Quantifying and demonstrating scrubbing H2S from farm-based anaerobic digestion systems

Quantifying and demonstrating scrubbing H2S from farm-based anaerobic digestion systems

Summary

Anaerobic digestion (AD) produces renewable energy in the form of methane (CH₄)-enriched biogas. Hydrogen sulfide (H₂S) is formed in the produced biogas in trace quantities (0-2% by volume). H₂S can cause corrosion damage to biogas-fueled engine-generator sets (EGS), resulting in high maintenance costs and/or lost revenues for farmers. There is a need for thorough understanding of optimal H₂S scrubbing operational and maintenance parameters, quantifiable advantages and disadvantages of different types of scrubbing systems, and the capital and operational costs associated with H₂S scrubbers, and for this information to be conveyed to farmers using or interested in using AD systems. To address these knowledge deficiencies, the efficacy of H₂S scrubbing systems on five existing dairy farms in PA, MD and NY is being evaluated, with financial analyses conducted on each AD scrubber operation.

Biogas production and H₂S, CO₂ and CH₄ concentrations in the biogas were quantified on existing H₂S scrubbing systems (biological scrubber, physical-chemical scrubber, and air injection) on four out of the five dairy farms included in the study. We have completed 10 months out of the proposed 20-month evaluation period using two project custom assembled continuous biogas monitoring/data recording systems. Bi-weekly to monthly maintenance was conducted to ensure the robustness of the gathered data. Farmers were informed on how to maintain scrubber performance and how their operations affected the composition of the biogas. Financial data on capital costs and on-going maintenance is being collected.

An annual advisory meeting was held, with valuable input given for future extension programming. Two Facts Sheets topics were published, with each topic containing four Fact Sheets (parts 1-4). The topics were: 1) Hydrogen sulfide removal from biogas, and 2) Emissions from biogas-fueled distributed generation units. Two informational sessions/field days were conducted/contributed to; one in Vermont and one in New York.

Objectives/Performance Targets

Project Objectives from Revised Proposal:

• Evaluate biogas scrubbers operating on five NE farms to document: a) efficacy of different H₂S scrubbers b) effect of H₂S on EGS operation, c) determine the extent of differences in on-farm sulfur concentration, d) economic cost/benefit of scrubber operation, and e) practical aspects of daily operation and maintenance of AD systems and biogas scrubbers;
• Create a Farmer’s Guide to Dairy-Derived Biogas: Production, Scrubbing and Utilization;
• Host five field days in the NE;
• Develop and deliver extension/outreach materials including at least 4 Fact Sheets, 5 Case Studies, 6 popular press article/newsletters, journal articles, and field day presentations to ≈ 150-200 dairy farmers, installers, government agents, and NGOs, with all materials available free on Cornell’s PRO-DAIRY Environmental Systems program website, extension and Livestock Learning Center.

Performance Targets from the Revised Proposal:

By project conclusion, biogas scrubbing performance and cost benefits will be determined on five dairy farms with digesters. By project conclusion, 10 dairy farmers with 5,000 total cows will install new biogas scrubbers and 10 additional farmers (with 2500 cows) will improve/update their existing scrubbers. As a result, these 20 farms will realize a total of 6 MW of additional generation capacity with a value of $4,500,000/year. The results and recommendations from the study will be written into the Biogas Guide, Fact Sheets, Case Studies, and popular press and journal articles. The results and recommendations will be used by at least 10 dairy farmers (>5,000 total cows) to guide new scrubber installation and/or improvements to existing units, resulting in enhanced engine-generator set output, i.e. higher capacity factors.

Challenges to Meeting Performance Targets:

The price of milk is currently at all-time lows, with many/mots dairy farmers in the NE losing money every year. The 30% federal tax break for renewable energy installations, which many digester operators use (including one of our collaborating farmers that submitted for upgrade his H₂S scrubber and EGS), expires at the end of 2016 and is not expected to be renewed with the new political climate. The largest H₂S scrubber installer and operator in New York went out of business last year, leaving many farmers with existing H₂S scrubbing systems without any technical support or periodic calibration capabilities. These challenges are beyond the control of the grant PIs, but are very real challenges to the entire AD industry.

