Scrubbing up nicely

In the belief that it will be financially beneficial, PPL is investing over $600 million in scrubbers for three of its largest coal fired units, despite the fact that their emissions are already below the legal limits.

George Lewis, PPL Corporation, USA

With new environmental rules and regulations and a strict corporate environmental policy bringing mounting pressure, PPL finds itself with the question that is familiar to many, how to maintain availability and competitiveness while generating electricity with as little impact on the environment as possible?

PPL Generation, a subsidiary of PPL Corporation, generates around 12 000 MW of electricity at sites across the US. In its power plant portfolio, the most vulnerable to environmental pressures are its coal fired facilities that have a total output of 4500 MW. In 2003, these power plants produced 29 million MWh.

Since the Clean Air Act was passed in 1990 several plant improvements and operational changes have reduced emissions from PPL’s power plant portfolio. Sulphur dioxide (SO2) emissions have fallen by 40 per cent, nitrogen oxide (NOx) emissions have been reduced by 60 per cent, and the carbon dioxide (CO2) emissions rate has dropped by 15 per cent.

Decision process

Despite these emission reductions, PPL decided to conduct a Clean Air Strategy update in 2003 as part of its ongoing environmental stewardship. This update included a review of control technologies for compliance with Clean Air Act requirements. The economic analysis determined that the addition of flue gas desulfurization systems (scrubbers) at PPL’s three largest coal fired units in Pennsylvania – Units 1 and 2 at the Montour power plant and Unit 3 at the Brunner Island power plant – would provide significant environmental benefits to the region, reduce PPL’s exposure to the increasing volatility in the SO2 emission allowance market, and maintain the economic competitiveness of these generating units.

All three units in total generate 775 MW and operate with supercritical, once-through pressurized boilers. The Montour plant is located in Montour County in north central Pennsylvania. The Brunner Island plant is located in York County in south central Pennsylvania.

Figure 1. The options for water supply and wastewater treatment are the biggest issue that scrubber designers have to contend with at the Montour power plant in north central Pennsylvania
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During the scrubbing process SO2 is removed from flue gas leaving the boiler, before it passes up the chimney. The update report concluded that that the addition of scrubbers would enhance the existing pollution controls on these generating units, which have low-NOx burners and electrostatic precipitators to remove particulates (ash). The two Montour units also have Selective Catalytic Reduction systems.

Aside from the most obvious benefit of installing scrubbers i.e. a reduction in SO2 emissions of about 168 000 tonnes annually, PPL found that other factors such as fuel supply strategy and a fall in mercury emissions also made the installation of scrubbers an appealing prospect.

By adding scrubbers the generation company discovered it would also have greater flexibility in the supply of coal to these three base load units. PPL’s current compliance strategy for SO2 emissions means that it has to use low sulphur coal, but the use of scrubbers will increase its future coal supply options.

An additional benefit factored into the evaluation of scrubbers is the expectation that they will be part of a strategy for reduced mercury emissions. The US Environmental Protection Agency has issued a proposed rule for mercury emissions.

In February 2005, PPL chairman, president and chief executive officer, William F. Hecht announced the company would be spending at least $630 million on scrubber projects and engineering procurement and construction contracts for wet limestone scrubbers at the three units were awarded to Shaw Stone & Webster just three months later. Construction will begin in 2006 and PPL expects all three scrubbers to be operational by late 2008.

Project economics

In evaluating the business case for scrubbers, PPL wrestled with some very basic questions. When to build? What will it cost? What will be the value of emission allowances when the scrubber is in service?

The analysis considered anticipated new requirements for SO2 and NOx reductions outlined in the US Environmental Protection Agency’s Clean Air Interstate Rule (CAIR) proposal. Applying to 28 eastern states, including Pennsylvania, CAIR addresses non-attainment of the EPA’s ozone and fine particulates (related to SO2 and NOx) standards.

PPL’s research determined that any reductions in SO2 emission allowances in 2010, and further reductions in 2015, would have a significant impact on large, coal fired generation plants in Pennsylvania. Continued operation of the Montour and Brunner Island plants without scrubbers would require the purchase of a growing number of emission allowances.

As such the company’s Risk Management group conducted a sensitivity survey using low case and high case SO2 emission allowance price curves. The results indicated significantly higher downside economic exposure from relying on market purchases of emission allowances when compared with installing scrubbers.

Given the steady increase in the value of SO2 emission allowances – to as much as $800 per tonne in the early months of 2005 – it became clear that the scrubbers at Montour and Brunner Island should be placed in service sooner rather than later.

Initial studies based on economics at the time assumed a sulphur removal rate of 95 per cent with the use of scrubbers. With the price escalation in SO2 emission allowances, PPL re-evaluated the economics discovering that increasing the sulphur removal rate to 97 per cent was justified economically based on reduced use of emission allowances.

Execution and risk management

As a result of the economic analysis, the challenge for PPL became bringing the in-service dates forward. So the company turned to its Engineering and Construction Management Group, which was just wrapping up a five-year Peaking Power building programme. And it was good timing in more than one respect for PPL as the Project Management Office was able to integrate lessons learned from previous projects and refine project management processes and procedures especially for these scrubber installation projects. The challenge for the team was to examine all facets of the design, planning, and construction in order to accelerate the schedule and get all three scrubbers in service by the end of 2008.

The team put together a comprehensive Risk Management Plan that identified the construction risks which could jeopardize the accelerated in-service dates. The Plan highlighted gaining air permit approval from the Pennsylvania Department of Environmental Protection (DEP), as the critical point which could delay the installation. PPL has thus far had a series of meetings with DEP officials to review scrubber plans and schedules, and to ensure that PPL’s application includes all of the information the DEP needs to process the Montour air permits by January 2006 and the Brunner Island air permit by August 2006.

