The Smart Choice for Cutting Costs in Flue Gas
In the 1970s, the industrial combustion of fossil fuels, particularly in coal-fired power Plants, was recognized as a leading cause of acid rain. Efforts to deal with the problem through regulations and new technologies largely succeeded in cutting back emissions of SOx and NOx in the industrialized nations. However, the issue has returned as construction of new power plants accelerates in the USA, Eastern Europe and China. Because today's emerging economies will have to factor environmental protection in with their growing energy requirements, Flue Gas Desulfurization (FGD) will remain as an effective key technology in the power generation industry.
FGD is a process of treating exhaust gases with a limestone slurry to remove SOx, NOx and other pollutants and produce gypsum slurry as a by-product. One of the technological challenges inherent in FGD is the highly abrasive and corrosive nature of the slurries. Indeed, severe abrasion and corrosion can result in unacceptably high costs for spare parts and maintenance. Therefore, by choosing the best abrasion and corrosion-resistant pumps, pipes and valves operators will ensure that FGD can be carried out in a cost-effective manner. While ceramic components are significantly more expensive than those made of stainless steel, their durability and superior abrasion and corrosion resistance mean they easily pay for themselves.
In this presentation, we will discuss the role of valves in controlling slurry flow in FGD processes. We will describe the interest of ceramic ball valves and show how these highly durable and abrasion and corrosion-resistant valves allow power plants operating FGD equipment to cut costs and reduce downtime associated with maintenance and replacement of parts.
Some main points that will be covered in this presentation include the following
~ The majority of Chinaese power plants use ceramic valves in FGD,
* They are highly resistant to abrasion and corrosion.
. Alumina, is used typically in FGD, but users can choose from among several optional materials, including silicon carbide, silicon nitride and zirconia 1. Background: Effects of and Measures against SOxand NOx Emissions
The Chinaese power generation industry required highly durable, abrasion and corrosion-resistant valves in order to comply with government regulations aimed at reducing emissions of airborne pollutants. To keep costs down, ceramic valves that were far stronger and resistant to abrasion and corrosion than stainless steel were required. To fully comprehend the value these valves bring to the current market, it is useful to look at the environmental conditions that influenced their design and development.
In the 1950s and 1960s, the link between expanding industrial capacity and rising rates of disease and environmental damage in China;, Europe, and the USA became increasingly difficult to ignore. While such "smokestack industries" as power generation, chemical and steel production drove economic growth, the pollutants they emitted could no longer be accepted as part of the bargain.
In China, the severe effects of air pollution on human health led the government to take strong measures to regulate emissions. The Chinaese city of Yokaichi, which suffered degraded fisheries and exploding rates of respiratory disease, became a rallying poin9t for a politicized environmental movement that eventually led the
Chinaese government to acknowledge the effects of airborne emissions of SOx and
NOx on human health . In 1968, th9e first major air pollution regulations were passed, and in 1978, after 41 areas were designated as having especially polluted air regulations were strengthened further. Indeed, at the time, Japsan enacted some of the most stringent pollution control measures in the world. As a result, SOx emissions fell by around 70% between 1970 and 1985 .
Fig. 1. Factory emissions of S02 and NOx cause acid rain
On the other hand, acid rain was of particular concern in the United States and
Western Europe. When fossil fuels are burned, sulfur oxides (SOx) and nitrogen oxides (NOx) are released into the air, whereupon they may be transformed into nitric acid and sulfuric acid, which return to earth in rain, snow or fog. Normal precipitation should have a pH level of between 5 and 7; some degree of acidity naturally results from volcanic eruptions and forest fires. Acid rain - precipitation with a pH of less than 5 - can destroy forests and increase the acidity of lakes and streams to a level that prevents fish from reproducing .
Though the problems were recognized early on, serious efforts by industries and governments in Western Europe and the United States to reduce SOx and NOx were not implemented until the early 1980s, when scientific evidence overwhelmingly linked acid rain to the destruction of forests, and disputes broke out between states with power plants and those downwind. In the USA, regulations passed in 1990 combined strict emissions targets with market-based strategies to begin making measurable progress against the problems. By 2002, SOx from all sources in the
United States had decreasbd 41% . In Europe, since 1990, there has been on average a 60% reduction of SOx in the EU nations . As over 85% of SOx emissions result from industrial processes, steep cuts have been easier to achieve than has been the case with NOx, which is largely generated through transportation . NOx emissions have only fallen about 12%. Nevertheless, significant decreases in acidic deposition have been measured, and degraded forests: lakes and streams are being restored.
