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Biogas purification at Mapocho WWTP

2021-10-04 11:34:03

Biogas is a mixture of gases, the main components of which are methane and carbon dioxide, produced as a result of the fermentation of organic matter in the absence of air by the action of a microbial consortium (anaerobic digestion).

 

The composition of the biogas depends entirely on the nature of the waste used and the conditions under which anaerobic digestion is carried out. Generally, the biogas is used for energy recovery, which is determined by the methane concentration. However, a purification of certain contaminants with abrasive and corrosive character, such as siloxanes and H2S, is necessary to ensure a correct operation of the cogeneration engines.

 

The Mapocho-Trebal wastewater treatment plant (WWTP), located in Santiago de Chile, treats 30% of the town's water and is a benchmark facility. The facility uses biogas energy by means of three cogeneration engines, each with a power of 1,000 kW. Normally two of these are in operation, while the third is reserved for maintenance stops.

 

The strict legislation of the trans-Andean country regarding emissions of volatile organic compounds (VOCs) and nitrogen oxides, and the need to obtain an effluent free of mainly siloxanes and H2S for a correct use of the purified biogas, led those responsible for the WWTP to contact BION.

Our technical department studied the case in depth and proposed to implement a competitive alternative both in economic and technical terms.

 

Plant data

 

The biogas purification system consists of two parallel analogous lines composed of three filters in series.

  • Biogas flow rate: 3600 m3/h.
  • H2S concentration: H2S concentrations, due to biogas pre-treatment, range from 40 to 60 ppmv. In specific cases where the pre-treatment does not work properly, peaks of up to 500 ppmv have been measured. The objective is to reduce these values to concentrations below 1 ppmv.
  • Siloxanes: An exhaustive characterization of the siloxanes present in the biogas stream revealed significant concentrations of cyclic siloxanes type D3, D4 and D5 among others. Values of up to 10 mg/m3 were measured for the D5 type. 
  • Others: in addition to the compounds, significant concentrations of mercaptans, cycloalkanes, chlorinated compounds and aromatic hydrocarbons were measured.

Solution

 

After a thorough characterization of the biogas to be treated and an assessment of the working conditions, the solution proposed by BION was as follows:

 

High quality Active Carbon for the removal of high molecular weight VOCs and siloxanes.

High quality Bion KOH to improve the removal of H2S, certain VOCs and mercaptans.

The solution proposed by BIOCONSERVACION was successful, generating biogas with H2S levels below 1 ppm.

 

BION's collaboration continues once the solution has been implemented and the results have been verified, since it has the possibility of carrying out an analysis of the condition of the socks prior to their replacement. By evaluating the remaining capacity of the media, it is able to optimize the process to achieve maximum utilization of the potential of each installation.

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