Standing on the shoulders of giants5 July 2002
Retrofitting SCR plant 230 feet above the Gibson power station in the USA requires a titanic effort physically and logistically, involving the world's biggest tower crane at the USA's second biggest coal fired plant.
Cinergy-PSI's Gibson generating station, located near Evansville in Southern Indiana, was built in the mid 1970s and is the second largest (3250 MW) coal fired power station in the USA. The station is equipped with five GE 650 MW steam turbine generators and Foster Wheeler supercritical boilers operating at 3850 psig, 1005°F superheat and 1005°F reheat. Gibson station has a combined coal burning capacity exceeding eight million tons annually, which is procured mainly from mines within the local Indiana and Illinois basin coal fields. Existing environmental facilities at Gibson included SO2 scrubbing on units 4 and 5, in addition to electrostatic precipitators and low NOx burners on all five units. Gibson utilises a 3600 acre man-made lake for condensate cooling and manages over 160 acres of dedicated wetland refuge at the generating station site.
US clean air regulations
Driven by complaint petitions filed by the north-eastern states, the US Federal NOx State Implementation Plan (SIP) call is the most pertinent set of environmental regulations currently affecting capital investment by US power utility companies. Under 1997/98 legislation the US Environmental Protection Agency (EPA) found that NOx emissions from sources in 22 states significantly contributed to non-attainment of the ozone standard. In 1998, the EPA finalised its ozone transport rule, known as the NOx SIP call.
The original NOx SIP call applied to 22 states in the eastern portion of the US, including Indiana, and required that these states reduce NOx during the 1 May to 30 September ozone season, primarily from industrial and utility power sources, to a level of 0.15 lb/MBtu (lb per million BTU) by 31 May 2004. Under the SIP call, each affected state, including Indiana, is given a NOx budget, starting in the 2004 ozone season. States then assign electric generating units a share of that budget based on the 0.15 lb/MBtu emission rate. The EPA originally gave the states until September 1999 to incorporate NOx reductions (and under the direction of the state a NOx trading programme) into their SIPs. Following numerous federal appeals the Indiana Air Pollution Control Board adopted the finally amended NOx SIP call rule in June 2001. This meant that Cinergy-PSI were required to comply with the Indiana SIP rule specifying an emission limit of 0.15 lb/MBtu, the compliance deadline being 31 May, 2004.
The EPA's SIP call included a model NOx allowance cap-and-trade programme that states could adopt in their final SIP, which would allow sources to buy and sell allowances to meet their budgeted NOx emission levels. However, as of 2001 it was unclear how the emissions allowance market would develop, or the extent to which Cinergy-PSI could rely on such a market to comply with the legislation. Failure to comply could result in a variety of sanctions including excess penalties of up to $27 500 per day for each ton of excess NOx emission. The NOx budget allocation to Cinergy-PSI proposed by the Indiana state regulator for the 2004-2006 ozone season was set at 12 343 tons. To meet this budget, Cinergy-PSI needed to reduce its overall NOx emissions by 63 per cent.
PSI's NOx compliance programme is driven by the need to comply with environmental legislation. The prime economic issues influencing project definition are the cost of non-compliance penalties, project capital costs, resultant O&M costs and the cost (or value) of NOx credits, within a future NOx trading market.
The costs of executing SCR retrofits are largely dependent on site specific issues, critically configuration and layout of existing facilities. According to the EPRI paper Technical feasibility and cost of SCR NOx control (GS-7266), SCR capital costs were estimated in the range 96-140 $/kW (1991 prices) for retrofit to coal-fired units, depending on furnace type and reactor location. The EPRI publication projects wide levelised (ie from a uniform base) cost variations, in constant 1990 dollars per ton of NOx removed, dependent on process conditions, ranging from $2750 to nearly $4000/t for coal firing.
Together with architect-engineer consultants, Cinergy-PSI evaluated the major available NOx control technologies:
• Selective catalytic reduction (SCR) is capable of removing in excess of 85 per cent of the NOx by a catalytic chemical process that reduces NOx to nitrogen and water. SCRs are the most capital intensive of the control technologies and take 24 to 36 months to design, procure and construct. SCRs also require the longest unit outages for installation 'tie-in', up to twelve weeks depending on unit configuration.
• Selective non-catalytic reduction (SNCR) is not as capital intensive as SCR technology but is only capable of reducing 20 to 30 per cent of the NOx. SNCRs do not require as long a lead time as SCRs and are effective on smaller units.
• Boiler optimisation programs are computer control programs which enable improved monitoring and control of the combustion process towards reduced NOx emissions; this technology is generally used in conjunction with other technologies such as low NOx burners and can help reduce NOx in the range 5 to 10 per cent.
• Overfire air is a process of injecting a portion of the burner supply air above the burners in a boiler, reducing temperatures and NOx emissions; this technology is typically used in conjunction with low NOx burners.
• Low NOx burners are burners designed to lower combustion temperatures in a boiler, reducing temperatures and NOx emissions by approximately 35 - 50 per cent.
