by Bioconservacion August 16th, 2016 0 comments
The Northern Wastewater Treatment Plant (WWTP) is one of the biggest in the central region of Portugal, servicing various municipalities. The infrastructure is located in the area of Coimbra, Leiria, taking up 6 hectares of land; it receives effluents from some 250,000 inhabitants across multiple municipalities. 
It has the capacity to treat around 38,000m3 of wastewater on a daily basis, 77% of which is domestic, 18% industrial and 5% corresponding with other types of effluents.
The resulting biogas from this treatment is used as a fuel for a cogeneration plant, where energy recuperation is maximised through a combined production of heat and electricity.  Thermal power is used to heat the sludge digesters, the building and the hot water supply. The electrical energy produced, which can be up to 1.2 MW, is sold to the national grid. 
Biogas desulphurisation
To reuse biogas energy iron filings is used; one of the most widely practiced traditional methods. However, the high labour costs and low efficiency led to Simlis searching for other technologies to improve this process
As an alternative, Bioconservacion, together with its distributor in Portugal, Equifluxo, posed the use of Bi-On Fe, through a renewable filter with ambient air, capable of reaching an elimination capacity of H2S of up to 45% p/p. The use of this method would provide the following advantages over iron filings: 
  • There is no need to add an iron chloride solution as a step prior to desulphurisation.
  •  It would reduce the average number of changes thanks to the high desulphurisation capacity (45% p/p of the Bi-On Fe versus 12% p/p of iron filings).
  • There would be higher efficiency rates.
  • It would reduce energy consumption thanks to the low pressure drop values. 
  • Depending on the final composition of the product spent and the regulations of each country, Bi-On Fe could even be renewable. 
Furthermore, the elimination of biogas water, as well as favouring the optimal operability of Bi-On Fe, has other advantages. Gas drying allows for the optimisation of the combustion process in the cogeneration engine, reaching increased energy generation values of between 2% and 5%; it increases the motor's efficiency and reduces the consumption of gas/fuel; it allows for the reduced contamination of motor oil with condensate, prolonging the service life of the oil from 400-500 hours to 2000-3000 hours. Finally, it should be highlighted that, when treating biogas with siloxanes, this dehumidification stage will help to lightly decrease the concentration of some of these obstinate compounds.
Bioconservacion solution
Bioconservacion designed a reactor for the treatment of biogas with the following characteristics:
  • Flow rate: 25 m3/h
  • Temperanture range: 35-40 ºC
  • Humedad relativa: 95-99 %
  • Composition: CH4 66%, CO2 30%, H2S 2500 ppm
In order to carry out the desulphurisation of Biogas, Bioconservacion/Equifluxo designed a highly versatile piece of equipment with a double glass-fibre wall to reduce the effects of condensation, with a plenum that avoids the creation of preferred paths, and thus makes the most of the whole system, as well as generating high pressure drop values. 
Desulphurisation system in ETAR do Norte ( Portugal)
Over a 12-month period, Bioconservacion/Equifluxo carried out exhaustive monitoring of the equipment's results, with technical visits every 45 days during the first 3 months and, following this, every 3 months. 
The results obtained were highly positive, with efficiency levels of more than 99%, allowing for the energetic power of Biogas to be fully harnessed.
Fig. 2 Evolution of H2S concentration  at the entrance / exit of the reactor