydrogen sulfide from the methane stream. As a secondary goal, if it could be feasibly achieved, is the removal of carbon dioxide from the methane stream.

There exist several methods by which hydrogen sulfide can be removed from the methane stream. Each is summarized below:

Water scrubbing

Water scrubbing involves using water as the medium through which the biogas is passed through. As the biogas is passed through the water, the higher solubility of hydrogen sulfide causes it to be absorbed in the water, leaving the methane in the biogas. Using this method, the methane can be purified up to 95%.11 Benefits of this method include that it is equally suitable to be used to remove carbon dioxide as well and so if it is desired, then this implementation can filter out both hydrogen sulfide and carbon dioxide. The main disadvantages are the overall lesser efficiency compared to other processes in terms of methane gas loss and energy consumption.

9 Cornell University Renewable Bioenergy Initiative. “Cornell University Renewable Bioenergy Initiative (CURBI)

Feasibility Study.” January 2010. 10

Kapdi 2005. 11

C. Ofori-Boateng and E.M. Kwofie (2009) Water Scrubbing: A Better Option for Biogas Purification for Effective Storage World Applied Sciences Journal 5 (Special Issue for Environment): 122-125, 2009 ISSN 1818-4952 © IDOSI Publications, 2009


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Figure 3. Water scrubbing process to remove carbon dioxide from biogas without regeneration.

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Pressure swing absorption

Both carbon dioxide and hydrogen sulfide can be removed from the gas by this process depending on the specific use and type of molecular sieves. Pressurized biogas is pumped into a sequence of columns containing molecular sieves which preferentially absorb carbon dioxide and water. After being passed through the columns, the biogas is scrubbed to be water free and cleaner than gas produced by techniques such as water scrubbing. However, this process requires much more sophistication and process control, including re-circulation of the gas in order to avoid losing considerable amounts of the methane gas.13

Figure 4. Schematic of a pressure swing absorption system with carbon molecular sieves for upgrading biogas.

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12

Western United Dairymen. “Biomethane from Dairy Waste: A Sourcebook for the Production and Use of Renewable Natural Gas in California.” July 2005. <http://www.suscon.org/cowpower/biomethaneSourcebook/Full_Report.pdf> 13

Ibid. 14

Ibid.


8 Iron oxide bed

The use of a foundation of iron oxides reacts with hydrogen sulfide to remove hydrogen sulfide from the gas and form solid iron sulfide compounds. In a typical application, the iron oxide can be spread over inexpensive wood chips in order to increase the surface area and increase the effectiveness of the scrubbing process. 20 grams of hydrogen sulfide can be removed for every 100 grams of chips that are present.

Liquid phase oxidation

Similar to water scrubbing, this process uses water as the solvent but incorporates the addition of chemicals which react with the hydrogen sulfide. Solutions such as iron chloride are used which then combine with the hydrogen sulfide to produce insoluble precipitates which can then be removed from the stream and incorporated into the waste slurry for disposal. This system is ideal for small scale digesters such as the one for the SRF.

Air/oxygen injection

The addition of air into the anaerobic methane stream enables the bacteria that are present in the biogas stream to oxidize the sulfides that are present, thus reducing the concentration of hydrogen sulfide. Air is injected into the digestor, which allows thiobacilli bacteria to oxidize the hydrogen sulfide and reduce hydrogen sulfide concentration up to 95% to less than 50 ppm. Air is inserted to typically create a 2% to 6% air to biogas ratio and hydrogen gas and clusters of elemental sulfur are created by the process. This method is likely the least expensive and easiest to maintain method of scrubbing when no upgrading is required. However, if there is not careful control over the air injected, explosive gas mixtures can be created and problems can arise if the gas is later upgraded by other scrubbing processes.15

Biological filter

The use of water scrubbing in combination with desulfurizing organisms is another method that enables hydrogen sulfide to be removed from the biogas stream. However, this method also requires the introduction of air into the biogas stream and as such is not preferred. In addition, much of the technology is not proven to work on a small scale. As one review concludes, “biological methods are less well known, and more intensive research activities are needed… in situ, compact, one-stage S-removal has to be optimized, especially for small-scale applications.”16

Given that the described scrubbing systems all accomplish essentially the same goal, the cost of each system is a major consideration. Ofori-Boateng and Kwofie estimate the cost of each unit for a biogas plant that produces 210 m^3/day of biogas. (This is roughly 10 times the amount of biogas that our system would produce, and the cost scales linearly with capacity.)

