No Separation Anxiety
Biogas upgrading and the production of biomethane is a state-of-the-art-process of gas separation, and a number of proven technologies currently exist to fulfill the task of producing a biomethane stream of sufficient quality. These commercially available technologies have proven to be both technically and economically feasible.
Lars-Evert Karlsson, global product line manager for Bremen, Germany-based Purac Puregas, which designs and delivers biogas upgrading plants, says biogas production is growing around the world and there is an increasing demand for upgraded biogas to be used as vehicle fuel or injected to the natural gas grid. “To enable the efficient use of biogas in these applications, the gas must be upgraded—for example, the carbon dioxide, which constitutes a large part of the raw biogas from the digester, must be separated from the methane,” he says. “Methods commercially available today include amine scrubbers, water scrubbers, pressure swing adsorption (PSA) units, organic scrubbers and membrane units.”
Ricardo Hamdan, management consultant for Greenlane Biogas Limited, Vancouver, British Columbia, says amine and cryogenic methods are also used, but are less common than water scrubbing, pressure swing absorption and membranes due to their higher cost. A recent Swedish Gas Technology Centre (SGC) report written by Fredric Bauer, Christian Hulteberg, Tobias Persson and Daniel Tamm shows that for midscale applications, the most common options are all viable. “The scrubbing technologies all perform well and have similar costs of investment and operation,” says Karlsson, who contributed to the SGC report. “The simplicity and reliability of the water scrubber has made this the preferred choice in many applications, but the high purity and very low methane slip from amine scrubbers are important characteristics.”
The SGC report finds that the investment costs for PSA and membrane units are about the same as they are for scrubbers, yet recent developments of the membrane units have made it possible to reach low methane slips.
Understanding the Choices
Hamdan and Greenline contributed to the recent American Biogas Council’s interactive report, “Biogas to Biomethane/Renewable Natural Gas,” which explains how each of the methods work. “It is an OPEX vs. CAPEX game,” he says. “Most are comparable, but while membranes are the least expensive on capex, the operating cost is way higher than the water scrubber or PSA, for example.”
Stephanie Thorson, business development leader with the Biogas Association, explains that the biogas will include substances that will need to be removed in order to inject it into the pipeline, including carbon dioxide, water, hydrogen sulfide, oxygen, nitrogen, ammonia, siloxanes and particles. “Concentrations depend on the compositions of the substrates used to create the biogas,” she says. “To prevent corrosion and mechanical wear of the equipment, it can be advantageous to clean the gas before upgrading.”
The most widely used technologies for biogas upgrading are the following, as described by the International Gas Union.
Pressure swing adsorption: This technology purifies the gas by way of adsorption of impurities on active coal or zeolites.
Physical absorption: Water or another liquid such as alcohol can be used to bind carbon dioxide. This is called water scrubbing or pressurized water wash.
Chemical absorption: Chemical absorption is comparable to water absorption. A liquid such as amine is chemically bonded to the carbon dioxide. In order to recycle the solution, a heat treatment is applied.
Membrane separation: Methane can be separated from carbon dioxide using semipermeable membranes. The force can be a pressure difference, a concentration gradient, or an electrical potential difference.
Cryogenic separation: Trace gases and carbon dioxide are removed by cooling down the gas in various temperature steps.
“Because of the high cost of upgrading, it is important to choose a system that has low energy consumption and high efficiency, giving high methane content in the upgraded gas,” according to Thorson, who authored “Farm to Fuel: Developers’ Guide to Biomethane,” to help farmers determine if biomethane production is a good fit for their farm and operations. “The best technology choice is based on the parameters of your plant, such as the prices of electricity and heat. It is possible to lower the methane loss, but at the expense of higher energy consumption.”
For biomethane projects, the size, level of automation and the complexity of the system determines the amount of hours per week required to operate the system. This can range from one part-time operator for several hours each day to full-time operators. Arthur Wellinger, managing director of Triple E&M, an internationally operating consulting company located in Aadorf, Switzerland, and general manager of the Swiss Biomass Association, says in Europe, water washing is the most commonly used method.
“It is—next to PSA—the oldest technology and still reasonably good,” he says. “Other technologies are increasing fast like chemical absorption and membranes. Chemical absorption has the lowest methane emission without any further treatment with the lowest electricity consumption, however, a high heat consumption that has to be produced in most cases by renewable energy.”
Just as the methods are different, according to Hamdan, the equipment needed for each technology is drastically different. “Most will have a booster blower, a compressor to work under pressure, vessels with an absorber (either water or a chemical), dryers and a booster compressor to send to either pipeline or vehicles,” he says.
Looking at amine scrubbing, Karlsson says the use of reactive systems for removing CO2 from biogas is not a brand new notion, but it is less common compared to other technologies such as PSA and water scrubbing. “The synopsis of features of the technology is to use a reagent that chemically binds to the CO2 molecule, removing it from the gas,” he says. “This is most commonly performed using a water solution of amines (molecules with carbon and nitrogen), with the reaction product being either in the molecular or ion form.”