Accomplishments/Milestones

Milestone 1: Initial meetings with Advisory Board and farmers within four months of contract execution to: 1) receive feedback on project commencement and the developed logbook to record generator performance, maintenance efforts and costs, and 2) determine project baseline for capital costs, maintenance time and maintenance expenses.         

Milestone 1 Update: Part 1 of this Milestone has been completed. A meeting was held with each farmer at the start of the project to explain the project, objectives and milestone. Additional informal meetings have been held with the farmers as we collect data to update them on our progress and gather information on their system operations and expenses. One New York dairy farm with a H₂S scrubber on the project was replaced with a different farm with an H₂S scrubbing system from the same manufacturer, due to AD system shutdown at the original farm.  Eight months of biogas data was collected from the new farm.

An Advisory Board was held in April 2016, with all Advisory Board members participating (in person or by phone). During the meeting, we received feedback on the project commencement and possible improvements and discussed current scenarios of anaerobic digestion in Northeast US. In addition, Advisory Board members discussed different dairy groups they are associated with in their area (VT, NY and PA) that could be targeted for extension events. In addition, new information on permitting for air emissions implemented in various states that have implications for digester operators was discussed and subsequently, and as a result, in addition to the series of four (4) H2S scrubbing Fact Sheets, a series of four (4) Fact Sheets were written on EGS emission. All eight (8) Fact Sheets have been posted on the Cornell PRO-DAIRY Dairy Environmental Systems web site (see:  http://www.manuremanagement.cornell.edu/Pages/Popular_Pages/Fact_Sheets.html) and the information will be incorporated into future extension efforts.

 Future Work on Milestone 1: We will continue to collect information on the baselines for capital costs of the scrubbers, as well as the maintenance time and maintenance expenses. We have developed expense form for the farmers to use to assist in continual collection of this data.

Milestone 2: Provide quantified information to five diary farmers on their biogas- scrubbing performance and cost-benefit, specifically for two farms with external biological scrubbers, one with an in-vessel system using air injection, one physical-chemical scrubber, and one combined bio-air injection system. The five farmers will receive information on their H₂S scrubbing evaluation, their economics, and improved scrubber and ADS maintenance/optimization within 24 months of contract execution. Dozens of farmers will also receive this information during field days hosted on their farms in order to impart this knowledge to fellow farmers by April 2017.

Milestone 2 Update:

Overall: Data was collected over 10 months at four out of the five farms involved in the project. The farmers were updated on how to improve scrubber performance and how their practices affected the composition of the biogas. In addition, the presence of the project biogas monitoring systems demonstrated the importance of properly maintaining and calibrating the equipment used to measure gas quality; particularly H₂S. Often, farm owned monitoring equipment was either not working at all, or generally under reporting values of H₂S when compared to project measured calibrated values.   

Pennsylvania Systems

One of the PA farms had issues with clogging of their air injection pump for scrubbing out the H₂S (which they were unaware of before project commencement), and we explained how they could observe the data gathered by the biogas monitoring unit to predict if and when they might have further clogging issues. The farmer noticed that after fixing the pump, they were able to change the oil in the EGS less frequently due to higher quality biogas. We had discussions with the farmers, approximately monthly, while the monitoring units were on their farms, to discuss the collected data and its implications in order to help them gauge the performance of their scrubbing systems and the effects of a cleaner biogas stream on their EGS units.

As an example, the data from one PA farm with only air injection pumped at a constant flowrate as their scrubbing process is shown in Figure 1. The average H₂S concentration (1822 ± 23 ppm), CH₄ (56.2 ± 0.1%) and O₂ concentration (0.048 ± 0.004 %) were determined. The variability in H₂S concentration correlated with the quantity of air injected into the digester. On October 15^th, the H₂S concentration decreased to 100 ppm (0.01%), while the O₂ concentration rose to 0.5%, and the CH₄ concentration dropped to 50%, likely due to N₂ introduced into the biogas stream with air injection and possible interference to methanogenesis due to excess oxygen. There was also variability in the feedstock sulfur levels, which affected the concentration of H₂S produced in the biogas. During the study period, the AD system received glycerol as a feedstock (18^th – 19^th October), which led to an increase in CH₄ production after 20^th October, with gradual rising of H₂S levels after substrate introduction.