Cinergy-PSI developed the project front-end definition, working with architect-engineers Sargent & Lundy and Stone & Webster to plan, model and verify the available technology options. Conceptual designs were prepared and sensitised for the various compliance options and cost estimates were developed for use in the model. The options were ranked on the basis of the marginal cost per ton of NOx removed. On the basis of the foregoing modelling and analysis the decision was made to utilise SCR technology at Gibson station.
Owing to timing of the NOx reduction deadlines within the original SIP call, Cinergy-PSI was faced with a compressed execution time period, driven by the outage programmes of the subject generating Stations. In recognition of this constraint Cinergy-PSI decided to execute the Gibson SCRs on a fast-track basis utilising an alliance of engineering and construction companies. The architect engineer for the alliance is Sargent & Lundy. Foster Wheeler Energy is EPC contractor for ammonia system(s); Foster Wheeler is also procurement services contractor, whilst the construction contractor is Foster Wheeler Zack.
The total programme duration for all five Gibson SCRs was set at four years, with tie-in and completion of the final unit scheduled for May 2004. The Primavera construction control tool based EPC schedule provides for tie-in of each of the five SCRs during either a spring or fall outage, ie within periods of relatively low power demand, SCR tie-ins being concurrent with planned outage maintenance.
Process and scope
The Gibson SCRs are located in new flue gas ducts added between the economiser outlets and the air pre-heater inlets. Vaporised ammonia is injected into the flue gas at a sufficient distance upstream from the catalyst to ensure mixing. When the flue gas and ammonia mixture passes over the catalyst a reaction occurs which reduces the NOx to nitrogen and water. The SCRs are designed to be operated during the summer months and at variable loads (of up to 30 per cent of peak) by means of an economiser bypass which maintains the catalyst above minimum temperatures. Owing to the additional pressure drop caused by the SCRs it was necessary to revamp capacity in the induced draft fans; to provide operating flexibility variable speed drives were added to the ID fans. The major components of the SCR installation are shown in Table 1.
The scale of the Gibson SCR project is impressive. Each SCR is elevated at 230 ft above ground level; the overall SCR programme entails 20 000 tons of structural steel and 15 000 tons of catalyst reactor and flue duct components. FW and Cinergy-PSI procurement is international. Main process components have been sourced from either the USA or Europe whilst fabricated high grade structural steel is supplied from the Middle East. The design required large pilecaps in a four tower configuration to bridge over existing air heater and ID fan ductwork, above which the reactor box is mounted at a height of 230 ft from the ground.
Construction work started in May 2000 with surveys and preparation for piling, and mechanical enabling work on unit 2. The enabling work included relocating operating equipment, re-routing existing electrical and I&C equipment, structural modifications and re-routes of high pressure boiler piping. Owing to site congestion an early decision was made to pre-assemble the steel and major duct sections into modules of weight up to 120 tons. Erection of these components made necessary the use of gigantic cranes, in the case of units 2 and 3 the Kroll 10000, the world's largest tower crane, with a capacity of 120 tons at a 300 ft radius. Fifteen were built in all, 13 of them going to the Soviet Union in the 1980s to build its nuclear reactor fleet. Assigned to unit 4, which is the most constricted of the Gibson SCRs, is a Demag heavy duty 90 ft base 'platform ringer' crane with a capacity of 120 tons at a 400 ft radius. In support of the major cranes during rigging and de-rigging is a fleet of attendant cranes, which includes a crawling Demag CC4800.
Labour for the SCR project is expected to reach 800 men at its peak in 2003; the total craft labour man-hour projection is over 4 million hours. Safety management is a primary objective to both Cinergy-PSI and Foster Wheeler, and Foster Wheeler Zack in April 2002 achieved its first one million man-hour free of lost time accidents milestone.
All construction work must be progressed in close co-ordination with the Cinergy-PSI operating facility, with no unplanned generating disruption. Maximum opportunity is realised from short scheduled outages and de-rates to facilitate critical construction activities. To achieve these objectives a high degree of construction planning is required; the Primavera control tool includes numerous pre-outage 'finest hour' activity milestones to eliminate the potential for Cinergy-PSI business disruption. SCRs are tied-in during twelve week outages, which require construction critical path activities to work around the clock on an hourly schedule basis.
Construction status of the overall Gibson SCR project was approximately 50 per cent complete as of April 2002. Unit 2 SCR had already been tested successfully at design (85 per cent) NOx reduction whilst unit 3 SCR is in the advanced stages of commissioning. Thus Cinergy-PSI will be able to operate SCRs on units 2 and 3 in test mode during the 2002 summer ozone season, which commenced on 1 May 2002.
Information on clean air regulations was based on the testimony of William Tyndall, a PSI Energy v-p, to Indiana State Regulatory Commission in January 2002. Information on technical options was based on the testimony of John Roebel, a PSI Energy v-p, to the same Commission. Information on the project status came from Cinergy-PSI construction manager Randy Findlay.
TablesTable 1. Major components and services