15

Ibid. 16

Abatzoglou, N. and Boivin, S. (2009), A review of biogas purification processes. Biofuels, Bioproducts and Biorefining, 3: 42–71. doi: 10.1002/bbb.117


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Method Capital Operational Maintenance

Water scrubbing $28050 $2995 $595

Chemical absorption $28055 $3719 $779

Biological method $22251 $3277 $648

Table 2. Cost of biogas purification unit for different methods, in USD.17

This cost analysis indicates that water scrubbing is about the same initial cost as chemical absorption, the only other category of reliable scrubbing methods at our disposal, and actually saves money on operational and maintenance costs.

Regardless of which system is employed in the digester setup, the specialized nature of these scrubbers means that specific vendors have to be contacted in order to obtain an implementation of a scrubber that is suitable for the purpose and scale of the system that will be implemented in the SRF.

Storage

As biogas cannot be easily liquefied at ambient temperatures, it is commonly compressed in order to reduce storage volume and increase output pressure. Typical storage methods for various pressures are shown in the table below.

Table 3. Most commonly used biogas storage options.

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For small, on-site use of biogas, low pressure systems are preferable due to low cost and ease of use. The additional energy use, safety concerns and scrubbing requirements for medium and high pressure storage systems can often be costly and high maintenance for small, on-site applications. However, some of these extra costs may be mitigated by the increased output of higher purity biogas due to its increased heat content. Since the biogas that will be produced is planned for direct consumption rather than storage and transport, the storage methods can be temporary in nature. The storage methods used must be able to handle excess gas that is not immediately being used when the rate of production is greater than the rate of consumption.19

Low-Pressure Storage—Biogas can be stored at low pressures by using inflatable gas holders that usually operate at pressures of less than 2 psi. These inflatable gas holders are usually made of flexible fabrics such as high and low density polyethylenes and are made in a wide

17

Ofori-Boateng and Kwofie 2009. 18

Kapdi 2005. 19

WUD 2005.


10 variety of sizes that can be easily added to the system. These bags can also be protected from puncture damage by using them as liners in steel or concrete tanks.20

Medium-Pressure Storage—Biogas can also be stored at pressures between 2 and 200 psi through the use of commercial propane tanks. Before being stored in these tanks, the biogas must be cleaned by removing hydrogen sulfide to prevent corrosion and the biogas must be compressed. Additionally, around 10% of the energy content of the biogas will be spent in compressing the gas to these levels if the biogas contains around 60% methane.21

Ballonbau is an example of an ideal storage reservoir that boasts a 10 year life span. The spherical storage device is made of a highly durable polyester cloth fortified with a rubber coating, and has a steel-pipe framing. The 10 m3 reservoir will cost $8,300, and will be able to store 1 days’ worth of gas (approximately 35 m3).

Figure 5. Ballonbau Biogas Storage Balloon.

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Structural Integration

Pre-processing

Food waste must be passed through a shredder or grinder before it is suitable for anaerobic digestion. Quality grinding of input food waste can increase biogas yield by up to 28% compared with disposer treatment.23

20

Ibid. 21

Ibid. 22

Ballonbau. Company Website. Accessed 16 Dec 2011. <http://www.ballonbau.de/> 23

Kouichi Izumi, Yu-ki Okishio, Norio Nagao, Chiaki Niwa, Shuichi Yamamoto, Tatsuki Toda, Effects of particle size on anaerobic digestion of food waste, International Biodeterioration & Biodegradation, Volume 64, Issue 7, October 2010, Pages 601-608, ISSN 0964-8305


11 The grinding system consists of a mincer and a homogenized grinder, but whether the latter is necessary depends on what kind of food waste will be fed to the digester. The grinder would be mostly useful for fruit and vegetable digestion or other hard foods or foods with hard seeds. The mixing unit mixes everything going through the reactor in order to create a uniform mass of feedstock. The mixing unit is a crucial part of the digester since many U.S. digesters have failed in the past and poor mixing being the main reason. The biogas produced by the digester will be collected at the upper end of the reactor and there will be a hydraulic check. The data-acquisition system consists of a device to monitor the temperature and humidity of the plug-flow reactor. There will sensors for that placed at the top, middle, and bottom of the reactor.

