Monitoring and control of biogas production Los Gatos Research (LGR)

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1 Application note AN/ANALYTICAL/005 EN Monitoring and control of biogas production Los Gatos Research (LGR) The LGR-ICOS process analyzer is an excellent choice for real-time monitoring of gas contaminants and heating value at biogas facilities Measurement made easy Introduction In a 2 1 /2 month trial, the ABB LGR-ICOS process analyzer outperformed a traditional gas chromatograph for real-time measurement and control of a commercial biogas production facility. Both analyzers measured carbon dioxide (CO2), methane (CH4) and high heating values. The ABB LGR-ICOS analyzer provided a faster response time with similar or better accuracy and reliability. Faster real-time response times offer the possibility of tighter process control. In addition, the LGR-ICOS process analyzer does not require carrier gases and considerably reduces ongoing maintenance. The ABB analyzer also outperformed an electrochemical sensor for measuring hydrogen sulfide (H2S).

2 Monitoring and control of biogas production Los Gatos Research (LGR) Background Biogas is produced by the anaerobic fermentation of carbohydrates in organic material, including plant matter, animal waste and sewage. The biogas plant typically feeds waste into an anaerobic digester a large, oxygen-free tank. Digester temperatures range from 30 to 38 C (86 to 100 F) to optimize bacterial digestion. Such digesters typically decompose waste in a few days to produce biogas. In addition to digesters, landfills provide a similar function on a large scale. The resulting biogas consists mainly of methane (50 to 75 %), carbon dioxide (25 to 50 %), nitrogen (0 to 10 %) and hydrogen (0.01 to 5 %). The exact concentrations depend critically on the digester feedstock. This biogas can then be burned directly as fuel for boilers and electricity generation. Alternatively, it can be scrubbed to remove carbon dioxide and other non-methane compounds. In this case, the resulting gas (almost pure methane) can then be pressurized and injected into the existing natural gas infrastructure or used to power natural gas vehicles. Advantages of biogas Biogas' greatest advantage is that it is virtually carbon-free. Converting carbon in such materials as plant matter and animal waste to fuel essentially recycles the carbon. For example, energy generated from biogas typically produces less than 50 g of CO2/kWh, compared to 550 g of CO2/kWh for conventional natural gas-generated energy and 1000 g of CO2/kWh for coal-generated energy. The very small carbon output makes biogas a renewable energy source and unlike some other renewables such as solar and wind, biogas occurs continuously, providing a constant raw material for energy. Finally, biogas plants can be modified easily and economically to address new feedstocks and output needs. These advantages have resulted in more than 2,000 biogas production sites in the US and an additional 10,000 in Europe. Estimates set the worldwide number of biogas sites at over 100,000 with large usage in Asia and developing countries. Biogas is expanding rapidly as many countries seek to reduce their carbon footprint. 2 AN/ANALYTICAL/005 EN Monitoring and control of biogas production Los Gatos Research (LGR)

3 Measuring contaminants and heating value Anaerobic digestion can also produce unwanted contaminants, including hydrogen sulfide (H2S) and ammonia (NH3). Moreover, some of the biogas constituents (for example, CO2, H2O, O2) must be monitored accurately to meet specific applications. Hydrogen sulfide Elemental sulfur in the digester can be readily converted to H2S by bacterial action. The exact amount depends on the feedstock and ph of the digester. Since the high levels of hydrogen sulfide sometimes found in biogas reactors (1000 to 6000 ppm) can lead to rapid death, this gas poses severe safety risks. Even lower levels (<10 ppm) can cause negative physiological effects. A new OSHA eight-hour time-weighted average limit approaches 1 ppm. In addition to the safety risks, this contaminant also leads to piping corrosion, engine pitting, and other infrastructure damage. Many biogas engines specify less than 90 ppm of H2S, and most natural gas pipeline applications less than 4 ppm. Biogas producers use a variety of techniques to remove H2S, including biological desulfurization, dry oxidation and liquid phase oxidation. Oxygen Since production uses anaerobic digestion, the biogas created has little or no oxygen. But some biogas producers add air to enhance the production of sulfur-reducing bacteria and to lower H2S levels. The resulting increase in oxygen and nitrogen can result in an explosive condition, poison the anaerobic bacteria in the digester, or lower the heating value of the biogas. Carbon Dioxide As noted above, carbon dioxide is a natural by-product of anaerobic oxidation of organic waste. However, it lowers the biogas heating value and requires removal if the biogas is going to be used for vehicle fuel or inserted into a natural gas pipeline. In addition to monitoring these contaminants, biogas producers and users also must measure methane and heating value. Methane represents a direct measurement of digester output and the methane fraction must often be optimized for biogas use. In combination with carbon dioxide and oxygen, the CH4/CO2 fraction can also be monitored to confirm proper anaerobic digestion, as well as assure anaerobic conditions and minimal air leakage. The level of heating value determines the commercial value of the biogas and is crucial in pricing the gas for sale. Ammonia Depending on the amount of nitrogen in the feedstock together with the ph and temperature in the digester, biogas may also contain elevated levels of ammonia. This can result in NOX emissions when the biogas is burned in boilers and engines. Water Upon production, biogas is always saturated with water at an elevated temperature. This water vapor can condense in pipelines and corrode infrastructure. Moreover, biogas injected into a natural gas pipeline must contain less than 200 ppm of water. Biogas producers actively remove water via a series of water traps. Monitoring and control of biogas production Los Gatos Research (LGR) AN/ANALYTICAL/005 EN 3

