Quality control

Cost-effective clean air requirements can be met by use of natural gas

Published: Wednesday 15 January 1997

Compared to the energy it cons (Credit: Shri Krishan)natural gas or 'marsh gas' is associated with petroleum deposits and is obtained from wells dug in oil-bearing regions. It has a very low amount of carbon and is almost entirely combustible. Mobile engines that burn natural gas generate relatively low rates of polluting emissions. B R Oza, A D Vadhel and R W Gaikwad of the department of chemical engineering, Pravara Rural Engineering College, Loni (Uttar Pradesh), proved that use of natural gas as vehicular fuel emit less carbon dioxide (co2), carbon monoxide (co), nitrogen oxides (nox), sulphur and non-methane hydrocarbons than current gasoline engines or even electric vehicles (Indian Journal of Environmental Protection, Vol 16, No 9).

Around 60 to 70 per cent of greenhouse gases are released from highway motor vehicles which normally run on gasoline, whereas the use of natural gas as vehicular fuel can offer a least-cost solution to growing air quality problems in the world's major cities ( see table).

Select use of natural gas is also an environmental air quality control method. In this, natural gas is burnt, alone or as a part of a mixture of fuels. The technologies for select use application consist of cofired applications and boilers fired in parallel. Select gas use with fuel oil or coal can help users to control emissions of any number of air contaminants including sulphur dioxide (so2), particulates or other air pollutants resulting from burning oil. Moreover, its high ignition temperature and limited flammability range makes accidental combustion most unlikely, making it the safest fuel to deal with.

In many applications, select gas use offers a way of simultaneously controlling emissions of several different pollutants -- so2, nox, particulates and even hydrocarbons. Conventional fuel combustion results in three major types of emissions: nox, so2 and particulates. Substituting conventional fuel (like coal) with natural gas will result in corresponding reduction in these emissions. Two new gas combustion technologies have been introduced by Oza et al for emission control -- reburn technology for nox control and dry sorbent injection for so2 control.

'Reburn' involves diverting a portion of the fuel into a second fuel rich 'reburn' zone to create conditions for nox reduction. Air is added in the final 'burn out' zone to complete the overall combustion process.

Dry sorbent injection for so2 control involves three steps. Firstly, injecting limestone (a dry sorbent) into a high temperature environment to form chemically active particles and then mixing this calcined sorbent with the products of primary combustion. Finally, sulphation takes place in which so2 is captured by reaction with the calcined solvent.

Natural gas supplies are widely distributed but highly underutilised. They are likely to be adequate for the rest of this century and most of the next. It has, therefore, a vital role to play in the eco-friendly growth of countries all over the globe, especially one like ours.

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