Lems, R. and Dirkse, E., DMT Environmental Technology, The Netherlands(free)
More and more effort is being put in the utilization of bio-solids. Bio-solids can be an important source for useful products like fertilizers and biogas. DMT has been developing biogas treatment plants for over 20 years following the market developments closely.
Biogas was first seen as a nuisance at e.g. landfills, creating odor problems and methane emissions. A flare is, therefore, a cheap and simple solution. But with time more and more biogas was produced intentional from bio-solids to generate heat and/or energy in a boiler, gas engine or Combined Heat and Power unit (CHP).
The next step is upgrading biogas to natural gas quality to utilize the maximum amount of energy. The “green gas” can be injected in the gas grid or used as vehicle fuel in e.g. cars, busses, ferries and agricultural machinery.
In this article a presentation is given about the start-up and first results of two biogas upgrading plants. One full-scale plant in Zwolle (NL), where 550Nm3/hr of biogas, produced by digesting of bio-solids, is upgraded to natural gas and injected in the national grid. The other plant is a small scale unit in Zalaegerszeg (HU) at a WWTP where 85Nm3/hr of biogas is upgraded to over 97% methane concentration and, thereafter, compressed to 220bar. The compressed biogas (CBG) is used as vehicle fuel for the company’s fleet of cars.
Biogas, Bio-methane, CBG, CO2 removal, Green gas, Grid injection, Water scrubbing, Renewable energy, Upgrading.
Depletion of fossil fuels causes demand for renewable fuels. Biogas is one of the most promising green fuels because it is produced out of waste streams. The use of biogas causes no global CO2 emission and hardly any local emissions on e.g. fine dust and NOx. This makes it one of the cleanest fuels on the market (CROB 2007; Jönsson and Persson 2007). Another important fact about biogas is that it can be stored quite easy compared to wind or water energy. Nowadays most biogas is burned in gas engines to produce electricity and heat in which energy losses up to 60% are common when the heat can’t be used in full potential (Hegaret et al. 2008). By upgrading biogas this energy loss can be significantly reduced. Moreover, potential utilization of biogas is enhanced. Upgraded gas is comparable to natural gas and therefore it can be injected into the gas grid or used as vehicle fuel.
Upgrading of biogas is done by increasing the methane concentration of biogas by reducing the concentration of CO2. Various techniques can be used to remove CO2 from biogas of which pressurized water scrubbing has many advantages over the other processes, being: simple, robust, small footprint, flexible in treating fluctuating incoming flows, high energy efficiency, low investment and operational cost (Hullu et al. 2008; Weidner et al. 2008).
The driving force of the process is the difference of solubility for CH4 and CO2 in water. CO2 dissolves reasonable good in water were CH4 does not. By elevating the process pressure this difference becomes bigger and the CO2 is absorbed more quickly. An advantage of water scrubbing is that H2S which is often present in biogas is washed out as well. Figure 2 shows the theoretical solubility of CH4, CO2 and H2S at various pressures (Perry 1984; Lide 1990). The used process water can be regenerated by stripping with air at atmospheric pressure in which case the solubility of CO2 and H2S decrease again.