How one of the UK’s oldest coal plants avoided potentially damaging breaches of its prescribed air quality strategy by developing a predictive action plan based on a thorough SO2 audit and local weather knowledge.
Environmental credentials are a fundamental indicator of company performance and one in which good business – let alone status – depends on a willingness to go beyond regulatory compliance. UK power generators, subject to ever more stringent emissions limits, are a focal point for reducing industry’s impacts on its surrounding environment and communities. One plant, Ironbridge B power station in Shropshire, part of E.ON UK’s portfolio, has demonstrated its commitment to ‘go the extra mile’ in solving an emissions phenomenon, with a proactive response that is believed to include a first in the UK generation market.
The station, working with Power Technology, E.ON UK’s energy industry asset management and scientific consultancy, and in consultation with the Environment Agency, developed an action plan to tackle a potential sulphur dioxide issue that could, under certain circumstances, result in the need to take one or both of the station’s units off-load at short notice. Ironbridge, a 970 MW coal-fired plant built in the 1960s, had in January 2005 recorded an unexpected number of high concentrations of sulphur dioxide at ground level through its ongoing monitoring procedures.
While there was no breach of the Air Quality Strategy (AQS) SO2 objective set by the Department for Environment, Food and Rural Affairs, a continuation of the pattern could have increased the risk of a contravention towards the end of the calendar year, and introduced a corresponding risk to the station’s production.
The root cause of the recorded SO2 increases was ultimately traced back to extreme weather conditions – a period of consistently gusty wind at high windspeeds and from a relatively constant direction – during the winter. A compounding factor was the plant’s location, in a steep-sided, wooded valley on the River Severn where it forms the Ironbridge Gorge, a place associated with the birthplace of the industrial revolution.
The plant’s immediate neighbours are the Gorge communities, including the tourist and educational facilities based on its heritage and, slightly further afield, the new town of Telford.
This gives its name to the two ambient air quality monitoring sites – Telford Aqueduct and Telford School – which the station operates under its air quality management plan (AQMP). The station is not fitted with flue gas desulphurisation plant and is opted out of the Large Combustion Plant Directive.
During a six-day period in early January 2005, the Telford Aqueduct site recorded 18 exceedences of the SO2 threshold concentration of 100ppb measured as a 15-minute mean. The exceedences, which were confirmed by quality assurance checking of the ambient gas analyser, varied from 103ppb to 147ppb. The monitoring site is just over 2.6 miles east-north-east of the station stack and at the time was downwind of Ironbridge which was operating normally. No exceedences were recorded at the Telford School monitor which is 1.8 miles due east of the stack.
The SO2 compliance criteria set down in the AQS limits the number of exceedences of the 15 minute threshold to a maximum of 35 times at any monitoring point in a calendar year. In 2005 Ironbridge, in common with other large coal-fired power stations, had just introduced a risk management framework (RMF) as part of its AQMP.
The RMF requires SO2 non-compliance risks to be managed within each year, taking account of incidents as they occur. Quarterly risk indicators, based on the number of exceedences for each monitoring site, are assessed using a green, amber and red ‘traffic light’ system. Any risk status above green requires action by the station.
To manage the issue and minimise the potential for non-compliance, the station, together with Power Technology, and E.ON UK’s Trading, Coal Operations, Asset Management and Environment and Corporate Responsibility teams worked with the Environment Agency. In tracking down the precise circumstances of the Telford Aqueduct incident, Power Technology examined:
• the sulphur content of the station coal;
• the role played by the exceptional weather;
• the pattern of station generation.
The coal studies carried out established that sulphur content alone was not the cause of the incident. Modelling and analysis of monitoring data and sulphur contents indicated that burning fuel with a maximum 1.0 % sulphur for the entire year could be expected to result in compliance with the 15-minute mean AQS objective. Ironbridge mainly burns three types of fuel: imported Russian coal usually with 0.3 % sulphur; other coal deliveries typically in the range 0.7 to 1.0 %; and stock reclaim coal averaging 0.9 %. Additional calculations, including historical meteorological data from 2001-2003 and 2005, showed that there would have been no exceedences during the January period had 0.3 % sulphur coal been used. It did not, however, guarantee that exceedences would never occur when 0.3 % coal was burnt in the future.
Furthermore, there are issues related to continued use of 0.3 % sulphur coal and supplies from a single source. These coals can cause technical problems, import capacity may be limited and interruption of a single source coal may pose risks to the security of electricity supplies.
Research into how the meteorological conditions affected the atmospheric dispersion of SO2 was against a background of strong winds across the country with utility lines disrupted and trees uprooted. At Ironbridge one of the effects was the rare event of the weather grounding the cooling tower plume.
