Retrofitting selective catalytic NOx reduction for gas turbines
Selective catalytic reduction is an effective alternative to combustion management technologies to achieve NOx compliance with existing and new gas turbines.
ADRIAN JONES and MIKE RIMMER
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Emissions of oxides of nitrogen (NOx) produced by the combustion of fossil fuels are widely accepted as having significant adverse health and environmental impacts. This is illustrated by recent data from the UK which, according to the Committee on Medical Effects of Air Pollutants, estimated that NOx emissions were responsible for between 28000 to 36000 premature deaths per annum.1
The majority of NOx emissions in developed countries stem from road transport; however industry, particularly the energy and manufacturing sectors, are also major contributors. With the general decline in coal and oil fired power stations, the sources of industrial NOx emissions are increasingly arising from discrete combustion processes such as from gas turbines, for example small scale power generation or mechanical drives used with compressors and pumps in the energy and process industries.
In response to the health and environmental threats posed by NOx and other aerial pollutants, governments and regulatory agencies have introduced increasingly restrictive emissions legislation. For instance, in Europe operators must comply with the pollution emission limits set out in the Large Combustion Plant Directive (LCPD)2 and the Medium Combustion Plant Directive (MCPD).3
These directives require all new gas turbines to comply with strict emissions limits, with older systems being subject to an adoption/grace period, which for most non-compliant gas turbines will end between 2023 and 2025.
If gas turbine systems can not meet emission limits they will need to be replaced or modified, which is likely to be both costly and disruptive. An alternative is to install an ‘end of pipe’ emissions abatement system such as selective catalytic reduction (SCR), which achieves emissions compliance by removing NOx from exhaust gases. Such technology can be a practical and cost effective upgrade measure for non-compliant systems and can normally achieve NOx performance superior to a modern gas turbine operating without SCR.
In the UK, by 2017 NOx emissions had fallen by 72% from a peak in 1970; this is principally due to improvements in vehicle emissions technology and the closure of coal or oil fired power stations. Emissions from energy industries and manufacturing are the next biggest emitters of NOx, with gas turbines used as mechanical drives and for small scale power generation being an increasingly significant contributor (see Figure 1).
NOx itself is an overarching term used to describe the mixture of nitric oxide (NO) and nitrogen dioxide (NO2) produced in combustion processes, mainly as a result of thermal dissociation of nitrogen and oxygen at high temperatures, which combine to form NO or NO2. This reaction can be summarised as follows:
2N2 + O2 –> 2NO + 2N
N + O2 –> NO + O
In most combustion processes, the NO:NO2 generation ratio is high at around 7:1 to 9:1;5 however, NO is rapidly oxidised to NO2 in the atmosphere and therefore all NOx emissions are expressed in terms of NO2.
NOx presents a number of health and environmental problems including:
• Respiratory inflammation and infection, and increased sensitivity to allergens
• Generation of ground level ozone and other photochemical pollutants
• Formation of acid rain
• Eutrophication of aquatic environments.
Legislation and regulation
In terms of geographical impact, Europe arguably leads the way in regulation of industrial combustion processes. Historically, combustion units with a net thermal input (NTI) greater than 50 MW have been controlled by the LCPD. (As of 2012 this directive was incorporated in to the Industrial Emissions Directive [IED],7 which is a wider reaching and more comprehensive legislation that brings together several existing directives under one regulation.) The MCPD was introduced in 2016 to establish emission limits for combustion process with NTI below 50 MW down to 1 MW.
The LCPD and MCPD both set an absolute NOx limit of 50 mg/Nm3 for newly installed gas turbines. This reflects the fact that new systems operating with dry low emissions (DLE) combustion controls should readily achieve NOx emissions below 30 mg/Nm3, as detailed in the Large Combustion Plant Best Available Techniques Reference (BREF) document.6
For existing gas turbines, NOx emission limits are higher, at 75 mg/Nm3 (LCPD) and 150 mg/Nm3 (MCPD). However, older gas turbines without emission reduction technologies may not achieve NOx emissions lower than 150 mg/Nm3. In many cases, older gas turbines can be upgraded to incorporate modern combustion technologies, but this can be costly and is not technically viable for all machines. For such installations, SCR abatement offers a practical and economic solution for emissions compliance.
SCR technology and operation
SCR is a well-developed and proven technology which has been widely adopted to control emissions of NOx from combustion processes in many parts of the world. This includes gas turbine drives (both combined cycle and simple cycle); for example, as of 2009, 650 combined cycle gas turbine systems were operating with SCRs in the US.8
Rather than limiting the generation of NOx by combustion, SCR reduces the amount of NOx emitted to atmosphere via the exhaust system. Typically, reductions of 90% or higher8 can be achieved, and thus combustion systems generating NOx concentrations of up to 400-500 mg/Nm3 can achieve emissions of 50 mg/Nm3 or lower.
Installation and configuration
SCR works by injecting a chemical reductant into the combustion exhaust gas flow upstream of a catalyst bed, which converts nitrogen oxides into nitrogen, water vapour and carbon dioxide.
Figure 2 shows the main components of a SCR system as it may be installed with a simple cycle gas turbine exhaust system. Essentially, a SCR system comprises exhaust ductwork and a catalyst bed with a reductant storage and injection system. In some systems, depending on the catalyst being used and the exhaust gas temperature, an air tempering system may also be required.
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