On June 2, 2014, the U.S. Environmental Protection Agency (EPA) proposed a commonsense plan to cut carbon pollution from power plants. EPA is proposing the state goal approach under Section 111(d) of the Clean Air Act by 30 percent from 2005 levels, while starting to make progress toward meaningful reductions in 2020. With this new rule, energy producers are required to develop alternatives to reduce carbon dioxide (CO2) emissions by 2020.
Plant heat rate improvement projects are one of the low cost methods to gain reductions in CO2 emissions given quick implementation time. Fuel flexibility needs, tube erosion related forced outages, and flue gas back end temperature compatibility issues are mainly the key challenges that coal fired power generation industry had to overcome in the midst of ever stringent emission compliance requirements.
There have been several publications and case studies showing the potential performance improvements of different cleaning technologies in the past such as deposit weight monitoring, water cannon cleaning, heat flux monitoring, etc. However, there is still a lack of conclusive decision criteria and a set of tangible guidelines. The objective of this article is to perform a comparison of the best available boiler cleaning technologies in order to develop guidelines for the expected profit and emission impact on coal fired power plant operations. This article presents a collective study in an effort to establish guidelines for asset owners and plant operation engineers to quantify the return potential for their specific case. The simplified model uses key inputs that will provide quick insight on how much opportunity is available with each cleaning technology to improve boiler performance and emissions.
The results can be further used as a decision tool for the Coal Fired Power Industry to help identify the alternative cost effective solutions resulting power generation cost reduction, reduced emissions, and improved operational reliability. The guidelines are driven from first principle models and validated using actual case studies on both new and existing boilers.
BEST AVAILABLE CLEANING TECHNOLOGIES
The best available cleaning technology available in the furnace can be broken into 3 pieces:
- Cleaning Device
- Feedback Device
Figure 1 illustrates the typical effective radiant heat transfer area in the furnace section. Due to the limitations within the boiler structure, cleaning devices may not be located in a way to have 100 percent cleaning coverage in the furnace (obstructions around the boiler, burners, air ducts, etc). The most effective cleaning coverage can be achieved with the SMART Cannons technology as the cleaning pattern of cannon is not limited to a rotational fixed coverage area as it is with a wallblower.
Figure 2 shows a comparison of SMART Cannon and rotational cleaning device (wallbower & water lance) cleaning zones. As cleaning with a wallblower will be limited to its own wall that and limited with a circular coverage, furnaces cleaned with wallblowers will leave more dead space between adjacent cleaning devices and often time the cleaning coverage will be limited to 40 percent of the available heat transfer area in the furnace. Water Cannon will be able to clean the opposing wall and side walls with a large coverage trajectory leaving no dead space between cleaning zones. As a result, the cleaning coverage with Water Cannons can range up to 60 to 80 percent of the overall available heat transfer area in the furnace. Clyde Bergemann’ s SMART Cannon WLB100A is shown at the left side of Figure 2.
- Smart Flux Sensor: The most common and reliable feedback device used to detect the cleanliness status of a water wall is Heat Flux sensors. Clyde Bergemann’s SMART Flux sensor technology is used in all types of coal, waste to energy, and solar applications providing key data for plant operations and SMART Clean optimization algorithm.
Heat flux is defined as the heat transfer quantity through a unit area within 1 hour. The industry standard unit used for heat flux is kbtu/ft2hr. A SMART Flux sensor measure the heat flux by measuring the temperature differential between the fire side and the water side of the tube wall metal. A successful cleaning operation should result with a heatflux improvement between 10-30 percent from the dirty base line condition.
As 95 percent of the heat transfer in the furnace is through radiation, convective heat transfer can be ignored for simplicity purposes. Heat transfer through a small section of water wall can be derived as:
Q = ξσAω(T14 – T24)
PRB contains high levels of calcium and magnesium compounds, which are major sources of the reflective property of PRB ash deposits. Calcium compounds can be in the 24 percent range for PRB coal compared to 1 to 2 percent for eastern coals. Also, magnesium is in the 5 percent range for PRB coal compared to about 1 percent for eastern coals. The normal emissivity of a boiler tube with a coating of iron oxide is from 0.85 to 0.89. However, a slag deposit can cut this value to 0.5, and thus have a significant effect on the amount of heat absorbed by the furnace. From the radiant heat transfer equation it can be seen that this reduction in emissivity (or increase in reflectivity) would reduce heat transfer by almost half.
As the ash thickness and exact ash conduction coefficient is not known in real time, Clyde Bergemann SMART Flux sensor technology simplifies the heat flux calculation by directly measuring the temperature differential across the tube metal wall:
- Membrane Sensor: The water wall cleaning should be managed within the predetermined boundaries by the system design. This will ensure that the measured thermal impact represents the actual stress conditions on the overall zone area being cleaned. Membrane sensors are used to detect any over spraying and keeping the zone cleaning within the zone boundaries.