Readings from the Telford Aqueduct monitor showed strong winds and a high level of correlation with Meteorological Office wind speed and directional data. The nearest observational station, Shawbury, which is about 13 miles to the north-east, had recorded winds predominantly from the south-west. Shawbury wind speeds during the period of the exceedences were in the range 7.7 to 14.4 metres per second (m/s), higher than the average Shawbury figure of 6.2 m/s during exceedences a few years previously. Wind direction data from Telford Aqueduct and Shawbury were plotted against the 15-minute mean SO2 concentrations.
Telford Aqueduct wind directions most likely to lead to exceedences were in the range of 255º to 270º, while the Shawbury directions were in the range of 210º to 240º. The difference between the two is attributed to terrain effects affecting the Telford Aqueduct wind direction when it originates from the south-west quadrant. A high ridge near Ironbridge tends to create a funnel effect and direct winds towards the power station. Further analysis by Power Technology showed that more than 90 % of the Telford Aqueduct winds had a bearing in the range of 180º to 284º over the period of the high concentrations. It concluded that the occurrences were local to the site.
To check whether the station load and operation of either or both of its units were factors in SO2 exceedences, detailed comparisons were carried out between generation data from January 2005 and the two preceding years.
Power Technology concluded that there was no clear indication that certain station loads or number of units operating were more likely than others to cause exceedences and that adjusting load was not a realistic method of attempting to reduce the risk of exceedences in the future.
As no assured solution had emerged from the coal-sulphur and generation analyses, Power Technology, working with the station, turned to developing an action plan to be added to the AQMP. It comprised three elements:
• greater control of coal sulphur content;
• checking weather forecasts for one day ahead and on the day to help plant scheduling decisions;
• developing a trigger system for taking the station off load when preset concentration levels of SO2 are recorded at Telford Aqueduct.
Deliveries of Russian coal, typically 0.3 % sulphur, were increased to Ironbridge and a new coal quality control system was implemented with the aim of ensuring, as far as possible, that other coal deliveries were limited to sulphur content of 0.7 to 1.0 %. The two sets of weather forecasts were used to determine the exceedences risk arising from wind speed and direction.
The coal sulphur content, wind speed and direction were brought together in a second use of the traffic light principle to provide plant operators with a daily, quick reference check on the likelihood of exceedences. Each factor is colour rated, producing an overall risk factor or green, amber or red.
Formulating a trigger system under which the station would be taken off load at short notice was a serious step with cost implications. More importantly, at times of high electricity demand, the removal of Ironbridge from the network could present a risk to security of electricity supplies.
A number of other considerations were also weighed-up, including that it would be impossible to prove whether taking the station off load had prevented an exceedence. There was also the possibility of removal from the grid when an exceedence would not, in fact, have resulted had the station continued generating. However, the over-riding conclusion was that shutting down, while it might not prevent one or more exceedences, would certainly prevent a large number occurring in a single day.
The trigger scheme was based on analysis by Power Technology of historic monitoring data, searching for patterns in the concentrations one to three hours prior to an exceedance occurring.
The findings were tested to pinpoint the triggers most effective at minimising exceedences and prompting fewest false alarms. The trigger alarms were activated from the new live feed that was set up between the monitoring stations and the control room to provide operators with data for real-time 15-minute mean SO2 concentrations. Previously, data from the monitors was collected daily and not immediately available to the operators.
Audible alarms were set to trigger at pre-set SO2 concentration levels. In response to an alert, the station took steps based on the type of coal being burnt.
• When burning purely low sulphur coal – 0.3 % or less – the station would come off load immediately, in a controlled manner, if an SO2 level of more than 100 ppb was recorded at Telford Aqueduct.
• For the coals with sulphur content greater than 0.3 %, the station would be taken off load if the pre-set trigger levels lower than 100ppb were exceeded within fixed time periods. The plant would not resume generation before 5.00 am the following day at the earliest. The trigger levels were formulated as part of a system which can be relaxed or tightened if required, following each quarterly review of station impacts which produces the green, amber or red risk status.
No further exceedences of the SO2 limit were recorded during the remainder of the year but the station implemented the action plan and shut down units in three instances during the period.
On the first occasion the single unit in operation was taken off load and on the second occasion both units were closed down. In the third instance, the unit burning the highest sulphur coal was shut down, with the second continuing to generate accompanied by a fall in SO2 concentrations at Telford Aqueduct.
The steps taken enabled the station to return to green status in the RMF after the second quarter and retain it for the balance of 2005. The way in which the incident was tackled was highlighted by the Environment Agency in its current 2006 report, Spotlight on Business, which reviews the environmental performance of British industry.
Ironbridge featured in a case study headed ‘Working together for cleaner air’. The EA said the station had faced a ‘high’ risk of breaching its yearly maximum of exceedences, and had “improved short-term weather forecasting and, at significant expense, modified the way they operate under high-risk weather conditions.” It went on to describe Ironbridge’s avoiding an SO2 breach as ‘a significant achievement, given events at the start of the year.’