The reactor can be loaded as often as once a day or much more rarely depending on the volume of feedstock that it receives. This needs to be estimated once it is clear how big the eatery of the SRF will be and how much food waste can be taken in from other sources.

The input feedstock can also be treated with wastewater by reusing greywater captured in the SRF. The wastewater management team has informed us that systems will be in place to separate greywater (water from bathroom sinks, showers, etc.) from blackwater (water from toilets, kitchen sinks, etc.). We initially eliminated the possibility of a combined digester that utilizes blackwater due to serious risk of pathogen spread. While more research needs to be done, case studies demonstrate that combining greywater with kitchen waste in an anaerobic digester increases biogas yields.24

Placement

Ideally, we would have a plug flow digester, which would be a long tubular shape, placed underground. Placement underground would reduce temperate fluctuation. It would also have to be easily accessible for maintenance. If we are unable to place the plug flow digester underground, we can also put in it a maintenance room.

A plug flow reactor also needs to be tilted so that a smooth flow is ensured throughout it. This means that the reactor will need some vertical and a lot of horizontal space. Furthermore, there needs to be space allocated to biogas storage, but that is flexible as long as there is a channel going from the reactor to the storage container. Additionally, the reactor needs to be easily accessible by facilities for cleaning and maintenance. If the room is cramped, it might be unsafe for personnel to clean it given the high temperatures and the large amounts of waste and bacterial.

For a continuously stirred-tank reactor, it would be placed in a mechanical room, possibly in the basement. Size would depend on the amount of waste the reactor would have to process, which would depend on the amount of waste produced by the café. The figure below demonstrates how tank size can be estimated based on input food waste.

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Zeeman G, K Kujawa, de Mes T, L Hernandez, de Graaff M, L Abu-Ghunmi, A Mels, et al. 2008. "Anaerobic treatment as a core technology for energy, nutrients and water recovery from source-separated domestic waste(water)". Water Science and Technology : a Journal of the International Association on Water Pollution Research. 57 (8): 1207-12.


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Figure 6. Sample sizing estimate assuming 400 lbs/day of food waste.

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For ease of transportation, it would be ideal to minimize the distance between the anaerobic digester and the kitchen. This would enable the use of a direct food chute to transport food waste into a holding unit in the digester room, and would allow the heat produced from the biogas to be directly pumped into the kitchen.

Stored biogas would have to be moved into a storage tank, to be placed where the energy storage team sees fit. A suggestion would be in the same room in which the digester is contained, for the purposed of conserving space.

Input Food Waste (lbs/day) 100 400 800

Volume (gallons) 150 1000 1000

Floor Area (ft^2) 26 75 75

Approx Cost ($) 630 1790 1790

Biogas Yield (ft^3/day) 250.75 963 926

Table 4. Sizing estimates for various levels of input food waste.26

Temperature

Anaerobic digesters are typically operated at a temperature between 86 and 140 degrees Fahrenheit.27 The precise temperature is determined by the size and type of the reactor. This

25

Biorealis Systems Inc. “Anaerobic Digester Calculator.” 2010. Accessed 15 Nov 2011. <http://biorealis.com/wwwroot/digester_revised.html> 26

Ibid.


13 temperature requirement means the anaerobic digester will need a dedicated heat source, as its heating requirements are drastically different from other parts of the building.

One option for heating explored is to use hot water provided by a potential geothermal installation for the SRF. To test this approach on a prototype digester, we plan to use hot water provided by Cornell’s district heating system, which uses steam cogeneration to maximize energy efficiency. In the facility, this water source would be replaced by water heated by a heat exchanger that draws heat from a closed-loop geothermal system, which heats water simply because of the temperature difference between the air and the ground hundreds of feet below the surface. To our knowledge, such a system exists in only one other location in the world, a municipal facility in San Bernandino, CA, where the combined system created an annual cost saving of over $30,000.28

As of this writing, the geothermal team intends to utilize radiant floor heating in the SRF. This works perfectly for the anaerobic digester, as the piping can easily be attached to the exterior of the digester to provide heat.

