Biogas desulphurization and siloxanes abatement. Case study : MCFC project by CETAGUA (Mataró – Spain)
Reducing the carbon footprint of our society is imperative. This can be achieved by capturing and confining anthropogenic CO2 emissions as well as by replacing fossil-based fuels with renewable or waste-derived fuels. MCFCs (Molten Carbonate Fuel Cells) are unique in being able to do both these things. Thanks to their operating principle, CO2 can be extracted from a gas stream on the cathode side and hydrocarbon fuels like biogas can be converted to electricity on the anode side.
However, the degradation caused by the contaminants in these gases must be addressed. In this sence, the project “MCFC catalyst and stack component degradation and lifetime: Fuel Gas Contaminant effects and Extraction strategies” aims to tackle this problem from two sides:
1) investigating poisoning mechanisms caused by alternative fuels and applications and determining precisely MCFC tolerance limits for long-term endurance;
2) optimizing fuel and gas cleaning to achieve tailored degrees of purification according to MCFC operating conditions and tolerance.
CETaqua (Centro Tecnológico del Agua), a company belonging to Suez group was involved in the project, where one of the main objectives was to test at pilot scale innovative adsorbent materials for biogas treatment alternative to conventional activated carbons which were being used at most of the wastewater treatment plants (WWTPs).
Figure 1. Biogas purification system located in the WWTP in Mataró
Thus, CETaqua contacted with Bioconservacion and carried out the project in a WWTP located in Mataró (Barcelona). Biogas purification was performed in a two steps system. The system was composed by two identical lines composed by a first vessel packed with 3mm Bi-On SIGMA (before Bi-On Fe) aim to eliminate H2S and a second step with a vessel packed with 4 mm Bi-On AC for VOCs and siloxanes removal (Figure 1).
The tests last around half a year and the results were assessed at a rate of 10Nm3/h, while the pollutants concentrations were in a range between 700 to 2000 ppmv and 4-5 mg/m3 for H2S and siloxanes, respectively.
Bioconservacion proposal results
In reference to H2S elimination, average results are summarized in Figure 2.
Figure 2. H2S concentrations along the purification system.
As it can be seen, H2S was successfully eliminated in both lines and just during certain episodes where peaks were measured, significant amounts of H2S were detected at the outlet of the first step but always maintaining removal efficiency values over 95% (Figures 2). Either way, the second vessel packed with Bi-On AC, not only eliminated siloxanes and VOCs, but acted as a polishing step which led to negligible H2S concentrations after the overall purification system.
Figure 3 clearly shows that Bi-On SIGMA was exhausted, since both colors can be distinguished before and after use.
Figure 3. DS B packed with Bi-On Fe (a b). Bi-On Fe in DS B (c). DS A packed with Bi-On Fe (d). Bi-On Fe in DS B (upper part).
Regarding siloxanes, Bi-On AC hardly achieved removal efficiency values around 30%. Even it might seem a low media performance, it exists a wide variety of siloxanes and not all of them are effectively removed with the same carbon quality. Thus, depending on the siloxane that should be removed one or other type of carbon would be suggested. Considering that no siloxanes speciation was done, results obtained are satisfactory.
Summarizing, biogas purification system performed successfully and biogas could be employed at the fuel cells.