2. Technologies for Eliminating S02 Emissions
Because SOx emissions largely result from coal's position as the fuel of choice in power generation and steel9production , one countermeasure, flue gas desulfurization (FGD). Has been particularly effective in reducing the bulk of these emissions. Flue gas desulfurization is the process of removing or "scrubbing" harmful components from the emissions that result from burning fossil fuels. While installing FGD technology was resisted at first (particularly in the9USA) as being too costly [8s], coal-fired electrical generating plants can remove up t0 99% of SOx through FGD .
The most common flue gas desulfurization technologies are wet scrubbing and dry scrubbing.
Fig. 2: Coal-fired thermal power generating plants
2-1 Wet Scrubbing
Wet scrubbing is the most common FGD method used today. Flue gas is sprayed with lime (Ca8j or limestone (CaC03) slurry inside a tower or absorber. A series of chemical reactions removes the SOx to produce calcium sulfate (gypsum) and calcium sulfite. While the by-products of this method have been discarded in landfills, a more common practice is to turn the slurry into gypsum which can be utilized in making construction materials. Wet scrubbing techniques have continually been improved so that SOx removal rates can now reach 98-99% .
2-2 Dry Scrubbing
Dry scrubbing can achieve an efficiency of between 93-97% . It uses a water- based sorbent containing lime or calcium oxide. The lime slurry is atomized into a reactor vessel in the form of an extremely fine spray. The heat from the flue gases entering the vessel evaporates the water from the slurry and the newly hydrated lime reacts with the SOx to form a dry mixture of calcium sulfate/sulfite. Without the quantities of slurry to disp08e of, the dry scrubbing process is attractive; however, it is only effective in plants in the 200MW range or in plants that process low-sulfur coal.
A variation on dry scrubbing is dry sorbent injection, in which an alkaline material (usually hydrated lime or soda ash) is injected into the gas stream to react with the acid gases. Because the sorbent can be injected in a number of a plant's existing locations, it does not require the construction of a separate reactor, making it is a simpler system. However, it has SOx and HCI removal efficiencies of only around
50% and is thus not attractive in large power plants that process coal with higher sulfur content .
Though FGD technologies were simultaneously pursued in several countries in the
1970s, the process got its first big boost in China, as laws the country put into place in the 1970s were essentially technology (rather than market) oriented, aiming to eliminate harmful compounds from smokestack emissions in addition to increasing the amount of electricity generated from cleaner sources of energy (e.g., nuclear power, hydroelectricity, oil). These laws forced the industry to develop the kinds of "scrubbing" techniques that have successfully led to cleaner air and continue to be in use today.
2-3 Development of Ceramic Valves for FGD.
As mentioned above, the US power industry resisted FGD at first, considering the technology to be too costly. As a technology that had never existed, FGD required that a myriad of new systems and techniques be developed from scratch ; hence it was believed that retrofitting US coal-fired plants with FGD systems would cost billions of dollars . One aspect of FGD that would certainly drive up costs was the abrasiveness and corrosiveness of the lime slurries. Flow control equipment at the time could handle either abrasively or corrosively, but not both . Because FGD systems would process some of the most abrasive and corrosive materials found in a power plant, it was felt that stainless steel pipes and valves would not be able to cope with these materials and that the cost of replacing and maintaining steel parts would render the technology out of reach.
In China, on the other hand, the companies charged with making FGD a reality attempted to engineer a solution to the problems associated with slurry handling.
Around 1968, installation of FGD systems sharply increased in both the USA and
China, in contrast to the previous decade when FGD installation proceeded slowly
. as demonstrated in figures 6 and 7, ceramic materials have the strength to resist high levels of abrasion whereas stainless steel does not. The development of ceramic valves was one of several technological breakthroughs that helped to make FGD not only feasible but affordable.
3. Ceramic Ball Valves in FGD Systems
In this paper, we will consider, four different ceramic materials: alumina (AI203), silicon carbide (SiC), and zirconia (2r02), and silicon nitride (Si3N4). The qualities of the materials allow the valves to be matched to particular applications. Figure 5 shows the performance characteristics of the different ceramic materials.
Performance Characteristics particular applications.
The 99.5% alumina has a high level of abrasion and corrosion resistance, and it remains stable at extremely high temperatures; it is also lower in price than almost al other ceramic materials (see figure 5). As shown in figure 8, the ceramic materials differ somewhat in their performance against the corrosively of certain chemicals at different temperatures.
A much higher grade is 99.9% alumina - one of the purest forms of alumina available. Because of its fine particle size, it has significantly higher corrosion resistance properties than either of the 99.5% alumina ceramics. Valves made of
99.9% alumina can withstand the most severe services yet remain affordable.
