WO2009022060A1

WO2009022060A1 - Method for improving the performance of a circulating bed reactor, as well as circulating bed reactor capable of implementing the method

Info

Publication number: WO2009022060A1
Application number: PCT/FI2008/050464
Authority: WO
Inventors: Seppo Ruottu
Original assignee: Einco Oy; Ruottu, Lauri
Priority date: 2007-08-16
Filing date: 2008-08-15
Publication date: 2009-02-19
Also published as: FI20075574A0

Abstract

The invention relates to a method for improving the performance of a circulating bed reactor, in which at least some of the heat transfers into a fluidized material. The method comprises making up a set of return channels of at least one cooled return channel, in which some of the thermal energy, possessed by the fluidized material passing therethrough, is recovered, as well as of at least one uncooled return channel, in which a flow of the fluidized material passing therethrough is adaptable to become substantially adiabatic and conveying the fluidized material into the set of return channels, such that the difference between a total flow of the fluidized material passing through the riser channel and a flow of the fluidized material returning through the cooled return channels is capable of being guided into the uncooled return channel.

Description

Method for improving the performance of a circulating bed reactor, as well as circulating bed reactor capable of implementing the method

The invention relates to a method for improving the performance of a circulating bed reactor, in which circulating bed reactor at least some of the heat, possessed by combustion gases evolving in the circulating bed reactor, transfers into a fluidized material adapted to circulate within the circulating bed reactor, and which circulating bed reactor comprises air distribution means, fuel supply means, a fluidization chamber, a riser chamber, a separator means for separating the fluidized material from combustion gases, a set of return channels by way of which the fluidized material is returnable back into the fluidization chamber, as well as heat transfer elements for the vaporization of water and/or for the superheating of vaporized water. The invention also relates to a circulating bed reactor capable of implementing the method.

In this application, the circulating bed reactor refers to a system which comprises at least air distribution means, fuel supply means, a fluidization chamber, a riser chamber, as well as a cyclone separator and a set of return channels for a fluidized material adapted to circulate within the circulating bed reactor. In addition to the circulating bed reactor, it is also possible to speak about steam boilers provided with a fluidized material recycling feature, when dealing with a system intended for the vaporization and superheating of water, which is also known as a circulating fluidized bed furnace.

In the fluidization chamber and in the riser channel of a circulating bed reactor there are chemical reactions, especially oxidation reactions, taking place while the upward flowing gas carries along solid particulate fluidized material. By means of a cyclone separator in connection with an upper part of the riser chamber, the particles making up a fluidized material are separated from the gas flow and returned by way of a set of return channels, provided for the fluidized material, back into a lower part of the riser channel or into a so-called fluidization chamber.

As a rule, the walls of riser channels in circulating bed reactors, and generally in steam boilers, are in a cooled condition. What is meant by this is that the walls are fitted with heat transfer elements to receive heat from combustion gases, as well as with steam generators and superheaters. It is an objective to have the riser channel and the heat transfer elements dimensioned in such a way that, with a nominal power of the boiler and with a certain fuel or fuel mixture, the temperature would settle within a desired range in the riser chamber's upper part. Thus, when the operating conditions change, the temperatures in all parts of the riser channel will also change. Indeed, in steam boilers, the highest temperature of a riser channel is usually established between upper and lower parts of the riser channel in a difficult- to-predict manner. The riser channel's temperature fluctuations are a source of many problems, such as a deterioration of the combustion result, as a consequence of increased emissions and the soiling of heat transfer surfaces.

A special problem evolves when harmful substances are able to make a contact with heat transfer surfaces at excessively high temperatures. A poor management of temperatures is particularly adverse in the process of burning materials classified as waste. In particular, the exposure of superheater pipes to harmful substances contained in a combustion gas has been found to cause serious corrosion problems. Typically, it is the case of so-called heat corrosion. The unacceptably rapid corrosion of superheater pipes incurs high costs also in terms of repairs and downtimes.

Various proposals have been presented for reducing the corrosion of heat transfer surfaces. In some cases, the corrosion rate has been successfully reduced by changing the feeding ratios of fuels, but oftentimes this is neither possible nor economically viable. The reduction of corrosion has also been attempted by supplying the combustion chamber with an anti-corrosive inorganic agent. However, the poor manageability of temperatures, along with corrosive compounds contained in a combustion gas and in the ash traveling therewith, continues to be a serious problem and thus far, aside from restricting the superheating temperature, no other working solution has been found to this problem. Decreasing the superheating temperature nevertheless deteriorates substantially the electricity efficiency of a facility and is therefore a poor option in terms of energy economy. For the above reasons, and particularly in the process of incinerating wastes, the temperature of superheated steam must nevertheless be limited to a level which is too low from the standpoint of efficiency, typically to less than 500⁰C.

