Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2003-07-09
2004-11-16
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S653000, C060S671000
Reexamination Certificate
active
06817181
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a method for operating a heat-producing plant of the type which comprises a chamber for burning chlorine-containing fuels and a channel connected with said chamber, in which a plurality of consecutive heat-transferring devices are located, through which fluids, such as steam, water or air may pass with the purpose of being heated by hot flue gas which flows through the channel in the direction from the combustion chamber towards an outlet, the flue gas being conditioned by addition of a sulphur-containing additive, which is injected into the flue-gas flow in a channel section located, on one hand downstream a combustion zone, in which substantially all fuel is burnt, and, on the other hand, upstream and at a distance from that heat-transferring device positioned downstream the combustion zone being the first to be hit by the flue gas, and which additive, after the entrance thereof in the flue-gas flow has the purpose of, during the way thereof to said first heat-transferring device, sulphating above all gaseous alkaline chlorides comprised in the flue gas, with the aim of reducing chlorine-induced corrosion on the heat-transferring devices.
PRIOR ART
By U.S. Pat. No. 4,043,768 is previously known how to add a sulphur-containing additive to the type of flue gas that is obtained by burning coal of a varying sulphur content. As preferred additives, finely powdered ammonium sulphate or ammonium bisulphate are mentioned, which should be inserted into the flue gas when the same has a temperature within the range of 590-900° C. The primary object of the addition of the sulphur-containing additive is to improve the efficiency of an electrostatic dust separator. Thus, what forms the basis of the technique being presented in U.S. Pat. No. 4,043,768 is the understanding that the dust separator works most efficiently in burning coal of a relatively high sulphur content (3-5% sulphur) as a consequence of said fuel providing a resistivity of the flue-gas particles of 10
8
-10
10
ohm cm, while coal of a low sulphur content (1%) provides a considerably higher resistivity (10
13
ohm cm) of the particles, and thereby a considerably lower precipitation capacity of dust separators. By adding suitable amounts of a sulphur-containing additive, the resistivity of the flue-gas particles can be optimised at burning of coal of a low sulphur content; all with the purpose of attaining a good dust separation. In the described plant, the dust separator is located immediately upstream the flue-gas outlet or chimney, but down stream a plurality of heat-transferring devices, such as a secondary superheater, a reheating superheater, a so-called “ball-room”, a primary superheater, an economizer and an air preheater. In the preferred embodiment, which is described in U.S. Pat. No. 4,043,768, the sulphur-containing additive is inserted at a position upstream the dust separator, but downstream a number of heat-transferring devices. More precisely, it is preferred that the additive is injected into a section located between the inlet to said “ball-room” and the primary superheater, where the temperature is within the range of 900-550° C.
BACKGROUND OF THE INVENTION
Solid fuels in the form of bio and waste fuels are becoming an increasingly established energy source for combined power and heat production, among other things as a consequence of such fuels being long-term available and providing an energy-efficient combustion. The category of bio fuels includes, among other things, wood chips, bark, straw, sawdust, black liquor and the like, while waste fuels may, for instance, contain sorted domestic waste, industrial waste, demolition wood, sludge. Principally, waste fuels should be understood as such materials that already have been used for another purpose, while bio fuels are such plant materials that are utilised from the nature without other purposes than producing energy.
Since bio and waste fuels have been put into operation on a large scale, such fuels have, however, in several aspects proven considerably more difficult to burn than coal. Among other things, this is due to the fact that the ash of the bio and waste fuels has a different composition and different melting characteristics than the ash of coal. One of the most expensive problems is corrosion and ash deposits on the super heater tubes and other parts being comprised in the heat-transferring devices of the plants. Consequently, severe high-temperature corrosion has been detected in a large number of heat-producing plants after a few years of operating time with 100% bio fuels. At wood-fuel burning, the corrosion begins at steam temperatures in the superheater of approx. 480° C., and increases along with the temperature up to approx. 500-600° C. This equals the steam temperature of the hottest superheaters in modern combined power and heat plants. There is a trend in the industry to mix in demolition wood and sorted waste fractions in the fuel mixture, which may further accentuate the above-mentioned problems and extend the corrosion also to heat-transferring surfaces of lower material temperature. By those skilled in the art, chlorine is regarded as the principal corrosion accelerator. It is assumed that the chlorine is transported to the surface of the superheater tubes in the form of gaseous-phase alkaline chloride (at wood fuel mainly potassium chloride), alternatively as very small aerosols of alkaline chloride having condensed just upstream the superheater. On the tube surface, reactions between the alkaline chlorides and, for instance, ferrous oxide take place while forming free chlorine, which in this form is highly corrosive. The exact mechanism for the corrosion is not entirely clarified, but it is beyond all doubt that chlorine plays a central role.
By those skilled in the art, the relation of sulphur/chlorine or the S/Cl ratio of the fuel has been proposed as a parameter indicating the risk of chlorine-induced high-temperature corrosion. It is previously known that addition of sulphur to the fuel reduces chlorine-induced corrosion at waste burning. It has been assumed that an S/Cl ratio higher than 4 would be sufficient in order for the superheater corrosion to become acceptably low. Wood fuels have, however, an S/Cl ratio within the range of 1-2, and unsorted domestic waste approx. 0,2-1.
Also fuels of a relatively high S/Cl ratio may give rise to problems related to alkaline chlorides, viz. if they are burnt under conditions which mean that a major share of the sulphur is bound in the form of sulphate compounds already in the burning process. Such conditions prevail, for instance, at the burning of coal in a fluidized bed at addition of lime-stone, dolomite or other compounds that form basic oxides in the combustion chamber with the purpose of decreasing the emission of SO
2
. Especially at a relatively high dosage or the presence of the sulphur-binding additive, in order to meet stricter environmental requirements, chlorine corrosion has occurred on superheater surfaces situated downstream the combustion zone.
The hypothesis concerning the ability of sulphur to decrease the chlorine content of the superheater deposits is that the alkaline chlorides of the flue gas is converted into alkaline sulphate before they reach the superheater according to the following sum formula:
2KCl+SO
2
+½O
2
+H
2
O
K
2
SO
4
+2HCl (1)
the equilibrium of which is displaced to the right at flue-gas temperatures below approx. 800° C.
Test measurements have shown that a fairly high concentration of SO
2
is required in the flue gas in order for KCl to be significantly reduced in flue-gas temperatures at about 700-900° C., which equals the temperature range within which the superheaters in the most common types of boilers operate. For instance, measurements at burning wood chips have shown that an SO
2
content corresponding to approx. 50-150 mg S/per MegaJoule (MJ) fuel is required in the flue gas in order to achieve a 50% reduction of the potassium-chloride c
Nguyen Hoang
Vattenfall AB (publ)
Young & Thompson
LandOfFree
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