Scrubbing water
The ratcheting down of emission levels for sulfur dioxide has sparked a mini-boom in the market for flue gas desulfurization (FGD) systems, or scrubbers. NETL estimates that the size of the U.S. FGD market is expected to increase by more than 100,000 MW over the next 10 years. Although water requirements for scrubbing are a fraction of those needed for cooling purposes, FGD units require a significant amount of water to produce and handle the various process streams (limestone slurry, scrubber sludge, and the like). NETL’s 2005 “Power Plant Water Usage and Loss Study” found that makeup water requirements for the FGD island at a 550-MW (nominal) subcritical coal-fired power plant are about 570 gallons/minute (gpm), vs. about 9,500 gpm for cooling water makeup.
Flue gas scrubbing can be accomplished with either dry or wet systems. Wet scrubbers entrain the flue gas in a water spray, capturing sulfur dioxide and other pollutants, which are then removed by creating an alkaline slurry. Dry scrubbing injects the alkaline particles directly into the flue gas stream, obviating the need for water, but the more limited contact between reactants in the absence of water results in lower pollutant removal efficiencies.
New technologies that reduce or recover evaporative losses from scrubbing flue gas, or increase the removal efficiency of dry scrubbing, could reduce water use and associated costs. Another way to quantify the water requirements for a typical wet scrubber is to determine the amount of water that a plant could save by shifting from wet to dry scrubbing, or by capturing all of the evaporation produced by wet scrubbing. NETL came up with a figure of 25 gal/MWh. Again using $1 to $4/kgal as the range of total water costs, the savings would amount to 2.5 to 10 cents/MWh. For our 350-MW baseload plant operating year-round, the potential annual savings from shifting from wet to dry scrubbing ranges from $75,000 to $300,000, with a midrange value of $220,000.
If all three loss processes (evaporation from cooling towers, blowdown, and flue gas scrubbing) could be simultaneously reduced at an existing 350-MW coal-fired plant, the total annual cost savings would be $875,000 to $2,700,000 (depending on climate and the cost of water), with a midrange total of $1,950,000. Figure 3 shows the potential savings for each process, assuming an intermediate cost of $2.82/kgal for total water use. Most of the savings are from reducing blowdown and evaporative losses, with the elimination of losses from wet scrubbing a minor contributor.
3. Saving water, and dollars. Potential savings from reducing three process water losses at a 350-MW coal-fired power plant, assuming a total water cost of $2.82/kgal. Source: EPRI, 2007
Other sources of water
Where clean water is unavailable at a reasonable cost, lower-quality nontraditional water supplies may be good substitutes, as long as depreciation of cooling systems can be minimized by limited pretreatment of intake waters. Potential sources of degraded water include treated urban wastewater, storm water, mine drainage, quarry dewatering, and water produced by oil and gas extraction (see POWER, March 2007, “Reclaimed cooling water’s impact on surface condensers and heat exchangers”).
Wastewater from public treatment works can be very affordable, at the low end of the treatment/disposal costs shown in Table 2, because such water has already been treated. This water source will also grow sustainably, because growing populations that require more electricity also generate growing wastewater flows. New sewage flows, just from domestic water use alone, can be expected at a rate exceeding 40 gal/day per capita. About 16 gal/day per capita are sufficient for new power generation, assuming current average rates of 33 kWh per day of electricity demand per capita and water consumption for power generation of 480 gal/MWh.
Where population growth is insufficient for increasing wastewater flows, advances in technologies that enable the use of degraded waters may also present substantial opportunities for cost savings. As Figure 4 shows, the cost of treatment required to safely use degraded waters can exceed $4/kgal for produced waters and agricultural return waters, making it the largest component of the cost of water. At such a high cost, use of these degraded waters is not often competitive. However, advances in the ability to use degraded waters without extensive pretreatment—such as spray-enhanced dry cooling—could reduce the overall cost of cooling water, making degraded water competitive with more traditional groundwater and surface water sources.
4. The cost of using degraded water. Representative water treatment costs per 1,000 gallons from various sources. Source: EPRI, 2004
To roughly estimate the potential saving from advances in the use of degraded waters, we can assume a reasonable decrease in the cost of pretreatment, based on the range of current costs. Water resulting from oil and gas extraction, and agricultural return waters, cost $4/kgal or more to treat—about four times what it costs to treat fresh water. It is unlikely that treatment technologies and/or the development of materials compatible with degraded waters will eliminate the gap. It is possible, however, that the difference in treatment costs could be significantly reduced, by as much as 25 to 75 cents/kgal. For our 350-MW baseload plant that requires 480 gal/MWh, the savings would amount to $370,000 to $1,100,000, with a midrange value of $740,000.
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