Biogas Desulfurization at ETAR do Norte (Portugal)
The Wastewater Treatment Plant (ETAR) do Norte is one of the largest in the central region of Portugal, serving several municipalities. The infrastructure is located in the area of Coimbra, Leiria, occupying 6 hectares of land; it receives effluents from about 250,000 inhabitants of several municipalities.
It has the capacity to treat some 38,000 m3 of wastewater daily, of which 77% is domestic, 18% from industry and 5% is other effluents.
The biogas resulting from treatment is used as fuel for a cogeneration plant, where energy recovery is maximized through combined heat and power production. The thermal energy is used to heat the sludge digesters, the building and the domestic hot water. The electrical energy produced, which can reach an output of 1.2 MW, is sold to the national power grid.
Biogas Desulfurization
Iron filings, one of the most widely used traditional methods, were used to reuse biogas for energy recovery. However, high construction costs and low efficiency led Simlis to look for other technologies to improve its process.
As an alternative, BION together with its distributor in Portugal, Equifluxo, proposed the use of BION Fe, a filter media regenerable with ambient air, capable of reaching a H2S removal capacity of up to 45% w/w. The use of this media would provide the following advantages over iron filings:
Save adding an iron chloride solution as a step prior to desulfurization.
Reduce the number of media changes due to its high desulfurization capacity (45% w/w of BION Fe versus 12% w/w of iron filings).
Achieve higher efficiencies.
Reduce energy consumption due to the low pressure drop values generated.
Depending on the final composition of the spent product and the regulations of each country, BION Fe could even be recoverable.
On the other hand, the removal of water from the biogas, besides serving to promote the optimal operation of BiON Fe, has other advantages. The drying of the gas allows optimizing the combustion process in the cogeneration engine, reaching values between 2 and 5% more energy generation; it increases the efficiency of the engine and reduces the gas/fuel consumption; it allows reducing the contamination of the engine oil with the condensate, extending the life time of the oil from 400-500 hours to 2000-3000 hours.
Finally, in the case of treating biogas with siloxanes, this dehumidification stage will help to slightly reduce the concentration of some of these recalcitrant compounds.
Bioconservacion Solution
BION designed a reactor for biogas treatment with the following characteristics:
- Flow rate: 25 m3/h
- Temperature range: 35-40 ºC
- Relative humidity: 95-99 %.
- Composition: CH4 66 %, CO2 30 %, H2S 2500 ppm
To carry out biogas desulfurization, Bioconservacion/Equifluxo designed a very versatile unit with a double fiberglass wall to reduce the effects of condensation, with a plenum that avoids the generation of preferential paths and therefore the use of the entire medium, as well as the generation of high pressure drop values.
Over a period of 12 months, Bioconservacion/Equifluxo carried out exhaustive monitoring of the equipment's results, with technical visits every 45 days for the first 3 months and every 3 months thereafter.
The results obtained were very positive, reaching efficiencies of over 99%, allowing full use of the energetic power of biogas (Figure 2).
Evolution of H2S concentration at the reactor inlet/outlet over time.