Montour issues

A project of Montour’s magnitude presents many technical issues for consideration. For this power plant, water supply and wastewater treatment are the most critical issue in the design of the scrubbers and auxiliary systems. The plant is located about 19 km from its main water source, the West Branch of the Susquehanna River. Water from the river is pumped to the plant through a 106.7 cm diameter pipeline. A nearby reservoir, which PPL maintains for public recreational use, serves as a backup water source.

Because of limits on river water withdrawals, especially during periods of low river flow, and high turbidity when it rains, PPL plans to use the plant’s detention basin for the scrubbers’ process water. The detention basin flow consists primarily of water taken from the cooling tower basins for sluicing bottom ash and pyrites.

Using water from the detention basin requires important decisions to be made on scrubber operating conditions. Normal water demand for the scrubbers will be about 15 million litres per day. In addition, the wastewater treatment process will require about 2.8 million litres per day. Effluent flow with both units operating is 22.7 to 26.5 million litres per day.

Figure 2. SO2 is removed from flue gas leaving the boiler, before it passes up the chimney
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PPL is considering two options for scrubber water demand. One option is to restrict the scrubber cycle blowdown rate to 946 litres per minute, which would result in chloride concentration of 30 000 ppm based on the chemical composition of coal used at Montour and restrict materials that can be used in the absorber and downstream equipment. PPL has focused on Stebbins Semplate tile lined absorbers, which can withstand the chloride concentrations expected at Montour. While several other alloys are available, only C276 has demonstrated the ability to operate on a daily basis at chloride levels as high as 30 000 ppm with no adverse consequences.

A second option is to increase the blowdown water flow to 2650 litres per minute. The higher flow rate would reduce chloride concentration enough to broaden the range of materials while leaving it still high enough to require cooling with blowdown water prior to biological treatment for removal of other constituents from the wastewater. With the second option, increased water demand of about 3.8 million additional litres per day would result in limited flow margin for droughts or extended periods of dry weather when the plant must curtail river water use.

PPL will continue to evaluate the environmental, technical and economic impacts of both options for blowdown water rate before making a final decision.

To minimize the environmental impact of scrubber operations, a state-of-the-art wastewater treatment process will be used at Montour. The treatment process will include metals precipitation and solids removal, as well as treatment for nitrates and selenium compounds. The biological treatment process for nitrates and selenium requires dilution of the effluent from the metals precipitation step with 2.8 million litres of water per day.

Figure 3. The aged and decayed condition of the electrostatic precipitators at Brunner Island has meant new fields will have to be installed before the scrubbers can be fitted
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The Montour plant currently discharges effluent from the detention basin into a small stream with a flow that makes up a small percentage of the total flow after the plant discharge. Because the discharge flow is so large, standard treatment technology for the scrubber effluent may not be adequate to meet water quality criteria.

The two options for effluent handling are to pipe the water back to the Susquehanna River by constructing a second 19 km pipeline, or install an evaporator-crystallizer process to remove chlorides and return the water to the scrubber. The purification process results in no liquid discharge but is unproven in this application, requires a significant amount of power to operate and produces a potentially hazardous solid waste that must be disposed of safely.

Brunner Island issues

Brunner Island is a run of river plant along the Susquehanna River south of Harrisburg, Pa. While the scrubber will have access to as much water as it needs, PPL is considering wastewater treatment options similar to those at Montour to minimize the environmental effects of the scrubber operation.

A technical challenge at Brunner Island is the condition of the electrostatic precipitators. Particulate collection equipment is made up of the original Joy-Western precipitation units arranged in a chevron design downstream of the air heaters, followed by Lodge Cottrell rigid frame precipitators, which were added in the 1980s. However, as the original boxes are reaching the end of their useful life due to deterioration of both the casing and the internals, PPL decided to add some additional fields on the inlets of the Lodge Cottrell boxes and demolish the original Joy-Western precipitation boxes.

The new fields will be assembled adjacent to the existing precipitators, then rolled into place during a planned outage. PPL must complete the precipitator upgrade, including demolition of the original boxes, before it can install foundations for the scrubber and new chimney. A fast track project is under way to complete the precipitator upgrades during a planned outage in 2006.

Booster fans and benefits

Booster fans were added in previous pollution control projects as the boilers for all three units operate with the furnace pressure positive. Scrubber construction will require additional booster fans in order to handle the pressure drop through the scrubbers. To limit the upstream negative pressure these additional fans might produce, PPL plans to use the existing chimney as a ‘damperless bypass’, bleeding a small amount of air into the gas flow, thereby serving as a vacuum relief.

In addition to the environmental benefits, the project will create about 800 temporary construction jobs between the two locations, and an unspecified number of permanent jobs at both plants for scrubber operations.

PPL also plans to reuse the scrubber byproducts. It already uses fly ash collected by pollution control equipment on these generating units. The particulate is processed and reused as fill for construction projects.

SO2 in the flue gas reacts with the limestone slurry inside the scrubber absorber to produce calcium sulphite, which cannot be dewatered and used beneficially. Adding oxygen to the pool of liquid in the reaction tank at the bottom of the absorber promotes the formation of calcium sulphate, or artificial gypsum. This gypsum byproduct can be removed, dewatered and reused.

The byproducts of the Montour scrubbers could be sold to a wallboard manufacturer and could also be used for reclaiming abandoned mine sites in eastern Pennsylvania. PPL is designing the absorber and auxiliaries, including the gypsum drying equipment, to produce purity levels that wallboard manufacturers require and it is currently negotiating with a potential partner regarding the sale and reuse of scrubber byproducts.

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