Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2000-08-09
2001-08-07
Wong, Edna (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
C204S157510, C423S164000, C423S640000
Reexamination Certificate
active
06270631
ABSTRACT:
BACKGROUND OF THE INVENTION
It is well known that in the combustion of carbonaceous fuels, such as coal, the sulphur contained therein is oxidised, usually largely to sulphur dioxide, although under certain conditions some sulphur trioxide can be formed. Whilst a small proportion of this sulphur dioxide can be chemically combined as sulphite (or sulphate if sulphur trioxide is present) into the ash formed largely from the non-combustible materials in the fuel, most of the sulphur dioxide is vented from the furnace combustion zone as part of the exhaust gasses. In the past these furnace exhaust gasses containing sulphur dioxide have been vented to the atmosphere through a stack, but this is no longer possible, due to ecological damage caused by such acidic emissions, including the formation of acid rain. The only known effective way to reduce the release of sulphur oxides to an acceptable level is to capture the sulphur oxides chemically from the furnace exhaust gasses before they are vented to atmosphere.
One method that is extensively used to capture sulphur oxides is to add an alkaline reacting solid additive to the hot furnace gasses to react with the sulphur oxides. The most commonly used additive is a particulate material derived from limestone, and it is usually added to the combustion zone of the furnace. Four reactions involving the limestone material are theoretically possible:
(a) CaCO
3
→CaO+CO
2
,
(b) CaO+SO
2
→CaSO
3
,
(c) 2CaO+2SO
2
+O
2
→2CaSO
4
, and
(d) CaO+SO
3
→CaSO
4
.
In the conditions that exist in the combustion zone of a fluidised bed combustor furnace, or FBC, sulphur trioxide is generally present in only relatively low levels, so that the last of these reactions is of little importance. As a result, the furnace ash residues typically comprise a heterogeneous mixture of CaO, CaSO
4
, limestone, unreacted carbonaceous char derived from the fuel and fuel derived ash materials, which are primarily inorganic compounds. This technique is particularly suitable for modern fluidised bed combustors (FBC's).
All of these known processes utilising a more or less dry particulate additive show poor utilisation of the added reagent, in the sense that in order to reduce significantly the amounts of sulphur dioxide, and of sulphur trioxide if present, in the furnace gasses prior to venting to the atmosphere, a substantial excess of the reagent has to be used, above the theoretical requirements of the reactions set out earlier. This is particularly true for the most commonly used reagent, which is either limestone as such (substantially CaCO
3
), lime (CaO), or hydrated lime (ca(OH)
2
plus some CaO). Although limestone is a relatively low cost material, both limestone, lime and hydrated lime are poorly utilised in the sulphur dioxide capture process, with utilisation figures in the range of 30%-40% being considered good.
Poor utilisation of the lime or limestone with its adverse effect on furnace operation costs, also provides a furnace ash which poses disposal problems. Since sulphur dioxide capture in the furnace is generally inefficient, FBC ashes commonly contain up to at least about 20% of free CaO. An FBC ash containing this amount free CaO cannot simply be dumped. It can generate dangerously high temperatures in contact with water, and landfill sites containing it are both unstable and generate a water leachate with an unacceptably high alkaline pH in the range of between 11 and 12. This leachate too requires treatment before it can be safely discharged. Further, over extended time periods in such a landfill site these ashes are found to be subject to considerable expansion, which both affects dump stability and produces yet more alkaline leachate requiring treatment.
In order to mitigate these difficulties, FBC ashes are generally subjected to a two stage CaO hydration procedure. First, the ash solids are mixed with water, generally in a pug mill. Then the wet solids are treated with further water at the disposal site, in part to complete the hydration process and in part to achieve optimum solids density. The second addition of water allows cementitious reactions involving the other components in the ash to go to completion, which should improve the overall strength and durability of the landfill site.
This method suffers from several disadvantages. Chemical analysis of the hydrated ash shows that at the end of the two stage process the hydration reaction is not complete, and at most only about 70-80% of the CaO in the ash is hydrated. It is also found that the water losses encountered due to steam formation in hydrating the ash are quite high, and can range as high as 40-50% by weight of the ash being treated, even though the theoretical water requirement for an average ash containing about 18% free CaO is only approximately 6% by weight of the ash being treated. It is also found that the hydration reaction at ambient temperatures is slow, and may take hours, or even days, to reach a reasonable level of completion.
Several methods have been proposed whereby better hydration of FBC ashes may be obtained.
It has been proposed to increase the reaction rate by increasing the water temperature. In the so-called Pyropower method, water at 98° C. is recommended. In the so-called CERCHAR process a pressurised hydration reactor is used. Both of these methods whilst proffering a better level of hydration, increase significantly the cost of the hydration process.
A need therefore exists for a faster, less expensive and more effective way of hydrating at least a major portion of the CaO content of FBC ashes.
SUMMARY OF THE INVENTION.
This invention seeks to provide a process for hydrating CaO residues in FBC ashes that both achieves a better level of hydration, which does not add significantly to the cost of the hydration process, and which reduces the consumption of water in the hydration process. Further, as an adjunct to the hydration process, it is possible to trap at least some of the CO
2
in the furnace gasses, by using some of it to convert the hydrated CaO to CaCO
3
. This both reduces the amount of CO
2
released to the atmosphere, and converts the potentially dangerous CaO in the ashes into an effectively inert and benign material. In the hydration process according to this invention the reaction between the solid ashes and the liquid water is activated sonochemically. By exposing the ash/water mixture to sound under the correct conditions of frequency and power input it is possible to improve both the rate of hydration, and the level of hydration, of FBC ashes. The process can be carried out in a single step, and does not require an extended time period.
DETAILED DESCRIPTION OF THE INVENTION
In the process according to this invention the rate of the solid/liquid reaction between the hydration water and the CaO in the ashes is enhanced by sonic irradiation. It is known that ultrasound irradiation can enhance chemical reaction rates by up to two orders of magnitude in some cases. In some cases this appears to be as a result of better mixing. In others it appears to be as a result of a cavitation phenomenon which has been postulated to involve the production of micro scale transient bubbles with extremely high temperatures, and which generate shockwaves on implosion. It is also known that ultrasonic irradiation can cause pitting of solids, and strong shearing forces at the liquid/solid interface which can significantly enhance mass transfer processes across the interface. Very little is known about the influence of sonic radiation, that is sound radiation at frequencies within the audible range, on chemical reactions.
It has now been found that even though the CaO in FBC ashes is only one component of several in a complex mixture, sonic activation of the hydration reaction is feasible, and appears to be both effective and economical in comparison with the currently used or available methods. Sonochemical activation appears to provide a hydration process that can achieve hydration levels in excess of 80% in a re
Her Majesty the Queen in right of Canada as represented by the
Wilkes Robert A.
Wong Edna
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