Menu

FILTER LEAK MONITORS FITTED TO CREMATORS WITH FLUE GAS TREATMENT PLANT

Introduction

The Secretary of States Guidance Note PG5/2 (2012) covers the operation of plant designed for the cremation of human remains. The Note covers both abated and unabated cremators. The purpose of this document is to explain how PG5/2 (2012) deals with continuous particulate monitors fitted to plant that have flue gas treatment plant installed (abated cremators).

Instrument Selection

The nature of an abated cremator installation will mean that the particulate burden in the waste gases after the bag filter will be very low, and typically < 5 mg/Nm³. This is a very low dust burden to measure accurately both on a continuous basis using instrumentation, as well as for the test teams conducting the annual emissions tests.

The old methods of measuring dust using a form of optical transmissometer or light scattering measurement device that basically detect the loss of light in various ways from a regulated source cannot be used for the very low emissions from a bag filter plant, as the lower limit of detection is too high. It is for that reason that almost all optical transmissometers have been removed and replaced with instruments using tribolectric or electrodynamic principles that have much reduced lower limits of detection, and are suitable for this duty.

A filter leak monitor is the type of instrument typically selected by the cremator manufacturers to meet the continuous monitoring requirements of PG5/2 (2012) for abated cremation plant, and this meets the requirements of PG5/2 (2012) in full.

This type of instrument is not intended to monitor particulate matter emissions in terms of an actual dust concentration, but is used to detect upward or downward trends in emissions, or highlight a step change in emissions resulting from plant failures such as a broken filter bag, or a by-pass damper that has not fully closed for example.

It is very difficult to even attempt to calibrate such a monitor due to the very low dust burden in the flue gas after the bag filter. In order to attempt calibration, the results of the annual emissions tests are normally used. PG5/2 (2012) gives details of what the annual emissions tests should comprise, and in the case of total particulate matter, is usually three, discrete tests of 60 minutes duration.

If a 60-minute test is carried out when the dust burden is very low, then the tiny mass of material deposited on the test teams filter results in high level of uncertainty (reflected in the ± figure given against the reported emission value). This normally is not relevant, as the emission level stated is usually so far under the emission limit value of 20mg/Nm³ (even taking into account a large uncertainty) for a correctly operating bag filter, that it doesn’t really matter.

If it was decided to attempt to calibrate the continuous particulate monitor to give an indication of the actual dust burden, rather than acting as a filter leak monitor, then the duration of each calibration particulate test would need to be extended considerably in order to meet the test requirements regarding meeting minimum detection limits. When considering that the statutory PG5/2 annual emissions tests for three 60-minute tests would have to be done in addition to the much longer duration calibration tests, then this would increase the overall cost of the annual tests to each crematorium substantially, and is not required to meet the requirements of PG5/2 in any case.

The Filter Leak Monitor in Use

It is typical to report an emission from the particulate monitor in terms of a concentration in mg/Nm³, as this followed the pattern set for unabated cremators since the first PG5/2 Note was published in 1991, when there were no abated cremators in the UK.

It was always made clear that the results from the particulate monitor were only indicative at an abated site, as the instrument could not be reliably calibrated from the results of the statutory annual emissions tests, as the dust concentrations in the flue are so low.

In typical use, the system will report particulate emissions of zero, or close to zero for most of the time, as in reality, true emissions are usually close to zero for correctly operating plant.

An unexpected consequence of this approach is that in order to achieve the extremely low readings from the instrument consistent with actual particulate emissions, this could in some cases de-sensitise the instrument to an extent such that unexpected increases in particulate emissions were suppressed. As bag filter leaks or non-sealing by-pass damper problems are an extremely rare occurrence, picking up on instruments that were not sensitive enough can prove to be very difficult.

In order to improve how particulate emissions are reported, and to eliminate the possibility of de-sensitising instruments to suppress readings, it would be better for the particulate monitor to act purely as a filter leak detector, and no longer report unreliable particulate readings in terms of a concentration.

The system should monitor in two ways:

The system should alarm in the event of a sudden, step change in emission that indicates the bag filter efficiency has been compromised, and requires immediate attention.

A lower level monitoring advisory alarm will warn when an upward trend in emissions has occurred that may be symptomatic of a by-pass damper not sealing as well as it used to for example. Whilst this by no means indicates that the plant is breaching the emissions limits set down in PG5/2 (2012), it is an early warning that something has changed, and the equipment needs checking.

We advise that you should contact the manufacturer of your cremation equipment to ascertain if this is possible, as well as your Regulator (Environmental Health Officer in England & Wales, SEPA in Scotland).