Three editions of an official memorandum on chimney height calculation have been issued since the Clean Air Act 1956. The last was published in 1981. All of these dealt exclusively with conventional boilers running on fossil fuels.
The thinking behind HMIP's new guide is explained in a report by Warren Spring Laboratory (WSL).2 This points out that the existing guidance could not readily be applied to newer processes using fossil fuels, such as fluidised bed combustors and combined heat and power units, nor to waste incinerators - and even less to the multiplicity of industrial processes which are now controlled by local authorities under the Environmental Protection Act 1990.
The guide is intended primarily for use by local authorities, and is suitable for most processes which they now control. Exceptions are processes generating odorous emissions, on which separate guidance is being prepared by HMIP, and those where emission dispersal is likely to be affected by hilly topography.
The purpose of the guide is to provide a chimney height calculation method which will ensure that ground-level concentrations of a pollutant do not exceed a guideline value for that substance for more than about five minutes in meteorological conditions which are likely to be encountered 98% of the time.
The first key step in calculating a chimney height will be to establish a Pollution Index for an emission. This is derived from a knowledge of its emission rate and the guideline ambient concentration of the pollutant, corrected by the background concentration of the pollutant in the area concerned.
A key problem in calculating the Pollution Index is the lack of official guidelines on acceptable levels of almost all pollutants. As WSL's report notes, "there is currently no list of short-term guideline concentrations suitable for the general populace issued by any authority in the UK. There is little assistance world-wide - limited lists are issued by some countries, but comparison between them often shows large variations in guideline concentrations."
The common practice in the UK has been to use a fraction - generally between one-fortieth and one-hundredth - of the limits laid down for workplace exposures for this purpose. "This practice is, for good reasons, much disapproved of by those with toxicological and epidemiological expertise," notes WSL, but out of sheer necessity it forms the basis of the new guide.
Another thorny problem is how to treat air pollutants of a similar type, or those which are dissimilar but which have similar physiological effects, when assessing emissions. WSL's advice, accepted by HMIP, is that concentrations of pollutants of a similar type, such as acid gases or organic solvents, should generally be summed when calculating a Pollution Index. However, synergistic health effects of different pollutants are not covered by the guidance, again because of a lack of official guidance on how they should be treated.
The guidance goes on to explain how chimney heights should be calculated once a Pollution Index is available, and then corrected for the presence of nearby buildings and other structures.
The guide concludes by listing a series of factors which should override the results of these calculations, such as proximity to buildings. It also notes that on plants operating to a wide range of throughputs the more serious pollution problems may occur at low rather than high load factors, and chimney heights for both circumstances should be calculated.
Finally, the guide notes that low stack heights may be calculated for plants with highly abated emissions. However, because abatement equipment may malfunction, or because secondary emissions such as odours or visible plumes may be more important than a primary pollutant, or because unabated emissions may be passed to "bypass" stacks in the event of a plant failure, higher stacks may in fact be needed to ensure that local pollution problems do not arise.