November 19, 2007 | General

Gas Utility Taps Into Farm Digester Biogas

BioCycle November 2007, Vol. 48, No. 11, p. 57
Adjusting to a fluctuating feed rate and more frequent quality monitoring are among the hurdles jumped as dairy sells upgraded biogas to Michigan utility.
Charles Hauska

MICHIGAN Gas Utilities, a natural gas utility serving more than 165,000 southern Michigan customers, is part of Integrys, a Midwest energy company with an interest in power production from renewable resources. The company’s “NatureWise” renewable energy program supports generation of electricity from a blend of wind and biomass, specifically gases produced from landfills and anaerobic digesters on farms.
About a year and a half ago, Michigan Gas Utilities (MGU) was approached by Scenic View Dairy Farm in Fennville, Michigan, asking if MGU would be interested in working with the farm to take its cleaned up biogas into its distribution system. MGU had just been acquired by Integrys, and wasn’t familiar with its parent company’s policies and philosophies regarding farm biogas. Integrys gave MGU the green light to explore this possibility. Its only words of caution were to avoid financial risk, as ratepayers support the utility. Another consideration was to not put a gas into the distribution system that could ultimately cause damage.
MGU previously worked with Scenic View, having installed service to a grain dryer in 2001. The dairy’s initial contact regarding the biogas project was in June 2006. An agreement to proceed was worked out fairly quickly, and by August 2006 MGU installed a high-pressure (HP) service line from its distribution system to serve some boilers needed to heat the anaerobic digesters to get the process started. Design of station requirements began the following month, and the station equipment and connection to the HP distribution system (at 135-150 psig) were installed in mid-January 2007. By this past September, the interconnect station and monitoring equipment (gas analysis, shut in valve and automatic open valve) was fully automated and implemented to where the system is working pretty much on its own without a lot of manual intervention.
Players involved in the project are Scenic View Dairy; Phase 3 Renewables of Cincinnati, Ohio, a contractor to Scenic View; system component developers including Vilter Manufacturing and QuestAir Technologies; and within MGU, the engineering department, telecommunications group and the operations and gas supply group. The biogas from Scenic View feeds into MGU’s 150 lb high-pressure distribution system that feeds the towns of Fennville, Saugatuck and Douglas, Michigan. There are a couple thousand customers on that system. The supplier for the transmission system is ANR Pipeline. Because ANR serves several of MGU’s systems with a large overall daily volume, the small volumes associated with the biogas did not create reliability issues from a volume nominating perspective. MGU treated this interconnect contractually in the same way that it treats natural gas production from local producing wells. The supply contract calls for pricing based on the published natural gas index price minus $0.20/mmbtu.
Scenic View Dairy installed a complete mix anaerobic digester system to process manure from 2,200 head of cattle. The system initially employed two 870,000-gallon mesophilic digesters with a design solids content of five to 20 percent. The dairy recently completed a 50 percent expansion and is now cofeeding heifer manure with lactating cow manure on a 25/75 percent basis. The dairy codigests the manure with syrup stillage from a nearby ethanol plant. Design biogas output is 475,000 cubic feet/day. Scenic View has two 350 kW Caterpillar 3412T reciprocating gas engines; electricity is sold to Consumers Electric Company.
To accommodate the additional gas production, Scenic View Dairy and Phase 3 Renewables decided to install a biogas upgrading system that incorporates a QuestAir Technologies, Inc. pressure swing absorption (PSA) unit (the M-3200 system). The biogas upgrading system was fully operational in February 2007 to convert biogas into pipeline-grade natural gas. The dairy first utilizes the untreated biogas to operate electricity generation engines, which provide all the electric power to the existing farm and new operations. After those power needs are met, the dairy makes an economic decision to either generate power and sell excess electricity to the grid or sell upgraded biogas directly to MGU. The price paid for electricity changes each hour based on market indices, while the gas price index changes once per month.