US patent 6,293,781 Bl discloses another method and apparatus for solving the foregoing problem. It concentrates on preventing compounds, which are problematic for heat transfer surfaces, from getting in contact with heat transfer surfaces by positioning the heat transfer surfaces in a bubbling fluidized bed located in the return channel of a circulating bed reactor. The solution is based on the fact that, at this stage, the fluidizing gas does not contain corrosive compounds as yet. In the method according to the cited publication, the fluidized material circulating in a circulating bed reactor, i.e. a sand material intended for this purpose and having a certain grain size, is conveyed prior to a heat transfer element into a fluidized bed which does not include heat transfer surfaces. The sole purpose of this fluidized bed is to reduce the concentration of corrosive compounds upstream of a heat transfer element. According to the cited patent, the fluidized gas flow can be divided in various ways in said fluidization chambers, also in vertical direction, for an optimum result.

However, the solution presented in the cited US patent involves one fundamental drawback, which is due to the fact that the heat transfer surfaces are therein disposed within a fluidized bed present in the return channel of a circulating bed reactor. First of all, across the fluidized bed exists a pressure difference which carries a lot of combustion gas to a fluidization heat transfer element. The operation of such a solution in a desired manner requires that, in order to dilute the concentrations of corrosive compounds, the fluidized bed must be supplied with a very large amount of a pure fluidizing gas. This results in numerous further drawbacks. The method leads to a complicated and expensive apparatus. On the other hand, the heat transfer surfaces set in a fluidized bed experience rapid mechanical wearing in perpetual sand blasting, as the fluidized bed has its particles grinding intensely against the surfaces of a heat transfer element. Another downside is due to the fact that the fluidized beds of a return channel increase operating and investment costs and hamper the management of gas flows in the reactor.

One task of the present invention is to eliminate or alleviate these drawbacks found in the US patent 6,293,781 Bl. Another essential objective is to avoid the above-mentioned corrosion problems and to enable a higher-than-before final temperature of superheated steam for improving electricity efficiency, as well as to enable superheated steam and reactor temperatures to remain more steadily than before at the set values thereof in diverse operating conditions. Hence, the objective is to provide a solution much easier to control than before.

This multipurpose task is now accomplished by a method according to the present invention, which is characterized by what is presented in the characterizing clause of claim 1. On the other hand, an arrangement of the invention has its characterizing features set forth in the characterizing clause of claim 13. In addition, numerous preferred embodiments of the invention are presented in the dependent claims.

Thus, the method of the invention provides a way of protecting the heat transfer surfaces of both a vaporizer and particularly a superheater from harmful substances and compounds in a simple and simultaneously an energy-efficiency enhancing manner. The solution is structurally simple, attractive in terms of its costs, and provides a distinct improvement in the efficiency of an energy production process, especially in the performance of electricity production. In addition, the mechanical stress applied to heat transfer surfaces is decisively lesser than before and consequently the apparatus has a service life which is substantially longer than before.

A practical implementation of the inventive idea has been successfully provided as a working entity enabled by and consisting of a few essential features. Regarding the invention from the standpoint of its functional aspects, it is first of all preferred that the circulating bed reactor's riser channel be given a substantially adiabatic design. In this context, the meaning of this is that, in the riser channel intended for combustion gases and located downstream of the fluidized bed, heat transfer to a significant extent only occurs between combustion gases flowing therein and a fluidized material, specifically its particles, rising up along with said gases. Thus, the circulating bed reactor's riser channel is substantially heat-insulated, such that the thermal energy, contained in combustion gases flowing in the riser channel as well as being released in response to combustion reactions still going on therein, becomes engaged primarily with the combustion gases and with the fluidized material circulating by way of the riser channel. Consequently, the fluidized material is capable of being returned by way of a set of uncooled return channels at a temperature which is at least more or less constant, i.e. no loss of thermal energy is allowed to happen. Of course, it is still possible to provide heat transfer elements in the riser channel but, from the standpoint of energy economy and because of the above-described corrosion problems, this is not advisable after all.