Maryland System

The farm in MD has an iron oxide scrubber that the farmer filled with rusted scrap iron and steel scrapings. Due to the low surface area of the scrap metal and the high volume of biogas passing through the scrubber, the media was saturated within a short period of time. There were no major difference existed between the H₂S concentrations in the pre-scrubbed (676 ± 3 ppm) and post-scrubbed biogas (698 ± 3 ppm), with an average CH₄ concentration of 66.1%. On November 17^th 2016, we and replaced the saturated media with fresh steel wool to test the effects of the higher surface area on the scrubbing efficiency of the scrubbing unit and evaluated the time taken for the media to be saturated and lose efficiency, as shown in Figure 2. There was a noticeable difference in the pre-scrubbed and post-scrubbed H₂S concentration for the first few days of the experiment (11-18 to 11-20), with low H₂S concentrations in both the pre-scrubbed and post-scrubber biogas after this time, likely due to the temperature drop in the unheated digester at this time. We plan to conduct further tests to confirm this observation. The farm produced a total of 47158 kWh of energy from the EGS run on the biogas during this period.

New York Systems

An example financial table with the characteristics, performances and costs of the two NY farm systems over the course of the data collection period are shown in Table 1. The two biogas scrubber systems in NY are both biological scrubbers, the system at Farm 1 performs better. On Farm 1 there is an advantage in that the digester supplies two EGS, and so biogas flow through is maintained during routine EGS maintenance as one EGS would still be working during this time.  In addition, Farm 1 has a solid cover over their digester, while Farm 2 has a flexible cover that posed problems during the winter months when snow and rain would build up on top and restrict the collection and flow of biogas, resulting in system slowdowns or shutdowns.  This difference in system performance can be verified by examining the Capacity factor (how much power was actually produced divided by how much power could have been produced over the same period) of the two farms shown in Table 1.  The capacity factor is less than one due to system shutdowns for maintenance or outages, and also due to operating the systems at below design capacity due to inadequate biogas flow or other maintenance problems (overheating of engine heads, etc.)

Future Work: Data will be gathered from the fifth farm, a second PA dairy farm that has both air injection as well as a separate biological scrubbing system starting in 2017. In addition, a second data collection period from each farm could occur to better understand seasonal effects on H₂S scrubbing.  Labor, maintenance and utility (heat and electricity demand) costs for all systems will continue through the duration of the project to provide the best possible measure of the costs associated with operating these systems.           

Changes or Issues in Work plan: There was a one to two-month delay in getting money dispersed to University of Maryland and Cornell University (subcontract), which resulted in initial delays in project commencement, but the project has commenced and is working within a new timeline that incorporates those initial delays. There were additional delays due to issues with the H₂S sensors in the Siemens Biogas analyzers. Due to the high levels of H₂S in the biogas, we have had to replace the sensors multiple times, which resulted in breaks in biogas collection periods. We will continue to monitor the H₂S sensors and replace them, as necessary, and as the budget permits. Siemens is aware of the issue and has been working with us in troubleshooting the units.

Milestone 3: Provide > 700 people, including famers, essential information on scrubbing and biogas performance through the publication of a Biogas Guide, four Fact Sheets, five Case Studies, 6 popular press article/newsletters, and two journal articles by August 2018. The farmers will be able to use the Guide to determine: (1) factors affecting biogas composition, (2) biogas scrubbing materials and capabilities for different types of scrubbing systems, (3) operating costs for five ADS, and (4) managing and operating guidelines for EGS with different scrubbing system types. This comprehensive farmer-friendly guide currently does not exist, nor are publically developed data available on H₂S scrubbing efficiencies and operating costs for maintaining ADS. This will be a welcome resource by current and future digester owner/operators. A preliminary outline of the Biogas Guide will be circulated to ten NE farmers for review and input.

