April 3, 2008

Wet Limestone is Not the Only Option - Conclusion of Hot Topic Hour April 3

The Calcium Reagents Hot Topic discussion yesterday revealed that the rules of the game are changing and that using calcium in different ways may be the winning strategy in the future. A prediction for the need to scrub small plants with highly efficiency is the main game changer. Magnesium enhanced wet lime scrubbing may be a better choice in the new game.

Bill Ellison of Ellison Consulting said that magnesium enhanced lime inhibited oxidation provides high efficiency, low capital cost, safe disposal of mercury, reduced wastewater treatment, NO_x reduction, and lower corrosion. This system design has been around for many decades. Dolomitic lime or lime with magnesium additions can be utilized to obtain an efficiency of more than 99 percent. The sludge with 50 percent moisture is fixed with lime to make a very leach-adverse landfill. With the rising cost of limestone systems, the capital cost advantage of the more compact lime system is substantial.

Forced Oxidation Lime Scrubbing

Operation in the inhibited oxidation mode is not the only option for use of lime in wet scrubbing. APS Cholla and PacifiCorp. Huntington are utilizing forced oxidation with lime. The long PacifiCorp distance between the quarry and the Huntington site means that transportation cost is more significant than the differential in materials cost. Duke Zimmer uses magnesium enhanced lime inhibited oxidation but with a subsequent oxidation stage to produce wallboard gypsum.

Demand Factors

The CAMR vacature will result in every power plant installing scrubbers. The vacature of CAMR has the effect of relisting power plants as air toxic emitters. Since most plants emit more than 10 tpy of HCl or more than 25 tons per year of combined toxics, each will have to meet BACT. It would be difficult to remove HCl without removing SO₂. Since ambient particulate rules will eventually force the small plants to scrub SO₂, it will make sense to comply with MACT by scrubbing both HCl and SO₂.

Lots of small power plants can use lime and cofire biomass. Many of the presently unscrubbed plants are between 30-300 MW. Local sources of biomass can provide for 5 to 10 percent of the total fuel requirement. Ball mills for small power plants are expensive. Many of these small  power plants are quite old. They should be replaced by new coal-fired power plants within 15 years. It therefore makes sense to spend more on operating costs (lime) and less on capital cost

Coal-fired power generation can expand if waste heat is used to make ethanol. There are lots of ways to reduce the net greenhouse gas contribution of coal-fired power plants. Using the waste heat to make ethanol is very attractive. Over 50,000 MW equivalent would be needed for 50 billion gpy of ethanol. This potential is large enough to significantly increase lime demand. Making coal green will open the door to building new power plants. Lime demand will grow in all industries. Without the use of lime for wet scrubbing, that segment will be a small part of the lime market which McIlvaine predicts to rise to 36 million tons in 2015. However, if an additional 10 million tpy were consumed for wet lime scrubbing, FGD would represent 40 percent of the total lime market. Market predictions are shown below.

Total Demand (million tpy)

Lime and limestone can be used for SO₃ reduction. In addition to SO₂ removal, lime is also proving to be useful for capturing SO₃. You can inject hydrated lime ahead of the lime scrubber and not only capture the SO₃ but utilize the unreacted lime in the downstream scrubber.

Efficiency Factors

CO₂ capture will require highly efficient pre scrubbing. For one reason or another all plants want to claim that they are CO₂ ready. Scrubber system designs are leaving room for a CO₂ scrubber. But any SO₂ entering the CO₂ scrubber will degrade the amine or ammonia. So to be truly CO₂ ready a scrubber with 99 percent SO₂ removal will be preferable to one with 98 percent or 95 percent SO₂ removal. In the first case the amine loss is half of the second case and 1/5 the third case. Lime is more competitive the higher the SO₂ removal efficiency. The pumping power and number of nozzles doubles as the efficiency requirement increases from 95 to 99 percent when limestone is the reagent. Lime scrubbing liquor requirements are modest at 99 percent efficiency. Ninety nine percent SO₂ removal is coming. The alternative is renewables. There is a “business as usual” attitude which seems to ignore facts such as TXU cancelling eight coal-fired power plants because of emission problems. The alternatives are really “clean coal” or alternative fuels”. If the SO₂ emissions can be cut in half (2 percent to 1 percent) for only 0.1 mils/kWh, then the utilities should be out in front with this initiative. This sure beats 100 mils/kWh for renewables.

Processing Issues

Regional grinding of ground limestone for SO₂ and SO₃ capture will be increasingly attractive. With small power plants or with many plants in a region it makes sense to grind at a central location and ship the limestone to multiple sites. Dry grinding is cheaper and more efficient than wet grinding. Individual mills made by Loesche and others can supply all the stone need for a number of power plants. If small power plants have to install scrubbers, the economics are unfavorable for ball mills at the plant site.

Bill Ellison’s power points can be viewed directly in the FGD Decision Tree as follows:

Continuing Decision Process For: Wet

Growing Attractiveness and Versatility in Application of Generic Wet Magnesia Enhanced, Pebble Lime Scrubbing, etc: presented by Bill Ellison, Ellison Consultants - Hot Topic Hour April 3, 2008.

http://www.mcilvainecompany.com/FGD_Decision_Tree/subscriber/Tree/DescriptionTextLinks/Growing Attractiveness and Versatility in Application of Generic.htm