Managing and Measuring Pulp Mill Odour Emissions
One of the major environmental issues facing kraft pulp mills is odour produced by the kraft pulping process. Odour is a consequence of the use of sulphur compounds as cooking chemicals for the wood chips. These convert to reduced sulphur compounds. The four main reduced sulphur compounds, known collectively as TRS, have an unpleasant odour and can be smelt at very low levels. Helen Jenkins from the Tasman mill at Kawerau reports on managing pulp mill emissions.
The nature of odorous emissions is that they can be transported over long distances. While not considered hazardous to human health or to the environment, pulp mill odour can have a high nuisance effect in surrounding communities.
For the Tasman site, climatic conditions have a particularly large influence on the impact of odour from the site. In winter, inversion conditions are common across the Rangitaiki Plains in the Bay of Plenty where the mill is situated. As a result, emissions from the site that would normally be very well dispersed can be trapped close to ground level. This means that while emissions of odour from the site are at relatively constant levels, the odour experienced in the community varies.
Emission reduction
At the Tasman mill, several capital projects have been carried out over the past six years with the aim of reducing TRS emissions and hence odour. Two common methods of control for odour emissions in kraft mills are to scrub the emission using an appropriate chemical, or to incinerate the odorous gas. Both methods are used on site, in different applications. However the company is always looking for new and cleaner odour reduction technologies.
Figure 1: Kraft Pulp Cycle
There are various sources of TRS emissions within the kraft process. High-concentration sources of odour are present in the recovery area, where spent cooking chemicals from the chip digesters are converted back to a form that is reusable as a cooking chemical. Another area of high emissions is the lime kilns, which are another stage of the chemical recovery process. On the Tasman site emissions from both these sources are treated in wet chemical scrubbing systems to reduce the odour. The largest scrubber on the site was installed in the recovery area in 1994, and has reduced odour from the source it scrubs from 3.5 kg/hr to 0.1 kg/hr on average.
Two very high concentration sources of odour are known as NCG and SOG. NCGs, or non-condensible gases, are generated from the digester area. SOG is the stripper off-gas from the foul condensate stripper. This stripper, commissioned in 1996, removes methanol and volatile and odorous compounds from one of the process streams. Doing this reduces the overall odour of the effluent from the mill and hence the odour from the treatment pond system. Both NCGs and SOGs are incinerated in the lime kilns on site to remove these odour sources from the mill emissions. However there is occasionally a need to vent these sources for short periods of time, when kiln conditions prevent combustion.
Some other sources of odour are less concentrated. Low-concentration odour is emitted around the fibreline area, which contains the pulp washing stages, and from the chip bins, which contain chips to be fed into the digesters. A wet chemical scrubber was installed on the chip bin for the No. 2 Digester in 1998, and has proved highly efficient.
The combined impact of these and other reduction projects on TRS emissions has been substantial _ the reduction in mill TRS from 1994 to 2001 has been over 95 percent. TRS emissions from the mill compare well internationally, as shown in Figure 2.
Figure 2: Benchmarking Graph
Odour measurement
TRS is relatively easy to measure. The mill uses a combination of measurement methods. Minor sources of TRS are sampled manually and analysed using gas chromatography. Sources with a high contribution to the overall mill TRS emissions are monitored in real time using continuous emission monitors. In addition to point source measurements on-site, the mill operates two ambient TRS monitors. These are sited in local communities, in the direction of the prevailing wind.
However, while TRS contributes to mill odour, TRS is not a true measure of odour. Currently the best measurement instrument for odour is still the human nose. A method of odour measurement that uses the sensitivity of the human nose has been developed - olfactometry.
In olfactometry, a point source emission is diluted to the point where it is barely detectable by the human sense of smell. The number of dilutions needed gives a measure of the initial strength of the odour. In an olfactometry exercise, a panel of people is used to establish the average level of detection for the panel, since individual sensitivity to odour varies. The number of dilutions needed to get to the level of detection for any particular sample is known as the number of odour units, and quantifies the strength of that sample prior to dilution.
An olfactometry exercise was carried out for the Tasman site in 1996 and again in 1999 to establish the level of odour emitted by point sources on site. Results were encouraging, showing a drop of about 35 percent for the total mill odour emission.
One of the advantages of olfactometry is that the number of odour units for a point source is a variable that can be used in modelling programmes to give estimates of the local ambient odour. This was done in 1996 and again in 2000 for emissions from the Tasman site. TRS emissions were also modelled - this allowed the model to be checked against the ambient TRS monitors to ensure that the model did indeed reflect real conditions.
Figure 3: Modelled TRS Concentrations on 2 January 2000
An example of a "contour map" for TRS emissions is shown above for one of the worst case days modelled for summer. The location of the mill is marked with a star. Units for the isobars are mg/m3 of TRS. There is no limit of detection for odour for the group of TRS compounds as a whole, however the main species present, hydrogen sulphide, has a limit of detection of 7 mg/m3. Similar contour maps were done for odorous emissions.
The olfactometry work established that point source emissions of odour had reduced from 1996 to 1999, and the modelling survey for 2000 predicted that ambient levels of odour would have reduced correspondingly. However, this information is meaningless unless it can be shown that people in the surrounding communities have noticed the difference. To verify that people in the community were actually experiencing the reduction in odour from the mill, the company carried out odour surveys in 1996 and again in 1999.
An odour survey provides a direct feedback from the community, and is a measure of the level of annoyance caused by odour. A survey gives a measure of effects over the past 6-12 months, and also serves as a benchmark for future surveys. Surveys are becoming widely used as a tool to measure odour effects.
The two Tasman surveys used seven annoyance categories for odour ranging from "definitely not annoying" up to "extremely annoying". The aim of the surveys was to establish the percentage of the community who were "at least annoyed" by odour emissions. Results from the 2000 survey showed an encouraging result in comparison with the 1996 survey (Figure 3). The
threshold value of 15 percent relates to the findings of studies of control groups for surveys. Typically 5-15 percent of people in unaffected communities report annoyance due to industrial odour.
Figure 4: % at Least Annoyed Graph
Looking ahead
Results from all the work carried out in 1996 and 1999 on odour emissions from the mill are encouraging and establish that odour from the mill has reduced significantly. The company intends to survey at regular intervals to measure odour reduction from the site.
The challenge going forward is to look for new technologies and innovative approaches to continue to reduce odour emissions from the site.
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Helen Jenkins Carter Holt Harvey Tasman Kawerau Helen is the Environmental Engineer for the Carter Holt Harvey Tasman pulp mill and has been at the mill for six years, mainly working in the area of air quality. She is a chemical engineer by training and previously worked for Auckland University and Auckland Regional Council. |
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Amber Duncalfe
Editor - RM Update
Ministry of Agriculture and Forestry
PO Box 2526
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NEW ZEALAND
Tel: +64 4 894 0710
Fax: +64 4 894 0745
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