The technology consists of an absorber, in which the CO2 is removed from the biogas, and a stripper, in which the CO2 is removed from the amine solution.
While not yet commercially available, Karlsson notes that new process designs have been suggested, in which double absorption columns will be used, one of which is pressurized to increase the solubility of carbon dioxide in the solvent and thus increase the separation of the gases.
As described in the SGC report, pressure swing adsorption is a dry method used to separate gases via physical properties. Explaining PSA on a macro level, the raw biogas is compressed to an elevated pressure, and then fed into an adsorption column, which retains the carbon dioxide but not the methane. When the column material is saturated with carbon dioxide, the pressure is released, and the carbon dioxide can be desorbed and led into an offgas stream. For a continuous production, several columns are needed as they will be closed and opened consecutively. PSA unit characteristics include feeding pressure, purging pressure, adsorbent, cycle time and column interconnectedness among other things.
Karlsson says that a common design for PSA units includes four columns with one of the columns always engaged in adsorption, while the other three are in different phases of regeneration.
Membrane separation has been around since 1990, and the ability to combine high methane recovery with high methane concentration requires selective membranes and suitable design, as the membranes used for biogas upgrading retain most of the methane, while most of the carbon dioxide permeates through the membrane.
According to “Membrane Technology and Applications,” written by R.W. Baker, the permeation rate through a typical membrane (made of a glassy polymer) used in biogas applications is mainly depending on the size of the molecules, but also on the hydrophilicity.
There are several membranes on the market today used for biogas upgrading, including two types of polymeric (glassy polymers), hollow fiber membranes (Air Liquide MedalTM and Evonik Sepuran) and one carbon membrane (manufactured by MemfoACT AS). The membranes are continuously improved to get higher selectivity, higher permeability and cheaper manufacturing.
Hamdan describes a water scrubber as a physical scrubber that uses the fact that carbon dioxide has much higher solubility than methane in water. In a water scrubber, carbon dioxide is separated from the raw biogas and dissolved into the water in the absorption column by using high pressure, normally 6 to 10 bar. Carbon dioxide is then released from the water again in the desorption column by addition of air at atmospheric pressure. Some water scrubbers are also equipped with a heat recovery system that can be used to heat the digester.
Pros and Cons
Hamdan emphasizes that regardless of the technology chosen, it could fit in a wastewater treatment plant, a large landfill, or a simple farm. “The versatility of the product is important. Also the market potential, how big it is and how the switch to natural gas in transportation fuel and the deployment of this natural gas infrastructure will help biogas projects flourish as customers find value on renewable natural gas—the purified biogas, which is equivalent in composition with regular natural gas,” he says.“In the U.S., transportation fuel and the RIN market is driving the deployment of RNG.”
He explains the pros and cons of each method as follows:
Physical Solvent (other than water): “The pros are a high absorption rate, high-CH4 yields are possible, and it can deliver biomethane at low pressure. The cons include the solvent is dangerous to handle, it’s complex with a difficult control system, there’s a prohibitive capital cost for new equipment and the biogas/landfill gas contaminants cause foaming.”
Physical Membrane (high-pressure process): “The benefits include its low capital cost, simple plant and experience upgrading LFG. Meanwhile, the disadvantages are a low biomethane purity, high energy consumption and membranes foul and require replacement.”
PSA/VSA (pressure swing absorption, vacuum swing adsorption): “The good things about this method are it can remove some inert gasses, often with an additional process module and a low-efficiency version is cost-effective for the small-scale operator. Cons include the media could become fouled and require replacement, problems maintaining a high CH4 recovery, bed fluidization causes “dusting” of media, and upstream H2S removal is required.”
Water Scrubbing: “The pros are that it offers excellent safety and proven performance, it’s reliable, simple and easy to maintain, it has low capital and operating costs, siloxanes are effectively removed and it can take high levels of H2S. The bad is that it cannot remove inerts such as oxygen and nitrogen.”
Hybrid Water Scrubbing plus VPSA: “Benefits for this method are it can remove inert gasses, is reliable and easy to maintain, can meet stringent regulation—such as California’s Rule 30—and has excellent safety and proven performance. The biggest disadvantage is that it requires a larger footprint.”
In the report, “Biogas to Biomethane,” prepared by the Vienna University of Technology, it is determined that providing a universally valid comparison of the different biogas upgrading technologies is difficult, because the multitude of essential parameters strongly depend on local circumstances. Furthermore, the technical possibilities of a certain technology (for example, regarding the achievable biomethane quality) often do not correspond with the most economic operation.
“The technical development of most biogas upgrading methods nowadays is typically sufficient to meet any needs of a potential plant operator,” the report states. “It’s only a question of finding a plant design providing the most economic operation of biomethane production. As a result, it is strongly recommended to perform a detailed analysis of the specific biomethane costs to be expected and to account for all possible upgrading technologies.”
Author: Keith Loria
Freelance Writer, Biomass Magazine