Should geothermal heating not be available, another innovative option is to use direct steam injection, which has been proven to be effective as a heat source for anaerobic digestion.29 Cornell already produces steam for heating in a sustainable manner, using cogeneration to capture exhaust from turbines at the recently-opened Combined Heat and Power Plant.30

Insulation material should be used to ensure maximum efficiency. Reflectix provides one economic and readily available option.31

Odor

Many of the issues relating to odor surrounding anaerobic digestion concern large-scale industrial setups that process animal waste on farms. However, the use of anaerobic digestion to process pre-consumer food waste in a sealed environment creates significantly less odor than keeping the food waste on site or composting it.32 The process itself will also be virtually sealed, as food waste would be transported in sealed containers or through chutes, and the digester itself is completely sealed.

27

van Lier JB, A Tilche, BK Ahring, H Macarie, R Moletta, M Dohanyos, LW Pol, P Lens, and W Verstraete. 2001. "New perspectives in anaerobic digestion". Water Science and Technology : a Journal of the International Association on Water Pollution Research. 43 (1): 1-18.

28 Lund, John. “Examples of industrial uses of geothermal energy in the United States.” Geo-Heat Center Bulletin,

September 2003. <http://geoheat.oit.edu/bulletin/bull24-3/art1.pdf>

29 Huchel, Jim and Van Dixhorn, Lee. “anaerobic digesters heated by direct steam injection: Experience and lessons

learned.” Water Environment Foundation. 2006. <http://www.environmental-expert.com/Files%5C5306%5Carticles%5C8527%5C032.pdf>

30 Cornell University Press Relations Office. “Cornell marks opening of Combined Heat and Power Plant.” 15 Jan

2010. <http://www.pressoffice.cornell.edu/releases/release.cfm?pageid=38522>

31 Reflectix, Inc. Company website. Accessed 15 Dec 2011. <http://www.reflectixinc.com/>

32 Michigan Farm Bureau. “Frequently asked questions about Anaerobic Digesters (ADs).” 2011.

<http://www.mi.gov/documents/mda/MDA_AnaerobicDigesterFAQ_189519_7.pdf>


14 New research in thermophilic and mesophilic anaerobic digester designs also demonstrates that two-phase digesters can virtually eliminate odors.33 We could accomplish this with a plug flow digester by ensuring that the last stage of our digester is mesophilic by operating it at a slightly lower temperature.

Biofilters can also be used to remove odors from air that might circulate throughout the building.34 They are a biological treatment method that removes and oxidizes odorous compounds present in the air-stream. Such setups are commonly used in many municipal facilities across the United States.

Flare

As a safety precaution for our system, a flare will need to be incorporated into the SRF. The biogas collected from the anaerobic digestion tank will need to be stored in either tanks or biogas bags in the digester room. This gas will most likely be pressurized to minimize the size of the required tank and to facilitate the flow of the gas from the storage tank to its end use. This poses a danger to the facility, as pressurized gas is highly flammable and requires special storage. In order to avoid possible explosions or damage to the system, a flare is necessary to dispose of excess biogas. Biogas that can be neither used nor stored will be directed out of the digester room and storage system by this flaring system.

There are a number of different flare systems that could be used within the SRF facility. One in particular, a stainless steel Semi-Enclosed Candlestick Flare (SEF-10), is 20 feet tall and 10 inches in diameter.35 This flare burner is constructed from high temperature alloy steel and has a flame shroud designed for two main purposes: to prevent wind from extinguishing the flame and to reduce the visibility of the flame. Both of these functions are quite essential to the flaring process as the flame must be kept lit to avoid releasing methane to the atmosphere (methane has a global warming potential 21 times as potent as that of carbon dioxide, making it significantly strong greenhouse gas) and the visibility of the shroud should be minimized as to prevent negative responses from the community.

As the SRF facility will be used for educational purposes, it will be quite dangerous to store combustible biogas on-site. Many students, faculty and staff would be uncomfortable knowing that a flammable biogas was being stored within this educational facility. As such, it is imperative that a flare be incorporated into our digester design and also be hidden from public view. We hope to integrate this flare strategically in the external design of the SRF so that it is not immediately noticeable to onlookers. We do not anticipate having to use the flare; however, it will be added to our design as an extra precaution.

