Environmental Control Devices
SO₂ Control Technologies

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Participant
Bechtel Corporation

Location
Seward, Indiana County, PA (Pennsylvania Electric Company's Seward Station, Unit No. 5)

Plant Capacity/Production
73.5 MWe equivalent from a 147 MWe plant

Coal
Pennsylvania bituminous,
1.2%-2.5% sulfur

Technology
Bechtel Corporation's in-duct, confined zone dispersion flue gas desulfurization (CZD/FGD) process

Additional Team Members
Pennsylvania Electric Company
cofunder and host

Pennsylvania Energy Development Authority
cofunder

New York State Electric & Gas Corporation
cofunder

Rockwell Lime Company
cofunder

Project Funding

*Additional project overrun costs were funded 100% by the participant for a final total project cost of $12,173,000.

Project Objective To demonstrate SO₂ removal capabilities of in-duct CZD/FGD technology; specifically, to define the optimum process operating parameters and to determine CZD/FGD's operability, reliability, and cost-effectiveness during long-term testing and its impact on downstream operations and emissions.

Technology/Project Description
In Bechtel's CZD/FGD process, a finely atomized slurry of reactive lime is sprayed into the flue gas stream between the boiler air heater and the electrostatic precipitator (ESP). The lime slurry is injected into the center of the duct by spray nozzles designed to produce a cone of fine spray. As the spray moves downstream and expands, the gas within the cone cools and the SO₂ is quickly absorbed on the liquid droplets. The droplets mix with the hot flue gas, and the water evaporates rapidly. Fast drying precludes wet particle buildup in the duct and aids the flue gas in carrying the dry reaction products and the unreacted lime to the ESP.

This project included injection of different types of sorbents (dolomitic and calcitic limes) with several atomizer designs using low- and high-sulfur coals to evaluate the effects on SO₂ removal and ESP performance. The demonstration was conducted at Pennsylvania Electric Company's Seward Station in Seward, PA. One-half of the flue gas capacity of the 147-MWe Unit No. 5 was routed through a modified, extended straight section of duct between the first- and second-stage ESPs.

Confined Zone Dispersion Flue Gas Desulfurization Process Flow Diagram
Larger jpeg version

Results Summary

Environmental

• Pressure-hydrated dolomitic lime proved to be a more effective sorbent than either dry hydrated calcitic lime or freshly slaked calcitic lime.

• Sorbent injection rate was the most influential parameter on SO₂ capture. Flue gas temperature was the limiting factor on injection rate. For SO₂ capture efficiency of 50% or more, a flue gas temperature of 300 °F or more was needed.

• Slurry concentration for a given sorbent did not increase SO₂ removal efficiency beyond a certain threshold concentration.

• Testing indicated that SO₂ removal efficiencies of 50% or more were achievable with flue gas temperatures of  300-310 °F (full load), sorbent injection rate of 52-57 gal/min, residence time of 2 seconds, and a pressure-hydrated dolomitic-lime concentration of about 9%.

• For operating conditions at Seward Station, data indicated that for 40-50% SO₂ removal, a 6-8% lime or dolomitic lime slurry concentration, and a stoichiometric ratio of 2-2.5 resulted in a 40-50% lime utilization rate. That is, 2-2.5 moles of CaO or CaO•MgO were required for every mole of SO₂ removed.

• Assuming 92% lime purity, 1.9-2.4 tons of lime was required for every ton of SO₂ removed.

Operational

• About 100 ft of straight duct was required to assure the 2-second residence time needed for effective CZD/FGD operation.

• At Seward Station, stack opacity was not detrimentally affected by CZD/FGD.

• Availability of CZD/FGD was very good.

• Some CZD/FGD modification will be necessary to assure consistent SO₂ removal and avoid deposition of solids within the ductwork during upsets.

Economic

• Capital cost of a 500-MWe system operating on 4% sulfur coal and achieving 50% SO₂ reduction was estimated at less than $30/kW and operating cost at $300/ton of SO₂ removed (1994$).

Project Summary
The principle of the CZD/FGD is to form a wet zone of slurry droplets in the middle of a duct confined in an envelope of hot gas between the wet zone and the duct walls. The lime slurry reacts with part of the SO₂ in the gas and the reaction products dry to form solid particles. An ESP, downstream from the point of injection, captures the reaction products along with the fly ash entrained in the flue gas.

Bechtel's demonstration showed that 50% SO₂  removal efficiency was possible using CZD/FGD technology. The extended duct into which lime slurry was injected is in the foreground.

CZD/FGD did not require a special reactor, simply a modification to the ductwork. Use of the commercially available Type S pressure-hydrated dolomitic lime reduced residence time requirements for CZD/FGD and enhanced sorbent utilization. The increased humidity of CZD/FGD processed flue gas enhanced ESP performance, eliminating the need for upgrades to handle the increased particulate load.

Bechtel began its 18-month, two-part test program for the CZD process in July 1991, with the first 12 months of the test program consisting primarily of parametric testing and the last 6 months consisting of continuous operational testing. During the continuous operational test period, the system was operated under fully automatic control by the host utility boiler operators. The new atomizing nozzles were thoroughly tested both outside and inside the duct prior to system testing.

