Increasing profits for low-cost biogas producers

Sept. 24, 2015 - PRLog -- A biomethane plant from Alfagy, a UK renewable energy company, is delivering cost effective solutions for converting biogas into natural gas. The upgraded gas can be injected into the natural gas grid and pumped into homes and restaurants for use in heating systems and cookers.

Alfagy’s bio-methane plant has more than 35,000 hours (more than 4 years) of operation at 99% uptime with minimal maintenance costs. In this industry, this level of performance is considered well demonstrated. Other solutions from Sweden and the USA are sold at comparatively equal prices, but the operational costs are very different compared to the UK solution. As such, Alfagy’s upgrading plant produces one m² for half the cost of some other systems. The low cost is achieved by a technology relying on power consumption alone without expensive chemical or biological additives and membranes.

‘Our plant provides a more profitable business plan for new biogas-to-grid producers,’ says Peter Kindt, chairman of Alfagy. ‘The low cost producer will not only earn more but could also become the market leader. Meanwhile this technology allows for greener living in urban areas.’

Power consumption, the main operational cost, of the Alfagy’s plant is reduced to 0.154kWh per m³ feed gas. This low electrical consumption includes the pre-treatment of the feed biogas and the oxidisation of the off-gas.

Biogas purification through adsorption

Methane is concentrated from biogas by using molecular sieves as adsorption agents. They adsorb nitrogen, water vapour, and carbon dioxide to a much higher degree than methane. This means that a product flow comprising essentially only methane can be removed from the process air being passed through the adsorption bed. The compressed biogas enters the adsorbers and the carbon dioxide is adsorbed while the methane product leaves the vessels. After a certain time the adsorption is interrupted and evacuation by a vacuum pump desorbs the enriched carbon dioxide. The biogas product flow is compressed to the required discharge pressure. The product gas has a dew point of -65°C and is therefore ready for grid injection without a gas dryer.

Generally, higher recovery leads to more biomethane or synthetic natural gas in the grid and the size of the equipment is given by the required feed. Alfagy offers a slow PSA system, because the molecules need time to enter the sieve and time to get out as well. A small and fast system deliver poor biomethane production and worse financial results.

The upgrading process

The detailed process of upgrading biogases from landfills and digesters is simple, cheap and, environmentally friendly. The biogas feed contains mainly methane (CH₄), carbon dioxide (CO₂), nitrogen (N₂), oxygen (O₂), hydrogen sulphide (H₂S), water (H₂O), traces of hydrocarbons, and hydrochlorofluorocarbons (HCFCs). In order to upgrade the biogas to a natural gas (NG) quality the H₂S, HCFCs, and most of the CO₂ need to be removed. The Alfagy process uses activated carbons and molecular sieves for removal of the impurities.

The raw biogas is collected from the landfill or the digester plant in a main header. The biogas is fed with a blower at approximately 20-40 mbarg to the upgrading plant. The first step is cooling and then reheating for ideal desulphurisation before the gas is led through two adsorbers in line with the activated coal filter to remove H₂S to near 0ppm.

The removal of H₂S will be achieved in a chemisorption process with activated carbon. The activated carbon is impregnated with a catalyst which helps the process. The H₂S in the gas reacts to elemental sulphur according to the following reaction: 8H₂S + 4O2 --> S₈ + 8H₂O. The elemental sulphur is adsorbed by the activated carbon. The biogas is then cooled to about 5°C. During cooling, the available water will condensate and is separated in a water separator.

The Alfagy’s system applies pressurised gas to the separation process and also applies a vacuum to the purge gas. This system is amongst the most efficient systems available, measured on customary industry performance indicators, such as recovery - product gas out/product gas in - and productivity - product gas out/mass of sieve material – which leads to much lower production costs.

The CO₂ is adsorbed on a molecular sieve and the methane achieves natural gas quality. After a certain cycle time, the adsorber vessel is saturated and a regeneration step is required. The regeneration is achieved by reducing the pressure to below atmosphere with a vacuum pump. At this low pressure, the CO₂ will desorb from the molecular sieve and will be sent to a regenerative thermal oxidiser (RTO) and a clean exhaust without any harmful greenhouse gasses is blown to the atmosphere. In order to improve the methane yield, one or two pressure equalisation steps are introduced. With Alfagy’s technology, the methane recovery figures can be reached up to 98%. Methane content in the product gas is strongly dependent on the feed gas concentration.

Since natural gas is an odourless gas, an odorant need to be added for safety reasons. In Europe, the most commonly used odorant is tetrahydrothiophane (THT) or tertiary butyl mercaptan (TBM) in the UK.

Alfagy is a leading global supplier of biogas related technology in gas conditioning, power- and biomethane upgrade plants.

For more information:

Alfagy UK London  +44 208 123 7400
Alfagy USA East Coast  +1 (718) 618-9710
Alfagy France  +33 970 465 100
Alfagy Scandinavia  +45 65 74 57 00
Alfagy South Africa  +27 11 0836000

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