The fluidized material, which is heating in the riser channel and, moreover, separated from the combustion gas flow by means of a separator device, such as a separator cyclone, disposed in engagement with an upper part of the riser channel, is delivered into a set of return channels provided for the fluidized material. The fluidized material return channels, disposed circumferentially around the riser channels, are now divided according to the invention into cooled and uncooled ones. A desired portion of the fluidized material flow is returned back into the fluidization chamber by deflecting it to flow through the cooled channels. The hot fluidized material delivers some of its heat to heat transfer elements disposed in these channels. The fluidized material can be regulated in terms of its flow rate and dwell time in the channels in order to achieve a desired heat flow from the fluidized material to the heat transfer elements.

That portion of the fluidized material, which does not fit in to flow through the cooled return channels or which is just deflected to bypass the cooled return channels, can be simply returned in the form of an overflow into the uncooled channels and further directly back into the fluidization chamber at its hot temperature.

Hence, both vaporization and especially superheating take place by guiding the hot fluidized material, separated in the cyclone, to flow in contact with vaporizers and superheaters located in the set of return channels. The cooled return channels can be designed in such a way that the hot fluidized material packs in the channels effectively in response to gravity alone and proceeds in the way of a homogeneous fluid flow downward at a desired rate controlled by appropriate regulation elements. Ensured at the same time is an efficient transfer of heat between fluidized material and heat transfer surfaces.

One of the decisive differences between the method according to the invention and the US patent No. 6,293,781 Bl lies in the fact that the return channels, which have heat transfer surfaces included therein, have the fluidized material flowing in a tightly packed condition, such that similar unnecessary flowing movements of sand, making deviations from its principal traveling direction and abrading the heat transfer surfaces, does not exist to an adverse extent. Thus, the flow of a fluidized material is comparable to a linear fluid flow in which the flow proceeds smoothly in a single course essentially without unnecessary turbulences or similar flow disturbances. In other words, the packed fluidized material does not contain, in a harmful degree, for example gas bubbles appearing in a bubbling fluidized bed, which would have an impact on its fluid-like behavior and would inflict disturbances and confusion in the flow of sand material. In addition, the packed sand is adaptable to flow by the action of gravity alone. A smooth flow of fluidized material can also be assisted by the disposition and design of heat transfer elements. Preferably, the heat transfer surfaces are arranged in a parallel relationship with the fluidized material flowing direction. Likewise, heat transfer surfaces can be provided particularly on the wall faces of return channels. It is further possible to employ for example a lamellar type of heat transfer elements or traditional tubular heat exchangers outfitted for example with a tube-fin assembly, with the tubes extended in the fluidized material flowing direction. Thus, from the standpoint of a fluidized material's flowing characteristics, it is preferred that the resistance of flow established by heat transfer surfaces be as insignificant as possible. This also enables the minimization of a mechanical wear effect.

The consequence of a fluidized material being in a packed state is that the flow of combustion gas reaching the heat transfer elements along with the fluidized material is a negligible fraction of what it would be to heat transfer elements installed in the fluidization space. This alone is an effective means of preventing the access of combustion gases to heat transfer surfaces to cause the above-mentioned numerous problems. The combustion gas flow into cooled return channels is as such meaningless in all situations, but the access of combustion gases to return channels along with sand can be eliminated even more effectively by further supplying a lower part of the heat transfer elements with a small amount of a pure flushing gas, for example normal combustion air.

A plurality of other benefits of the invention shall appear in the following as described by means of one exemplary embodiment of the invention. The invention will be described more precisely with reference to the accompanying drawing, in which Fig. 1 shows a circulating bed reactor of the invention in a view of vertical section and Fig. 2 shows a set of return channels of fig. 1 in a view of horizontal section along a sectional line A-A, and

Fig. 3 shows a second embodiment of the invention in a view of horizontal section consistent with fig. 2.

Referring to fig. 1, there is illustrated one circulating bed reactor 1 embodying a solution according to the invention. Therein, the combustion air is supplied by way of an assembly 31 into an air cabinet 34, from which it is delivered through a grate 30 into a fluidization chamber 4. The fluidization chamber is also supplied with a fuel by way of a feed assembly 26, and non- fluidizing material is discharged therefrom by way of an outlet assembly 25. A fluidized material 2, having established a fluidized bed 33 on the fluidization chamber's floor by means of the combustion air supplied through the grate 30, bubbles in a fluid-like behavior and, at the same time, the fluidized bed releases particles of the fluidized material along with combustion gas flows up towards a riser channel 7. Proceeding simultaneously in the fluidized bed 33 is a process of drying and igniting a fuel mixed therewith, as well as ultimately also, to a large extent, of burning a residual carbon of the fuel.

