Environmental innovations seen in 1997 `Project of the Year` winners
All projects feature innovate emissions control system use for reducing NOx, SO2 and particulates
Douglas J. Smith
Although the United States electric power market is in the process of massive restructuring, two of this year`s “Project of the Year” winners are U.S. projects: the Roanoke Valley Energy Facility owned by Westmoreland-LG&E Partners and the Birchwood Power Facility owned by Southern Energy Inc./Cogentrix. The only project to receive an award outside of the United States was Singapore Power`s extension of its Senoko Power plant.
Power Engineering and Power Engineering International 1997 “Project of the Year” award winners were:
– Roanoke Valley Energy Facility, submitted by DB Riley Consolidated Inc., Worcester, Mass. USA;
– Extension of Senoko Power Plant, submitted by Siemens AG Power Gen- eration Group (KWU), Germany; and
– Birchwood Power Facility, submitted by ABB Combustion Engineering.
In addition to the awards for the project owners, the magazines` editors have also recognized the major vendors involved with the winning projects. These include:
– Senoko project, Siemens Power Generation Group and Austrian Energy & Environment;
– Birchwood, ABB Environmental Systems and ABB-CE; and
– Roanoke Valley, DB Riley Inc., Environmental Elements Corp., Nalco Fuel Tech and ABB Environmental Systems.
Simple-cycle to combined-cycle
From 1990 through 1991, Power Senoko Ltd. installed four Siemens V94.2 gas turbines at its Senoko power station in Singapore. In July 1994 Power Senoko, a subsidiary of Singapore Power, awarded Siemens of Germany a turnkey contract to convert the gas turbine power plant into a combined-cycle plant. The combined-cycle plant, located in the north of Singapore, was synchronized with the Singapore grid in June 1996.
Because the gas turbines had run for approximately 30,000 hours since entering service, a major overhaul of the machines was included as part of the Siemens contract. Overhaul of the gas turbines included installation of diverter flaps in the bypass stacks which are used when switching from simple-cycle to combined-cycle operation.
Each gas turbine has its own heat-recovery steam generator (HRSG). The two- pressure stage, natural circulation HRSGs were supplied by Austrian Energy & Environment of Vienna, Austria. Superheated steam is raised in the high-pressure (HP) and low-pressure (LP) stages of the HRSG. The LP stage of the HRSG has condensate preheater, evaporator and superheater stages while the HP stage includes an economizer, evaporator and a superheater.
In addition to the HRSGs, two 163 MW condensing steam turbines were installed. The single-shaft steam turbines have a single flow HP section and a double flow LP section. Tables 1a and 1b show the performance data for the gas and steam turbines.
HP superheated steam from the HRSGs is first expanded through the HP stage of the steam turbine and then through the LP steam turbine. Additional LP superheated steam is supplied to the HRSG`s LP section. A seawater condenser, in addition to condensing the LP steam, also serves to deaerate the condensate for turbine loads above 50 percent. At loads below 50 percent the feedwater is deaerated in the feedwater storage tank.
Ahead of schedule
Even before the conversion of Senoko power station into a combined-cycle plant the site was congested with limited space between the existing buildings. As a result, there was little storage area, especially for large pieces of equipment needed in the conversion. In addition, before any work could commence a substantial amount of demolishing and rerouting work had to be done. While this work was being carried out, the plant`s gas turbines continued to generate power.
The plant is split into two blocks: Block 10 and Block 20. Each block has two gas turbine-generators, two HRSGs and one steam turbine. A single water treatment plant supplies both blocks.
One of the first systems to be commissioned, in December 1995, was the water treatment plant. Pressure tests on Block 10`s HRSGs were performed in February and March 1996 and the first hot gas to the HRSGs followed in May 1996. The steam turbine-generator was synchronized with the Singapore grid at the beginning of June 1996.
With the completion, commissioning and commercial operation of Block 10, work commenced on the commissioning of Block 20. This unit entered commercial operation on Dec. 6, 1996. Although many obstacles were encountered during the conversion to combined-cycle the Senoko project was completed two months ahead of schedule.
On the environmental side a thermal impact study was carried out to determine what effect, if any, the plant`s increased thermal discharge would have the marine life around the plant. Results of the study concluded that construction of the plant`s thermal discharge point and the extension of the inner thermal plume would not have an effect on the marine habitat around the site. With combined-cycle operation, the Senoko plant increased its electric output by 61 percent.
Innovative dry scrubber
The contest judges selected Roanoke Valley Energy Facility as an award winner primarily for the innovative use of environmental controls. This facility was the first plant in North America to install and operate a circulating dry scrubber (CDS). Roanoke Valley Energy Facility is an independent power producer located in Weldon, N.C. Westmoreland Energy Inc. and LG&E Power Inc. jointly own the plant. A subsidiary of LG&E Power, LG&E Power Services is responsible for the plant`s operations.
There are two units at Roanoke: Unit 1, with a net load rating of 182 MW, and Unit 2, rated at 50 MW. The facility has two DB Riley Inc. pulverized coal-fired boilers both of which are fitted with low NOx CCV burners and advanced overfire air systems. Unit 1 has a GE steam turbine while Unit 2 has a Mitsubishi steam turbine. Both units cogenerate process steam, sold to Patch Rubber Co.