One alternative to a flaring system is the use of a biofilter in which microbial communities could oxidize the methane. Wood fibers could be used in the biofilter design to prevent clogging of the filter system. While this proves a viable alternative, biofilters do not offer the added safety control system that flares do; furthermore, they have large size constraints and are known to

33

Kim J., and Novak J.T. 2011. "Digestion performance of various combinations of thermophilic and mesophilic sludge digestion systems". Water Environment Research. 83 (1): 44-52. 34

Columbia Biogas. “FREQUENTLY ASKED QUESTIONS.” 2010. <http://www.columbiabiogas.com/faqs/index.html> 35

Flare Industries, Inc. “SEMI ENCLOSED CANDLESTICK FLARE.” Accessed 16 Dec 2011. <http://www.flareindustries.com/products/pdf/2%20SEF%20Flare%20.pdf>


15 be quite odorous. We would recommend using a flaring system over this methane oxidizing biofilter.

Education

Due to limited accessibility and location of the digester, there cannot be many people that will see the digester in person. We propose to develop an informational video that explains the way our digester functions along with a live feed of what is happening inside the digester at the very moment. We could also keep a counter of how much waste has been processed in a certain time period.

This information should also provide data on how much biogas is produced and how much is fed to the reactor in order to obtain that. This structure would allow both for practical use of the digester and practical maintenance, as well as a good way to educate the public on food waste treatment, renewable energy, and sustainability in general.

This will require installation sensors and energy-monitoring equipment to ensure a net positive production of energy, while keeping track of the costs of transportation of food waste. This equipment should also keep track of the usage of the equipment itself, to ensure that we are using equipment with the longest lifespan and to avoid costly replacements later on.

The SRF should also measure visitors’ response to the presence of the anaerobic digester to ensure that they do not perceive any negative impact stemming from its location in the building the prototype is housed in, and to see if the educational components of our designing are translating into changed viewpoints on sustainability. This can be accomplished primarily through surveys, tours, and independent assessments of the system.

Off-site

We considered the possibility of locating the digester somewhere other than the SRF. The CURBI report proposed several alternative site locations, including Stevenson Road (near a central composting facility), Guterman Laboratory (a bio-climatic facility), and Game Farm Road (near the CCVM dairy facility).

However, CURBI’s proposal was for a much larger biomass facility, which would have larger space and maintenance requirements than the anaerobic digester for the SRF. The relatively small nature of our system removes the necessity to locate the digester far away from central parts of campus. Even taking that into account, CURBI seriously questioned whether these sites were ready to absorb the stored biogas, writing that “Until future thermal needs such as greenhouses or additional buildings are constructed, excess heat … would need to be released to the atmosphere.”36

Additional concerns include the logistical difficulties with procuring an additional site on top of the SRF, and the disconnect that an off-site location would present with the core mission of SRF. An off-site location would preclude innovative ideas such as combining the digester with geothermal and wastewater treatment systems at the SRF itself.

36

CURBI, 2010.


16

Feedstocks

Food waste at Cornell

Figure 7. Composting travel routes on Cornell's campus.

37

Currently, Cornell’s dining hall staff takes charge of food collection since they are trained in the differentiation between organic and inorganic products. The pre-consumer waste, which will be used, is safer than post-consumer waste because employees do the sorting. This reduces the risk of contamination in the anaerobic digester system.38 Next, after sorting the food in roll-off containers, it is put into dump trucks. The dump trucks then transport the dining hall waste to the existing composting area labeled in the Stearns & Wheler map above.

Types & quantities of waste

37

Ibid. 38

Ibid.


17 Source Feedstock Component Tons/YR Collected Tons/YR (Dry)

Crops Dedicated energy crops baled (4 x 4 x 8) 8000 6800

Vertinary Hospital Animal manure and bedding 2886 2020

Forests Culled forestry cuttings 1250 825

Forests Woody biomass 6000 3600

Animal Science Animal manure and sawdust 116 70

Polo Fields pre-gound pallet waste 2189 1313

Various Chicken manure and kraft paper 1350 715

Poultry buildings Plant material and soil 60 30

Greenhouses Plant material and soil 274 82

Plant Science Plant material and soil 320 96

Plantations Plant material 36 11

Dining Halls food waste 458 141

Animal Science Large animal manure 208 64

CCVM Dairy Complex Animal manure and bedding 5000 500 Table 5. Feedstock data at Cornell.

