HOT TOPIC HOUR

HOT TOPIC HOUR

Dry Scrubbing Issues and Options reviewed in November 12 Hot Topic Hour

The session yesterday benefited from the insights of the lime suppliers as well as Paul Farber who is coordinating with McIlvaine as an expert on dry scrubbing. Two power plant owners who are presently making scrubber selections also participated. A set of slides summarizing all the options was used as a basis of the discussion.

Dry Scrubbing slides

The slides were extracted from a special Dry Scrubbing website which is part of 44I Power Plant Air Quality Decisions. Participants were encouraged to send additional papers and support material for inclusion in the Decisions program. By the end of the session we had received some good data from Mississippi Lime.

McIlvaine is making an effort to provide total cost of ownership analyses.

DSI, wet lime, CDS and other options will be compared using consumption quantities and costs (e.g. $200/ton) and compared to SO₂ harm per ton, e.g. $10,000/ton.

Also mercury at $60,000,000/ton will be included.
So a system that eliminates an additional pound of mercury a year generates $30,000 on the positive side of the evaluation.

The stoichiometric ratios and consumption of reagent are a very important factor in evaluating total cost of ownership. McIlvaine factors were reviewed by the participants.

Assumptions:

Average of 2.5% sulfur Appalachian coal on wet lime/limestone systems which are mainly located in mid-west. Most of the real high sulfur (>3%) plants in the Ohio Valley region already have scrubbers.
Wet limestone systems tonnage is 1.764 times the wet lime systems tonnage.
Average of 1.3% sulfur Appalachian coal on dry lime systems in East.
Average of 0.6% sulfur PRB coal on dry lime systems in West.
Need to differentiate between the East and West dry lime systems because the ratios are significantly different. The difference in sulfur is amplified by the difference in the stoichiometry (1.4 on 1.3 % sulfur vs. 1.1 on 0.6% sulfur).
Lime is 93% available. CaO and limestone is 95% available CaCO₃.
Removal efficiencies are 95% for wet lime/limestone systems and 93% to 94% range for dry lime systems.
Average plant load factors are 70%.
SO₃= 1% of SO₂ but 2% if SCR is installed.

Ratios:

Tons Lime per MW	Tons	Efficiency	Comments
Dry Lime East	80	93%
Dry Lime West	45	93%
Wet Lime	110	95%
Wet Limestone	195	95%

Tons Lime per Ton SO₂
Dry Lime East	1.40	93%
Dry Lime West	1.10	93%
Wet Lime	0.95	95%
Wet Limestone	1.70	95%
Tons of Hydrated Lime/Ton of SO₃	3	80%
Tons of Hydrated Lime/MW/SO₃	3	80%
Tons of Hydrate/Ton SO₂ – DSI	4	95%	DSI requires 3 times the lime.
Tons of Hydrate/Ton SO₂ – DSI	2.5	70%	Existing Precipitator

With the high efficiency hydrates now available it is possible to achieve up to 95 percent removal just with DSI injection ahead of a fabric filter. The new hydrates have steep increases in efficiency with small increases in sorbent. So 2 lbs./lb. of SO₂ might achieve 80 percent removal while doubling the ratio improves efficiency to 95 percent. So the exact quantity of sorbent needed will be difficult to predict.

Many plants are considering DSI because of the short possible remaining life of the plant. The cost of sorbent will be a major factor in comparing the DSI with little capital cost to dry scrubbers with substantial capital cost. However, if a new fabric filter is needed then the capital cost of DSI will be quite high.

The least capital investment is for a DSI + existing precipitator. Dry scrubbing + existing precipitator is another option. DSI plus a new fabric filter would equal or exceed the dry scrubber/precipitator combination. The dry scrubber + fabric filter will require the highest capital investment.

If the efficiency requirement is low then the DSI+ precipitator is a possibility. Sorbacal achieves 70 percent removal at a 2.5 ratio.

DSI is utilized at the air heater inlet for both corrosion control and reduction of the acid dewpoint. This allows the owner to extend the air heater and recover enough heat to improve boiler efficiency by more than 1 percent. The SO₃ averages about 1 percent of the SO₂ without SCR and up to 2 percent of the SO₂ with SCR. If lime requirements for 80 percent reduction are 3 lbs./lb. of SO₃, then the additional lime needed at this point is only a few percent of that needed for SO₂ removal.

The reduction of the flue gas temperature entering the precipitator improves the efficiency of the device. In Japan, heat exchangers to reduce the temperature from 325°F to 195°F resulted in emission decreases from 30 mg/Nm₃to 10 m/Nm₃. So the DSI at the air heater plus DSI prior to the existing precipitator may be attractive for older plants.

McIlvaine invites input as we work through the total cost of ownership for various alternative.

DSI, wet lime, CDS and other options will be compared using consumption quantities and costs (e.g. $200/ton) and compared to SO2 harm per ton, e.g. $10,000/ton.

Also mercury at $60,000,000/ton will be included.

So a system that eliminates an additional pound of mercury a year generates $30,000 on the positive side of the evaluation.

DSI, wet lime, CDS and other options will be compared using consumption quantities and costs (e.g. $200/ton) and compared to SO₂ harm per ton, e.g. $10,000/ton.