Controlling emissions from the Roanoke facility is of major concern. An ABB Environmental Systems dry SO2 scrubber and reverse air baghouse have been installed on Unit 1. Unit 2 is fitted with a Nalco/Fueltech NOx OUT urea injection selective non-catalytic reduction (SNCR) system and an Environmental Elements Corp. circulating-bed dry SO2 scrubber and baghouse. The CDS was supplied under license from Lurgi GmbH.
According to John Halloran, DB Riley president and COO, the most significant innovation at the Roanoke plant is the CDS technology used on Unit 2. The Roanoke plant is the first application of this technology in North America. According to the suppliers, dry scrubbers have the following advantages:
few material handling problems;
able to meet all performance requirements;
easy to control during changes in fuel and boiler operation; and
lower operational and maintenance costs than competing technologies.
It is reported that since startup of Unit 2 the CDS has operated very well with almost no maintenance required between outages. In addition, the system has used 10 percent less lime than guaranteed. As mentioned previously, NOx emissions on Unit 2 are controlled by a SNCR system. This system reduces emissions of NOx from a limit of 0.300 lb/MMBtu to less than 0.170 lb/MMBtu through urea injection into the combustion zone. A record of the Roanoke Valley Energy facility`s operating criteria for the years 1994-1997 is shown in Table 2.
Since commercial operation, the Roanoke facility has made significant economic and social impact on the local community. Today, the facility is the largest tax base for Halifax County. As part of the Roanoke project, the owners funded an expansion of the Weldon wastewater facility, including replacement of the city`s aging municipal wastewater system with a new pumping station and pipeline. Since being upgraded the wastewater system`s processing capacity has doubled to five million gallons per day. Another benefit to the community is an ash monofill storage facility. Although the facility was developed by the owners of the Roanoke plant, it is owned and operated by Halifax County and is another revenue stream for the county.
Birchwood meets emission limits
As a coal-fired power plant, the Birchwood facility in Virginia was designed to meet difficult on-demand power needs within the confines of some of the most difficult power plant emission limits in the world, says Frank Richters of ABB. The Birchwood cogeneration plant is designed to handle a rigorous twice-per-day cycling duty with load ramps of 5 MW/minute. The plant is required to achieve full load in hot startup in 2 3/4 hours, a warm start in 4 1/4 hours and 5 1/2 hours for a cold start.
Located in a rural area east of historic Fredricksburg, the plant is jointly owned by Southern Energy Inc., a unit of Southern Co., and Cogentrix. The plant started commercial operation in late 1996 when it dispatched 240 MW to Virginia Power. Steam is sold to Greenhost Inc., a 38-acre greenhouse complex.
A controlled circulation boiler fitted with low NOx tangential firing system, supplied by ABB-CE, minimizes the NOx emissions. This is achieved without increasing other combustible emissions of CO, hydrocarbons or unburned carbon, which are normally associated with in-furnace NOx reduction techniques. To meet the existing emissions standards for Virginia, it was also necessary to provide a selective catalytic reactor (SCR) to further reduce emissions of NOx below what is possible with combustion techniques alone.
Birchwood uses a dry, hot, high dust SCR designed for high NOx reduction with low ammonia slip. The SCR (Figure 1) is located between the secondary and primary boiler economizers. A grid of ammonia nozzles injects a controlled flow of ammonia/air mixture into the gas steam entering the SCR. According to ABB-CE, splitting the economizer around the SCR allows for high steam generator efficiency across a wider operating range.
A single-train, dry flue gas desulfurization system and baghouse control emissions of SO2 and particulates at Birchwood (Figure 2). The scrubber incorporates three flue gas inlets. Each inlet incorporates a spiral-type gas distributor with a rotor atomizer mounted in its center. These distributors and atomizers are sized so that any two can scrub the full volume of the flue gas. As a result, on-line maintenance of the scrubber`s atomizers can be carried out while maintaining optimum emission limits.
Particulates in the flue gas are controlled by a single baghouse. The baghouse has 5,000 fiberglass filter bags mounted in 12 individual compartments. Its design allows the baghouse to treat the full design volume of the flue gas with one compartment out of service. A filter cake that accumulates on the bags contains approximately 30 percent lime. Because of the lime, the baghouse contributes significantly to SO2 removal.
By optimizing the baghouse`s cleaning, the plant has been able to achieve maximum SO2 removal while minimizing the pressure drop. In addition, a steady flow of ash from the hoppers to the ash handling system has also been achieved. The end result is very low emissions.
Because the plant`s location, near the Shenandoah National Forest, plus Virginia`s stringent air emission regulations, the project`s air emission goals were very tight. These are outlined in Table 3. The owners of the plant report that based on actual test data, the Birchwood power facility has the cleanest stack emissions of any pulverized coal-fired facility in the US. They also say that emissions of NOx are the lowest of any type of coal-fired plant in the US.
It is important to note that Birchwood`s net plant heat rates, shown in Figure 3, are the average net heat rates for a given load projected over a one-year period. The heat rate also includes the supply of process steam to the steam host.
Presentation of awards
Senoko Power, Siemens Power Generation Group and Austrian Energy & Environmental received their awards on Sept. 9, 1997, during the keynote session of POWER-GEN Asia `97. Roanoke Valley Energy Facility and Birchwood Power Facility owners will receive their awards during the keynote session at POWER-GEN International in Dallas, Texas on Dec. 9, 1997. The major vendors involved with the two U.S. projects will receive their awards during the show on the exhibition floor.
Roanoke Valley Energy Facility, Weldon, N.C., USA
Senoko Power Station, Singapore