THE BIG THREE - ON-LINE MONITORING OF GREEN HOUSE GASES

ABSTRACT
National emissions regulations have historically targeted the common stack gas pollutants which have long been known to cause damage to health or ecology, but there is growing concern about the long term effects on the world’s climate from what were hitherto less obvious pollutants. The Kyoto Protocol has identified six gaseous pollutants as “green house gases” and the top three in particular, carbon dioxide, methane and nitrous oxide have become subjects of some interest. Monitoring for emissions of these gases requires appropriate analyser hardware, and infrared techniques are the most relevant. Although various transducer designs suitable for measuring these components have existed for some time, recent advances in gas filter correlation techniques now provide sensitive and accurate measurements of nitrous oxide and methane with low residual cross-interferences. For the effective measurement of percent levels of carbon dioxide a new development in ultra-compact, solidstate, single beam IR techniques, utilising a pulsed source, is now able to provide this analysis in flue gas in a miniaturised, high performance and cost-effective package.

1. INTRODUCTION
Environmental regulations defining limits for the emissions of industrial pollutants to the atmosphere have long been in place in most nations, and historically these have targeted those acidic or toxic pollutants associated with causing direct damage to ecology or health, for example via acid rain or ground level ozone formation. Such regulations have contributed to the growing pressure for firm environmental measures world-wide, which in turn have created a major global business in the supply of environmental products and services, forecast to grow at substantial rates over the next few years [1]. As part of the overall environmental business, the market for continuous emissions monitoring systems (CEMS) for the usual gaseous pollutants has seen fairly constant growth, with increasing requirements for more sensitive measurements at lower concentration levels. More recently, since the identification of the global warming issue and the establishment of the United Nations Framework Convention on Climate Change (UNFCCC), further gases have begun to attract particular attention from the international community, the most notable being carbon dioxide (CO₂), the primary “green house gas” (GHG), held responsible as a major cause of global warming. The 1997 Kyoto Protocol (the result of COP-3 - the third Conference Of Parties to the UNFCCC, held in Japan) identified this and five further gases as critical GHGs whose emissions to atmosphere must be reduced in order to reverse the global warming trend, and consequently started the process of international negotiations on reduction targets. Within the European Union (EU) this has recently resulted in major CO2 emissions reduction targets being agreed by member states, to be achieved by 2008-2012. Countries committed to big reductions include Germany (21% reduction from 1990 levels), the UK (12.5% reduction) and Italy (6.5% reduction). In addition, EU member states are now required to submit annual reports on their progress in meeting their targets for GHG emissions reductions [2].

The second most important GHGs were identified as methane (CH4) and nitrous oxide (N2O). Whereas the major cause of anthropogenic CO2 emissions is combustion, the two biggest sources being power generation and road transport, the sources of CH₄ and N2O are various. In the UK it is estimated [3][4] that of the total 1996 CH4 emissions (78 million tonnes as CO2 equivalent), 46% came from landfill sites, 25% came from bovine agriculture, 10% from natural gas grid leakage and 9% came from coal mines. In contrast, the UK’s N2O emissions during that year amounted to 58.6 million tonnes (as CO2 equivalent) and were believed to arise mainly from arable agriculture (49%), nitric acid and adipic acid process plants (37%) and road transport (5%). For comparison, the UK’s CO2 emissions for the same year amounted to 593 million tonnes, calculated to be about 2% of the global total. 

The top three GHGs in particular (CO2, CH₄ and N2O) have therefore become the subject of interest, especially in emissions from combustion processes and in gases produced by landfill sites or certain chemical processes. Each of them, by virtue of the fact that they are GHGs, have strong infrared (IR) absorbances and hence are entirely amenable to instrumental analysis by IR means. The IR technology appropriate for their analysis is clearly dependant on the measurement sensitivity required, however in addition to sensitivity there are many other measurement parameters which should be considered as necessary for ensuring not only high performance but also reliable practical operation. The increasing recognition of the need for detailed assessment of various parameters for CEMS analysers in particular has resulted, for example, in the recent UK MCERTS certification scheme performance standards [5] which are a valuable yardstick by which to evaluate the selected measurement technology.  

In the case of CH₄ and N2O their emissions, from whatever source, are generally at trace levels and a high sensitivity technique is needed when monitoring is considered. IR gas filter correlation (GFC) techniques have become very well accepted for monitoring trace level pollutants in flue gases, giving the advantages of high sensitivity and low residual cross-interferences. Both CH₄ and N2O have a sufficient degree of rotational fine structure in their IR absorbances to enable the application of GFC techniques, and this paper reports recent developments in GFC transducer technology to produce high performance measurements of these two gases. 

The quantification of high volume percent levels of carbon dioxide in flue gas is inherently less arduous, but nevertheless requires an appropriate form of technology to be applied to achieve it in an accurate, yet cost effective, way. The recent development of a simple, single beam IR transducer with no moving parts, using a novel pulsed IR source, now enables such performance to be achieved. This paper will also discuss the principles of this IR technology, and the factors governing the achievement of high measurement performance, low crossinterferences, reliability and compact size.