The key to a successful Furnace Cleaning intelligence is the ability to predict the impact of furnace cleaning on other boiler parameters. In early 2000’s the furnace cleaning systems and intelligence used for furnace cleaning was driven by only local heatflux feedback. Initiating cleaning solely on local heatflux may result in not optimum performance in other sections of the boiler. Too clean furnace may cause low reheater temperatures or lower than desired main steam temperature, both resulting in penalty on Plant Heat Rate. Clyde Bergemann intelligence module used for Furnace cleaning is called SMART Furnace. This module determines when would be the best time to clean the furnace given the key performance indicators and current boiler and plant performance.
When water walls are cleaned by water, the amount of stress put on the boiler tube is calculated by measuring thermal impact. TI is defined as the largest dip in tube surface temperature when a sensor area is cleaned. Water cannon cleaning pressure and jet progression velocity is adjusted based on the Thermal Impact measured from the sensor being cleaned. If the TI is greater than 110 F, the aggressiveness is driven down and tube life is optimized. If the TI is small, the SMART Cannon system is required to clean more, the aggressiveness is increased until the cleaning target is made again. This will optimize tube life while cleaning water wall tubes.
Above figure illustrates the decrease in FEGT and EEGT as a function of “Furnace Area Cleaned” by using SMART Furnace systems along with SMART Cannons for an average supercritical boiler. The data used for the graph is consolidated results from Clyde Bergemann installations for PRB boilers using SMART Furnace Technology using Water Cannons.
Impact of Heat Rate Improvement on CO2 emission
Using the existing Clyde Bergemann fleet data and the model simplified average expected heat rate improvement with the best available technology in the furnace is 0.96 percent.
The CO2 emission of Coal can be simplified by neglecting the CO release from incomplete combustion as shown in the following equation:
C(12) + O2 (32) à CO2 (44)
Therefore, there will be 44 tons of CO2 to be released for each 12 ton of fuel Carbon being burnt. For the case of PRB, the CO2 release by burning 1 ton of coal can be calculated as follows:
C% of PRB = 40% (typical fixed Carbon as received)
MC = Total coal flow from an average performing PRB fired 500 MW supercritical boiler without Best Available Cleaning Technology can be calculated as follows:
Gross Unit Heat Rate = 10,000 btu/kwh
Qin = Heat Input at 500 MW generation
HHV = 8,800 btu/lb (as received)
Qin = UHR [btu/kwh] x 500 [MW] x 1000 [kw/MW]
MCoal = Qin / HHV = 10,000 x 500 x 1000 / 8,800 = 568,180 lb/hr
MCoal = 258 T/hr
Mc = MCoal x C% = 103 T/hr
The corresponding CO2 emission for this fuel carbon flow rate once CO is neglected;
MCO2 = MC x (44/12) = 379 T/hr
DUHR%furn = 0.96 percent is the average Unit Heat Rate improvement is realized with the Best Available Furnace Cleaning Technology from the base line defined, the reduction in CO2 emission can be found as:
DMCO2 = MCO2 x DUHR% furn = 3.64 T/hr
If a typical Capacity Factor of 80% is used, the annual reduction in CO2 emissions can be calculated as;
DMCO2 = C.F.[%] x MCO2 [T/hr] x DUHR%furn x 24 [hr/day] x 365 [days/yr] = 25,484 T/yr
The problem areas in the convection pass is typically around the bull nose and division panels as these locations are subject to excessive flue gas temperatures resulting in molten deposit accumulation and Clinker Formation. The latest cleaning device technology within the past decade provides effective options to deal with such tenacious deposits. Following are some examples for the possible alternatives to improve convection pass cleaning:
- Nozzle Upgrades: Nozzle cleaning efficiency can be improved by implementing fully expanded nozzles where the cleaning jet pressure is completed converted into kinetic energy. With such nozzle upgrades, the typical required cleaning flow can be reduced by 20-30 percent for sootblowing. If the cleaning is handled by air, 20 percent reduction in air flow may result in 20 percent reduction in parasitic load to be delivered to the air compressors. This is an easy upgrade and can be done when the unit is on line. However, compressor turn down ratio needs to be suitable to support the 20 percent reduction in air demand. The expected effect of 20 percent compressor load reduction for an average 500 MW unit can be calculated as follows:
By using the adiabatic Power Equation above and inputs from Figure 10, average compressor power required for 1 cubic feet of compressed air can be calculated as 0.45 kW/ft3
For the case of our sample boiler with 10,000 btu/kwh heat rate using air for both convection and furnace section, 20 percent reduction in cleaning flow would result in 5,5M sft3/day reduction in cleaning air consumption. The resulting elimination in parasitic power is 0.3 MW. The corresponding improvement in Unit Heat Rate is 0.07 percent just by a simple nozzle upgrade project.
