BioCycle July 2009, Vol. 50, No. 7, p. 57
Reasonably steady gas flows are an important component of a gas treatment operation.
AT the Greenwood Farms Landfill in Tyler, Texas, methane gas previously flared is now collected, upgraded and injected into natural gas pipelines. The facility, officially opened in early June, is collecting and treating 1,200-scf/minute of landfill gas to produce 800,000-scf/day of methane. Gas for the system is collected from 60 wells, averaging 80 feet deep, over a 60-acre area.
Southtex Renewables in Midland, Texas, designed, built, owns and operates the gas treatment facility at Tyler and three other landfill gas plants around the country. The company is part of Southtex, a 25-year old company that is one of the largest manufacturers of gas treatment plants in the country.
As with biogas from an anaerobic digester, cleanup of landfill gas requires the removal of hydrogen sulfide (H2S), water and carbon dioxide (CO2). “Landfills actually have more CO2 than digesters,” explains Luke Morrow, president of Southtex Renewables. Unlike digester gas, treatment systems for landfill gas need to remove nitrogen and oxygen as well. “In a landfill, you are going to pull 40 to 60 inches of water vacuum on PVC wells that are put down into the landfill,” he says. “So there is a potential to pull air through the system, which is made up of nitrogen and oxygen.” Other impurities, such as hydrocarbons from items such as paint thinners disposed in the landfill, also need to be stripped from the gas.
The treatment process starts by compressing the gas and running it through sulfur treatment beds to remove H2S and other sulfur species. “Then the gas goes through our pretreatment system where it is stripped of any remaining sulfur species, hydrocarbons and potentially any halogens or VOCs,” Morrow adds. Impurities stripped from the gas are incinerated. In the final step, the gas is sent to a physical solvent-based treatment system, to remove CO2. “You are left with a stream that is 95 to 96 percent methane that is pipeline quality,” he says.
Southtex also builds systems that remove CO2 with an amine treatment. These use aqueous solutions of various alkanolamines. “We use a physical solvent any time you have more than 20 to 25 percent CO2 and oxygen,” says Paul Morrow, president of Southtex Treaters.
Amine treatment was employed for a large-scale digester gas processing system designed by Southtex two years ago. “We did not have the oxygen and we were able to treat the gas at a low pressure to avoid a lot of compression,” he explains.
KEYS TO SUCCESSFUL TREATMENT
Southtex is applying its experience and expertise treating natural gas to treating renewable gas streams. Runtime for Southtex’s landfill facilities is better than 97 percent, well above the industry average. The key to a successful gas treatment, whether it is at a landfill or a digester, is having people experienced with start-up and the ongoing operation of the plant, Luke Morrow says. “That is why we get the high run time.”
Adds Paul: “An operating facility it is not like a dishwasher. You just don’t turn it on and everything works. It takes a lot of applied knowledge and experience to run these things efficiently and effectively.”
Reasonably steady gas flows are another important component of a gas treatment operation. “Our treatment plants are built to run essentially at a steady-state operation consistent with the gas stream composition and volume,” Morrow explains. “With landfills, there will be some variance due to temperature, humidity and barometric pressure, but gas flows are reasonably consistent and do not swing widely.”
Diane Greer is a Contributing Editor to BioCycle.
Sidebar p. 58
Hydrogen Sulfide Removal Using Digester Effluent
A NEW biogas treatment system uses oxygenated effluent as the reactor medium to remove hydrogen sulfide from the gas stream. The BioScrub-130, manufactured by Energy Cube, LLC, was installed at the Tollenaar Dairy in Elk Grove, California to reduce sulfur dioxide emissions and extend the life cycle of its power generating equipment. Tollenaar has an in-ground heated concrete digester with a floating cover, supplied by RCM Digesters. As the biogas passes through the BioScrub reactor, the H2S is absorbed into the activated effluent and converted to elemental sulfur. “The product of this chemical reaction is a suspended sulfur solid which is flushed back to the lagoon,” explains Jere Martin of Energy Cube. Ambient air is used to pump oxygen into the effluent in the first chamber of the unit; in the second chamber, biogas is pumped through the oxygenated effluent. “It is important to note that oxygen is not injected directly into the biogas stream,” adds Martin. “The 2-stage design requires less safety equipment and prevents excessive oxygen in the scrubbed biogas.”
The BioScrub-130 (130 cfm biogas) is 7-feet wide, 27-feet long and 7-feet high. A 300 cfm unit is being developed. The effluent is cycled at the rate of a few gallons/minute to keep the sulfur flushed from the system. “This varies with the biogas volume and the H2S levels we are treating,” he explains. “We monitor the oxygen content and pH to determine the refresh rate.” Depending on the effluent quality, there is a potential for foaming. At Tollenaar Dairy, this is controlled with a foam trap, but could also be done by adding a defoaming agent.
July 21, 2009 | General
Landfill Gas Treatment Innovations
BioCycle July 2009, Vol. 50, No. 7, p. 57