Bltn0045 PDF
Bltn0045 PDF
Bltn0045 PDF
CO2
Application Bulletin
A World of Protection
The following is a quote from the January, 1988, issue of POWER Magazine: Electric Power Research Institute (EPRI) data reveals that pulverizer explosions occur at least once a day in USA utility power plants. Average cost per event is around $3 million. Looking at the data another way, there are five fires every four years and three explosions every 10 years at the average coal fired unit. Annual cost to the industry exceeds $1.0 billion. As many as 85% of USA coal fired units are not equipped with fire detection systems and less than 20% use some form of inerting type protection system. Chemetron has provided CO2 fire suppression and inerting systems for pulverizers for many years. Experience gained, along with recent changes in the code, are covered by this bulletin. Changes in inerting recommendations and developing technology have necessitated revisions in this bulletin originally written in 1988. A coal mill (pulverizer) reduces raw coal to small particles, which can be conveyed by air directly to a boiler, kiln, etc. (direct firing), or to intermediate storage (indirect firing). There are a number of different types of mills which either grind the coal or smash it with steel balls. The drawing accompanying this bulletin shows a common type mill used in power plants. Raw coal enters at the top and drops onto the grinding table, where the rollers reduce it to fine particles. Heated air picks up the coal and conveys it up to the classifier, where the larger particles are separated and dropped back down onto the grinding table, while the lighter particles are conveyed through the coal pipes to the boilers.
4801 Southwick Drive Third Floor Matteson, IL 60443 Telephone: 708/748-1503 Fax: 708/748-2847 email: info@chemetron.com
1999 CHEMETRON FIRE SYSTEMS, all rights reserved. Issued 1988 - Rev 1/1999 CARDOX IS A REGISTERED TRADEMARK OF CHEMETRON FIRE SYSTEMS.
Fire Protection
Paragraph 3-5.3.2 of Standard No. 8503 outlines procedures for fighting fires. If CO2 is used as a fire extinguishant (not just to inert), paragraph (a) or (b) below would apply. They state: (a) If sufficient inertant flow capacity is provided (at least 50% by volume of the primary air flow of the system), inert the pulverizer air/fuel flow, shut off the fuel feed, empty the pulverizer of fuel, shut down and isolate the pulverizer. (b) Stop the primary air flow, trip the pulverizer and feeder, isolate the system and inert. Avoid disturbing any accumulation of dust
When CO2 is used as the inertant, the more the air movement can be stopped, the less CO2 that will be required. The minimum CO2 level required for inerting is such that oxygen levels will be below 15%. Chemetron recommends 65% CO2 (7.5% oxygen) as a design level. When calculating CO2 flooding in coal, remember that the coal adsorbs considerable CO2, which is thus lost and will not help reduce the oxygen concentration. Paragraph 3-5.2.1.2 says: The inerting procedure shall be as prescribed by agreement between the pulverizer equipment manufacturer and the purchaser. They should consider fuel characteristics, pulverizer temperature, size and arrangement of the pulverizer. We strongly recommend that designers bring Chemetron into the planning and design process so we can calculate proper CO2 quantities and flow rates to insure that adequate systems are specified. Remember, if inerting is done under different conditions, different CO2 rates will be needed and the CO2 system must be designed accordingly. Several of the pulverizer manufacturers have already identified the CO2 flow rates required for inerting different mill sizes.
Inerting
The most common use for CO2 is for inerting the mill as an explosion prevention means. The Standard for Pulverized Fuel Systems, NFPA Standard No. 8503, paragraph 3-5.2.1.1, says: A pulverizer tripped under load shall be inerted and maintained under an inert atmosphere until confirmation that no burning/smoldering fuel exists in the pulverizer, or the fuel is removed. Note that paragraph 3-5.2.2.2 says: Due to the danger of an explosion when opening and cleaning, pulverizers shall not be cleaned manually until they and their contents have been cooled to ambient temperature. The use of CO2 as an inertant allows the maintenance of the inert atmosphere until the mill cools and can be opened for cleaning. Your attention is called to the other various requirements for inerting that appear in Standard No. 8503, based on other conditions that can occur.
Combustion Engineering
Combustion Engineering has considered CO2 inerting as an auxiliary to the steam system and as a stand alone system. In the auxiliary mode, the CO2 would be required only after the unit trips. With the primary air fans off, the pressure differential across the mill is low, the air infiltration low, and the CO2 use low. For a stand alone CO2 system, coal pipe shutoff valves, capable of sealing against full primary fan pressure, are needed. Tests have shown that the application of CO2 in the above conditions will inert a mill in a matter of seconds, but if the CO2 is shut off, it is lost quickly.
Volume (Cu.Ft.)
44K 44N 44G 49N 49G 56K 56N 56G 67K 67N 67G
470 470 470 610 610 750 750 750 1,500 1,500 1,500
312 390 468 511 611 667 744 822 890 1,112 1,334
75K 75N 75G 89K 89N 89G 104K 104N 104G 118K 118N
2,000 2,000 2,000 2,250 2,665 2,665 4,400 4,400 4,400 6,200 6,200
Table 2 CO2 Requirements for Inerting Ball Mills Pulverizer Series D2 D3 D4 D5 D6 D7 D8 D9 D10 Volume FT3 300 500 750 950 1,200 1,500 2,000 3,100 3,900 Carbon Dioxide Lbs 43 72 100 136 172 215 286 443 558 *Lb/Hr 1,300 2,200 3,000 4,100 5,200 6,500 8,600 13,300 16,900
Ball Mills
Figure #2 shows a roller-type mill, while Figure #1 shows a ball-type mill in which a drum rotates, causing steel balls to crush the coal. Foster Wheeler has this type mill in their D Series. Table 2 provides information for inerting with CO2. The CO2 injection points are shown at the classifiers and at the exhausters, and since methane gas can accumulate in the ducts above the mill in case of fire, CO2 is injected there as well. For long term inerting, an external discharge is applied through these upper nozzles, and a procedure is established to periodically roll the mill while it is inerted to facilitate cooling for fire extinguishment.
Volume (Cu.Ft.)
Mill
Mill
Reliability
There is material in the literature questioning the reliability of all gaseous agent inerting. In order to maximize the effectiveness of the inerting system, one should understand:
1
Homemade systems are potential problems. Those experienced in CO2 system design can calculate proper agent quantities and application rates, and do the flow calculations that ensure the required rate is achieved. It is mandatory that this be done. Systems should be given full discharge tests to check that design concentrations are obtained. Various expected conditions should be included in the test procedure and multiple tests run, if necessary. Operating conditions change and there may not be one fixed agent discharge rate that will handle all desired conditions. The system can be arranged to handle two or more application rates; but only if the operating procedures identify the need for same and it is designed into the system from the start.
Safety Aspects
In addition to the normal safeguards necessary when dealing with CO2 systems see NFPA Standard No. 12, Carbon Dioxide Extinguishing Systems it is required that procedures be developed to positively prevent inadvertent introduction of the inerting media when personnel are present. Chemetron can be consulted as to how best to accomplish this.
Paragraph 3-5.3.4 of NFPA Standard #8503 states: In the event of fires detected in components such as cyclones, dust collectors, pulverized fuel bins for the pulverized fuel systems other than direct fired systems, the affected components shall be isolated and inerted.
Figure #1 - Low Pressure CO2 Coal Pulverizer Inerting/Fire Protection for a Ball Mill
Page 6
Figure #2 Low Pressure CO2 Coal Pulverizer Inerting/Fire Protection for a Roller Type Mill
Page 7