Performance Tests of Water Cannon Furnace Cleaning Systems

Transcription

1 Performance Tests of Water Cannon Furnace Cleaning Systems Electric Power 2003, Houston, Texas Charlie Breeding, P.E. Clyde Bergemann, Inc. Atlanta, GA

2 Table of Contents INTRODUCTION 3 FURNACE SLAGGING 3 FURNACE CLEANING 4 INTELLIGENT SOOT BLOWING 5 TEST PROGRAM 5 TUCSON ELECTRIC SPRINGERVILLE 6 ALABAMA POWER, MILLER 9 BELLE RIVER, DETROIT EDISON 9 SMARTGAUGE TESTING 11 CONCLUSION 14 REFERENCES 14

3 Introduction Clyde Bergemann has installed water cannon furnace systems in over 60 utility power plants. This paper presents the results of performance and start up test from a number of those plants. These tests quantified the changes in furnace exit gas temperature, unit output, effect on steam flow, boiler efficiency, NOx production, and other plant parameters due to furnace with water cannons. Special testing of a new technology utilizing strain gages to identify fouling of convection pass elements is also presented. Data from special instrumentation such as HVT probes and emissivity instruments is included in the analysis along with data from permanent plant instruments and heat flux monitors. A special technique of utilizing the heat flux monitor as an indicator of thermal impact is described and results are presented. Most of the water cannon installations use heat flux sensors as a feedback mechanism to determine the timing and frequency of. These instruments can also be configured to infer the thermal impact of the water spray on the furnace wall tubes. Most coal fired power plants are no longer burning the fuel that the boiler was originally designed for. In many cases the current fuel is vastly different than the design fuel. This is especially the case with boilers that have converted to Powder River Basin (PRB) coal. Often the change in coal is accompanied with increased slag formation and deposits in the furnace. Cleaning the furnace with water cannons can improve the heat absorption of this section of the boiler and return it to near the design conditions of cleanliness. Furnace slagging The ash component of coal can leave significant deposits on the heat transfer surfaces of the furnace. In the high temperature range of the furnace slagging can be particularly tenacious and cannot always be removed by steam and air blowers. Manual lances using water had been used to clean deposits off furnace walls. The success of this technique lead to the mechanization of the lance into the water cannon. Water was also the medium used in water lances, which are a adaptation of the steam wall blowers. In a typical coal fired power plant burning 800 lbs of coal per Mwh a 500 Mw unit would combust ~ 200 tons of coal per hour. Ash contents of the various coal types range from 6 to 40 precent. If we use 20% as an average this unit would handle 40 tons of ash per hour. In this 500 Mw example 20% to 30% of the ash is typically bottom ash, and the remaining 70% to 80% (or 28 tons per hour) is fly ash entrained in the gas stream. Deposits are formed when the ash in the flue gas is at a temperature above its melting point. The laboratory term for the initial melting point is the initial deformation temperature. Typical values of initial deformation temperature for PRB coal are in the 2100F to 2200F range. With typical gas temperatures of 2500F in the furnace, the ash is in a semi-molten condition and when it comes in contact with furnace water walls that are at a relatively cooler temperature, a deposit is formed. The majority of the ash passes through the boiler and is collected in the ash removal equipment (electrostatic precipitator or bag house). However, even a small portion collecting on the heat exchange surfaces can have a dramatic impact on plant performance. Not only is there an insulating effect from the deposit, but there is also a change in the emissivity of the surface. Surface emissivity is critical to heat transfer in the furnace area since most of the heat transfer is from radiant heat. The general equation for radiant heat transfer is:

4 Q = σ ε S ( T where : Q = Btu / hr S = area, sq. ft. T = temperatureof gas T σ = ε = emissivity T 4 2 Btu / sq ft, hr, T = temperatureof surface ) 4 Some coal such as PRB contains high levels of calcium oxide and magnesium oxide, which are major sources of the reflective property of PRB ash deposits. Calcium oxide can be in the 24% range for PRB coal compared to 1 to 2 % for eastern coals. Also, magnesium oxide is in the 5% range for PRB coal compared to about 1% for eastern coals. The normal emissivity of a boiler tube with a coating of iron oxide is from 0.85 to 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 emissiviity (or increase in reflectivity) would reduce heat transfer by almost half. This reduction of heat absorbed in the furnace results in higher furnace exit gas temperatures. Less heat is absorbed by the water walls and therefore more heat must be absorbed by the superheater and reheater sections in the convection passes to maintain full load steam conditions. It is often the case that this upset in boiler balance is enough to reduce the capacity of boilers. The higher furnace exit gas temperatures usually result in higher stack gas temperatures and therefore lower boiler efficiency. A reduction in boiler efficiency increases heat rate and therefore increases the cost of generation. The elevated temperatures of the gas leaving the furnace often results in ash entering the convection passes containing ash that is above its fusion temperature. Thus this ash will deposit on the superheat and reheat surfaces a further reduce the ability of the furnace to make steam conditions. High temperatures that exist in the furnace for longer time periods also result in more production of thermal NOx. Furnace The traditional method of furnace is the retractable wall blower. This device is inserted into the furnace wall through an opening. The blower tube rotates while a jet of steam or air is sprayed on the furnace wall to clean off deposits. A blower typically can cover a 10 foot diameter area. Wall blowers are often not adequate to clean boilers that are burning PRB, lignite, or other coals that form furnace deposits. Water lances were developed to clean larger areas and use a water jet as the medium. This device is a modification of the retractable sootblower. It sprays a jet of water back on to the wall it is mounted on in a spiral pattern as it is inserted into the furnace. Water lances cover about a 20-foot diameter area. Thus, a 500 Mw size boiler my be outfitted with 40 or more in an attempt to keep the furnace clean. They have small nozzle area and require high purity water. Lances can be bent if they are hit by falling slag while inserted into the boiler. Water cannons spray a jet of water from an opening in the furnace wall to the opposite wall. In many installations a cannon can clean from the nose arch to the bottom slope tubes and from one side to the other. Thus 4 cannons, one on each wall, can clean an entire furnace. A concentrated water jet, which is produced by a special nozzle, crosses the boiler inside and impacts on the slagged wall surface. The effect is based on the fact that the impacting water which penetrates the topmost layer and expands into steam. In this way the slag is broken up and removed from the surface.

5 Water cannons offer a number of advantages over conventional wall blowers and water lances. The large number of wall blowers and/or water lances requires an extraordinarily high maintenance expense. The use of steam, air, or high purity water as agents results in high operating costs. Water cannons can use filtered water with a clear reduction in cost. The water cannons also allow areas to be cleaned that cannot be outfitted with wall blowers and water lances. Division walls, nose arches, and lower slope tubes are examples of areas routinely cleaned by water cannons. In a typical furnace the area cleaned can often be doubled. This increase in has the effect of reducing furnace exit gas temperature (FEGT) by increasing the amount of heat absorbed in the furnace. An additional benefit is the reduction in thermal NOx production. Reductions of 100 F in FEGT are common and NOx reductions of 10% or more have been documented. Intelligent soot blowing The water cannon mechanism allows for 90 degrees of movement in both the vertical and horizontal planes. Also, variable speed motors drive the mechanism providing for constant speed of the water sweeping across the furnace wall. The speed of the water jet moving across the wall is referred to as Jet Progression Velocity (JPV). EPRI studies and actual experience indicates that the JPV is directly related to the thermal impact on water wall tubes. A JPV of 300 ft/min. results in about 100 deg. F metal temperature excursion. Computer control of the cannon operation allows for any area of the boiler to be bypassed by the cannon spray. Thus, over fire air ports, manholes, burners, and any other special area can be excluded from the water spray. Also, special zones can be created, to clean above burners, or bottom slope tubes, or any other troublesome surface. Traditional boiler was done on a time or condition basis. Operators monitored steam temperatures and operated the devices to control drops in steam temperature. Blowers were set up to run in a set sequence. Often blowing was set on a time basis, every 12 hours, every shift, once per night, etc. This type of operation often resulted in the blowing operation following a build up of deposits, it did not act to clean deposits at they occurred. When computer control is applied to boiler, we often find the performance, measured by heat flux, can double. Water cannon systems are combined with heat flux sensors in the furnace walls to detect deposits and operate the sequence as needed to keep the furnace in a clean condition at all times. This type of computer controlled automatic system is often called Intelligent Soot Blowing. Heat flux sensors operate with embedded thermocouples in selected water wall tubes. Paired thermocouples detect the heat flux gradient in the tube. This is monitored and any reduction in heat flux at constant load is associated with deposits forming on the furnace surface. The sensor is also used to determine when an area is clean. With this type of system the furnace cleanliness can be optimized and steady predictable operation is assured. In addition to monitoring the heat flux, the thermal impact on boiler tubes can be monitored. With this system of monitoring and control precise can be accomplished with minimal impact on tubes. Test Program Most of the water cannon systems are provided with start up testing and fine tuning. Some systems have additional performance tests that determine specific accomplishments and compliance with contract guarantees. The data presented in the following sections of this paper are taken from papers that have been presented previously or from recent tests. Clyde Bergemann is continually conducting research into improved methods and methods of controlling equipment. Data from field tests is used to improve equipment and control systems.