39

These values came from the CURBI report which prorated values from a December 2007 Cornell University class project report prepared by Joan Casey, Leigh Gerson, and Annie Smith (Casey, et al.) for similar feedstocks for manure digestion.

Transportation

CUSD’s current potential sites are within 25 miles of all the important feedstock sources and have a multitude of costs. The cost schedule is listed in the table below. Given current composting infrastructure, digested food waste could easily be absorbed by current routes, and output digestate could easily be transported to locations managed by Cornell Farm Services.

Table 6. Cost schedule for waste transportation.

40

Limiting parameters

Feasibility and limiting factors are immensely important in describing the design process. According the CURBI report, location is the limiting factor. The site location must be suitable

39

Ibid. 40

Ibid.


18 and a local range of input streams must be within a maximum threshold of 25 miles. The reason it must be close is that transportation and preparation costs are very high.

The waste stream mass balance diagrams below display the dry and wet digestion possibilities and their raw input and output amounts. Dry fermentation process will refer to anaerobic digestion of higher solids biomass in a batch treatment process. Wet anaerobic digestion will refer to processing of materials, which can be pumped and treated continuously with closed containers.

The feedstock varies and is not constant which introduces the variable input problem. Dining Halls produce 5 – 6 tons of waste per day in fall and spring semesters but in the summer it drops significantly to 1 ton per day.41 The Waste Vegetable Oil from dining halls drop off in January, May, and June 6,000 gallons. Lastly, the Alkaline hydrolysate is available in batches of 3,000 gallons each.

Figure 8. Waste stream mass balance diagrams.

42

Table 7. Wet vs dry feedstock availability.

43

41

Ibid. 42

Ibid. 43

Ibid.


19

Therefore from the above table, the maximize amount of wet anaerobic digestion input/year is 8,000 tons/year which results in 25,000 cubic feet of biogas per day. Dry digestion, which may also be an option, maxes at 13,400 tons/year, which results in 1,200 cubic feet of biogas per day.

Seasonal Factors

Dining hall waste production is proportional to the resident student population. Waste production is fairly consistent through the year, but drops significantly in January, early June and early June through mid-August. Since dining hall wastes cannot be effectively stored due to odors and pests, this waste must be processed on a daily basis throughout the year.

Alkaline hydrolysate waste production varies widely from as little as 3, gallons/week to more than 20,000 gallons/week, depending on animal mortality at the CCVM. Since this waste is very odoriferous and decomposes quickly, it should be treated when produced or stored in sealed vessels with odor control facilities for a short period of time.44

Energy crops such as grasses are harvested in the summer months and can readily be baled and stored for year-round use. Similarly, woody biomass can be chipped and stored for year-round use. Storage issues are discussed below.

Contacts

Steve Beyers ([email protected]) is the Services team leader for Cornell's Environmental Compliance and Sustainability and sent the initial complete CURBI Report and leads to find additional contacts and information.

Spring Buck ([email protected]) manages Cornell’s R5 Operation for Respect, Rethink, Reduce, Reuses, and Recycle. He assisted in researching and answering tough questions.

System Analysis

Energy Estimates

Some components of our system have energy demands. The grinder uses energy to physically process input food waste. A pump is necessary to ensure flow of the digester, and an air compressor is used to facilitate storage of the output biogas. We estimated the combined energy usage of these components using sample models. The results are shown in the table below.

44

Ibid.


20

Table 8. Estimated energy usage of the system.

Taking this into account, we can estimate the net energy gain from our system depending on several factors. Varying the amount of input waste per day, we assumed 5mm particle diameter grinding, biogas yield of 0.6 m3 (20 ft3) per pound of waste, 65% methane content, and an energy potential of 2.28 kW/ft3 methane/minute. This produced the following net energy schedule:

Table 9. Estimated net energy.

We also estimated the carbon offset of our system assuming that the heating potential produced would offset the use of natural gas. Using current data on the carbon equivalent of natural gas, we produced the following natural gas offset schedule:

Table 10. Estimated natural gas offset.

