The guidance was originally issued just before the local authority air pollution control system established by the Environmental Protection Act 1990 was brought into force on 1 April 1991. Eventually more than 70 guidance notes on specific processes were issued, and the DoE has promised to review these every four years in a rolling programme. One note, on cement storage and handling operations, has already been revised (ENDS Report 222, p 35 ).
The first of the new revisions deals with wood combustion processes between 0.4-3.0MWth. The main amendments are:
The original guidance contained "spot" emission limits for particulates and organic compounds. The numerical limits have not been changed, but their impact has been attenuated because the new version allows excursions above the limits where continuous monitoring is used. In such cases, compliance will be achieved if 95% of the 15-minute average readings for each rolling 24-hour period do not exceed the limits laid down, and if the peak emission does not exceed twice the limits.
An emission limit of 400mg/m3 for carbon monoxide has been added. However, this is to be disregarded in the 30 minutes after start-up from cold and when a boiler is idling pending demand.
The minimum frequency for quantitative monitoring of particulate emissions has been extended from six to 12 months. The same frequency will apply to carbon monoxide.
The revised guidance also clarifies how smoke emissions should be monitored and the circumstances in which emission limits for formaldehyde, hydrogen chloride and hydrogen cyanide should be set.
The second revised note deals with the manufacture of timber and wood-based products. The amendments in this case are more far-reaching:
The only specific emission limit in the original note - 50mg/m3 for particulates - has been deleted.
Companies have been given much longer to replace cyclones used to abate wood dust emissions. The original guidance required existing processes in their entirety to be upgraded to new plant standards by 1 October 1996. However, where cyclone replacement is necessary, this will not have to be done until 1 October 2000 for processes where exhaust air flows exceed 300m3/minute, and until 1 October 2003 for smaller flows.
The guidance adds that cyclones may remain acceptable after these dates even though it concedes that they are less effective than fabric filters. Specifically, it says that they may continue in use where wood with a water content of 20% or more by weight is worked, where they serve operations giving rise only to emissions of large particulates, and where a process is located "well away" from housing or other populated areas.
The initial guidance required continuous indicative monitoring for particulates. However, this is now required only in the event of "persistent failure" of fabric filters, and where total exhaust air flows from all abatement equipment exceed 300m3/minute.
Where cyclones are used, indicative monitoring of particulates, backed up by visual and audible alarms to indicate malfunction, are recommended - but not for those to be replaced before the upgrading deadlines laid down above, nor for those with an exhaust air flow of under 300m3/min, nor necessarily where a process is well away from populated areas.
In place of the previous emphasis on continuous monitoring of particulate releases, the revised guidance suggests minimum frequencies of inspection and replacement of filter bags where continuous monitoring is not practised. Filter bags should be inspected visually at least once a month, and in general replaced routinely every four years.
Total enclosure of wood dust storage and loading operations is now regarded as "inappropriate" for processes well away from populated areas.