In this case, the riser channel 7 is not provided with heat transfer elements and the flow therethrough proceeds in a substantially adiabatic manner. The extra thermal energy, which is already contained in combustion gases, as well as also releasing as a result of oxidation reactions and bonding to combustion gases, has a portion thereof transferring into a flow of fluidized material, which proceeds in the riser channel 7 and has become snatched from the bubbling fluidized bed 33 present on top of the grate 30 to join the combustion gas flows. The heat flow to the riser channel's walls 73 is insignificant, and a cooling action in the riser channel is provided exclusively by particles of the fluidized material flowing therealong. Another consequence of an adiabatic flow in the riser channel is that temperatures existing in the circulating bed reactor lie generally within the range between the temperature of the fluidization chamber 4 and that of the riser channel 7. When the flow of solids returning by way of the uncooled return channel 17 back into the fluidized bed 33, i.e. the mass flow of the hot fluidized bed material 2, occurs at a sufficiently high rate, the temperature of the entire fluidization element 4 remains sufficiently high. At the same time, all temperatures observed in the riser chamber 7 settle within a desired temperature window.

The mixture of combustion gases and the fluidized material 2 finds its way from the riser channel 7 into a fluidized material separator device provided at the top thereof, in this case, for example, to a separator cyclone's vane system 8 arranged in an annular configuration relative to the riser channel. Thence, the combustion gases and the fluidized material proceed, while rotating tangentially relative to the riser channel, into a cyclone chamber 9. In response to radial and tangential velocities, the fluidized material migrates onto a wall of the cyclone chamber 9 while the combustion gases maintain the vortex motion and climb towards a central pipe 28 present in an upper part of the cyclone chamber 9 for a final exit therethrough. Naturally, a circulating bed reactor of the invention can also be adapted to implement the recovery of heat from combustion gases directly from the combustion gases as well. This is preferably carried out by means of extra heat transfer elements included for example in the cyclone chamber 9 or in subsequent process steps.

The fluidized material separated onto the cyclone wall trickles by way of a guide funnel 29 to a preferably annularly configured top part 18 of the set of return channels and thence into various return channels 17, 20, 21 of the set of return channels, which in this case are configured axial-symmetrically as a ring around the riser channel 7. The return channels 17, 20, 21 are arranged to provide at least two, preferably mutually heat-insulated zones around the riser channel 7. In this embodiment, the disposition of channels is such that an inner position is occupied by the uncooled return channel 17 established by a continuous ring. Outer positions are in this case occupied by the cooled return channels 20, 21 in a sector-like configuration, one 20 of which is here provided with the heat transfer surfaces of vaporizers 11 and the other 21 is provided with the heat transfer surfaces of superheaters 13. In view of blocking a transfer of heat from the uncooled return channel 17 to the steam-generating and superheating return channel 20, 21, there is further interposed between the same, in this case, a preferably annular thermal insulation 14. It should further be noted that even though the cooled return channels are fitted in a preferably sector-like configuration within a single ring, it is obvious that, for special reasons, it is of course possible to install cooled return channels in several rings as well. Likewise, the number of vaporizers, as well as that of superheaters, is by no means limited to any particular number or return channel 20, 21. Neither is the shape of the riser channel 7 restricted to a shape of circular cross-section, which is just one highly preferred embodiment of the invention.

According to a method of the invention, the flow of fluidized material is distributed in desired proportions into the uncooled return channel 17, the steam-generating return channel 20, and the superheating return channel 21. In order to control said proportions, the steam-generating and superheating return channels have bottom ends thereof fitted with actuators 55 and 56, which are used for regulating the rates of fluidized material flows proceeding through said return channels in line with desired heat transfer rates. The actuators 55 and 56 for the cooled return channels can be mechanical or pneumatic control equipment of the prior art.

A fluidized material flow in the uncooled return channel 17 will be determined as a difference between a total flow of fluidized material received in a preferably self-guided manner in the guide funnel 19 and the flows of fluidized material proceeding through the cooled return channels 20, 21. Thus, the flow of material along the return channel 17 can preferably be implemented as a simple overflow, i.e. the portion of a fluidized bed material flow, which does not fit in the cooled return channels, is guided as such preferably either gravitationally or in an assisted manner into the uncooled return channel and returns at its hot temperature directly into the fluidization chamber 4. It is also possible that, for the flow of fluidized material, the set of return channels be provided also at its inlet end with actuators, such that a flow rate into the channel 17 or a portion of the total flow can be adjusted irrespective of the amount of fluidized material being guided into the cooled return channels 20, 21 or the degree of fullness of the channels 20, 21.