- Air to steam conversion: The ideal steam extraction for sootblowing is the Cold Reheater (CRH) line for any air to steam conversion project. As the CRH steam is superheated yet its temperature is not too hot as main steam, such conversion projects do not require de-superheater stations reducing the cost of implementation to a reasonable level. The relatively low enthalpy of CRH steam also makes this extraction point beneficial with Heat Rate benefits. Using our 500 MW conventional base line case, if the air to steam conversion is made, approximately 0.72 MW net power generation is recovered resulting in 0.14 percent improvement in UHR.
- SMART Sootblower: SMART Sootblower is a specialized cleaning device designed to deal with tenacious or non-uniform deposit in problem sections of a boiler. The sootblower design can allow changing aggressiveness along the cleaning path of insertion and retraction with the use of specialized motion control technology and variable frequency driven dedicated motors, 1 for rotation and 1 for insertion and retraction of the sootblower. With the combination of high efficiency nozzles and proper feedback technology, SMART Sootblower can remove deposits at critical sections and keep the boiler operation stable without any clinkers or pluggage at high firing rates.
- SMART Explosion: SMART Explosion provides an efficient solution to increase the operation of boiler sections covered with dry ash by controlled explosions. Air heater and Economizer are ideal sections for the use of this technology. Each explosion induces vibrations into the heating surfaces, thereby removing fouling especially in areas where it is hard to reach or locate cleaning devices such as sootblowers or other cleaners. Challenging designs prone to fouling and pluggage such as tubular air heaters is an ideal location for this cleaning device. Given air heaters account for between 10 to 15 percent of the overall heat absorption in a boiler, adding SMART Explosion can improve plant heat rate by 1-3 percent if the current tubular heater is not cleaned an effective cleaning method.
- Thermodynamic Model (TDM): The best available feedback technology widely used in the industry is Thermodynamic Modeling using inputs from the available boiler instrumentation. Such models are essential to predict and optimize the heat transfer distribution in the boiler. Clyde Bergemann uses a Unit Heat Rate optimization model to predict the impact of cleaning on the overall Power Cycle Efficiency and optimize cleaning for the best local Heat Rate at any given time.
- SMART Gauge: One important feedback technology developed within the past decade is the SMART Gauge technology where the specialized sensors located on the boiler hanger rods capture the change in pendant weight thus the change in deposit weight is calculated. Such feedback technology is essential for boilers dealing with heavy deposit and clinker formation.
SMART Clean is a cleaning optimization system Clyde Bergemann uses to optimize the boiler cleaning as a whole in the Furnace, Convection, and Air Heater sections of a boiler. The SMART Clean algorithm calculates the heat transfer targets dynamically to determine the optimum heat transfer distribution in each boiler based on the Key Performance Targets. See below targets as a sample :
- Main Steam Temperature = 1005°F
- Hot Reheater Temperature = 1005°F
- Economizer Exit (SCR Inlet) Gas Temperature < 760°F
- Furnace Exit Gas Temperature < 2200°F at maximum firing rate
The SMART Clean system uses a simulation model that would predict the impact of each cleaning operation on Unit Heatrate and Key Operator Parameters such as Main Steam Temperature, HRH Temperature and Spray flows. The Thermodynamic and Heat Transfer models are typically wired and how the simulation results are used for the decision making process.
Unit Heat Rate and Emission Impact of Convection Pass Cleaning Upgrades
The results summary is arranged based on the following base line conditions:
- Conventional wallblower cleaning in the furnace section with no feedback driven cleaning optimization control
- Conventional retractable sootblower cleaning in the convection section with no feedback driven cleaning optimization control
- There is no air to steam conversion impact on the calculation
Clyde Bergemann simplified model is used to measure the potential for improvement in Unit Heat Rate, thus reduction in CO2. Following base line inputs are required to calculate a budgetary improvement figure on Unit Heat Rate:
FEGT, APH Gas outlet Temp, Gross MW, Heat Rate, Boiler Eff, HHV, percent C
Using the simplified approach and key input data provided:
- Average expected heat rate improvement with the best available technology in the furnace ~ 0.96 percent
- Average expected heat rate improvement with the best available technology in the convection pass ~ 0.55 percent
- Average expected heat rate improvement with the best available technology in both convection pass and furnace ~ 1.35 percent
The cost savings and profit improvement as a result of the 1.35 percent Heat Rate improvement for the subject 500 MW boiler can be calculated based on fuel savings cost. With the proper feedback devices and optimized cleaning, it is expected to avoid over cleaning or clinker related forced outages. In this example, the cost of 1 day forced outage has been accounted to represent a typical case. The operating profit can be improved between $250k and $2.2M depending on the boiler size as a result of upgrading the cleaning technologies in both Furnace and Convection sections.
If the simplified emission method is used to predict the impact of 1.35 percent UHR improvement on CO2 release, the corresponding reduction in mass flow is between 15,000 and 110,000 Tons/yr.