The UK's first selective catalytic reduction system at a large coal fired power plant is under construction (at Ratcliffe, pictured right), while planning permission is being sought for a second, at SSE's Fiddler's Ferry.
Although well established elsewhere, there are as yet no “end of pipe” (ie, secondary or post combustion) NOx reduction systems in operation at any of the UK’s coal fired power stations, one reason presumably being that all of the UK’s existing fleet was built over twenty years ago, before technologies such as selective catalytic reduction were well established at large scale.
However, if coal plants wish to remain in operation beyond 2015 (ie have “opted in” under the Large Combustion Plant Directive, currently being replaced by the Industrial Emissions Directive, expected to be enacted by the end of this year), post combustion NOx control, most probably SCR (selective catalytic reduction), will be needed – to achieve NOx levels below 200 mg/Nm3. (Plants currently “opted in”, in addition to Ratcliffe and Fiddler’s Ferry, are: Drax; Eggborough; Cottam; West Burton; Rugeley; Ferrybridge (50%); Longannet; Uskmouth; and Aberthaw.)
The UK’s first coal plant SCR system (with an estimated cost of somewhere between £200 and £300 million, the actual number being said by the utility to be “commercially sensitive”) is currently under construction, at E.On’s 4 x 500 MWe Ratcliffe power plant, with the SCR installation on the first unit expected to be completed during a 20 week outage scheduled for 2011. Doosan Babcock is the main contractor on this project.
It now looks like Scottish & Southern Energy’s Fiddler’s Ferry plant, also 4 x 500 MWe, which became operational in 1973, could well become the second UK coal plant to be equipped with SCR. The plant is north of the River Mersey, near to Warrington and to the east of Widnes.
In February SSE submitted an application to install SCR at Fiddler’s Ferry, under Section 36 of the Electricity Act 1989. It was, incidentally, one of the last submissions under this regime, now superceded by what is intended as a more streamlined system involving a new body called the Infrastructure Planning Commission, which the Conservative party has said they will abolish if they win the May election). (Interestingly, Ratcliffe did not apply for permission for its SCR under Section 36, having been advised that such permission was not needed.)
The Fiddler’s Ferry SCR plan is part of SSE’s environmental improvement programme and follows the installation of three flue gas desulphurisation (FGD) units at the plant over the period 2006-2008, “reducing emissions of sulphur by 70%”, the company says.
Since 2004 some £231 million has been invested in the Fiddler’s Ferry power station, which is equipped for biomass co-firing. The SCR project, to be carried out over the next six years, represents an additional investment of about £200 million.
For NOx control, “primary” measures have already been installed at Fiddler’s Ferry, in particular low NOx burners and separated overfire air (SOFA) technology. These have resulted in NOx emission levels of around 500 mg/Nm3.
The Fiddler’s Ferry SCR installation is envisaged as a three unit retrofit which will include fitting of three catalyst towers between the existing boilers and FGD units and above the electrostatic precipitator structures.
Flue gas from the boilers will flow to the SCR towers then to the electrostatic precipitators, followed by FGD and stack.
Each catalyst tower will have a chamber about 20 m long, 10 m wide, and 20 m high, with ducting of around 50 m2 in cross sectional area.
Modifications to the existing plant will include boiler integration, ID replacement fan, economiser adaptation and installation of two ammonia tanks (capable of storing a total of 170-190 tonnes of anhydrous ammonia). These tanks will have their own dedicated unloading facility, fully separated from general plant traffic. As ammonia is a hazardous substance, the tanks, tankers, unloading area and chambers will be fitted with detectors, alarms and safety equipment. It is estimated that one to two tankers of ammonia per day will be required. The ammonia will be transferred from the ammonia tanks to the SCR catalyst towers via pipework. The ammonia will be delivered to the chambers as a vapour and will either be electrically vaporised at the tank end of the pipeline or vaporised by indirect steam heating at the chamber end.
Operating the SCR system will add to the power plant’s parasitic load, adding about 3.5 MW per unit, or 0.7% of the power station’s full load.
Dealing with catalyst
There are two options for the catalyst when it reaches the end of its useful life: disposal; or regeneration/refurbishment. Disposal of spent catalyst usually involves separation of the catalyst frame from the catalytic material. The catalytic material can then be incinerated or disposed of in landfill (non-hazardous). Regeneration is typically undertaken off-site by a specialist company. It can involve physical cleaning processes to remove surface deposits or chemical reactivation to restore catalyst activity.
The decision as to whether to regenerate or dispose of spent catalyst is dependent upon the proximity of facilities with the capability to regenerate the catalyst. Such facilities exist within central Europe, in particular Germany. But there are no such facilities in the UK at present. Since spent catalyst will not be generated until around 2017/2018 at the earliest, further options will be considered nearer the time.
Subject to the grant of Section 36 consent, the SCR project engineering contractor is expected to be appointed in the last quarter of 2010, with detailed engineering design work being carried out in 2010-2012 and main construction taking place between 2012 and 2015. The overall objective is to have the SCR equipment fully operational for the beginning of 2016.
Construction lay down requirements will be met by using existing areas used previously for installation of the flue gas desulphurisation system.
Apart from the installation of selective catalytic reduction, other NOx control options considered for Fiddlers’s Ferry included: selective non-catalytic reduction (SNCR); BOC’s LoTOx technology; and Doosan Babcock’s NOxStar.
SNCR promises much lower costs per kg NOx removed than SCR. However, it was determined that SNCR had not been proven at the scale required and output NOx levels would not meet the legislative requirements. Furthermore, SNCR uses significantly more ammonia than SCR, which would require either larger on-site storage or more frequent ammonia deliveries. Additionally, uncertainty about future legislation meant that SSE could not “be confident that NOx emissions limits will remain at 200 mg/m3 for the economic life of the SNCR retrofit.”
LoTOx and NOxStar were also judged “unproven at this scale” and believed to “constitute an unjustifiable technical risk.”
Three SCR configurations
Three alternative ways of incorporating SCR into the flue gas path at Fiddler’s Ferry were considered (see diagram above):
• Between the economiser and the air preheater – ‘high dust’ option;
• Downstream of a high temperature dust removal system – ‘low dust’ option; and
• Downstream of the dust removal and flue gas desulphurisation systems – ‘tail end’ option.
The ‘high dust’ option has been selected for Fiddler’s Ferry, based on the system power requirements for heat make-up, which is significantly higher for the other two configurations.
The ‘low dust’ option would have significantly higher operating costs and capital costs as the location of air heaters and precipitators would have to be changed. In addition to this, the operating temperature would be higher, resulting in a greater energy demand.
Source: information on the proposed Fiddler’s Ferry SCR project is derived from Fiddler’s Ferry power station: project to install selective catalytic reduction (SCR) to reduce emissions of nitrogen oxides (NOx), Environmental Statement, Environmental Resources Management (ERM) and Scottish and Southern Energy, 23 February 2010