At this point, it should also be noted that the channels 17 need not necessarily consist of totally independent flow channels as in the embodiment of fig. 1. What is essential about uncooled return channels is to organize a return of the fluidized material 2 back into the fluidization chamber 34 without adapting the fluidized material to deliver any significant amount of its bonded thermal energy to its surroundings or to have the fluidized material returnable by way of the return channel 17 without a delivery of heat in a substantially adiabatic manner. The return channels 17 can also be provided in a direct communication with the riser channel 7, as illustrated, for example, in the cross-section of fig. 3. In that case, the return channels 17 are separated from the riser channel 7 by means of separation walls 17b. It is even conceivable that an uncooled return channel for the fluidized material is provided by the riser channel 7 itself, yet in such a manner that the return flow is not, in any significant degree, able to disturb combustion gas flows in the riser channel or the behavior of the fluidized material therein.

Thus, in practice, the fluidized material flows purely by gravity alone into return channels, becoming tightly packed therein. At the same time, a consequence of the fluidized material becoming tightly packed is that the combustion gas flow into cooled return channels is in all instances already insignificant as such. Neither is there any possibility of any significant vertical blending of gases taking place in the return channels. By virtue of this, the employed pure eventual flushing gas, rising counter-currently with respect to the fluidized material, is able to clean the cooled return channel as effectively as possible. By additionally directing into a lower part of the heat transfer elements and into a lower part of the return channels a small amount of pure flushing gas 40, for example combustion air functioning as secondary or tertiary air, the access of combustion gases and adverse corroding compounds contained therein to the heat transfer surfaces of a vaporizer and a superheater can be denied virtually completely. The employed flushing gas may also consist of other gases or gas mixtures preferably as inert as possible. It is even conceivable to use a combustion gas, which has been cleaned of detrimental compounds and impurities. The flushing gas is preferably preheated for improving energy economy, in the case of combustion air for example in a LUVO.

Hence, by virtue of a method according to the invention, the amount of a necessary flushing gas is small and, if for example the above-mentioned actuators 55 and 56 for regulating the flow rates of a fluidized bed material are chosen to be pneumatic, just actuator air 24 needed thereby will be sufficient in most cases for blocking the admission of a combustion gas into the cooled return channels, thus eliminating the need for other flushing gas.

Furthermore, by virtue of a packed condition of the fluidized material, nor is there any need to use, as far as a flushing gas is concerned, pressure levels typical for a fluidized bed gas, but the use of distinctly lower pressure levels is enabled in addition to the fact that the required amounts of flushing gas are decisively smaller than before. It is sufficient, for example, that the existing gas pressure in lower parts 55, 56 of the cooled return channels 20, 21 be first of all established higher than in outlets 6 of the cooled return channels 20, 21 and, second of all, that the pressure existing in a flushing gas inlet area shall nevertheless become lower than what is the pressure of a supplied fluidizing gas required for a minimum fluidization of the solids or fluidized material present in the channels 20, 21. The amount and feeding pressure of a flushing gas is thus selectable, in terms of both the amount and the type of fluidized material present in the cooled return channels, in such a way that the flushing gas enables providing a controlled gas flow, which runs counter to the fluidized material flowing direction, yet, at the same time, retains its strength at such a level that, for example, the formation of gas bubbles typical of a fluidized bed, and the powerful upward migration thereof, is disabled at least to a substantial degree. What is ensured at the same time is a smooth flow of the fluidized material in the vicinity of heat transfer surfaces and also a mechanical wear of the heat transfer surfaces as little as possible.

At the same time, an option is provided for reducing the internal load of a circulating bed reactor as a result of avoiding a fan power required by fluidization vaporizers and superheaters for a fluidizing gas or an abundant flushing gas.

Moreover, since the recycling and returning of a fluidized material back into a fluidization chamber is now feasible by means of gravity alone both in cooled as well as uncooled return channels, the use of external utilities, such as a compressed fluidizing gas, or the need of mechanical conveyance, shall be avoided in this sense as well.

