Clogging is expensive. Proof enough are the figures gathered by GRETH and published in a 1990 study, stating that heat generator clogging, for steam or superheated boilers, cost France 50 millions Euro each year, not to mention the out put losses due to production stoppages generated by thermal drops, and cleaning or replacement of the corroded pipes.
Cleaning has a cost. Existing technologies, although claiming to be universal for a given boiler are not. There is a feasibility problem.
MAIN REASONS TO CLOGGING
Fuel specifications
Fossil fuels, whether solid or liquid, are not the same as 20years ago. Heavy fuel-oils are the result of elaborate mixes designed to make the marketing of heavy products with a high asphaltene count. French coals, not very ashy with a low sulfur count and regularly calibrated, are more often than not replaced by imported coals, controlled of course, but sometimes more difficult to burn.
Trying to make savings
It has been tried, not unsuccessfully, to reduce the weight and bulk of boilers, keeping a similar steam production, downsizing of the sectors of smoke passages and increasing the number of chicanes on the gas flow. This has increased the resistance of the generators to the passage of smoke (increased losses in loads) and, unfortunately, also the soot deposits in the zones less used by smokes (direction changes or angles).
Automation of boilers
Automated exploitation has driven away from the boilers the experienced trained personnel who could operate a fire by just looking at it. It depends, more than ever, on the measuring and testing instruments managing the boilers and, consequently, and on their maintenance and regular gauging.
Longer stoking campaign between two cleanings
For the same economical reasons leading to "compact" boilers and automatize their running, heating campaigns were made longer. Yearly visits are sometimes stretched to their 18 months legal term. Derogations exist for certain industries and running conditions going up to 40 months. But this noble aim will be difficult to attain, because in some cases, generator clogging can get worse very quickly and hard to handle in a matter of days and thus generate a non programmed stoppage for an emergency cleaning.
The ageing process of boilers
According to François Kennel, from the Paris APAVE, most recent study, 62 % of the existing boilers are over 20 years old. As years go by, are we certain that these boilers keep on as efficient as 20 or 25 years ago, when they were made, due to their being clogged, unless of course they are "stripped" periodically to get to all the heat-exchangers and clean them completely, which is very expensive and damaging for the owner.
AN INVENTORY OF ON-LOAD CLEANING MEANS: PRO AND CONS
To try and maintain or restore their efficiency to their boilers, a certain number of solutions have been imagined. Each one of them has its pros and cons.
Steam or compressed-air soot-removal.
These processes need the help of purposely built equipment which must be adapted to the boiler's technology or the heat exchanger's for which they are designed and used regularly according to the constructor's prescriptions.
Because of their limited range, they are expensive on: piping taps, access bridges, lagging. The smaller the boiler, the more the steam soot-removal air intake will disturb the downstream process or the running of the generator itself, however, this technology is the only technique with a proven record whatever the fuel in use or the nature of the deposits; viscous, powdery, etc.
Constructors for such equipment are few on the French market: a company, BPE Forest holds a strong position on the market for large plants boilers and industrial or tertiary boilers. A subsidiary of Clyde Plc, a British Group, this company, today, can cover all the needs of operators, including shot-blasting and soot handling.
To face problems resulting from clogging to the use of new fuels or new combustion techniques, BPE-Forest devotes a large part of its budget to R&D. The partnership contract signed with AIT is a part of their strategy.
Striking
It is meant for the sole use of boilers designed to be cleaned with this technique. It is limited to powdery clogging, to less powerful boilers. Its possible noxiousness on collectors and tube footing is not really known. Moreover, it is difficult to predict clogging speed which hinders exploitation planning.
Shot Blasting
This technique, damaging to the oxide layer recovering the pipes, must be avoided for skin temperatures above 450 C°. It cannot be applied to boilers producing more than 50 tons/hour except locally (economizers or air-heaters for instance). Finally, humidification of the ashes can induce serious blockings difficult to eliminate.
Acoustic vibrations
Constructors for this type of equipment carefully write in their technical notices that their systems are only efficient on dry ashes, i.e. non-viscous. Everybody knows that clogging processes are based on the mineral composition of the ashes as well as on diffusion processes and thermophoresis. A deposit, seemingly powdery can become locally viscous, even temporarily, vibrations will never eliminate it.
