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Low-Cost Combustion Technologies to Control NOx and PM Emissions from Oil and Gas-Fired Boilers

Anthony V. Conti and Dan V. Giovanni, Electric Power Technologies, Inc.
Ing. F. Ermolli, EniChem S. p. A.

SUMMARY:

Emissions regulations throughout the world are driving the need to modernize combustion equipment to reduce NOx and particulate matter (PM) emissions from power and steam generating plants. In some instances, new combustion systems are required to meet applicable emissions limits at a cost that could exceed $2,000 per tonne/hour (t/h) of steam capacity (i.e. $1,000,000 for a 500 t/h steam boiler). Alternatively, low cost combustion technologies, known as Reduced Emissions and Advanced Combustion Hardware (REACH), may be retrofit at a cost equal to one-tenth (1/10) of a completely new burner system. REACH technology has been applied to more than 125 oil fired boilers and more than 20,000 MW (electrical) of installed capacity. In approximately one-third (1/3) of these applications, NOx reductions were of critical concern and reductions from baseline levels of up to 50% have been realized. Concurrent reductions in PM emissions were also significant, with resultant emissions that range from 50 to 120 mg/Nm3, depending on the site.

This paper presents results from the first REACH applications in Europe to several wall-fired and tangential-fired boilers firing heavy oil. These boilers range in size from 140 t/h to 530 t/h steam flow. For wall-fired applications, the patented V-jet steam atomizer has been used to reduce NOx emissions from >1,000 mg/Nm3 to approximately 550 mg/Nm3. Simultaneously, PM emissions were reduced from approximately 300 mg/Nm3 to less than 130 mg/Nm3. For tangential-fired boilers, PM emissions of 50 to 70 mg/Nm3 were realized with NOx reductions of approximately 10% from baseline levels.

REACH retrofits are normally limited to the replacement of oil atomizers and flame stabilizers with specially designed components. This limited scope of modifications keeps the retrofit time and cost very low relative to alternatives that offer comperable emissions reductions.

Preliminary combustion trials are underway with prototypical GAS-REACH firing equipment. The same principles of emissions reduction through low cost retrofit to a wide range of existing burner types are being pursued. The goal is to reduce NOx emissions without gas recirculation or overfire air to less than 100 mg/Nm3. Combustion trials are expected to be completed in March, 1998.

INTRODUCTION:

Emissions regulations for fossil fuel combustion continue to become more stringent as limits are being lowered around the world. Concurrently, the privatization/deregulation of power generation and global industrial competition is increasing the pressure to minimize cost. In response to these pressures, the useful lives of existing plants are being extended through technology upgrades whenever possible. As new NOx and PM emissions regulations are enacted, owners and operators of existing plants must choose control technologies that range from combustion equipment upgrades to complex and expensive post-combustion controls. With the steady trend in more stringent emissions regulations, post-combustion controls may ultimately be required for all plants; however, the size, complexity, and cost of these controls are largely determined by the concentration of emissions leaving the combustion zone. Thus, economic forces point to the combustion process itself as the place to begin controlling emissions. With a solid foundation of low emissions from the combustion process, the investment and operating cost of post combustion controls can be minimized.

This paper describes the first applications in Europe of a low-cost combustion upgrade technology for reducing particulate matter (PM) and NOx emissions from firing heavy oil in wall-fired and tangential-fired boilers. The technology, known as Reduced Emissions and Advanced Combustion Hardware (REACH), was developed jointly by Electric Power Technologies, Inc. (EPT), the Electric Power Research Institute (EPRI), and several electric utilities in New York State. This joint program produced a technology that can be easily retrofit to existing burners to solve a variety of problems related to poor combustion conditions, including, high stack opacity, high unburned carbon and NOx emissions, acidic stack fallout, flame impingement, poor boiler turndown, and high excess oxygen[1].

REACH Technology
The main retrofit components of REACH technology are specially designed oil atomizers and flame stabilizers which are engineered to optimize combustion performance and reduce emissions. These components can be adapted to a wide range of burner and boiler designs to maximize applicability, while retaining as much of the original burner as possible. In the majority of retrofit applications, REACH flame stabilizers and oil atomizers replace existing stabilizers and atomizers, while other burner components including air registers, oil guns, flame detection equipment, ignitors, and control systems, are left intact. Capital costs, installation costs, and outage duration are kept low by this limited scope of modifications. As a result, REACH technology offers substantial cost advantages relative to other retrofit options (e.g., complete burner replacement).

Two versions of REACH have been applied in a number of commercial applications. Combustion Performance REACH (CP-REACH) is designed to reduce PM emissions and opacity and to provide operational improvements including increased burner turndown, reduced excess air requirements, improved flame stability, and elimination of flame impingement on furnace walls. Low-NOx REACH (LN-REACH) is specifically aimed at retrofit projects where NOx reduction is the major goal. The key difference between CP-REACH and LN-REACH is the design of the oil atomizer. Boilers equipped with CP-REACH can be easily converted to LN-REACH. Detailed descriptions of these technologies and commercial applications have been published elsewhere[3-6]. A brief description is provided below.

Flame Stabilizers
For flame stabilization and aerodynamic control of fuel and air mixing, REACH uses a compound-curved-vane (CCV) swirler for applications in both wall- and tangential-fired boilers[2]. The CCV swirler provides better performance than conventional diffusers and flat-bladed swirlers that are commonly in use, because the CCV provides more consistent and stable air flow patterns over the range of combustion air flow rates. The CCV flame stabilizers supplied with REACH are custom-designed to produce the proper entrainment and swirl of combustion air at the discharge plane of the burner, and to match the oil spray of the REACH oil atomizer.

Oil Atomization
REACH oil atomizers are custom-designed to adapt to the existing oil supply equipment and operating conditions (i.e., pressure, temperature, and capacity)[2]. For steam-atomized systems, CP-REACH uses internal-mix (I-Mix) atomizers, which produce superior spray quality compared to other common atomizer designs (The quality of the oil spray is characterized by Sauter Mean Diameter (SMD), which is defined as the diameter of a hypothetical droplet that has the same surface-to-volume ratio as that of the total spray. SMD is most often used to characterize oil sprays because it is relevant to evaporation and combustion of oil droplets.). Proper atomization, combined with effective fuel-air mixing, promotes nearly complete carbon burnout, minimizing PM emissions and improving combustion performance.

For reducing NOx emissions in steam-atomized systems, LN-REACH employs a novel atomizer design - the Segmented V-jet atomizer (patented) - which divides the oil spray into distinct segments at the base of the flame. In this design, high quality atomization is retained for complete carbon burnout, while this unique fuel spray pattern produces a radially staged flame to minimize NOx formation. Simultaneous NOx and PM reduction is obtained with LN-REACH.

For mechanically atomized burners, CP-REACH oil atomizers can be designed to operate at supply pressures from 14 to 90 barg. Special low-NOx mechanical atomizers that produce oil spray characteristics similar to the Segmented V-Jet atomizer are also available.

Application
REACH technology is applied to boilers through engineering analysis of the existing burner configuration and boiler operating parameters, followed by the custom design of retrofittable oil atomizers and flame stabilizers. The major components of the burner are retained and significant changes to air registers, burner auxiliary equipment, pumping and heating equipment, or combustion controls are avoided. In some instances, the burners have been changed from mechanical to steam atomization. EPT has designed and supplied REACH for more than 125 boilers totaling 20,000-MWe of generating capacity. In a typical retrofit project, REACH atomizers and flame stabilizers can typically be designed and supplied within 6-8 weeks, and installed during a 3-5 day boiler outage. In two instances, the REACH hardware has been installed with the boiler on-line. Alternatively, REACH technology may be incorporated in the design of new burners. REACH technology has been licensed from EPT by COEN Company, and Ansaldo Energia S.p.A.

Previous papers have described the application of REACH technology to a 550-MWe, opposed-wall fired boiler[3] and to a group of 70 t/h industrial boilers[5]. Results from recent (1997 & 1998) REACH applications at thirteen large (140 to 450 t/h) industrial boilers in Europe are described below.

RECENT INSTALLATIONS IN EUROPE:

In response to environmental and economic pressures described in the introduction, a large European petrochemical company and an oil refiner have undertaken actions to reduce PM and/or NOx emissions from several co-generation plants within large petrochemical and refinery complexes. REACH technology was chosen to simultaneously improve boiler combustion performance and reduce emissions at low capital and operating cost. Table 1, Boiler Description and Project Scope, summarizes the REACH modifications done at thirteen boilers. In most cases, modifications were limited to flame stabilizers and oil atomizers. In a few cases, modifications were also made to burner throats to reduce air velocity, and new oil guns were supplied to replace aging and under-performing equipment. None of the boilers listed in the table were equipped with flue gas recirculation (FGR) to the windbox, and only the tangential fired boilers had upper auxiliary-air nozzles which could be used for overfire air (OFA).

Note: TF = Tangential-fired, SWF = Single-wall fired.

Table 1, Boiler Description and REACH Retrofit Scope

By way of specific example, retrofits to single-wall fired boilers, in plants labeled A and B, and the retrofits to tangential-fired boilers, in plants labeled C and D, are described in more detail below. The retrofit scope and results obtained in plants E and F are similar to those of the plants described in detail.

Plant A:
Three boilers in petrochemical complex A have been retrofit with LN-REACH technology. Units A1 and A2 are Breda 270 t/h SwF boilers with four burners on each of three elevations, while unit A3 is a Breda450 t/hr SwF boiler with three elevations of three burners, each. All three units were previously equipped with constant steam-to-oil differential pressure controlled steam atomization. The existing burners were equipped with Y-jet type atomizers and bluff body diffusers. With this burner and atomization configuration, the units were producing PM emissions of 250 mg/Nm3 or greater and NOx emissions up to 1,200 mg/Nm3 (adjusted to 3% excess O2). For the LN-REACH retrofit to A1 and A2, the bluff body diffusers and Y-jet atomizers were replaced with CCV flame stabilizers and Segmented V-jet atomizers. Installation was completed with the boilers on-line.

Unit A3 had exhibited extreme boiler vibration under certain load conditions in addition to the undesirable level of NOx and PM emissions. Engineering analysis of A3 revealed that the throat velocity in the existing burners was too high and the exit contour was misshaped for optimum LN-REACH performance, and possibly the root cause of the boiler vibration. Thus, the burner throats in unit A3 were enlarged from 737 mm to 813 mm, and the existing atomizers and flame stabilizers were replaced as in A1 and A2. Along with the enlargement, the burner throat contour was changed according to designs supplied by EPT with the REACH retrofit technical package. No pressure-part modifications were required.

After completion of all REACH hardware installation and other unrelated planned outage tasks, the boilers were prepared for normal start-up. Just prior to start-up, airflow measurements through each burner throat were taken to detect any imbalances in flow distribution. Air registers were adjusted to obtain uniform airflow among all the burners.

Plant B:
Two boilers in a petroleum refinery, plant B, have also been retrofit with LN-REACH technology. Unit B1 is a 435 t/h SwF boiler of nearly identical design to unit A3, above. Unit B2 is a 220 t/h SwF boiler with two burners on each of three elevations. Both boilers were originally equipped with Y-jet steam atomizers and flat-blade swirler flame stabilizers. Historical values of baseline NOx emissions reported by the plant for both units were approximately 1,250 mg/Nm3. Baseline PM emissions from B1 were 400 mg/Nm3, while PM emissions from B2 were 250 mg/Nm3. A particular challenge was posed by the fuel oil burned at plant "B", which contained 2.75% sulfur and 0.47% nitrogen. Burner throats in boiler B1 were enlarged from 762 mm to 864 mm to reduce burner axial velocity. Installation was completed during a normally scheduled maintenance outage.

Plant C:
Plant C is a tangential-fired boiler manufactured by Franco Tosi, supplying steam for process heating and electric power generation in a petrochemical complex. The unit has a steam generating capacity of 365 t/h, and is equipped with a small, close-coupled OFA compartment above each burner in the top elevation. The boiler is capable of burning No. 6 fuel oil and natural gas, and has four burner elevations and 16 burners. Each burner has single fuel-air compartment and auxiliary-air compartments immediately above and below the fuel-air compartment. The auxiliary-air compartments between the 2nd/3rd and 3rd/4th elevations are bricked shut. To avoid excessive opacity (smoking) prior to the REACH retrofit, the unit was normally being operated with the fuel-air compartment dampers 100% open, and the functional auxiliary-air compartment dampers virtually closed (i.e., 10% open for cooling).

CP-REACH flame stabilizers and internal-mix oil atomizers were installed to reduce PM emissions. The retrofit included: (1) replacement of Y-jet atomizers with internal-mix atomizers, (2) replacement of diffusers with compound-curve-vane swirlers and extenders for flame stabilization, and (3) conversion of the atomization steam system from constant steam pressure at 10.3 barg to constant steam-to-oil differential pressure of 0.7 bard over the load range, and (4) reactivation of the close-coupled OFA. To increase swirler flow entrainment, extender assemblies (i.e., bluff-body rings) were attached to the exit of the fuel-air nozzles to increase airflow turbulence and promote the formation of a strong internal recirculation zone.

Plant D:
In plant D, a petrochemical complex, two 170 t/h tangential-fired, Franco Tosi boilers have been retrofit with CP-REACH technology. Both boilers, D1 and D2, have two elevations and eight burners. Plant D was operating the existing medium-pressure, spill-return mechanical atomization system with an oil-water emulsion (10% water) to provide secondary atomization. With this approach, the plant had obtained a baseline level of PM emissions of 250 mg/Nm3. The plant objective was to reduce PM emissions to less than 50 mg/Nm3. The plant chose to take a step-wise approach to combustion equipment upgrades. In the first step, the plant retained mechanical atomization because there was not sufficient time to plan and implement the steam supply for steam atomization before an upcoming outage when the mechanical version of REACH could be installed. In response, CP-REACH mechanical atomizers were designed to replace the existing atomizers and still work with the existing oil supply conditions of temperature and pressure. CCV swirlers and extenders were also designed and supplied to replace the existing conical impellers. The decision whether to convert to steam atomization is to be taken at a later date.

RESULTS:

Baseline and post-retrofit data are not available from all the plants, as of this writing. The available data are summarized below in Table 2, PM and NOx Emissions Reductions With REACH Technology. Emissions concentrations are normalized to 3% excess oxygen in the flue gas, and stated on a dry basis.

Table 2, PM and NOx Emissions With REACH Technology

Plant A:
Simultaneous reductions in NOx and PM emissions were accomplished with the implementation of LN-REACH Segmented V-jet atomizers and CCV swirlers. Boiler A1 NOx emissions were reduced to 552 mg/Nm3 (reduction of 50%) and PM emissions to 42 mg/Nm3 (reduction of over 80%). Emissions of NOx from A2 were reduced to 448 mg/Nm3 (reduction of 59%) and PM emissions to 49 mg/Nm3 (reduction of 80%). The plant reports unburned carbon levels of less than 30% in the ash. Excess oxygen was 3% in both cases.

Emissions form boiler A3 were reduced to 90 mg/Nm3 for PM and 580 mg/Nm3 for NOx. NOx emissions from this boiler are higher than boilers A1 and A2 because the heat release rate in the furnace is 60% higher than in A1 and A2. Excess oxygen was limited to 2% to help control NOx.

Plant B:
With LN-REACH in boiler B1, NOx emissions were reduced to 485 mg/Nm3 and PM emissions were reduced to 60 mg/Nm3 (reductions of 85% and 61%, respectively). With one burner out of service (air registers open to provide additional overall fuel-air staging of the combustion), NOx emissions were further reduced to 420 mg/Nm3. PM emissions data are reported with one burner out of service. No PM data are available with all burners in service, but opacity remained unchanged at less than 10% in either operating mode.

In boiler B2, NOx emissions were reduced to 575 mg/Nm3 (reduction of 54%). With the middle elevation of two burners out of service (air registers open), NOx emissions were further reduced to 400 mg/Nm3 and PM emissions were reduced to 130 mg/Nm3 (a reduction of 68% in PM emissions). Again, stack opacity remained less than 10% during the burner out of service test. Historically, this unit has had higher PM emissions, most likely due to relatively short residence time in the furnace. These simultaneous reductions in NOx and PM were achieved with fuel-bound nitrogen of 0.47%.

Plant C:
Particulate matter emissions with CP-REACH at boiler C1 were reduced by 63% to 56 mg/Nm3. NOx emissions remained essentially unchanged at 715-735 mg/Nm3, as expected for a CP-REACH implementation. As discussed previously, to avoid high opacity the boiler was normally operated with the fuel-air compartment dampers 100% open and the functional auxiliary air compartments 10% open (for cooling) on all eight burners. By setting the auxiliary air compartments above the top burner elevation to 50% open, NOx emissions were reduced by approximately 9%, from 725 to 660 mg/Nm3 with a very slight increase in PM emissions from 56 to 69 mg/Nm3. This result was not surprising, and suggests that further biasing of combustion air to the auxiliary-air compartments in the top elevation or application of LN-REACH would be effective in lowering NOx emissions. The combustion performance improvements accomplished with the CP-REACH technology enabled use of the auxiliary air compartments as OFA ports without a significant increase in PM emissions or opacity.

Plant D:
The performance of both boilers, D1 and D2, were similar after the installation of the CP-REACH swirlers, extenders and mechanical atomizers. Particulate matter emissions were reduced from 250 to approximately 130 mg/Nm3, a 46 to 50% reduction without oil/water emulsion. This higher absolute level of particulate matter emissions compared to the other values obtained with CP-REACH results from the difference in atomization technology. In these boilers, medium pressure mechanical atomization (P<38 barg) was in use, whereas internal-mix steam atomization was applied in the other cases reported in this paper. Emissions of NOx were reduced from 730 to 630 - 640 mg/Nm3 from the two units. This NOx reduction of 13% was accomplished with adjustment of the air registers to bias the combustion air to the top of the furnace, similar to the approach used at plant C1, without significant impact on PM emissions.

COST:

The scope of supply in all of the REACH implementation projects described in this paper included engineering analysis of existing hardware and operating conditions, design and supply of atomizers and CCV swirlers, and field technical service for installation, start-up, and acceptance testing. The cost of this scope of work is summarized in Figure 1, REACH Retrofit Cost vs. Steam Capacity. By way of example, a REACH retrofit involving supply of atomizers and flame stabilizers for a boiler with steam generating capacity of 500 t/h (metric) will cost about $200 per t/h of steam capacity, or approximately $100,000. Costs from other REACH retrofits with a similar scope of supply are also included in the figure to illustrate the cost of REACH technology over a wider range of boiler capacities. Since REACH technology is custom engineered for each application, Figure 1 provides only a rough guideline for REACH retrofit costs, as variation around the trend line is evident in the figure.

Complete burner replacement is an alternative to REACH retrofit; however, costs for burner replacement ranges from three to ten times the cost of a REACH retrofit for the same scope of supply (i.e. engineering, equipment supply, and field technical service). For small units with few burners, the cost of burner replacement approaches three times the cost of REACH hardware because the engineering component is a significant factor for both. As unit capacity increases, the relative simplicity of the REACH retrofit dominates the cost compared to a complete burner. Regardless of unit capacity or the number of burners involved, REACH retrofits cost significantly less than burner replacement.

Beyond the direct cost of the REACH technology, the installation and on-going operating costs must be considered. In each case included in this paper, plant personnel carried out the REACH equipment installation during scheduled maintenance outages. With the exception of burner throat modifications at boilers A3 and B1, the installation work was completed within one to three days per boiler. In contrast, complete burner replacement or installation of other emissions controls such as OFA or post combustion controls would require much more outage time. In the case of industrial plants, lost operating time means lost production from other high-value assets, among other costs. In many cases, users of REACH technology have realized operating and maintenance cost reductions. These reductions have come from reduced unburned carbon, lower excess O2, longer swirler and air nozzle life, reduced fouling of the boiler, longer boiler maintenance intervals, and easier atomizer cleaning.

