An anaerobic digester has been in operation at Freund’s Farm for 9 years and is used only for heat (i.e. no electric power output). This project aimed to track its operation and better understand its performance. The digester performance was measured nearly every day for 16 months, providing a database for review and analysis.
This project involved the installation of additional instrumentation and required diligent daily data collection by the farmer. The data was entered into a spreadsheet bi-weekly and performance calculations were completed. Quarterly review meetings allowed for the team to review the data over longer periods of time and attempt to identify problems and solutions.
Based on the information gathered the team was able to determine boiler burn rate (202-313 kBTU/hr), gas quality (60% CH4), digester system efficiency (55%), digester system reliability & availability (82% & 95% respectively), and boiler combustion efficiency (70-90%). The team also identified possible causes of foaming which has been an issue for the digester operation. Several lessons learned have also been documented and areas for further study are proposed.
This project was intended to provide Freund’s Farm, Inc. with increased understanding regarding the current performance of the methane digester installed on their farm in order to enhance the digester’s performance.
Freund’s Farm, Inc. is a 230 cow dairy in Northwest CT that has been operating the digester for 9 years with significant labor and maintenance burden. The digester is considered an integral part of the farm’s nutrient management plan and also provides thermal energy to farm buildings.
Additionally, the Freunds have embarked on the development of an innovative, value-added, off-farm product which could incorporate several digester outputs. This product family currently includes planting pots (“COWpots”) and weed barrier paper (“COWpaper”) which can be made from fibers, nutrients, and heat extracted from the digester.
The work described in this report has provided a database of digester knowledge that should support the consistent, reliable, repeatable performance of the digester.
The primary goal of this project was to support the Freund’s in achieving more predictable operation of their on-farm anaerobic digester. To that end the following objectives were proposed by the team:
1. Understand the operation of the digester (gas composition, reasons for fluctuation, etc.)
2. Develop methods to control fluctuation of performance
3. Identify areas for improving the power produced from the digester gas.
4. Assess impact of digester feed and operation on solid outputs.
The team planned to compile 12 months of performance data by the end of 2007.
Further consideration led to the realization that the scope of Objective 4 was more complex than this study could reasonably include. That work was conducted as a separate project.
The installed digester has demonstrated variable performance (gas production, gas quality, significant thermal input required, etc.) which demands disproportionate attention on the part of the Freunds. The team proposed a data collection approach that would help to understand the conditions of the current operation. Next steps following the data collection and interpretation included planning future operational targets and potential improvements.
In order to obtain basic performance data on the digester additional instrumentation was purchased and installed under this grant. The data previously collected by the Freunds was incomplete and did not allow for a more comprehensive assessment of the digester performance. This new instrumentation along with spreadsheet based calculations based on the collected data allowed the team to measure and track actual performance, not just data points.
The measurement plan included the components noted below.
Process Component Data to be Collected Significance
Environment Ambient Temperature and Pressure Heat loss, thermal load demand, gas production measurement
Cows Feed properties, supplements / medicines / other treatments, manure output and properties Directly related to the digester input quality
Digester Input flow properties and rate, temperatures, heat input, solid and liquid output properties and rate, gas output rate and quality Enables the development of a performance map of the digester for optimizing operation and identifying improvement potential
Boiler Gas feed rate, hot water production Part of determining the balance of gas production remaining for additional uses
Heat Loads Thermal loads on boiler Determining the balance of gas production remaining for additional uses
The approach to this project included daily data collection by Matt. He would compile one or two weeks of data on predefined data sheets (Figure 3 & Figure 4). He would then fax them (Figure 5 & Figure 6) to Chris who would enter them into a spreadsheet. As multiple months of data were collected calculations were conducted by Chris using the data to determine heat flows, trends in foaming, and other cumulative aspects of the digester operation. The measurement plan and measured results were reviewed quarterly with the team. Often, the team would identify areas of further inquiry based on these discussions. But mainly, the data review served to provide feedback to Ben and Matt to help with the operation of the digester. Essentially, the development of this database and regular review meetings served to “close the loop” on what had been an open ended process.
The main project accomplishments and milestones follow. Related objectives are noted in parenthesis.
1. Established a data collection protocol and completed preliminary performance calculations. (Objective 1)
2. Procurement, installation, and checkout of data gathering instruments during the period between March and August 2006 (thermometers, CO2 sensor, and flow meter). (Objective 1)
3. Repair of bio gas flow meter. (Objective 1)
4. Started collecting and assessing performance data in August 2006 despite hardware failures. (Objective 1)
5. Installation of insulated digester cover (under separate funding).
6. Purchase of handheld infrared thermometer to allow in-field checks on permanent thermometers. (Objective 1)
7. Purchase of handheld gas combustion analyzer. (Objective 2)
8. Development of a boiler burner tuning tool (spreadsheet), and assessment of boiler efficiency using it. (Objective 2)
9. Established baseline overall process efficiency. (Objective 1)
10. Determined availability and reliability of system. (Objective 1)
11. Regular data review meetings to share information and gather team input. (Objective 1)
12. Identified significant heat loss in supply and return pipe runs from boiler room to the digester. (Objective 3)
13. Identified a possible relationship between heat exchanger temperature (absolute & changes) and digester foaming. (Objective 3)
14. Completed tracking of a small scale, on-farm digester for 483 days (16 months) (Objective 1)
More detailed discussion of key accomplishments follows.