6 Tucson Electric Springerville Unit 1 at the Springerville plant is equipped with 2 water cannons. The graph below is typical of the events at the plant. Heat flux can be seen to improve from the 50 kbtu/hr/ft2 to almost 100 kbtu/hr/ft2 when the heat flux sensor is cleaned. The graph also displays the thermal impact to the surface of the tube. This is done by monitoring the thermocouple in the heat flux sensor that is closest to the fire side surface. The installation of cannons at Springerville also resulted in the reduction in variation in main steam and reheat steam temperatures. This is probably due to the cannons ability to control furnace exit gas temperatures to a relatively constant level and the reduction in spray flow necessary to control the steam temperatures. The charts below indicate the reduction in temperature fluctuation.

7 The reduction in spray flow is shown on the next chart. This data is over a 12 month period. Data was collected when the unit was at full load. Most of the data points represent a week or more of data. The reduction in average spray flow was from 69.3 klbs/hr to 47.6 klbs/hr. It is estimated that this reduction in spray flow decreased (improved) heat rate by almost 15 BTU.

8 Springerville unit 1 Reheat spray flow Water Cannon Operation Start Reheat Spray flow (klb/hr) /5/ /25/2001 2/13/2002 4/4/2002 5/24/2002 7/13/2002 9/1/ /21/ /10/2002 Date

9 Alabama Power, Miller Unit 1 at the Miller plant is equipped with 4 Clyde Bergemann SmartCannons. These cannons incorporate the latest technology available in furnace water. The following graph indicates the results of HVT traverse tests at the plant during the initial performance tests of the cannons. Before cannon installation there were large differences in temperatures. After the cannons were installed the profile at the nose of the boiler was much more even with lower peak temperatures. Peak temperatures were reduced from levels above 2300 o F to 2150 o F. Such a reduction reduces the tendency for slag accumulations in the convection pass of boilers. Without Water Cannons Alabama Power Company Miller Steam Plant Unit 1 HVT Test #1 February 18, 2002 Alabama Power Company Miller Steam Plant Unit 1 HVT Test #7 February 21, 2002 With Water Cannons Belle River, Detroit Edison Detroit Edison has reported on a long term study of water cannon operation at the Belle River unit 1 installation. The unit is equipped with 4 water cannons. It is a 640 MW unit with a B&W wall fire boiler. The unit is sub-critical and designed for western sub-bituminous coal with MPS pulverizers. The unit experienced frequent deratings to below the 600 MW level due to high furnace exit gas temperatures and deposits plugging the convection pass. 192 wall blowers were replaced with the 4 water cannons and the amount of surface cleaned increased. Data was recovered from plant data loggers for approximately a year of operation before the cannons were installed. This was compared to data from the first year of operation of the cannons. As can be seen from the following graphs FEGT was reduced by over and average of 100 o F. There was a reduction in NOx emissions of 8 to 10%.

10 Measured Average FEGT at 600 MW (Net) - Unit 1 Days After Boiler Cleaning FEGT (Deg F) 2,400 2,350 2,300 2,250 2,200 5 Deg F 2,201 2, Deg F 2,258 2, Deg F 2,268 2, Deg F 2,308 2,288 Cannon Problems 78 Deg F 2,335 2, Deg F 2,346 2, Deg F 2,341 2, Deg F 2,362 2, Deg F 2,372 2,264 Before Cannon After Cannon 2,150 2, days 36 days 57 days 65 days 81 days 91 days 106 days 114 days 124 days Belle River Unit 1 Hourly Average FEGT vs Main Steam Flow Pre vs Post Water Cannon Periods 2,375 2,325 FEGT (Deg F) 2,275 2,225 2,175 Pre Post Poly. (Pre) Poly. (Post) 2,125 2, Main Steam Flow (M lb/hr)