In terms of gas quality monitoring standards, the Michigan Public Service Commission (MPSC) has Technical Standards with mandated gas quality. The allowable BTU range is 950 to 1,100. Total sulfur content has to be less than 20 grains per 100 standard cubic feet (scf). Hydrogen sulfide has to be less than .3 grains/100 scf.
MGU has its own gas quality standards as well, which it applies to local production natural gas wells that feed directly into its system. From the very beginning of the project with Scenic View Dairy, MGU’s gas supply group decided to treat this like any of its local natural gas wells, and implemented the same set of rules that it has used over the years. These quality standards are incorporated into the gas supply agreements. Some are more stringent than the MPSC’s quality standards. MGU’s standards include:
o Hydrogen sulfide less than .25 grains per 100 scf
o Total sulfur of less than 5 grains per 100 scf
o Oxygen – 1 percent or less by volume
o CO2 – 2 percent or less by volume
o Nitrogen – 8 percent or less by volume
o Gas temperature cannot exceed 110°F
An additional standard is that the gas “shall be commercially free from objectionable odors, dust, hydrocarbon liquids, water and any other solid or liquid matter which might interfere with its merchantability or cause injury to or interference with proper operation of the equipment through which it flows and any substance that might be come separated from the gas in MGU’s facilities.” Essentially, that is a catch-all phrase for saying MGU wants clean gas, and that is the only way it will be accepted into the system.
The Phase 3 biogas upgrading system installed by Scenic View Dairy does online monitoring of that gas stream, both within the QuestAir PSA and with an independent device. Therefore, MGU did not find it necessary to buy duplicative monitoring equipment of its own. However, there were several challenges that had to be addressed. The cycle time for sampling on the dairy’s monitoring equipment – 10 to 20 minutes – was too long. MGU did not want to put its system at risk for 20 minutes if there were a change in the upgrading system’s operation that could upset the natural gas supply system. Even though the pipeline distribution system that the gas is fed into is at a relatively high pressure (150 lbs), there is not a lot of volume, particularly in the summertime, so the dairy’s feed into the system at its maximum (about 80 mcf/day) is a fairly high percentage of the total gas going into the system. Minimum demand on the system is about 320 mcf/day, so biogas would be about 33 percent of supply.
Furthermore, the dairy is on a dead end leg of the system, therefore the gas supplied does not have an opportunity to blend with the other natural gas. Blending provides an opportunity to compensate for an out of spec value with values that are within the specification. In this situation, customers on the dairy’s end of the system are getting biogas only, so if the biogas weren’t at pipeline quality standards, their service could be impacted. Therefore, the gas stream would need to be watched online continuously in very short cycle durations.
The situation was resolved by installing equipment to monitor the specific gravity of the gas stream. Specific gravity changes immediately when the gas composition changes. This specific gravity tester has a cycle time of 10 seconds. The station design includes quality monitoring, automatic shut down and odorization of the gas before it goes into MGU’s system.
Essentially the process works as follows: Scenic View samples the gas through its equipment, and then gives a “handoff” to MGU’s remote terminal unit (RTU). The gas flows through a sales meter so MGU knows the quantity it is buying. Then the utility’s specific gravity sensor also provides a signal to the RTU. The RTU controls the solenoid valves, so if the dairy’s or MGU’s quality sensors detect gas that is outside the acceptable parameters, it will trip the solenoid and shut off the biogas feed into MGU’s system.
On Scenic View’s end, if the quality begins to trend off-spec, the biogas upgrading system will close the MGU valve and recycle the biogas through the system until the quality is back within specification, at which time the MGU valve is reopened. If MGU’s specific gravity tester is the first to sense the off-spec gas, MGU’s valve will slam shut. The biogas upgrading system will then recycle biogas for a period of time. If sufficient quality cannot be achieved within that period, the biogas upgrading system shuts down and the operator must diagnose the situation. Gas that meets the quality standard is odorized and goes into MGU’s system and to market.