Gas Composition – A carbon dioxide (CO2) meter was purchased and installed on the biogas supply line in the boiler room [CONSPEC P2416, Nondispersive IR CO2 Sensor, $910]. This meter provided a measurement of the CO2 in the biogas and, by subtraction, estimated the methane (CH4) content. The CH4 is the combustible fuel that would otherwise be emitted to the atmosphere if not gathered in the digester. Measurements indicated that the biogas CH4 content was generally stable and approximately 60 vol%. The CH4 burned varied during seasonal movement of the herd between pasture and barn and depended on the heat loads being served which also change seasonally. These transition periods is when the digester experienced the most significant changes in manure loading. Seasonal temperature changes also affect the gas production and quality. Since these two factors occur simultaneously (e.g. cows in from pasture as temperatures drop in the fall), this is the time of the year when the Freunds have been most active in keeping the digester running reliably.
Gas Consumption – A gas flow meter has been in the system since it was first built [Dresser-Roots Model 1.5M88M, S/N 8821063]. This meter is rather old and required two repairs during this project. It has been used to provide a measurement of total biogas sent to the boiler’s gas burner. This measurement has been most important recently as the Freund’s are applying for carbon credits due to their use of bio-based methane in the boiler. Average biogas flow to the boiler burner is 7.4 ft3/min (5.8 ft3/min in Summer and 9.1 ft3/min in Winter). As a result, the typical firing rate for the boiler’s burner has been 255.6 kBTU/hr (202.2 kBTU/hr in Summer and 313.6 kBTU/hr in Winter) based on a digester feed from 230 seasonally pastured Holsteins.
Process Efficiency – The additional instrumentation installed under this SARE project largely focused on temperature. These measurements are important to understand the efficiency of the Freund’s digester system. We have identified some short-comings in some of the installed instruments in a farm setting and have developed alternatives to them that seem to work better (e.g. handheld infrared thermometers vs. dial gauges for hard to reach locations). For the first time we have been able to assess the efficiency of the digester quantitatively and typical results are as follows:
Measurement (kBtu/hr) Power (%)
Biogas Sent to Boiler 250.0 100
Boiler Losses (stack) 82.5 33
Other Loads (office heat) 37.5 15
To Digester (heat loss & heating) 130.0 52
Heat Loss in Plumbing 30.0 12
Actual Heat to Digester 100.0 40
The overall system can be said to be operating at 15% efficiency (useful heat used in offices vs. total energy in biogas). However, considering that the digester also enables the Freund’s to separate their manure, spread the liquids via pumps and compost the solids for value-added products a better indication of the efficiency is 55% (office heat plus heat to digester). Not only is the heat in the office utilized, but all heat going into the digester is utilized to change the form of the manure for ease of transport and support of other farm ventures. The results do highlight areas of inefficiency (boiler stack losses and heat loss) which can be improved in order to make more power available for other uses.
Handheld Thermometer – A hand held infrared thermometer was purchased to aid Matt during data collection [Palmer-Wahl DHS85XL, $49]. Many of the locations from which temperature data is required are difficult to reach and some of the new thermometers installed for this project were proving to be somewhat unreliable. The handheld thermometer proved to be useful for checking the readings on other thermometers and allowed Matt a quick way of checking otherwise hard to reach temperatures.
Boiler Efficiency – A boiler efficiency and burner tuning worksheet was developed as part of this project. Previously burner air adjustments had been made based on Matt’s qualitative judgment developed from years of looking at the burner’s flame and making adjustments to the air supply. While this works well most of the time, there are times when the tuning is far enough out of adjustment (resulting from drastic changes in gas quality, pressure, system shutdown, etc.) that a more quantitative and direct method may be useful. This worksheet was developed to provide an estimate of boiler efficiency, burner air to fuel ratio, and firing rate by measuring CO2 concentration in the biogas, total cubic feet of biogas going to the burner over a period of time, O2 concentration in the burner exhaust (stack), and stack temperature. These last two are provided with a standard, handheld combustion analyzer [In this case, Bacharach FyritePro125, MPN#24-8105, $1,458]. When the boiler’s combustion efficiency was checked just after an adjustment by Matt using this method it was nearly 90% efficient. Unfortunately, this performance is quite variable and depends on daily tuning to maintain that level of operation. Typically, the boiler is likely operating near 70% efficiency. Correcting this issue would make more gas available for other uses which would increase the value of the digester to the farm.
