GAS TREATMENT AND CONDITIONING IN SOUTH AFRICA

Until recently waste gas from landfills, biogas from anaerobic treatment processes and other combustible off-gases were flared or even just allowed to vent to atmosphere. This has changed over the last 30 years in European and developed countries but it is only quite recently that South Africa is beginning to see the potential of generating electricity from waste gasses.

Contaminants in Biogas

Typical biogas consists mainly of about 60 plus % Methane and 30 plus % Carbon Dioxide but when it comes to combusting this gas as a fuel for electricity generation (whether it be co-generation or some other form of CHP plant in reciprocating gas engine or micro turbine) it is important to ensure contaminants found within your particular gas stream are within the engine manufacturers’ specifications. Among other things, gas impurities affect the maintenance intervals and engine oil life – which are two of the the main expenses when it comes to generating electricity from gas.

Dealing with gas impurities

The first thing to do is ensure you conduct a detailed gas analysis to establish and quantify the gas conditions for your particular site. You need to take into consideration the required quality of the gas at point-of-use, following which, the relevant gas conditioning equipment can be used to achieve the desired affect – depending on your contaminants:

HYDROGEN SULPHIDE (H2S)

Depending on your level of H2S in the gas you can either remove this via means of filtration or by biological and sometimes chemical removal. Which method to use depends largely on gas flows and costs but a general “rule-of-thumb” is gas containing H2S of below 600ppm can be filtered by means of activated carbon filtration. It generally becomes more cost cost effective to remove H2S via biological sulphur removal or chemical treatment with levels exceeding 600ppm.
You can read more about gas cleaning here.

AMMONIA (NH3)

Ammonia is very soluble in water and is best removed via water scrubbing. The most common method is through chilling the gas to “knock-out” the bulk of moisture in the gas train.

WATER

Most engines are designed to operate with some water in a gaseous state and a relative humidity of up to 80% (non-condensed water) but typical biogas generally exits the digester or landfill site as saturated. The easiest way to remove the moisture is by cooling the gas sufficiently to below dew point. This will remove a large proportion of the water. Gas chilling is a vital process and some sort of moisture removal is found in all successful biogas projects in South Africa.

SILOXANES

Siloxanes can be an issue when combusting in gas engines and are removed via absorbing mediums (charcoal, silica gel etc.). Siloxane and H2S removal in dry gas can sometimes be removed in combination with the same medium as a more practical and cost effective solution.

 

gas moisture removal south africa

Above: Skid mounted gas chiller and activated carbon filtration system

References:

“Biogas – An Important Renewable Energy Source”. World Biogas Association WBA fact sheet. First published May 2013.

“Biogas comes of age” by Michael A. Devine. First published in Energize RE: Renewable Energy Supplement – June 2014 pp 59-62.