11 Unit 1 Measured Hourly Average NOx vs Main Steam FlowPre and Post Water Cannon NOx (lb/mbtu) Main Steam Flow (Mlb/hr) 99NOx 01NOx Expon. (99NOx ) Expon. (01NOx) SmartGauge Testing An installation of the patented SmartGauge deposit monitoring system was installed at a southern power plant. The unit is a 880 MW plant with a CE designed boiler. The boiler has a group of reheat pendants suspended just in front of the nose of the furnace. This location promotes slag and deposit formation. A set of strain gages were installed on the rods that suspend the pendants. The gages are specially selected to be able to measure the weight of the pendant and the relatively small change in the load due to slag and ash deposits. Also two gages were installed on the rods that suspend the economizer. There was a known accumulation of ash on the economizer. The gages were installed during a major outage, so the boiler was relatively clean when it returned to service. It is not necessary to have an outage to install the gages as they are installed in the area above the penthouse. The location of the gages is shown on the two charts below. Reheater Gage Location Economizer Gage Location 3 4 Reheater Blower No Economizer 10

12 Cable 5 Cable 4 Channel 2 Channel 1 Economizer rods Cable 6 Cable 11 Wireless Cable 7 Wireless Cable 10 Cable 9 Cable 8 Channel 3 Channel 8 Channel 4 With 2 gages Channel 7 Channel 6 Channel 5 Hot Reheater Rods Front Wall of Boiler Birds eye view looking down on boiler roof Data acquisition system Individual readings from each gage on the reheater pendants are shown on the chart below. Also shown is operation of retractable sootblowers. The operation of the blowers coincides with reduction of weight on the gages. SmartGauge Individual Support Rod Data 200 Retract No & & /8/03 0:00 1/10/03 0:00 1/12/03 0:00 1/14/03 0:00 1/16/03 0:00 1/18/03 0:00 1/20/03 0:00 Strain (micro in/in) Guage 6 Guage 7 Guage 8 Guage 9 Guage 10 Guage 11 The output from the gages was converted from strain to stress using the stress-strain relationship for steel. Then with the diameter of the rod know the stress was converted to load in pounds. The data from each gage was set to zero at its low point. Then the data was combined to give a total weight for the six rods that were measured. The following graph is the result.

13 Reheat Pendant Ash Load from SmartGauge &8 4 8R &2 8 Retract No Delta Load on gages (lbs.) /13/03 0:00 1/14/03 0:00 1/15/03 0:00 1/16/03 0:00 1/17/03 0:00 1/18/03 0:00 1/19/03 0:00 1/20/03 0:00 1/21/03 0:00 The gages installed on the economizer were able to detect the increase in the weight of the ash deposit. This weight gain was not affected by blowing of the retractable soot blowers or by the sonic horns that are installed in this area. There are known obstacles to flow in this area that allow the ash to accumulate. SmartGauge on Economizer 120 Retract No. 12R&L 12 R&L 12 R&L Strain (micro in/in) /7/03 0:00 1/9/03 0:00 1/11/03 0:00 1/13/03 0:00 1/15/03 0:00 1/17/03 0:00 1/19/03 0:00 1/21/03 0:00 1/23/03 0: Guage 4 Guage 5 Linear (Guage 5 ) Linear (Guage 4 )

14 Conclusion The various tests demonstrate the benefits of intelligent sootblowing. Water cannon of the furnace can reduce furnace exit gas temperatures. The tests also demonstrate improved control of both main steam and reheat steam temperatures. Reduction in reheat attemperation spray flow improves cycle efficiency and heat rate. Reduction in NOx production is demonstrated over long periods of time. The use of sensors in the convection pass is also demonstrated. This technology has future uses in control of retractable sootblowers and optimization of the devices in the convection pass. References Furnace Cleaning using Water Cannons at Detroit Edison, Belle River Station, Peter Kohlert, Presented at the 2002 Water Cannon Users Conference Furnace Cleaning Using Water Cannons, Charlie Breeding, Presented at Electric Power 2002 Service Report No. 2K2-003, Alabama Power Miller Steam Plant Unit One, High Velocity Thermocouple Testing, Marcus Mathews, Innovative Combustion Technologies, Inc., February 2002 Demonstration of Clyde Bergemann Water Cannons at Miller Unit 1, Charles Bookhaker, Brian Mead, Mike Carlisle, John Sorge; Southern Company, EPRI Heat Rate Conference, Birmingham, AL, January 2003 Results of Performance Tests of Water Cannon Furnace Cleaning at Springerville, Chris Patterson, Tucson Electric Power, Charlie Breeding, Sr. Engineer, Clyde Bergemann; Presented at the EPRI Heat Rate Conference, Birmingham, AL., Jan United States Patent Number 6,323,442 issued November 27, 2001