The function and overall performance of a solution according to the invention are significantly affected also by the choice of a basic circulating bed reactor design. In the above-discussed working example, the reactor is made in an axial-symmetric configuration. There are obvious advantages to be gained with this configuration. Firstly, the wearing of structural elements, as well as the demand of space for the apparatus, will be minimized. A fluidized material separated by the cyclone 9 shall be distributed axial-symmetrically, whereby the uncooled return channels develop on top of themselves an axial-symmetric, sloping bed of fluidized material, which is almost independent of fluidized material flows in the cooled return channels. The portion of fluidized material, which is not directed into the cooled return channels 20, 21, will be self-guided gravitational Iy as an overflow into the uncooled return channel 17. Being implemented like this, the flow of fluidized material, as far as the uncooled return channel is concerned, does not require any actuators. It does not need any sort of regulation and a sufficient supply of fluidized material for the cooled return channels is always ensured. In the uncooled return channel, most preferably, the fluidized material flows gravitationally and freely in an unpacked condition.

Hence, the fluidized material is able to fall freely by way of the return channel back in the fluidization chamber and into the fluidized bed. This particular disposition of return channels also makes it possible that the uncooled return channel can have its flow rate fluctuating freely over a wide range without causing any trouble to the process in its operation. At the same time, the system will be simplified in terms of managing its flow technology, as the flow of fluidized material proceeds along all of the cooled return channels preferably in a packed condition and along the uncooled return channel in an unpacked condition.

On the other hand, a transfer of heat from the uncooled return channel 17 to the cooled return channels can be impeded for example by one annular heat insulation 14. Instead, between the cooled channels 20, 21 there is no need for thermal insulations at all. It should also be noted at this point that the set of cooled return channels 20, 21 is adaptable to consist of just one, preferably annular channel, which has both superheating and vaporizing heat transfer elements disposed therein, or of several discrete channels, which only includes either vaporizing or superheating heat transfer elements or both.

The invention is able to provide also numerous other major benefits and improvements over prior known solutions. Since there is now provided a capability of effectively preventing the heat transfer surfaces from making contact with corrosive compounds contained in combustion gases, it is possible to increase a steam superheating temperature significantly from the current values, used primarily in the incineration of wastes. By virtue of this, the method according to the invention is capable of enhancing the electricity efficiency of a plant. In addition, because the fluidized material, which delivers heat to heat transfer surfaces, flows smoothly in its packed condition without applying a powerful abrasive grinding action to the heat transfer surfaces, the mechanical wearing of the heat transfer surfaces will be remarkably lesser than what it is in conventional heat transfer elements placed in a fluidized bed. The wearing is further reduced by the fact that the actuators 55, 56 enable setting up the flow of fluidized material in the channels 20, 21 in a pulsed manner, i.e. the fluidized material advances over a desired displacement at a time and holds its position for a desired period before the next displacement. The longevity of heat transfer elements can be significantly increased by means of solutions according to the invention.

Likewise, the complicated and bulky fluidization apparatus required by fluidization superheaters and vaporizers will now be replaced with a simple, inexpensive and compact actuator. Also, the entire circulating bed reactor can be constructed in a manner simpler and less expensive than the currently used ones, and with less space required. Moreover, since the heat transfer surfaces only end up in contact with dry fluidized material, the need of chimney sweeping and chimney sweeping investments is also obviated.

Advantages of the invention manifest themselves also in easier adjustability. Especially, in the process of incinerating waste, the riser chamber must have its upper part 71 at a temperature of not lower than 850⁰C for ensuring an incineration process as complete and pure as possible. One of the conditions for this is that the temperature of a fluidization chamber remains sufficiently high. According to the invention, the fluidization chamber's adequate temperature is ensured in such a way that the flow of hot fluidized material in an uncooled return channel is upheld at a sufficiently high rate in all conditions. In the method according to the invention, it is possible to make sure in a simple manner that a sufficient portion of the fluidized material migrating by way of the riser channel 7 shall be returned by way of the uncooled return channel 17 to a lower part of the riser channel 72. As described above, it is now possible to make sure, in a manner easier than before, that the temperature of a fluidized portion remains at not lower than 750⁰C for ensuring a quick ignition of the fuel supplied into the fluidized bed. With the method according to the invention, the temperature of a fluidized portion is readily maintainable within the range of >800°C, whereby the reaction kinetics of burning is rapid, the emissions are minimized, yet the melting of ash is avoided. In addition, the riser chamber in a method of the invention preferably does not include any heat transfer surfaces at all, meaning that such surfaces cannot become soiled or corroded under any circumstances. Because the vaporizer and superheater pipes do not end up in any sort of contact with a combustion gas, the temperature of heat transfer elements' external surfaces can be increased significantly from previously used values. By virtue of a method according to the invention, the temperature of superheated steam can thus be respectively increased to over 500° without problems.

