Industrial Applications

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Participant
Passamaquoddy Tribe

Location
Thomaston, Knox County, ME (Dragon Products Company's coal-fired cement kiln)

Plant Capacity/Production
1,450 tons/day of cement; 250,000 scfm of kiln gas; and up to 274 tons/day of coal

Coal
Pennsylvania bituminous, 2.5-3.0% sulfur

Technology
Passamaquoddy Technology Recovery Scrubber™

Additional Team Members
Dragon Products Company
project manager and host

HPD, Incorporated
designer and fabricator of tanks and heat exchanger

Cianbro Corporation
constructor

Project Funding

Passamaquoddy Technology Recovery Scrubber is a trademark of the Passamaquoddy Tribe.

Project Objective
To retrofit and demonstrate a full-scale industrial scrubber and waste recovery system for a coal-burning wet process cement kiln using waste dust as the reagent to accomplish 90-95% SO₂ reduction using high-sulfur eastern coals; and to produce commercial, potassium-based fertilizer by-products.

Technology/Project Description
The Passamaquoddy Technology Recovery Scrubber™ uses cement kiln dust (CKD), an alkaline-rich (potassium) waste, to react with the acidic flue gas. This CKD, representing about 10% of the cement feedstock otherwise lost as waste, is formed into a water-based slurry and mixed with the flue gas as the slurry passes over a perforated tray that enables the flue gas to percolate through the slurry. The SO₂ in the flue gas reacts with the potassium to form potassium sulfate, which stays in solution and remains in the liquid as the slurry undergoes separation into liquid and solid fractions. The solid fraction, in thickened slurry form and freed of the potassium and other alkali constituents, is returned to the kiln as feedstock (it is the alkali content that makes the CKD unusable as feedstock). No dewatering is necessary for the wet process used at the Dragon Products Company cement plant. The liquid fraction is passed to a crystallizer that uses waste heat in the flue gas to evaporate the water and recover dissolved alkali metal salts. A recuperator lowers the incoming flue gas temperature to prevent slurry evaporation, enables the use of low-cost fiberglass construction material, and provides much of the process water through condensation of exhaust gas moisture.

The Passamaquoddy Technology Recovery Scrubber™ was constructed at the Dragon Products plant in Thomaston, Maine, a plant that can process approximately 450,000 tons/yr of cement. The process was developed by the Passamaquoddy Indian Tribe while it was seeking ways to solve landfill problems, which resulted from the need to dispose of CKD from the cement-making process.

Cement Kiln Flue Gas Recovery Scrubber Process Flow Diagram
Larger jpeg or wmf version

Results Summary

Environmental

• The SO₂ removal efficiency averaged 94.6% during the last several months of operation and 89.2% for the entire operating period.

• The NOx removal efficiency averaged nearly 25% during the last several months of operation and 18.8% for the entire operating period.

• All of the 250 ton/day CKD waste produced by the plant was renovated and reused as feedstock, which resulted in reducing the raw feedstock requirement by 10% and eliminating solid waste disposal costs.

• Particulate emission rates of 0.005-0.007 gr/scf, about one-tenth that allowed for cement kilns, were achieved with dust loadings of approximately 0.04 gr/scf to the scrubber.

• Pilot testing conducted at U.S. Environmental Protection Agency laboratories under Passamaquoddy Technology, L.P. sponsorship showed 98% HCl removal.

• On three different runs, VOC (as represented by alphapinene) removal efficiencies of 72.3, 83.1, and 74.5% were achieved.

• A reduction of approximately 2% in CO₂ emissions was realized through recycling of the CKD.

Operational

• During the last operating interval, April to September 1993, recovery scrubber availability (discounting host site downtime) steadily increased from 65% in April 1993 to 99.5% in July 1993.

Economic

• Capital costs are approximately $10,090,000 (1990$) for a recovery scrubber to control emissions from a 450,000-ton/yr wet process plant, with a simple payback estimated in 3.1 years.

• Operating and maintenance costs, estimated at $500,000/yr, plus capital and interest costs, are generally offset by avoided costs associated with fuel, feedstock, and waste disposal and with revenues from the sale of fertilizer.

The Passamaquoddy Technology Recovery Scubber™ was successfully demonstrated at Dragon Products Company's cement plant in Thomaston, Maine.

Project Summary
The Passamaquoddy Technology Recovery Scrubber™ is a unique process that achieves efficient acid gas and particulate control through effective contact between flue gas and a potassium-rich slurry composed of waste kiln dust. Flue gas passes through the slurry as it moves over a special sieve tray. This results in high SO₂ and particulate capture, some NOx reduction, and sufficient uptake of the potassium (an unwanted constituent in cement) to allow the slurry to be recycled as feedstock. Waste cement kiln dust, exhaust gases (including waste heat) , and wastewater are the only inputs to the process. Renovated cement kiln dust, potassium-based fertilizer, scrubbed exhaust gas, and distilled water are the only proven outputs. There is no waste.