Besides, vibration side-effects on the studs holding the furnace's refractories, the tubes footing, the on-site probes and exploitation comfort finally must not be neglected.
Injecting Chemical reagents in the fuel
Fuel additivation is only possible with liquid fuels. Globally, if correctly chosen, results are satisfying.
Adding liquid or solid reagents to coals or domestic and industrial waste is a great mixing problem when trying to get a constant ratio between the chemical reagent and the gases or deposits needing treatment.
Clogging is part of a local physico-chemical process: an injected reagent in the fuel or in to the flame cannot, unless it is that universal product we are all still waiting for, have an adapted behavior to all local situations. At best it will be inefficient in certain areas, at worse, it will have negative side-effects (eggshell, etc.).
Finally, some powdery regents have a fusion point so low that they melt in the waste heating areas or in the coal hearth-cavity, making them ineffective.
Liquid reagents injected in the flame, have not, to this day, proven their efficiency. This is probably due to the fact that, to this day, dispersion of these reagents depends upon the preferential currents reigning in the combustion chamber and that a part of these reagents is uselessly lost when it would be necessary for the active particles to lay on the cluster they are supposed to be treating. However, preventive chemical treatment is the only process, together with soot-removal done with water or air, which can, if the proper reagents are being used, have a predictable effect. Several examples can be given: urban heating, incineration plants for industrial waste, industries, etc.
Experience has taught us that chemical treatment, by itself, cannot do without mechanical soot-removal, still necessary to "scrape" deposits made crumbly by chemical treatment. Conversely, the same chemical treatment can contribute for mechanical soot-removal to operate at the fullest of their capacities.
It is in this complementary spirit that BPE FOREST, a subsidiary of the Clyde Group, offering the whole range of on-load techniques, and AIT, have been working together for several months with, as aims :
- to improve diffusion of chemical regents in the smoke flow;
- to improve efficiency of FOREST systems;
and, consequently, save energy and help generators running longer.
In this field, as in others, there is no standard miracle solution. Each case is a new one, constructors as well as users must understand this, otherwise they are disappointed and it does not help progress.
AN INVENTORY OF OFF-LOAD CLEANING SYSTEMS: PROS AND CONS
Cleaning with water, under pressure or running
It is a well-known fact that in fuel-oil boilers, a good part of the deposits is water-soluble. Made with high-pressure water or a plain water-jet, it is the most commonly used solution.
Necessary in some cases, this method implies today to respect the 1975 law, complete with the 1992 decrees about the destruction in specialized centers of liquid effluents produced using this kind of cleaning.
In coal-boilers, elimination of hard and sticky deposits can only be done with highly pressurized water-jets (= 1000 bars).
In spite of all precautions, water infiltrations between the refractories or under the grids can still occur.
However, the law must be respected when it comes to recovery and elimination of liquid effluents.
In the past, these problems were a main topic for thought at AIT managerial level, having products capable of destructuring some deposits. What was missing, were the means to suppress streaming and introduce these products in the least accessible areas of some generators. Until the day when…came the idea to use the same products as foam.
Compared to the use of pulverized liquid products, foam has definite advantages :
- It does not stream and clings longer to the deposits until they are all destroyed,
- It infiltrates much better than any liquid and gets quicker and better to the core of the clusters,
- It allows staff to work in better conditions since there isn't any splashing,
- It does not generate more effluents than there are in the boiler.
Example: to clean economizers, fifty liters of product are enough, a boiler producing 100t/h of steam with heavy fuel-oil, needs only 300 liters of product; the boiler's refractories will not get wet, etc.
Used in association with harmless mechanical means allowing saving product and improving heat-exchanger cleanliness, the CECOMOUSS, process, developed by AIT and its partners, was presented to many industrial or tertiary boiler users, constructors or controlling authorities such as APAVE.
Today, several urban heating users as well as industrials have already decided to put their trust in this process.
Boiler constructors are also very interested, among other offers, by the CECOMOUSS, process. In the actual state French economy is in , their interest lies in offering their clients innovating solutions allowing them to keep their boilers in a good state for a longer period of time.
CONCLUSION
This process is not ready for use on all boilers or all fuels. Work is in progress, in a partnership with BPE FOREST, to improve the process while using soot-blowers to inject the product and optimize its efficiency.
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