Reliability and Availability – An assessment of digester reliability and availability was performed based on the notes taken along with the daily data. For the purposes of this report, reliability refers to the fraction of time that no single part failed requiring maintenance activity. Availability refers to the fraction of the time that the digester system was running (i.e. accepting manure feed and producing gas / hot water.) In other words there may be items requiring repair that do not, alone, result in the digester being out of service. Of the 483 days for which data was collected, repair work was required on 86 days, and the digester system was not functional on 22 days. Thus, the reliability is 82% and availability is 95%. Put another way a farmer considering a digester similar to the Freunds could assume the digester will be out of service 1 out of every 20 days and some sort of maintenance will be required on 1 out of every 5 days.
The single greatest impact this project has had on the farm’s operation has been the enhancement of the data collection process. The added instrumentation and discipline of daily data collection provides short-term feedback for the Freunds to understand the current status of the system. Less regular calculations by Chris helped to determine the overall performance of the system and areas for improvement.
In a more global sense, the measurement of digester performance at Freund’s farm allows for them and others to appreciate the impact of their system in reducing emissions of green-house gases and global warming contributors. A crude estimate of avoided CH4 and CO2 emissions during the life of the Freund’s digester based on data measured so far is 425 and 410 tons respectively. One ton of CO2 is roughly equivalent to 3400 miles of typical passenger car travel and CH4 is roughly 10 times more significant in global warming potential. Therefore the digester has avoided the equivalent emissions associated with 15.8 million miles of road travel. The bio-gas created by the digester since being built is also equivalent to roughly 144 thousand gallons of fuel oil.
As a result of data review and discussion, the team also identified lessons that the Freunds have learned from their experience with the digester. A major lesson learned was that the layout of the digester system contributes to a number of the performance problems. The digester is located far from the boiler room which results in long runs of hot water piping and, therefore, heat loss. Additionally, the ability to add heat to the digester and keep it at a desired temperature appears limited by the existing heat exchangers. As a result higher hot water temperatures are required and this is thought to result in problematic foaming of the digester.
Lastly, by assessing and reporting the efficiency of the system along with its availability and reliability it is hoped that other small-scale farms considering a digester will be informed about the reality of their operation.
Education & Outreach Activities and Participation Summary
Ben and Matt Freund are extremely active in the local, regional and national agricultural community. As a result there is no lack of direct outreach opportunities. Some of the more significant ones are summarized below.
Youth outreach has included on farm tours for several high school groups both from the agricultural education perspective and environmental perspective and 2 groups of youths from NYC staying at special camp held at Great Mountain Forest.
General public outreach has included a feature in Financial Partner magazine concerning carbon credits and digester operation. The Freunds regularly include reference to the digester when discussing the background on their novel product, CowPots. In fact, Matt and the digester were featured on an episode of Dirty Jobs on the Discovery Channel.
Farmer outreach has included description of the project and digester performance at regular dairy cooperative meetings. The Freunds have also hosted multiple tours of farm groups ranging from University Extension to Farm Credit. The Freunds have also been very active in the formation of a group pursuing a regional digester opportunity with local utility (accepted but pending).
Political outreach has included hosting a congressional educational summit with 2 representatives (Murphy and DeLauro) and staff as well as area farmers. Matt has also spoken at a regional EPA meeting in Boston;
Private Sector outreach has included a tour with Cargill family member who sits on that Board of Directors.
Chris will be presenting a poster based on this project at the National SARE meeting in Kansas City in March 2008. Callahan Engineering will also host a copy of this report and associated tools on the website www.callahan.eng.pro so others can find it using a standard WWW search engine and download it directly.
Areas needing additional study
The project identified the following areas of potential study.
1. Enhancement of the spreadsheet to allow easier on-farm data entry and review (by the farmer).
2. The development of a “check engine light” in the spreadsheet so that the data Matt collects daily can be used directly in immediate calculations of performance and a “red flag” can be raised if any parameter is out of line. The team would also like to brainstorm and plan experiments to better understand how the system can be best controlled and optimized.
3. Consideration of initial boiler selection (and subsequent replacement), including consideration of automatic combustion air control to avoid the need for daily manual adjustments.
4. Consideration of the impacts of site layout on the performance of the digester (e.g. long runs of pipe result in high heat loss since the digester is far from the boiler room at Freunds.)
5. Consideration of the influence of heat transfer approach on the digester performance. For example the use of large heat transfer surfaces at lower temperature is likely preferred to smaller heat transfer surfaces at higher temperature. The theory is that good digester performance prefers steady, well distributed temperatures and slow changes in that temperature when needed. This study identified a relationship between absolute heat rack temperature and foaming of the digester and also noted a similar relationship between changes in temperature and foaming.