Upholding a temperature consistent with waste incineration regulations in fluctuating process conditions can also be managed in a method of the invention in an easily controlled manner. A preferred way of performing the adjustment is such that a flow of fluidized material passing through the vaporizer is controlled as a set value adjustment for the temperature of the upper part 71 of the riser channel 7. As the flow of fluidized material, having cooled in the vaporizer and returning to the fluidized portion, is reheated in the riser channel to a temperature consistent with the set value, it creates in the riser channel 7 an adiabatic cooling effect precisely equal to the vaporization efficiency.

In a method according to the invention, in this particular embodiment thereof, the temperature of superheated steam is also maintainable in a fluctuating vapor stream at its set value by controlling a flow of fluidized material passing through the superheater 13 as a set value adjustment for the temperature of superheated steam. Like the flow of fluidized material returning from the vaporizer, the flow of fluidized material returning from the superheater to the fluidized portion also creates in the riser channel an accumulating, adiabatically effected cooling. The cooling processes have a total effect such that the temperature of the riser channel's upper part 71 remains at its set value. Thus, when a change occurs in the cooling action of a superheater, the vaporizer's automatic control system performs an opposite adjustment operation to compensate for whatever effect the change in the superheater has on the temperature of the riser channel's upper part. Accordingly, in a method according to the invention, the temperature of the riser channel's upper part 71 remains at its set value regardless of fluctuations in the superheater's vapor stream.

The solution according to the invention has a multi-beneficial effect also on the adjustability of a plant. The riser channel's temperature remains now, more easily than before, within a set temperature window while the process conditions fluctuate over a wide range. At the same time, this is managed without the use of a booster fuel. The activation of a steam boiler is facilitated as the superheater is not exposed to overheating prior to a commencement of the boiler's own steam generation, nor is there a separate auxiliary boiler needed for the protection of superheaters. Emissions are also minimized as temperatures existing in the fluidization chamber and in the riser channel remain at optimum values thereof and the combustion can be performed as completely as possible. Besides, since the riser channel does not include erosion-susceptible heat transfer surfaces, the flow rates of combustion gases can also be increased significantly with respect to rates used in traditional circulating bed reactors. This results in an expansion of the power control range available for the reactor. The transfer of heat can be adjusted precisely over a highly extensive performance range.

Thus, the flow of fluidized material in the vaporizing return channel 20 is preferably controllable as a set value adjustment for a temperature Tl of the upper part of the riser channel 7, as shown in fig. 1. If Tl>set value, the flow of fluidized material will be increased by, and otherwise maintained the same or reduced, by the actuator 55. Respectively, the flow of fluidized material in the superheating return channel 21 is controllable as a set value adjustment for a temperature T2 of superheated steam. If T2>set value, the flow of fluidized material in the superheating return channel 21 will be reduced by, and otherwise maintained the same or increased, by the actuator 56. The actuators 55 and 56 are most conveniently pneumatic, the controls thereof taking place for example by means of valves 22 and 23.

The above-described control system provides for a simple way of managing the flows of fluidized material in cooled return channels and in an uncooled one, and makes the system straightforward in terms of its configuration. According to one additional embodiment of the invention, the arrangement in terms of control is such that the fluidized material present in the vaporizer and in the superheater is in a packed condition, as indicated by means of an arrow 27, and that its flow is regulated by means of sub-vaporizer and sub- superheater actuators, which are controlled intermittently such that the flow of fluidized material fluctuates between zero and a selected rate. The described mode of control, namely, enables obviating a few discovered practical problems relevant to applying a method according to the invention. The most essential of such practically discovered problems consist of three phenomena as follows:

1. Because of the hysteresis of control response, a continuous-action regulation does not necessarily provide an easy way of gaining control of the reactor temperature, but it may remain fluctuating with more than a hundred-degree amplitude on either side of the set value. The problem has been discovered through experimentation with several various combinations of control modes and devices.

2. Especially when using small-volume flows of fluidized material, the regulating actuators 55, 56 are prone to accumulate coarse material, which undermines predictability of the actuators' control responses. Such enrichment is likely to occur both in mechanical as well as in pneumatic actuators.

3. The management of combustion is hampered by a fairly large-volume airflow in the continuous-action pneumatic control device.

On the other hand, replacing the continuous-action regulation with a pulsed regulation according to the additional embodiment of the invention provides an effective means of eliminating also the foregoing extra problems. The pulsed regulation is preferably implementable in such a way that the duration and strength of a control pulse are selected and the pulse rate is controlled as a set value adjustment of temperature. This method is highly applicable in terms of both a vaporizer and a superheater.