The SO₂ removal parametric test program, which began in October 1991, was completed in August 1992. Specific objectives were as follows:

• Achieve projected SO₂ removal of 50%;

• Realize SO₂ removal costs of less than $300/ton; and

• Eliminate negative effects on normal boiler operations without increasing particulate emissions and opacity

The parametric tests included duct injection of atomized lime slurry made of dry hydrated calcitic lime, freshly slaked calcitic lime, and pressure-hydrated dolomitic lime. All three reagents remove SO₂ from the flue gas but require different feed concentrations of lime slurry for the same percentage of SO₂ removed. The most efficient removals and easiest operation were achieved using pressure-hydrated dolomitic lime.

Environmental Performance
Sorbent injection rate proved to be the most influential factor on SO₂ capture. The rate of injection possible was limited by the flue gas temperature. This impacted a portion of the demonstration when air leakage caused flue gas temperature to drop from 300-310 °F to 260-280 °F. At 300-310 °F, injection rates of 52-57 gal/min were possible and SO₂ reductions greater than 50% were achieved. At 260-280 °F, injection rates had to be dropped to 30-40 gal/min, resulting in a 15-30% drop in SO₂ removal efficiency. Slurry concentration for a given sorbent did not increase SO₂ removal efficiency beyond a certain threshold concentration. For example, with pressure-hydrated dolomitic lime, slurry concentrations above 9% did not increase SO₂ capture efficiency.

Parametric tests indicated that SO₂ removals above 50% are possible under the following conditions: flue gas temperature of 300-310 °F; full boiler load of 145-147 MWe; residence time in the duct of 2 seconds; and lime slurry injection rate of 52-57 gal/min.

Operational Performance
The percentage of lime utilization in the CZD/ FGD significantly affected the total cost of SO₂ removal. An analysis of the continuous operational data indicated that the percentage of lime utilization was directly dependent on two key factors: (1) percentage of SO₂ removed, and (2) lime slurry feed concentration.

For operating conditions at Seward Station, data indicated that for 40-50% SO₂ removal, a 6-8% lime or dolomitic lime slurry concentration, and a stoichiometric ratio of 2-2.5 resulted in a 40-50% lime utilization rate. That is, 2-2.5 moles of CaO or CaO•MgO were required for every mole of SO₂ removed; or assuming 92% lime purity, 1.9-2.4 tons of lime were required for every ton of SO₂ removed. In summary, the demonstration showed the following results:

• A 50% SO₂ removal efficiency with CZD/FGD was possible.

• Drying and SO₂ absorption required a residence time of 2 seconds, which required a long and straight horizontal gas duct of about 100 feet.

• The fully automated system integrated with the power plant operation demonstrated that the CZD/FGD process responded well to automated control operation. However, modifications to the CZD/FGD were required to assure consistent SO₂ removal and avoid deposition of solids within the gas duct during upsets.

• Availability of the system was very good.

• At Seward Station, stack opacity was not detrimentally affected by the CZD/FGD system.

Economic Performance
Estimates show that the CZD/FGD process can achieve costs of $300/ton of SO₂ removed (1994$) when operating a 500-MWe unit burning 4% sulfur coal. Based on a 500-MWe plant retrofitted with CZD/FGD for 50% SO₂ removal, the total capital cost is estimated to be less than $30/kW (1994$).

Commercial Applications
After the conclusion of the DOE-funded CZD/FGD demonstration project at Seward Station, the CZD/FGD system was modified to improve SO₂ removal during continuous operation while following daily load cycles. Bechtel and the host utility, Pennsylvania Electric Company, continued the CZD/FGD demonstration for an additional year. Results showed that CZD/FGD operation at SO₂ removal rates lower than 50% could be sustained over long periods without significant process problems.

CZD/FGD lime slurry injector control system.

CZD/FGD can be used for retrofitting existing plants and installation in new utility boiler flue gas facilities to remove SO₂ from a wide variety of sulfur-containing coals. A CZD/FGD system can be added to a utility boiler with a capital investment of about $25-50/kW of installed capacity, or approximately one-fourth the cost of building a conventional wet scrubber. In addition to low capital cost, other advantages include small space requirements, ease of retrofit, low energy requirements, fully automated operation, and production of only nontoxic, disposable waste. The CZD/FGD technology is particularly well suited for retrofitting existing boilers, independent of type, age, or size. The CZD/FGD installation does not require major power station alterations and can be easily and economically integrated into existing power plants.

Contacts

Joseph T. Newman, Project Manager
  Bechtel Corporation
  P. O. Box 193965
  San Francisco, CA 94119-3965
  (415) 768-1014
  (415) 768-3535 (fax)

Victor K. Der, DOE/HQ, (301) 903-2700
  victor.der@hq.doe.gov

Thomas A. Sarkus, NETL, (412) 386-5981
  sarkus@netl.doe.gov