Digester performance tracking, analysis, and lessons learned review at Freund’s Farm,

Digester performance tracking, analysis, and lessons learned review at Freund’s Farm,

Summary

The digester performance tracking and improvement project has had a strong start in 2006. The team has been able to procure and install all necessary data collection instrumentation despite some technical setbacks. More detail is provided below in the appropriate sections.

Objectives/Performance Targets

The primary goal of this project is to support the Freund’s in achieving a 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 plans to compile 12 months of performance data by the end of 2007.

Accomplishments/Milestones

The primary accomplishment in 2006 was the procurement, installation, and checkout of data gathering instruments during the period between March and August. The team was also able to establish a data collection protocol and completed preliminary performance calculations. Since August the team has been collecting and assessing performance data despite some hardware failures. The following summarizes key accomplishments.

GAS COMPOSITION – A carbon dioxide (CO2) meter was purchased and installed on the biogas supply line in the boiler room. This meter provides a measurement of the CO2 in the biogas and, by subtraction, estimates 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 indicate that the biogas CH4 content is generally stable and approximately 60 vol%. The CH4 content varies during seasonal movement of the herd between pasture and barn. This is when the digester experiences 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. This meter is rather old and has required two repairs just since this project began. 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. Typical biogas flows to the boiler burner are between 4.0 and 4.6 ft3/min. As a result, the typical firing rate for the boiler’s burner has been between 200 and 250 kBTU/hr 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 Power (kBtu/hr) %
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.

These are preliminary results, and the team plans to continue monitoring the performance to assess seasonal variations.

Impacts and Contributions/Outcomes

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 very immediate feedback for the Freunds to understand the current status of the system. Less regular calculations by Project Leader, Chris Callahan, help to determine the overall performance of the system and areas for improvement. A next step in this area is the development of a “check engine light” so that the data Matt collects daily can be used directly in immediate calculations of performance and a “red flag” can be raised if one parameter is out of line. The team also plans to brainstorm and plan experiments to better understand how the system can be best controlled.

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.

Collaborators: