Save on Clean Emissions Costs with the Right Valve

The Middle East and North Africa are likely to see an annual growth in power demand of 6 to 8 percent—roughly three times the world’s growth rate. Satisfying this surge in demand will require regional governments to leverage diverse power sources such as solar, oil, natural gas and coal. Governments have increased incentives for renewable energy sources within the region. The Middle East produces approximately 40 percent of its own electricity from oil—one of the world’s highest percentages. The MENA region also contains approximately one-third of the world’s natural gas reserves. However, the lack of infrastructure prevents the effective delivery of natural gas and development of a natural gas power grid. Coal is one of the most widely distributed energy sources. MENA countries, such as Morocco, have invested heavily in coal power generation. Coal generates only 30 percent of global energy, but leads all other sources in carbon dioxide emissions.

The MENA region must invest more than $300 billion in energy sources and infrastructure—especially coal—during the next five years to meet growing power demand.

Lime Slurry Scrubbers

Climate change concerns have caused the coal power industry to invest in flue-gas desulfurization (FGD) scrubbers. These scrubbers remove harmful pollutants, but drain valuable financial resources from power plants. Remember: An FGD scrubber or similar technology does not produce a single kilowatt of energy for the plant. The scrubbers inject limestone slurry into exhaust gases, which removes approximately 95 percent of sulfur dioxide. Power plants often struggle to find an efficient and cost-effective method for handling the limestone slurry. Lime does not dissolve, but is held in suspension; as the slurry slows down, the lime begins to settle out of suspension. Valves and instruments virtually self-destruct when used in a lime slurry piping system. Any cavities or open areas will allow the lime to deposit and harden. A valve that has cavities may become inoperable, and the instruments may no longer closely monitor the piping system. Valves that have cavities will require cleaning or flushing.

Right Valve, Right Application

A wide array of valves controls the power generation process. Roughly 4,000 valves are used in the typical 1,500-megawatt (MW) power plant. High pressures and immense pressure drops are some of the most demanding applications for critical service control valves. Power plants often spend hundreds of thousands for one of these valves. The typical power plant has hundreds of valve types and thousands of global manufacturers from which to choose. Every valve has a niche—an application where it excels. Matching the valve to its preferred application results in the most return for power plants and their customers in the MENA region.

Worst & Best Valve Solutions

For FGD scrubbers in coal-fired power plants, the abrasive lime slurry wears rotating equipment, scales on surfaces and obstructs flow. To save on maintenance costs while managing emissions, the power plant must choose the best valve solution for this application. Three different valves can be used in lime slurry piping systems, but only one has the most performance and savings potential.

Ball Valve

A ball valve with carbon-steel body, stainless-steel ball and stem, and polytetrafluoroethylene (PTFE) seats would be less than ideal for lime slurry. When the ball valve is closed, the lime will deposit scale on the ball’s surface. When the valve is opened and then closed again, the scaling lime will drag through the PTFE seats. The ball scaling will quickly destroy the relatively soft PTFE. Only with significant alterations could a ball valve be effective for lime slurry. Cobalt-chromium alloys or a stronger material should be used for the seats, which should also include a scraping lip to clean scaling lime off the ball. The ball should also be coated with a cobalt-chromium alloy to protect from scaling. The ball and plug valves should have cavity areas and flushing ports where the lime can settle. The changes to the ball valve design will likely increase the cost 10 to 20 times more than the standard ball valve, but only by adding these features will the valves survive in this harsh environment.

Knife Gate

A knife gate is also not ideal for handling lime slurry. The abrasive slurry will require a specially designed knife gate that incorporates a push-through gate and rubber sleeves. When the valve is 100 percent open, the rubber sleeves are the only components in contact with the lime slurry. The two rubber sleeves form a seal that prevents leakage out of the valve and isolates the stainless-steel gate. When the gate closes, it separates the rubber sleeves and pushes any buildup or scale out of the bottom of the valve and into a flushing area. Without regular flushing with clean water, lime deposits will pack the flushing area and damage the valve. The stainless-steel gate may need to be coated with PTFE. The coating allows lime scale to be wiped clean by secondary seals on opening and closing. When choosing between the PTFE coating or stainless steel, operators should remember the hassle of protecting the knife gate from scaling.

Pinch Valve

A pinch valve is the best valve choice for handling lime slurry. Pinch valves perform well in environments where scaling and abrasive materials are common challenges. The valves effectively handle extremely abrasive flows and typically out-perform even hardened metals or ceramics. A pinch valve uses a rubber sleeve that simultaneously pinches both sides to close the valve. The rubber sleeve cleans itself as it opens and closes. The two bars stretch the rubber sleeve as they begin to pinch, flaking material from the inner surface of the valve sleeve. As the valve closes, the velocity of the lime slurry increases and naturally pressure-washes the inner surface. The valve sleeve has no pockets or cavities in which lime deposits could accumulate. When selecting a pinch valve, higher quality means better performance. Inferior rubber manufacturing methods can severely limit the self-cleaning features that make pinch valves a top choice for emissions scrubbers. Consumers should choose brands that have proper quality control and International Organization for Standardization (ISO) 9001 certification. Grades of stainless steel vary little between manufacturers, but natural rubber or styrene butadiene can vary greatly in its quality and performance characteristics.

Other Industries

Lime or lime slurry is not limited to the power industry. The substance, which can also be used to control pH, has applications in the steel manufacturing, food and beverage processing, glass, pulp and paper, chemical and mining industries. All these industries, along with the power generation industry, share a need for an effective valve solution to handle lime slurry. Coal-burning power plants can use special rubber-lined, push-through knife gates and pinch valves in their bottom ash systems. Burning coal produces spent ash that, when mixed with high-pressure water (for example, 25 bar), can be highly abrasive and damage valves. Push-through knife and pinch valves provide a low cost-of-ownership solution to managing coal ash and waste.

Looking Ahead

How nations generate power is crucial to their economic future. The delivery of reliable energy to many countries in the MENA region will bring added prosperity and economic development. Close attention to all forms of energy technology, including rotating equipment, will best serve these countries in the search for the right energy strategy.