This data was produced using an Excel spreadsheet with variable parameters, so we intend to update and revise our predictions as more information becomes available.

Cost Estimates

Using sample components and estimated expenditures, we produced the following cost breakdown for our system.

Component Energy Usage (kWh/day) Parasitic Load (3%) Total Energy (kWh/day)

Fleetwood Grinder 0.245 0.007 0.252

1hp IPT Submersible Shredder Sewage Pump 2.952 0.089 3.041

Mi T M Corp air compressor 1.490 0.045 1.535

Total 4.687 0.141 4.828


21

Table 11. Estimated cost of the system.

These costs represent estimated values for sample components, so if firm decisions are made to use different technologies described in this report, the costs are likely to vary. In particular, the choice of biogas treatment method can alter the cost schedule dramatically, as shown in the Biogas Processing section of this report.

Using the current cost of natural gas and the offset analysis shown earlier, we estimated the following gains:

Table 12. Estimated gains from natural gas offset.

The important conclusion of this analysis is that our system is unlikely to offer a complete payback in a reasonable timeframe. Therefore, funding will be required in the form of fellowships, grants, or donations.

Conclusions

Recommendations

Our research indicates that an on-site anaerobic digestion for the SRF is feasible with adequate funding sources. The digester would be energy positive, reduce greenhouse gas emissions from composting, and enable the facility to utilize a renewable energy source for heating. The

Item Cost ($)

Grinder System 500

Digester tank 1439 Energy Usage:

Submersible Shredder Pump 800 Machinery 4.83 kWh/day

Solenoid Valves 210 1762.95 kwh/year

Tank Insulation 205

Heating Coil 408 Electricity Costs:

Settling Tank 839 0.12 $/kWh

Biogas Treatment 400 211.554 $/year

Biogas Storage Balloon 8300

Natural Gas Compressor 350 Operation & Maintenance Costs:

Flare 7000 5% of Initial System Cost / year

Polyethylene Piping 19 1300 $/year

PVC Piping 63

Tank Fittings 120 Total Annual Costs:

1600 $/year

Initial Cost: 21000

Estimated Additional Costs: 5000

Total Initial Cost: $26,000.00

Costs

Economic Analysis


22 digester is unlikely to create significantly adverse effects on the livability of the facility, and will impose only minimal structural and spatial requirements on the design of the facility.

Timeline

Spring 2012

Develop designs for both PFR and CSTR systems, and select one as information about the design constraints of the SRF becomes available.

Update life cycle analysis and energy estimates using new design parameters.

Acquire precise data about usable food waste at Cornell.

Research possibilities for equipment providers and compile information into a comprehensive decision matrix.

Fall 2012

Create a test digester and conduct experiments to evaluate energy efficiency and suitability for inclusion in the SRF.

Work with architectural programming, geothermal, wastewater treatment, and energy storage teams to integrate the digester into the complete design for the SRF.

Spring 2013

Continue digester experiments.

Create test setups for shredder & biogas scrubbing components.

Incorporate food waste analysis for a range of input values from 0 to 800 lbs/day to ensure operation at varied food waste levels.

Evaluate possibility of including post-consumer food waste.


23

Appendix

These calculations were done for the ISAD design for Dr. Angenent’s class. They are used primarily for estimating energy output and natural gas offset.

Average food waste composted daily 100 lbs

Average students / day 1800-2000

Assume: same size as as Mattins, similar amount of waste produced

waste 100 lbs/day

density of food waste* 638 lb/m^3

daily volume of waste (100/638) 0.157 m^3/day

daily volume of grey water (for 20% solids) 0.078 m^3/day

total daily volume 0.235 m^3/day

Using a HRT = SRT = 25 days

Volume of CSTR =

daily volume * HRT = 5.878 m^3

Safety Factor 1.5

Total Tank Volume 8.817 m^3

* value from Tchobanoglous et al. (1993)

From Tchobanoglous, G., Theisen, H., Vigil, S. 1993:

% Moisture of Food Waste 70%

% solids in food waste 30%

Reactor Aim (% solids) 20% solids

daily volume of waste 0.157 m^3/day

daily solids volume (from waste) 0.047 m^3/day

daily water volume (from waste) 0.110 m^3/day

Total Volume for 20% solids 0.235 m^3/day

additional water required 0.078 m^3/day

Mattins Café:

SRF Facility

Volume approximately 9 m^3 = approximately 2,400 gallons


24

waste 100 lbs/day

Assume 65% VSS destruction --> 65 lbs/day

Assume 20 cf biogas / lb waste 1300 cf biogas/day

Biogas- 62% methane 806 cf methane / day

0.559722 cf methane/min

Assume 2.28 kW/cfm 1.276167 kW

@ 24 hours/day 30.628 kWh/day

Methane Produced

Average density of water in tanks

ρ water (lb/ft^3) 62.4

ρ food waste (lb/ft^3) 18.07

Assuming that our waste stream is 20 % solids

f waste 0.2

f water 0.8

Weighted average density of material in tank

= fwaste* ρ waste + fwater * ρwater

53.53

* Assume the specific heat of the liquid in the tanks is approximately the specific heat of water

Cp 1

* Assume the liquid entering the tank is room temp (20°C) and it will be heated to 35°C

T (change to heat) 15 °C

59 °F

Properties of Digester Slurry for Heat Calculations

Diameter (in) 95 Diameter (ft) 7.92 Diameter (in) 87 Diameter (ft) 7.25

Height (in) 91 Height (ft) 7.58 Height (in) 67 Height (ft) 5.58

Surface Area (ft^2) Surface Area (ft^2)

=pi * diameter * height + 2*(pi*diameter^2)/4 =pi * diameter * height + 2*(pi*diameter^2)/4

286.91 209.63

Volume (ft^3) Volume (ft^3)

= (pi*diameter^2)/4 * height = 2*(pi*diameter^2)/4 * height

373.09 230.38

Digester Tank Settling Tank


25


Surface Area (ft^2) 286.91 Surface Area (ft^2) 209.63

Reflectix Double Reflective Insulation

1 roll = (4ft x 25 ft) Cost of roll ($) 40.84

Area (ft^2) 100

Rolls needed = total surface area / area of roll

# rolls 4.965343 Total cost ($) 204.2

5 rolls

Reflective Foil Tape

1 roll = (2 in. x 30 ft) Cost of roll ($) 3.25

Area (ft^2) 5

Rolls needed 99.30686 Total cost ($) 325

100 rolls

Option 1 2

Cost insulation ($) 204.2 325

Cost heating coil ($) 300 300

Cost bulkhead fittings ($) 39.76 0

Total Cost ($) 543.96 625

R- value range from 3.7 - 17 for each option

U- value (1/R) = measure of heat transfer:

range: 0.27 0.06

Average U- value : 0.16

Conclusion: Using either of these Reflectix insulation products will yeild

a 16% heat loss from each tank

Comparison

Insulation Calculations

Conclusion: Option 1 is more economical than Option 2

Heating Option 1: Coil inside, Insulation outside

Heating Option 2: Coil outside, Insulation outside

Insulation effects on heat loss


26

Heat loss (Btu/hr) Heat loss (Btu/hr)

* at 100 F,

cte = 900

(Btu/hr/sq ft)

258215.94 188664.94

Accounting for insulation, heat loss = 0.16 * max heat loss

41314.55 30186.39


335920.94 212100.70

Digester tank Settling tank

125 75

Area (ft^2) Area (ft^2)

45.55 47.93

Using 3/4" piping

diameter (ft) 0.0625

circumference (ft) 0.405

Length pipe needed = area/ circumference Length pipe needed = area/ circumference

length (ft) 112.47 length (ft) 118.35

= surface area * constant relating

maximum heat loss at various temperatures

The U-value depends on if the tanks are agitated or not. Since we are using water, we find the U -values

based on the following chart

Surface area of coil needed in each tank = Q / (U*T)

where Q is total heat needed, U is defined above, T is temperature change (59°F)

Total Q value = Heat needed to bring tank to 35°C + heat lost

* These calculations taken from Products Finishing Magazine (Online).

URL: http://www.pfonline.com/articles/sizing-heating-and-cooling-coils

Heat Lost by Digester and Settling Tanks (Assume 16% of max due to Reflectix Insulation

Heat Transfer Coefficient: U-value


27

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t^2)

Anaerobic Digestion Fall 2011 Final...

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