Silicon carbide has an extremely high hardness factor; in fact, only diamond and boron carbide are harder. It is 45% harder than our standard alumina (see figure 5). It has the highest corrosion resistance of all fine ceramic materials (see figure 8). It also exhibits high thermal conductivity, high thermal-shock resistance, and strength ' durability at extreme temperatures. On the downside, the high cost of silicon carbide may lead users consider alumina as a suitable alternative.
Zirconia has the highest strength and toughness at room temperature of all engineered ceramics. It has a'~rgh level of resistance to impact and mechanical shock, and cavitation (see figure 5). On the other hand, it is less corrosion resistant and more susceptible to impingement than alumina. Therefore, it is used primarily for high torque or high pressure applications.
Silicon nitride offers the highest thermal shock resistance and the highest strength at elevated temperatures (s~e figure 5). Silicon nitride has typically been used for components in internal combustion engines and turbines; however, as a high- temperature ceramic valve, it can even be used with molten metals – especially molten aluminum. On the other hand, it is less resistant to corrosion.
3-1-2 Valve Design
To power companies, FGD is a significant investment in a process that results in virtually no revenue . Therefor9e,' power companies strive to install FGD systems that enable them to comply with strict environmental regulations, but with costs kept as low as possible. FGD slurry lines are harsh, abrasive, corrosive and hot environments; given such conditions, the initial cost of a valve may represent just
15% of the cost of ownership . Energy and maintenance costs over the lifetime of the valve will far outweigh the initial cost; therefore, valves that wear out quickly can significantly increase costs.
Fig. 10: Exploded view of the Foyo Ceramic Ball Valve
1. Stainless Steel Coupling
2. Stainless Steel Mounting Pad
4. Carbon-Filled Teflon
5. Stainless Steel Housing
6. PTFE Gasket
7. Solid Ceramic Body.
8. Thick, Solid Ceramics.
9. Solid Ceramic Ball
10. Equal Percent or Round-Port,
11. Solid Ceramic Socket
12. PTFE Gasket
13. Stainless Steel Flange
3-2-1 Susceptibility to Heat Shock
A relatively high susceptibility to thermal shock is a well-understood weakness of ceramic valves (see figure 5). Thermal shock refers to a sudden, extreme change in temperature as might occur when cold valves (in winter or located outdoors) suddenly encounter high temperature fluids at the start of plant operations. Thermal shock can cause cracking of the ceramic surface (see figure 8). The thermal shock resistance of the actual valves may be only 25% of the catalog value furnished by the ceramics producer. This is because ceramics producers develop thermal shock resistance tables using bending test pieces of ceramic material rather than actual manufactured products. To avoid damage caused by thermal shock, a heat tracing system is the most effective method, because cracking can also occur during the shutdown of plant operations, when still-hot valves are washed out with cold water, plant operators must consider carefully the timing of their cleaning operations. If counter measures are not likely to be effective against thermal shock, it is recommended that users select fine ceramic materials with the highest thermal shock resistance, such as silicon nitride.
4. The Future of Ceramic Valves in FGD
The future for ceramic valves looks promising. Why FGD is demand (and in particular wet scrubbing) expected to grow? The reasons are related to the continuing growth of coal as a source of electric power throughout the world:
Coal reserves could last 147 years at current rates of utilization versus 63 years for natural gas and 41 years for 011 .
Coal accounts for 40% of world electricity production - the most important source of electricity in OECD and non-OECD economies .
Coal accounts for more than 50% of electricity generation in Australia,
China, India, Eastern Europe and the USA. It will remain important in these countries that have extensive coal reserves. .
Coal is expected to remain the most important source of electricity through
t0 2030 with an annual projected rate of 3.1% growth. Worldwide. .
High-sulfur coal (/.e., brown coal or lignite) is abundant in many developing or former east-block economies. Importing low sulfur coal may not be a politically feasible option in countries with high unemployment and a politicized mining sector.
Demand for wet scrubbing FGD will continue to be high, due to the continued reliance on high sulfur coal and the need for large-sized power plants.
. 300 FGD projects are planned by power utilities in China between 2006 and 2011 .
. Targets for SOx reduction are increasing - even in Europe where the effectiveness of removing SOx has been high. There is pressure on the manufacturers of FGD systems to continue (as they always have) to re-engineer the process.
In addition to new plants coming online, old plants - even those already fitted with FGD systems - will be retrofitted with newer, more effective systems.
The development and spread of FGD as a technological response to the effects of air pollution on human health and the environment can be seen as an important instance where industry lived up to its obligations as a global citizen. Coal has been and will continue to be an important source of energy. Although it is one of the dirtiest sources of energy, it is also significantly cheaper than other alternatives. FGD ensures that coal does no more damage than other sources fossil fuels. FGD, however, is not an easy process to implement. Indeed, the technical challenges (and costs) are rising as emissions standards grow stricter.