Boiler Replacement and NOx
By Joe Zwers
Replacing old steam systems can improve efficiency, cut costs and reduce emissions. A few years ago, the U.S. Department of Energy recognized Houston-based TPC Group for the energy efficiency of a new steam turbine it installed to run a compressor, and recently TPC replaced aging boilers with two new units from RENTECH Boilers using Coen ultra-low-NOxburners.
“There was an immediate impact due to the higher efficiencies, which produced a cost savings,” Johnny S. Park, the plant’s production engineer for utilities, said. “There was also a significant improvement in our environmental performance and a reduction in NOx emissions.”
Due to changes in the plant’s process, steam requirements have dropped from 1.5 million pounds per hour to between 850,000 and 1.2 million pounds. The reduction allowed the plant to eliminate some of the original nine boilers, but the existing ones were far from meeting the current emissions requirements.
“The boilers are quite old now and use 1940s technology,” Park said. “Because of their age, it would cost too much to upgrade the boilers for the low-NOx requirements.”
Due to the size of the boilers needed, it was impossible to purchase a packaged boiler and ship it to the site. Completely building a unit on site, however, was cost-prohibitive so TPC contracted with RENTECH Boiler Systems, Inc., to prefabricate components and assemblethem on site.
“In the past, they could only get the capacity they needed—474,000 pounds per hour—with a field-erected unit,” Gerardo Lara, chief process systems engineer at RENTECH, said. “With our construction methods, they could get the boiler they needed at about half the cost.”
Further complicating the issue was the fact that reliability and emissions standards needed to be met while burning a mix of pipeline natural gas and process gasses from the plant.
The project consisted of two D-style field-erected watertube boilers. The 664 mmBtu/hr heat input produces approximately 474,000 lb/hr steam flow at 725 psig and 710 F superheat. Each boiler was equipped with dual burners, superheater, economizer, fan, ladders, platforms, stack and trim.
To avoid problems that traditional package boilers have had with firebrick and refractory, the furnace section of the boilers features 100 percent membrane wall construction, built with 2.0”OD x .135”MW tubes on 4” centers. The tubes are connected by ¼” x 1” carbon steel membranes to form a water-cooled enclosure.
The furnace was sized at 13.0’ W x 44.0’ D. The furnace volumetric heat release rate for these units was less than 60,000 btu/hr-ft3, which is conservative for gaseous fuels but is key in achieving ultra-low-NOx conditions. Total furnace heat fluxes were less than 38,000 Btu/hr-ft2 of furnace which in turn speaks of low abuse of the radiant.
The convection tubes are 2.0” OD x 0.135” MW SA-178A tubes. Continuous, high-frequency resistance welded finned tubes were used to increase efficiency of the convection section. The convection section was designed to accommodate possible future installation of sootblowers. The tubes in the screen and convection sections where attached to the drums by rolling and flaring. Each tube hole was serrated and carefully cleaned and polished just prior to tube installation. Longitudinal, vertical baffles have also been included throughout the convection section to eliminate problems associated with acoustic vibration.
Large boilers (over 250 KPPH steam capacity) that use boiler tubes as downcomers have the problem that some of these tubes will act as downcomers when there is a low load and as risers as the load increases. The continuous reversal of those tubes can potentially create stagnant circulation spots (hence hot spots and fouling) and also causes problems with drum level control. To eliminate such issues, the RENTECH boilers have dedicated unheated downcomers that ensure circulation and make more efficient use of the steam drum. The areas that are supposed to act as risers (steam plus water mixture) are spread evenly across the drum, while the downcomers feed water to the lower drum on the ends of the boiler. Dedicated downcomers also stabilize the drum level during rapid changes.
The 72” ID x 48’ L steam drum was sized conservatively for the steam flow and pressure of the unit, and each head has a 14” x 18” manway to provide access for inspection. The drum is provided with primary belly pan, chevrons and cyclones to provide steam quality of no more than 0.05 percent moisture carryover. The secondary chevron separator is designed to operate even if the water level should rise as much as 2” above the “high-high” level, further reducing the chances of water carryover that can occur when the water level rises. All other drum internal piping, including an internal chemical feed distribution pipe, was included as needed to make the unit operational.
The superheater is a horizontal tube and fully drainable with a convection-type design. It was constructed as a separate module and shop-hydrostatically-tested prior to installation in the boiler. The superheater is located behind the division wall and the screen tubes to protect it from direct radiation. This is a single-stage superheater that provides a fairly flat steam temperature from 50 percent to 100 percent load. The operating temperature curve is shown in Fig. 1. Steam temperature was guaranteed at 100 percent maximum continuous rating (MCR) to be 710 F,+/-15 F.
The RENTECH superheater includes 100 percent membrane wall construction all along the outboard side of the convection section of the superheater. This construction reduces the occurrence of casing leaks that are common with designs that attempt to seal the superheater section with carbon steel casing.
The unit uses dual Coen Delta NOx ULN ultra low NOx burners that are capable of fast ramp rates. The flame is anchored by a stable hot flame in the core of the burner, generated from the core gas gun and the core fresh air fan. This anchoring flame allows the burner to operate with high flue gas recirculation (FGR) rates and attain ultra-low-NOx levels.
Flue Gas Recirculation
Induced flue gas recirculation (FGR) is required to achieve the NOx emissions from the burner. FGR ductwork, dampers and expansion joints were included.
Forced Draft Fan
An Arrangement #7 forced draft fan, manufactured by Robinson Industries, complete with 1,750 hp motor, coupling, inlet silencer, dampers and Bentley Nevada system, are used for combustion air. Ductwork from the forced draft fan to the windbox and expansion joint are also included. Test block conditions for the fan were based on 110 percent of the flow and 121 percent of the static pressure. The fan was sized based on the combined combustion air and FGR temperature.