The scrubber was evaluated over three basic operating intervals dictated by winter shutdowns for maintenance and inventory and 14 separate operating periods (within these basic intervals) largely determined by unforeseen host-plant maintenance and repairs and a depressed cement market. Over the period August 1991 to September 1993, more than 5,300 hours were logged, 1,400 hours in the first operating interval, 1,300 hours in the second interval, and 2,600 hours in the third interval. Sulfur loadings varied significantly over the operating periods due to variations in feedstock and operating conditions.

Operational Performance
Several design problems were discovered and corrected during start-up. No further problems were experienced in these areas during actual operation.

Two problems persisted into the demonstration period. The mesh-type mist eliminator, which was installed to prevent slurry entrainment in the flue gas, experienced plugging. Attempts to design a more efficient water spray for cleaning failed. However, replacement with a chevron-type mist eliminator prior to the third operating interval was effective. Potassium sulfate pelletization proved to be a more difficult problem. The cause was eventually isolated and found to be excessive water entrainment due to carry-over of gypsum and syngenite. Hydroclones were installed in the crystallizer circuit to separate the very fine gypsum and syngenite crystals from the much coarser potassium sulfate crystals. Although the correction was made, it was not completed in time to realize pellet production during the demonstration period. After all modifications were completed, the recovery scrubber entered into the third and final operating interval--April to September 1993. During this interval, recovery scrubber availability (discounting host site downtime) steadily increased from 65% in April to 99.5% in July.

Environmental Performance
An average 250 tons/day of CKD waste generated by the Dragon Products plant was used as the sole reagent in the recovery scrubber to treat approximately 250,000 scfm of flue gas. All the CKD, or approximately 10 tons/hr, was renovated and returned to the plant as feedstock and mixed with about 90 tons/hr of fresh feed to make up the required 100 tons/hr. The alkali in the CKD was converted to potassium-based fertilizer, eliminating all solid waste. Exhibit 36 lists the number of hours per operating period, SO₂ and NOx inlet and outlet readings in pounds per hour, and removal efficiency as a percentage for each operating period. 

Average removal efficiencies during the demonstration period were 89.2% for SO₂ and 18.8% for NOx emissions. No definitive explanation for the NOx control mechanics was available at the conclusion of the demonstration.

Aside from the operating period emissions data, an assessment was made of inlet SO₂ load impact on removal efficiency. For SO₂ inlet loads in the range of 100 lb/hr or less, recovery scrubber removal efficiency averaged 82.0%. For SO₂ inlet loads in the range of 100-200 lb/hr, removal efficiency increased to 94.1% and up to 98.5% for loads greater than 200 lb/hr.

In compliance testing for Maine's Department of Environmental Quality, the recovery scrubber was subjected to dust loadings of approximately 0.04 gr/scf and demonstrated particulate emission rates of 0.005-0.007 gr/scf-- less than one-tenth the current allowable limit.

Economic Performance
The estimated "as built" capital cost to reconstruct the Dragon Products prototype, absent the modifications, is $10,090,000 in 1990 dollars.

Annual operating and maintenance costs are estimated at $500,000. Long-term annual maintenance costs are estimated at $150,000. Power costs, estimated at $350,000/yr, are the only significant operating costs. There are no costs for reagents or disposal, and no dedicated staffing or maintenance equipment is required.

The simple payback on the investment is projected in as little as 3.1 years considering various revenues and avoided costs that may be realized by installing a recovery scrubber similar in size to the one used at Dragon Products. In making this projection, $6,000,000 was added to the "as built"  capital costs to allow for contingency, design/permitting, construction interest, and licensing fees.

Commercial Applications
Of the approximately 2,000 Portland cement kilns in the world, about 250 are in the United States and Canada. These 250 kilns emit an estimated 230,000 tons/yr of SO₂ (only three plants have SO₂ controls, one of which is the Passamaquoddy Technology Recovery Scrubber™). The applicable market for SO₂ control is estimated at 75% of the 250 installations. If full penetration of this estimated market were realized, approximately 150,000 tons/yr of SO₂ reduction could be achieved.

The scrubber became a permanent part of the cement plant at the end of the demonstration. A feasibility study has been completed for a Taiwanese cement plant.

Contacts

Thomas N. Tureen, Project Manager
  Passamaquoddy Technology, L. P.
  1 Monument Way, Suite 200
  Portland, ME 04101
  (207) 773-7166
  (207) 773-8832 (fax)

William E. Fernald, DOE/HQ, (301) 903-9448
  william.fernald@hq.doe.gov

John C. McDowell, NETL, (412) 386-6175
  john.mcdowell@netl.doe.gov