The current flow has been running at about 4,500 cubic feet per hour. Scenic View Dairy recently acquired property across the street from its current operations where it plans to house 200 to 400 steers and 10,000 hogs. Manure from these barns will be combined with existing feedstocks to increase biogas production. Scenic View is evaluating expansion options for both gas and electricity production. MGU expects there is enough demand on its system to take all of Scenic View’s production, even during the warmest days during the summer.
Valuable lessons have been learned about how a natural gas utility can work with biogas streams from farm digesters. First and foremost is developing a partnership and information exchange between the utility and the farm’s project development engineers. All parties need to work together to be aware of what it takes to get a project like this up and running successfully.
Flexibility for the utility on gas purchases is important. The system pressure fluctuates drastically from summer to winter. The gas has to be able to get into the system during the summer without causing problems. By the same token, a utility needs to make sure that if the biogas supply is interrupted in the winter, there is enough supply going into system from other sources. MGU had some difficulty convincing its gas supply division that this concept was workable, as it is accustomed to having a set amount of gas coming into its system from a local production well. Standard practice is “all or nothing” – either the supplier has to contract to sell all of its gas or the utility will not take their gas. With this project, the dairy delivers relatively small quantities. MGU’s supplier for that system, ANR Pipeline, provides about 70 percent of all the volume that comes into the utility in southern Michigan. With this large contract with ANR, the little amount of biogas coming in on a daily basis isn’t even a blip on their radar screen if it is on or off. In this particular circumstance, it worked out quite well. But this is a factor to be aware of depending on the distribution system a digester project is tying into.
It also is important to plan for time in the implementation schedule to tune the gas processing operation. In the case of Scenic View Dairy, it took over seven months to fine-tune the process in large part because the system pressure isn’t constant. It is a 150-pound system, which is the maximum it can handle. The pressure on any given day might fluctuate 10 or 15 pounds, and their equipment has to be able to adjust for that and still flow gas into MGU’s system. Project developers need to determine and plan for utility system pressure variability.
Phase 3 Renewables and the owners of Scenic View Dairy are considering installation of additional farm digester sites throughout southern Michigan. They are analyzing the possibility where a digester system could serve multiple farms in one area, thereby creating an opportunity for one interconnect. MGU is open to assisting local utilities and biogas producers with similar energy production initiatives.
Chuck Hauska is Vice President for Michigan Gas Utilities Corporation, a regulated natural gas subsidiary of Integrys Energy Corporation.
P. 58
PHASE 3 Renewables worked with Scenic View Dairy to supply upgraded biogas to Michigan Gas Utilities. Scenic View Dairy currently has the capacity to generate 475,000 cubic feet/day of biogas. If operated at full capacity, the generators consume 260,000 to 300,000 cubic feet/day and the biogas upgrading system processes 240,000 cubic feet/day. Biogas productivity varies up to 15 percent during the year as the composition of the manure changes. In summer, sprinklers are used to keep cows cool, which dilutes the manure. Also, additional electricity is needed on the farm during the summer for powering 286 electric fans for additional cow comfort. Phase 3 designed the controls to allow biogas utilization to fluctuate hourly, daily and monthly.
During the first six months of supplying gas to MGU, the volume fluctuated as Phase 3 made design improvements to its system. It is anticipated that the volume of gas sold will stabilize at maximum throughput as long as the market price differential between electricity and natural gas remains at current levels (electric at about $0.04/kWh and gas above $6/mcf).
The existing QuestAir system has the capacity to generate about 75 cubic feet of natural gas/minute, or about 4.5 million BTUs/hour. That translates into about $315,000/year of natural gas if gas is selling at $8.00/mcf. The cost of the biogas upgrade system was $500,000, including additional expenditures unique to this demonstration plant. Equipment necessary to feed that gas into MGU’s system was about $90,000. – Norma McDonald, Phase 3 Renewables

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