The boilers utilize horizontal gas flow economizers. The tubes are horizontal and fully drainable. The economizer was designed for the possible future installation of sootblowers and full size removable header box end panels. It was also designed for counter current flow. The economizer casing is ¼” carbon steel and is gas tight. It is internally insulated with mineral fiber insulation and lined with a 12-gauge carbon steel inner liner. Galvanized economizer support steel is also included.
Flue Gas Ductwork
The boiler outlet transition, economizer inlet and outlet transitions were fabricated of 0.25” carbon steel material, stiffened as required. The flue gas ductwork design pressure is the same as the economizer, +15/-0” WC. The ductwork was internally insulated with mineral fiber insulation and lined with 12-gauge carbon steel inner liner prior to shipment. Access doors (14” x 18”) where included to provide access to each section.
Each boiler has a single wall freestanding stack, 78” discharge diameter, and extends to 180 feet above grade. The stacks are constructed of carbon steel, A-36 material, with four EPA test ports and three 4” continuous emission monitoring system (CEMS) ports. One 360-degree, 3’0” wide OSHA-approved test platform is provided with access via a 50-foot caged ladder with no rest platforms. Personnel protection is included between the access ladder and the stack at the CEMS platform level.
Fine-Tuning the System
The first unit, boiler No. 10, went live in mid-2006 and boiler No. 11 by the end of the year. The 750 lb steam coming out of the boiler drives a 38 MW GE turbogenerator. Most of the steam, however, is reduced to 150 lb or even as low as 15 lbs and powers a number of turbo-compressors as well providing process heat.
As with any custom design, there were some issues to be resolved after installation, particularly due to the nature of the fuel mix. Further complicating the matter, TPC chose to contract separately with different vendors rather than hiring one company to oversee all aspects.
“All the applications are different, and a lot of these applications mean making changes after the boilers are installed,” Park said.
To address flame failures, TPC met with RENTECH’s Lara, supervising engineer Kevin Anderson and tuning engineer Brian Foster from Coen as well as representatives from the engineering firm and other contractors involved in the project. They spent a week on site and made three major recommendations.
The first was a change in the control logic to linearize the fuel flow and air flow to the boiler master for all fuel compositions. Next, the tuning curves were changed on the core inner and outer gas valves. In the past, only air curves were changed with limited success. Finally, they modified the air hood design.
“The way the air hood was facing, when the wind blew a certain way we would notice an increase in the air,” Park said.
Making the Changes
In August 2008 the boilers underwent a scheduled outage, one at a time. The main problem to address was the flame failures. The Coen Delta Ultra ULN burners use a three-stage combustion system. The center flame is stable, but high NO—. The mid-radial section and the outer section are much cleaner, but rely on the center flame for stability.
One issue to address was that the winds blowing off the Gulf of Mexico range up to 40 miles an hour, affecting air pressure readings. The inlet silencers included Aeroacoustic Corp. EZ-Flow piezometer tubes to record the air pressure differential using a simplified version of Bernoulli’s Equation for any two points on the flow stream:
PV1 + PS1 = PV2+PS2 + Losses1-2
where the velocity pressure and static pressure at one point equals the velocity and static pressure at the second point, plus the losses between the two points as a result of friction or other sources.
Lara said that while the piezometer normally gives a good idea of the air inlet, when the boiler is turned down, wind gusts throw off those readings. The devices are used with ratings in the order of three to four inches of water column at full capacity of the boiler, but at low load on the boiler, it can drop to 0.16” of water column, he said.
“A wind gust of 30 to 40 miles per hour can easily give you an alteration of that reading of ± 0.1” to 0.2” of water column, so you have a 100 percent noise-to-signal ration,” he said. “The wind makes your signals bounce 100 percent so you do not have a realistic measurement.”
In addition to causing flame outs, poor combustion also leads to coking of the burner elements. RENTECH modified the fresh air and core air inlets so the incoming air had to make a full 180-degree turn rather than the typical 45-degree turn. This eliminated the wind gust effect. The Coen engineers implemented improved control logic for both boilers to linearize the fuel flow and airflow to the boiler master for all fuel compositions. For a given heat input, the combustion air flow and fuel flow can be different depending on the actual fuel composition. The composition of the plant gas varies, and the new logic is designed to more readily take this into account. They also tuned the positions for the core, inner and outer gas valves based on pressure readings taken by one of the Coen engineers.
Other actions taken during the outage included turning the fresh air, FD fan inlet/outlet and FGR dampers on both boilers, installing pressure taps, and relocating the furnace pressure transmitter lines to eliminate a low spot.
Since implementing those changes, there have been no further flame failures. The boilers are also easer to operate since operators make minimal changes to bias on air dampers or O2 bias to control NOx.
Previously, operators had to adjust bias two to three times per hour to control NOx emissions, Park said. “The control logic changes, and tuning changes allow minimal adjustment that meet NOx emissions limits without the operator constantly changing the bias,” he added.
To further improve the control, the plant changed the gas pressure regulator from a control valve configuration to a Fisher 1098 pressure reducing regulator that gives a faster response time. Previously, slow response time of control valve configuration would lead to two boiler trips from pressure swings. Park said he also wants to switch to a 2-out-of-3 voting logic on the steam drums, instead of the current 1-out-of-2 logic.
The new boilers are operating as intended. Park said they are 5 percent to 7 percent more efficient than the older boiler No. 9. There is also a savings due to the lower emissions. TPC will probably buy another low NOx boiler in the next five years to replace boiler No. 9. When that happens, Park said he would recommend buying another RENTECH.