Milestone 3 Update: We are in the process of writing the Biogas Guide.  Two of the four Fact Sheets topics (total of eight Fact Sheets) have been posted to the Cornell PRO-DAIRY website.

Future Work: We will continue writing the Biogas Guide, which will be circulated to the Advisory Group and our farmer stakeholders for review prior to publication.  We will start writing the last two Fact Sheets, the Case Studies for each farm, 6 popular press article/newsletters, and two journal articles to be completed by August 2018.

Milestone 4: Host five field days to allow producers to see biogas scrubbing technologies and learn general information on ADS operation, biogas production, and H₂S scrubbing options, and specific information on H₂S scrubbing performance and economic viability for >100 farmers and their advisors for five the field days combined by August 2017.

Milestone 4 Update: We have contributed to or hosted two workshops, one in New York and one in Vermont. The VT Digester Operator’s Group was at Rosie’s Farm in Middlebury, VT on December 8^th, 2016 with 60 participants. At this workshop, controlling H₂S was a presented topic, and our work on H₂S scrubbers was highlighted (see attachment that was distributed widely at the meeting). The other workshop was held at Sunny Side Farm in Venice, NY on January 7, 2016. This workshop was a hosted industry group meeting with 12 participants to discuss the implications of the closing of the largest H₂S scrubber operator (ABC Scrubbers). Following the workshop, site visits were held at the operators affected by the ABC Scrubbers closure from January 19^th – January 21^st, 2016.

Quantifying and Demonstrating Scrubbing H2S from Farms 2016

Future Work: The additional three Field days will be hosted in 2017 or 2018 to provide the gathered information to the farmers. It was suggested by the Advisory Group that we plan events in conjunction with PA dairy farm events that will occur in Summer 2017, and we will be part of the VT Digester Operator’s Group event in December 2017. Opportunities to collaborate in our extension programming to expand the number of participants are continually being explored, and future field days are currently being planned. 

Milestone 5: PDs will consult and assist > 25 farmers on implementing H₂S scrubbers on dairy farms or improving their existing scrubbing systems, work with collaborating farms to optimize their systems, and continue to implement advisory board suggestions, as appropriate, by project conclusion.

Future Work: We look forward to connecting with more farmers through our extension events and helping them with future H₂S scrubbing needs. Two of our collaborating farmers have already purchased a new scrubbing systems due to our collaboration (MD system) or improved their existing system (PA farm).

Impacts and Contributions/Outcomes

Through our assistance, the farmers at one PA farm were able to solve their clogging problems and were able to save $20,000 by not having to install a new air injection pump for their digester. Now, they are continuously maintaining their pump in order to prevent further clogging. The MD farmer has purchased a new scrubber to be installed in Spring 2017.

The two written Fact Sheets will help convey the initial information from this project to the public.

Accurately measuring the concentrations of H₂S over sustained periods on farm has proven to be challenging, even with industry leading equipment from Siemens. Frequent calibrations and diligent maintenance of the measuring equipment is essential to ensuring confidence in measured values. Siemens requires monthly calibrations and yearly replacement of the H₂S sensor. Both of the NY farms use Inca gas analyzers (Union Instruments GmbH), and their manufacturer requires annual replacement of the H₂S sensor, but does not specify a calibration schedule. 

Only the two NY farm systems have their own gas analyzing equipment installed, primarily to take advantage of an incentive program by the New York State Energy Research and Development Authority (NYSERDA), which rewards farms for maintaining an H₂S concentration below 300 ppm in order to receive the performance-based payments from NYSERDA. It was clear from comparing the measurements of the two systems (Project and existing on farm) that in one case (Farm 2) the Inca analyzer was rarely working at all. Farm 1 better maintained and monitored their equipment (replacing sensors etc.), however, it appeared that values measured by the farm owned equipment were lower than those measured by the project equipment. This is likely due to the with our calibrations.  It seems clear that NYSERDA should require a validation/calibration check to ensure that their goal of incentivizing reduced H₂S concentrations is actually met.   

Collaborators: