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Coal-To-Gas Conversions And Retrofits For NOx Emission Control In Power Plant Operations

By Zeeco |

Introduction

Operational standards throughout the power, petroleum, and petrochemical industries are continuously evaluated and updated in favor of practices that mitigate environmental impacts. New operational regulations and emissions initiatives often present serious challenges to the preferred practices of the fossil power and industry worldwide. The divide between goals and current practices provides an arena for innovation where engineering teams work to reconcile the industry and regulatory perspectives. The impetus for change often comes in the form of environmental regulations, but it can also stem from monetary drivers or public perception.

In the United States, the Environmental Protection Agency (EPA) continues to tighten standards requiring all coal-fired electric plants to cut carbon, NOx and other emissions by significant amounts. Doing so can be an expensive proposition, pushing many power companies to consider reducing coal firing and add natural gas firing, or change over completely to gas. Coal-firing has delivered roughly half of the United States’ electricity generation for decades, but in recent years the percentage of coal-fired power has declined. Power companies are faced with the potentially expensive choice of continuing to burn coal while trying to reduce emissions, or switching to natural gas. The move from coal to natural gas and the current global regulatory environment mean plant owners are seeking to optimize necessary capital investments in new equipment by retrofitting and/or adding burners to burn natural gas to realize the most efficient and lowest overall emission levels possible. In this article, we will discuss ways to determine whether retrofitting or upgrading a boiler burner makes sense, and also examine some applicable technologies for the evaluation process. Finally, we will detail an in-depth boiler burner retrofit case study for lessons learned and results achieved.

One combustion company with worldwide experience in coal-to-gas conversions and retrofits for NOx emission control in power plant operations is Zeeco, Inc. The company’s combustion engineering experts perform retrofit applications when transitioning complete firing systems from coal to gas-fired or adding gas-firing capabilities to existing plants. Gathering the right information in advance of a retrofit is critical to understanding the type of firing system needed and the operational parameters of the system. This process typically includes a detailed site-visit performance analysis; full steam generation and fuel system combustion and ancillary review; and post contract-award reviews, such as boiler performance, physical flow modeling and computational fluid dynamics modeling. These steps help quantify the existing systems and determine any necessary design and operational changes to the boiler. The combination of engineering and modeling approaches is used to accurately predict field performance and meet NOx emission standards after installation and commissioning.

Performance Analysis

A thorough performance analysis by combustion experts can identify boiler, burner, and other system issues that impede achieving the desired operational emissions and performance levels. These analyses are often performed by physically walking the facility to check operational parameters and controls and evaluate boiler operational and performance data; and inspect the boiler, burners, burner components (register, gas/oil tips, pilots, etc.), and ancillaries such as controls and emissions monitoring equipment as thoroughly as possible. An assessment should also include working directly with the plant personnel responsible for operation of the equipment so that operational complexities and desired processes or parameters can be incorporated in the retrofit or upgrade planning. It  is important that a combustion expert with knowledge and understanding of the power industry, including combustion and boiler operations, properly designs the new burner/system to fit the project goals, emissions requirements, and day-to-day operation of the boiler. Understanding the processes occurring both upstream and downstream from the boiler is a critical component of any plan for an upgrade, retrofit or revamp project.

In cases where the existing equipment is not sufficient to meet necessary emissions levels or efficiencies regardless of maintenance or revamps performed, the expert may instead recommend a retrofit . Plant operators benefit when the combustion expert  evaluating the boiler thoroughly understands combustion as well as the steam generation equipment and can suggest corrections based on visual inspection, current operation, overall condition, flame geometry, and boiler performance data, if available.

The analysis phase of an application can include process tools such as boiler thermal performance studies, emissions reviews, ancillary equipment audits, controls and BMS reviews, physical flow modeling and Computational Fluid Dynamics (CFD) modeling. These tools can reduce application time as well as overall startup time and conditioning of the burner. Companies such as Zeeco use these engineering and modeling practices to understand precisely where every pound of air and fuel enter the burner to achieve the ultimate goal of meeting emission and performance goals while maintaining the steam generation output and quality. These tools provide insight into a boiler burner’s performance in a variety of situations and scenarios.

Further details of the analysis performed for application specifics related to new and potential retrofit / additions for gas firing follow:

 Steam Generator Performance Analysis

In any boiler system assessment, a thorough understanding is necessary of how proposed changes to the boiler geometry or operational characteristics will impact boiler performance. Changes in fuel composition or flow, convection bank configuration and cleanliness, operating pressure, and steam conditions can be evaluated. In addition to thermal calculations, metrics such as combustion efficiency, boiler efficiency and heat balance, gas-side pressure drop, and air heater performance can also be evaluated.  Analysis can determine projected boiler efficiency overall, furnace exit gas temperature, firing rate, and steam and flue gas thermodynamic conditions. These projections can be used alongside physical flow and computational fluid dynamics modeling to create a clear picture of the current state of the system and the effect various modifications may have on it.

Modeling

Physical Flow Modeling (PFM) visually details the combustion airflow. It focuses on the following:

  • 95% of the mass flow through a burner is air

  • Solving distribution issues between burners

  • Removing tangential velocities around each burner

  • Proper system pressure drop to ensure optimal performance

  • End goal of proper distribution of air and fuel to meet all project goals and emissions requirements


Burner predictive models are dependent upon equal airflow to deliver accurate predictions. Calculations assume each burner receives the same amount of air and fuel, and in the same compositions. In reality, boiler burners don’t operate this way unless the system has been properly balanced. The design team builds an accurate, 3-D clear plastic model that contains all internal components, Venturi meters, orifice plates, etc. so engineers can visualize air flow to each burner. Static probes are used to measure the various pressures.  After obtaining an accurate picture of existing flow patterns, the design team corrects any air flow imbalances in the model with baffles and turning vanes. In a burner, 95% of the mass in the combustion reaction is air, with the remaining 5% fuel. Balancing the air is critically important to achieving the designed performance of a boiler burner system, as seen in Figure 1: Peripheral Velocity Distribution.

Figure 1: Example of Peripheral Velocity Distribution, Burners 1-4

In this specific instance, data from testing illustrated that the mass flow is within the acceptable standard of ±2% average of all burners. The peripheral velocity distributions are within the acceptable standard of ±10% of all burners. System area pressure drop was maintained and brought within Zeeco’s requirements for zero swirl. Physical modeling shows actual mass flow in a system and then shows actual mass flow after corrections are implemented, removing the guesswork.

Computational Fluid Dynamics (CFD) is used to visualize temperature, velocity, emissions, particle sizes and their interactions within the combustion process. CFD is a proven modeling tool that aids in the design process to ensure optimal firing systems and boiler performance.  When CFD is used in conjunction with physical flow modeling during a retrofit or performance analysis, combustion engineers can accurately predict  boiler performance taking into account fuel and air distribution after any solutions indicated by the physical modeling are implemented.

CFD modeling focuses on:

  • Flame fit within the required boiler confines

  • Flame behavior and interactions between fuels and respective burners

  • Emissions predictions

  • Firing system temperature profiles for the boiler and downstream heat transfer surfaces


Because steam analysis, physical modeling, and CFD modeling each provide different but important predictions of field performance, using a combination of approaches in every burner design makes sense. It shortens startup and commissioning time since potential problems or emissions issues can be corrected in the engineering and design phase rather than having to be field-corrected on site. Overall boiler performance is more efficient from the beginning since the system has been thoroughly tested and evaluated prior to installation. In retrofit applications, a tandem approach to modeling allows for more cost-effective and predictable results in shorter timeframes. The cost of physical modeling is typically less than 10% of the overall project cost, but using it effectively can reduce startup costs by 50% or more.

Retrofitting to Meet Global Emissions Targets

The United States, Canada, the United Kingdom, the Kingdom of Saudi Arabia, and many other countries are implementing or considering new regulations that would require industrial plant and power plant owners to reduce their overall plant emissions. According to the United States EPA,[1] “Boilers and process heaters, used within a wide range of applications such as industrial process heating, petroleum refining, and chemical manufacturing, consume approximately 37% of all gas used in [the U.S. in] the industry and contribute a significant percentage to overall NOx emissions.”

When power plants use gaseous fuels without fuel-bound nitrogen, the primary contributor to overall NOx production is the formation of thermal NOx. Thermal NOx is produced when flame temperatures reach a level high enough to “break” the covalent N2 bond apart, allowing the “free” nitrogen atoms to bond with oxygen to form NOx. See Figure 2.

Figure 2: Calculated Peak Flame Temperature vs. Thermal NOx Production

Burner and complete firing system retrofits in boilers are an economical solution to achieving lower NOx levels while utilizing the existing boiler. Retrofits for ultra-low-NOx emissions can be challenging in some cases and many facilities are concerned about the retrofit impacting steam generation. Zeeco’s GLSF Ultra-Low NOx Free-Jet burner was developed to operate with a flame pattern or profile fitting within any  space or back wall firing length limitations while achieving significant NOx reductions and not impacting steam generation.

Case Study in Low NOx Emissions Control

One example of a retrofit application from a recent Zeeco boiler burner case study involves a pair of boilers that were originally designed for service on a ship more than 70 years ago. The boilers were removed and land based at a Gulf Coast refinery. Each boiler was originally equipped with two conventional burners with corresponding firebox dimensions of 14 feet deep*, 20 feet tall, and 10.8 feet wide. Information on the boilers was limited due to age of the installation, as many of the details important to properly engineer next-generation burner retrofits were not relevant 70 years ago and were therefore not clearly documented. Additionally, some original design documentation was missing. Zeeco replaced each of the existing conventional burners with a ZEECO GLSF Ultra Low NOx Free-Jet burner. The heat release design of the new burners was set at 77.4 MM Btu/hr per burner and the combustion air pressure drop was set at 3.25” W.C. at 15% excess air with an ambient air temperature of 100°F.

*This dimension was not apparent during the application development phase. It was left as “to be determined.”

Zeeco and the facility operators probed the boiler, sampled the flue gas and conducted air and flue gas system pressure surveys. The following challenges and solutions were identified:

1. Boiler Leakage

Challenge: Boilers had air leakage issues (tramp air), causing air to enter the boiler through locations other than through the burner throat. Not all of the air was being used to help mix the gas and air together, resulting in longer flames.

Solution: Boilers were thoroughly sealed. Combustion air entered through the burner and not through the leakage in the boiler.

2. Boiler Forced Draft Blower Control

Challenge: The existing Forced Draft (FD) blower control contained older technology and lacked fine adjustments. Therefore, it was difficult to maintain the desired airflow rate through the burner. The Induced Draft (ID) fan was instead used to control the airflow, affecting the draft within the boiler, which increased tramp air leakage.

Solution: Forced Draft blower controller was adjusted to allow for better excess air control. This allowed the boiler to be operated in a more traditional balanced draft configuration, reducing tramp air and NOx.

3. Change Gas Port Sizing:

The size of the ports was altered to operate at a maximum fuel gas pressure of 32 psig as opposed to the original 25 psig. The high gas pressure provided more energy to mix the fuel and air together, resulting in a shorter flame and reduced emissions. The burner was also redesigned to mix the steam directly with the air stream prior to entering the boiler. The new rate for the steam was revised to 0.5 lb (steam injection was provided to allow for firing a wide fuel specification).

4. Increase Air Pressure Drop and Add Air Spin

The original Free Jet design was based upon maximum combustion air pressure drop of 3.25” W.C. with no air spin. Field testing confirmed there was a slightly higher air pressure drop available to help with the mixing of the air and gas. A swirler in the throat of the burner was employed to improve mixing of the air and fuel and change the flame shape from long and narrow to shorter and bushy. Airside pressure drop increased to 3.5” W.C.

Field Results

After the burners and boilers were modified as described in the Challenges and Solutions section of this paper, the following emissions were achieved:

  • NOx emissions -------------------------------0.03 lb/MM Btu (HHV Basis)

  • CO emissions, corrected to 3% dry O2-----50 ppmv


After initial startup, evaluation and redesign, the burners were able to operate from low firing rates with good flame characteristics. The burners utilize approximately 0.3 to 0.4 lb steam injected into the combustion air stream and are currently operating at or below the target NOx emissions requirements.

During the process of this retrofit, Zeeco identified the following areas that should be  considered when retrofitting a boiler burner:

  • Burner Design Conditions: It is necessary to receive all of the design information at the beginning of the project. This task was challenging due to the age of the boilers and the limited amount of information that was available. Small changes in conditions can have a large effect on performance.

  • Gas Port Sizing: With the Free Jet design, it is crucial to maximize the mixing energy of the near-field flue gas and the air stream, especially at low rates to increase flame quality and allow for the sometimes shorter fireboxes in older boilers.

  • Air Pressure Drop: It is critical to optimize the design for introduction of the gas and the air streams, especially at low rates to keep flame lengths short.

  • Boiler Forced Draft Blower Control: It is very important to be able to control the combustion air with both the Forced Draft blower (FD) and the Induced Draft (ID) blower to mitigate/eliminate tramp air, which affects performance.

  • Placement of Steam Injection: To maximize the mixing energy of the gas and the air when not using steam injection, a steam lance inserted into the air steam was used. With the steam lance, the addition or subtraction of steam will not affect gas pressure or the mixing energy of the fuel and air.


Zeeco installed burners that were designed to use 0.3 lb steam/lb fuel gas injected into the fuel gas stream for increased NOx reduction if needed since External Flue Gas Recirculation (EFGR) was not planned for this application. The use of a small amount of steam was less expensive than reworking the boiler to add approximately 14% EFGR to achieve the required NOx emissions level of 0.03 lb/MM Btu (HHV). The new burners were installed in the existing burner windbox configuration. The new burner footprint was approximately the same size as the existing burners, so boiler modifications for burner installation were minimized to reduce the overall installation cost of the retrofit.

Conclusion

With a continued increase in power companies considering the switch from coal to natural gas and continued pressure to achieve ultra-low NOx  and lower carbon emissions levels, plant owners are faced with the expense of retrofitting boiler burners to burn natural gas at the most efficient and lowest emission levels possible. Performance reviews and accurate modeling give engineers insight into a boiler burner’s performance and help maximize efficiency from the earliest engineering and design steps all the way through startup and commissioning. Emissions levels can be accurately predicted, fuel and air mixing and the combustion process can be properly planned, and plant owners realize reduced costs long term.

 
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