4 Monitoring and control of biogas production Los Gatos Research (LGR) The limitations of current technology Biogas producers and users currently have access to a wide array of different technologies to measure the compounds listed above. Gas chromatography (GC) is used frequently to measure methane, higher hydrocarbons, heating value, carbon dioxide and oxygen. These analyzers require frequent maintenance and consumables with an associated high cost of ownership. Moreover, GC readings typically require 5 to 10 minutes per measurement, making active industrial control difficult. Lead tape sensors are often used specifically for measuring H2S. In this case, lead acetate impregnated tape reacts with hydrogen sulfide to produce a black lead sulfate. This black residue blocks an LED and the reduced light throughput is a measure of H2S concentration. This method is slow (minutes), consumes a lot of lead tape and requires constant maintenance. Similarly, chilled mirror detectors sometimes serve to measure trace water vapor in fuel gases. These sensors rely on condensation of water onto the mirror and the subsequent decrease in light intensity. They suffer from cross contamination from other condensable gases as well as slow response times. Finally, a suite of electrochemical sensors are sometimes used to measure many of the contaminant gases. Although these analyzers are economical, they suffer from significant cross interference and drift, making them impractical for actual industrial usage. In the trial described here, the electrochemical H2S sensor showed considerable drift that extended beyond the alarm threshold. Deployment of the LGR-ICOS process analyzer The LGR-ICOS process analyzer marks a dramatic evolution in the measurement of biogas, natural gas and other fuel gases. This analyzer uses ABB's proprietary laser-based absorption technology called Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS). This technology can measure biogas contaminants and heating values simultaneously with high accuracy at speeds of faster than 15 seconds per reading. The sensor system can be extended to report Wobbe index, relative density, compressibility and theoretical hydrocarbon liquid content. The LGR-ICOS process analyzer was installed for more than 2 1 /2 months at a standalone valve control station sited upstream from a natural gas distribution network. The control station includes monitors that measure H2S, CO2, and O2 levels, as well as heating value and dew point. The biogas typically contains methane levels greater than 96 %, resulting in a heating value greater than 36 MJ/m 3. Contaminant levels are less than 2 % CO2, 10 ppm H2S and 0.4 % O2, with a dew point of less than 40 C ( 40 F). Based on the outcomes of the analysis, the biogas is either injected into the distribution network or recirculated back into the biogas plant for further processing. Such control stations can be adapted to a variety of different biogas applications, including biomatter, agricultural waste, and landfills. 4 AN/ANALYTICAL/005 EN Monitoring and control of biogas production Los Gatos Research (LGR)

5 Figs. 1 and 2 show the measured CO2 and CH4 concentrations respectively by both the LGR-ICOS process analyzer and the gas chromatograph for two days. The values are in excellent agreement over the entire deployment period, with the LGR-ICOS analyzer providing a substantially faster data rate (10 seconds per reading versus minutes per reading). Fig. 3 shows the calculated high heating value (HHV) in MJ/m 3. The two analyzers agree to within 1 % of reading, confirming that the LGR-ICOS process analyzer can measure and report this value. Fig. 3: High heating value measured by the LGR-ICOS process analyzer and the gas chromatograph Fig. 1: CO2 concentration measured by the LGR-ICOS process analyzer and the gas chromatograph Fig. 2: CH4 concentration measured by the LGR-ICOS process analyzer and the gas chromatograph over a 2 day period the readings agree to within 0.5 %. Monitoring and control of biogas production Los Gatos Research (LGR) AN/ANALYTICAL/005 EN 5

6 Monitoring and control of biogas production Los Gatos Research (LGR) Finally, Fig. 4 shows the LGR-ICOS process analyzer compared with an electrochemical sensor for measuring H2S concentration in the biogas. Here the two measurements differ markedly. The LGR-ICOS process analyzer shows a much lower H2S contamination level with only a small spike during the two days. Conversely, the electrochemical sensor shows a higher H2S level, more noise and several spikes, two of which approach the alarm level of 10 ppm for this installation. The electrochemical sensor is reporting erroneous H2S data caused by drift and cross interferences and would exceed the alarm level of 4 ppm for natural gas custody transfer. The electrochemical sensor used for these measurements is known to exhibit drift and cross-interference from other compounds in the natural gas. For example, it is very sensitive to humidity and can lead to erroneous results if the sample stream is too dry for an extended period of time. Presentation of these results to the biogas producer engineers led them to believe that the LGR-ICOS process analyzer provides a substantially more accurate measurement of H2S in the gas stream. This conclusion was further confirmed by artificially adding H2S to the biogas stream and observing an accurate measurement on the LGR-ICOS process analyzer. Customer testimonial Based on this very successful deployment, the customer commented: "We were looking for a potential solution that would yield a faster response time with similar or better accuracy / reliability than traditional gas chromatograph-based applications for monitoring H2S, CO2 and CH4. We found that in ABB's LGR-ICOS process analyzer. The analyzer provided our control system with real-time information and accuracy that was on par with our current gas chromatograph technology without the need for traditional GC carrier gases or maintenance. We will be seriously considering implementing this technology into our valve control station design in future biogas applications." Fig. 4: Measured H2S concentration by the LGR-ICOS process analyzer and the electrochemical sensor 6 AN/ANALYTICAL/005 EN Monitoring and control of biogas production Los Gatos Research (LGR)

7 Notes Monitoring and control of biogas production Los Gatos Research (LGR) AN/ANALYTICAL/005 EN 7

8 Contact us ABB, Inc. Process Automation 3055 Orchard Drive San Jose CA USA Tel: Fax: / measurement Note We reserve the right to make technical changes or modify the contents of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB does not accept any responsibility whatsoever for potential errors or possible lack of information in this document. We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents in whole or in parts is forbidden without prior written consent of ABB. Copyright 2016 ABB All rights reserved AN/ANALYTICAL/005 EN