The biogas generated by the digesters can be used in two ways. One is cleaning the gas through the gas clean-up plant and exporting the natural gas (CH4) into the gas grid, the other way is fuelling a CHP.
The biogas produced from the digesters can be injected into the national gas grid once certain contaminants and CO2 have been removed. The CH4 produced has to meet the national calorific value which changes daily. To achieve this we inject a small amount of propane.
A dutch company, DMT, has developed a membrane system that upgrades the biogas, cleaning the carbon dioxide from the methane. The DMT plant upgrades the gas up to 99% methane.
The gas emenating from the biodigesters is comprised of:
|Methane||45 – 70||–||mol %|
|Carbon dioxide||30 – 45||≤ 2||mol %|
|Hydrogen sulfide||≤ 2000||≤ 5||mg/m3 (STP)|
|Ammonia||≤ 1000||0||mg/m3 (STP)|
|Oxygen||≤ 2||≤ 0.5||mol %|
|Nitrogen||≤ 8||≤ 5||mol %|
|Water||37 at 1 bar||≤ 8 at 40 bar||°C|
|Combustion heat||6.7 – 8.4||10.7-12.8||kWh/m3(STP)|
|Wobbe index||6.9 – 9.5||13.3 – 15.7||kWh/m3(STP)|
The aim of the process is to remove most of the impurities from biogas thus obtaining a biogas with a concentration of >97% methane. Pure methane has a calorific value of 9100 kcal/m3 at 15,5°C and 1 atm.
In general, biogas treatment involves a cleaning process and an upgrading process. The cleaning process is to minimize corrosion problems by removing H2S, H2O and other trace components; which could corrode engines, pipelines; and is harmful to the natural gas grid. The upgrading process is mainly based on the CO2 separation, in order to adjust the calorific value and promote it to pseudo-natural gas quality.
Once the gas is clean and all the impurities have been removed in what is called the pretreatment, the gas goes to the second stage.
- The DMT is formed by two units and each one can upgrade 1000 m3/h of biogas.
The removal of CO2 from biogas can be performed by applying adsorption, absorption, cryogenic method, and membrane gas separation. The first three methods are more traditional compared to the membrane separation which has certain advantages such as low cost, high energy efficiency, easy operation, modular state and flexibility.
Once the gas is cleaned and compressed it passes to the multi-stage membrane system which in this case, is configured in three stages. The CO2 and CH4 are seperated because the gasses have different molecular sizes and the membrane system acts like a sieve.
- The last step is the propane injection to achieve the correct calorific value before it goes to the NEF (Network Entry Facility). The NEF has a chromatograph (gas analyser) that determines whether the gas is within specification. If it is, the ROV valve is opened allowing the gas to flow into the grid, when a little bit of odorizer is added. If the ROV should close the gas is diverted to the flare awaiting resumption of quality.