How to Remove Carbon-Dioxide from Biogas? (4 Methods)

Carbon dioxide being noncombustible constituent of biogas lowers its heat value. Removal of carbon dioxide is not necessary when gas is to be used for cooking or lighting purposes only. A number of methods have been developed for CO₂ removal (scrubbing) which depending upon the technique involved are called water-scrubbing, caustic-scrubbing, solid absorption, liquid absorption and pressure separation.

A brief description of these methods is as follows:

Method # 1. Water Scrubbing:

In this method gas is made to pass through water which absorbs part of CO₂. The inherent limitation of this method is that it requires large quantity of water. Based on studies carried out by H.M. Lapp, 7 ft³ (0.2 m³) of biogas at 68°F (20°C) and 1 atmospheric pressure (1.03 kg/cm²) requires 2.7 gallons (12.3 litres) of water for CO₂ removal. CO₂ is highly soluble in water. Spent water following absorption of CO₂ becomes acidic and hence unsuitable for several applications as it corrodes metallic surface it comes in contact with.

Method # 2. Caustic Scrubbing:

This method works on the principle that when caustic solutions are made to react with CO₂ bearing gas streams, an irreversible carbonate-forming reaction followed by a reversible bicarbonate forming reac­tion take place as per the following equations. This process involves use of hydroxides of sodium, potassium and calcium.

In most industrial applications, no attempt is made to regenerate spent bicarbonate solution due to high steam requirement for this process. Carbon dioxide absorption into alkaline solution is adversely affected by slow conver­sion of dissolved CO₂ molecules into more reactive ionic species. Mixing of liquid during absorption helps to achieve diffusion of gas molecules in liquid and prolongs their contact time which adds to former’s absorptivity.

Normality of caustic solution also affects rate of absorptivity. With sodium hydroxide solution (NaOH), for instance, it was found that rate of reaction is more rapid if normality lies between 2.5 to 3. Potassium hydroxide (KOH) is more commonly used in industrial scrubbing but it suffers from limitation that it is not readily available in rural areas where biogas plants are normally located.

Calcium hydroxide [Ca(OH)₂] on the other hand is generally preferred for biogas scrubbing as this chemical is more readily available and cost of operating a lime-water scrubber is also relatively less. The main limitation of lime-water scrubbing are difficulties faced in controlling solution strength, and removal of large amounts of precipitate from mixing tank and scrubber.

In most cases, sediment and suspended particulate matter need be removed in order to avoid clogging in pumps, high pressure spray nozzles, packing and bubbling ap­paratus. Sodium hydroxide has major advantage of being available in easily handled pellet forms that enables rapid and simple recharging of scrubber. However with NaOH solutions problems of suspended particulate matter are not totally eliminated.

Method # 3. Method Developed by the IARI, New Delhi:

T.D. Biswas, et al., developed another method for CO₂ scrubbing. It was found that biogas can be removed by bubbling it through 10 per cent aqueous solution of mono-ethanolamine (MEA). By single bubbling through a plain column of 6 cm height, carbon dioxide content in biogas was reduced to 0.5-1 per cent by volume from initial value of 40 per cent.

Scrubbing column was made of an inexpensive plastic bubbler of 5 cm diameter and 15 cm height with only one orifice. Maximum removal of carbon dioxide was observed when bubbles moved out individually without colliding one another to form continuous stream. Optimum gas flow rate to the regulator was estimated as 100 ml per minute which gave 60 ml of purified gas per minute in the reservoir.

Decrease in this rate of flow was not found to cause any further scrubbing. Initial pressure of the gas introduced into the bubbler was 10 cm of water column and drop in pressure head was about 5 cm of water column. Both caustic potash and monoethanolamine solution were effective in reducing the carbon dioxide content to 0.5 to 1 per cent.

Whereas spent caustic potash solution cannot be regenerated, MEA solution can be completely regenerated by boiling for five minutes and thus can be used again and again. Furthermore, MEA solution is far less caustic than other solutions used and therefore pose much less hazards for skin. This method is thus very practical and economic for biogas scrubbing.

Method # 4. Pressure Separation:

This method works on the principle of compressing biogas beyond the limit of critical partial pressure of impurities (CO₂) with temperature greater than critical temperature of methane but below those of impurities. For instance carbon dioxide liquefies when gas temperature falls below 89.6 F (32 C) after compressing beyond 1106 psi (77.76 kg/cm).