CONCLUSIONS:

The following conclusions can be drawn from the implementation of REACH technology in thirteen industrial boilers, ranging in capacity from 140 to 530 t/h of steam generating capacity:

• REACH technology provided significant reductions in particulate matter emissions, ranging from 45 to 85%, with absolute values of PM emissions after REACH installation ranging from 130 mg/Nm3 with mechanical atomization to as low as 28 mg/Nm3 with steam atomization.
• LN-REACH provided significant reductions in NOx emissions in all cases where it was applied, ranging from 50 to 60%, with absolute values of NOx emissions ranging from 450 to 670 mg/Nm3. In all cases, these NOx reductions were accomplished with a simultaneous reduction in PM emissions and reduction in unburned carbon.
• The improved combustion performance provided by both LN- and CP-REACH enabled further NOx reduction through the use of OFA without problematic increases in PM emissions or opacity.
• The small scope of modifications required with REACH retrofits enabled these emissions reductions with a fraction of the outage time and a fraction of the cost of burner replacement.

FUTURE WORK - GAS-REACH:

The development principles of reducing emissions through retrofittable modifications of key burner components are now being applied to the development of REACH technology for gas-fired burners. Novel gas distribution components and flame stabilizers currently under test are providing NOx emissions of less than 200 mg/Nm3 without flue gas recirculation or overfire air in a 60 t/h boiler. As demonstrated above and in other applications with LN- and CP-REACH [7], this new gas technology may be combined with FGR and OFA for further reductions in NOx emissions. This GAS-REACH technology is intended for scale-up to larger capacity boilers within the next 12 months.

ACKNOWLEDGMENTS:

The authors are particularly grateful to Engineers A. Vizziello, F. Cuccinella, M. Carrara, G. Bottini, F. Ranieri and G. Galdo of EniChem S.p.A., Engineers S. Piccoli, and A. Fazio of Agip, and Engineer R. Pizzoli of Ansaldo Engergia for their contributions and support in the implementation and testing of the REACH installations described in this paper. In addition, the authors acknowledge the technical support of Combustion Components Associates of Monroe, Connecticut.

REFERENCES:

1. Giovanni, D.V., M.W. McElroy, and S.E. Kerho, "REACH: A Low-cost Approach to Reducing Stack Emissions and Improving the Performance of Oil-fired Boilers," EPRI/EPA Joint Symposium on Stationary Combustion NOx Control, Kansas City, Missouri (May 1995).

2. Kerho, S. E., and D. V. Giovanni, "Atomizer and Swirler Design for Reduced NOx and Particulate Emissions," EPRI Workshop on NOx Controls for Utility Boilers, Boston, Massachusetts (July 1992).

3. Giovanni, D.V., R.C. Carr and S.E. Kerho, "Reduction in NOx Emissions by Retrofit of Low-NOx Atomizers on a 550 MW Oil-fired Boiler," Third International Conference on Combustion Technologies for a Clean Environment, Portugal (July 1995).

4. Carr, R. C., Marco Alberti, and Christopher J. Nagel, "Retrofit of Gas Combustion Equipment and Low-NOx Oil Atomizers at a 550-MW Oil-Fired Utility Boiler," International Joint Power Generation Conference, Houston, Texas (October 1996).

5. Conti, A.V., S.E. Kerho and J. Lucente, "Low Cost Retrofit Combustion Hardware for Emissions Control on Industrial Boilers," ASME/EPRI International Joint Power Generation Conference & Exposition, Minneapolis, Minnesota, (October 1995).

6. Conti, A.V. and J. Lucente, "Reduction of NOx Emissions on Oil Firing at Bowline Point Unit No. 2," ASME/EPRI International Joint Power Generation Conference & Exposition, Minneapolis, Minnesota (October 1995).

7. Giovanni, D.V. And R.C. Carr, "Applications of REACH Technology to Reduce NOx and Particulate Matter Emissions at Oil-Fired Boilers," EPRI-DOE-EPA Combined Utility Air Pollutant Control Symposium, Washington, DC (August, 1997).

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