By means of the method and arrangements according to the invention, the operability of a plant can be improved to a degree that enables also an unmanned running of the plant and thereby reduced operating costs. From the standpoint of economy, another advantage is a possibility of totally omitting a traditional injection adjustment of the superheater temperature. Energy efficiency is also improved by a reduced internal load, as it has been possible to completely omit the fan power required by fluidization vaporizers and superheaters.

Claims

1. A method for improving the performance of a circulating bed reactor (1), in which circulating bed reactor (1) at least some of the heat, possessed by combustion gases evolving in the circulating bed reactor (1), transfers into a fluidized material (2) adapted to circulate within the circulating bed reactor (1), and which circulating bed reactor (1) comprises air distribution means (23), fuel supply means (26), a fluidization chamber (4), a riser chamber (7), a separator means (8, 9) for separating the fluidized material (2) from combustion gases, a set of return channels (17, 20, 21) by way of which the fluidized material (2) is returnable back into the fluidization chamber (4), as well as heat transfer elements (11, 13) for the vaporization of water and/or for the superheating of vaporized water, characterized in that the method comprises making up the set of return channels (17, 20, 21) of at least one cooled return channel (20, 21), in which some of the thermal energy, possessed by the fluidized material (2) passing therethrough, is recovered, as well as of at least one uncooled return channel (17), in which a flow of the fluidized material (2) passing therethrough is adaptable to become substantially adiabatic, that at least some of the heat transfer elements (11, 13) are disposed in the cooled return channel (20, 21) for recovering thermal energy from the fluidized material (2), and that the fluidized material (2) is conveyed into the set of return channels (17, 20, 21), such that the difference between a total flow of the fluidized material (2) passing through the riser channel (7) and a flow of the fluidized material returning through the cooled return channels (20, 21) is capable of being guided into the uncooled return channel (17).

2. A method as set forth in claim 1, characterized in that the cooled return channels (20, 21) have lower parts (15, 16) thereof supplied with a flushing gas, and that the flushing gas pressure in the lower parts (15, 16) is higher than in an outlet (6) of the cooled return channels (20, 21) and/or higher than in inlets (18) of the cooled return channels (20, 21), and that the flushing gas pressure in the lower parts (15, 16) of the cooled return channels (20, 21) is lower than a pressure required for the minimum fluidization of a fluidized material (2) present in the return channels (20, 21).

3. A method as set forth in claim 1 or 2, characterized in that the fluidized material (2) is capable of being guided into the uncooled return channel (17) as an overflow of the cooled return channels (20, 21).

4. A method as set forth in any of claims 1-3, characterized in that a flow of the fluidized material (2), which proceeds through the return channel (20) housing a vaporizing heat transfer element (11), is controlled on the basis of a temperature (Tl) of the riser channel's (7) upper part (71) and/or a flow of the fluidized material (2), which proceeds through the return channel (21) housing a superheating heat transfer element (13), is controlled on the basis of a temperature (T2) of superheated steam.

5. A method as set forth in claim 4, characterized in that the adjustment regarding a flow of the fluidized material (2) is conducted as a set point control.

6. A method as set forth in claim 4 or 5, characterized in that said temperature adjustments of the cooled return channels (20, 21) are implemented by fluctuating the instantaneous flows of the fluidized material (2) proceeding through the cooled return channels (20, 21) in a substantially discontinuous manner between minimum and maximum flows, such that the duration and/or the frequency of maximum flow cycles are varied as set point controls of the cooled return channels (20, 21).

7. A method as set forth in any of claims 1-6, characterized in that the fluidized material (2) is adapted to flow in the cooled return channel (20, 21) in a substantially packed condition for a heat transfer between the fluidized material (2) and the heat transfer elements (11, 13).

8. A method as set forth in any of claims 1-7, characterized in that for the cooled return channels (20, 21) are provided actuators (55, 56) for regulating a flow of the fluidized material (2) through each relevant return channel (20, 21).

9. A method as set forth in any of claims 1-8, characterized in that actuators for the steam generator (11) and the superheater (13) are adjusted periodically by means of control pulses.

10. A method as set forth in any of claims 1-9, characterized in that fluidized material flows directed into the cooled return channels (20, 21) are regulated by the periodically controlled actuators (55, 56), whereby the portion of a total flow of the fluidized material (2), which is not directed into the cooled return channels (20, 21), is capable of being guided as an overflow into the uncooled return channel (17).

11. A method as set forth in any of claims 8-10, characterized in that the gas, which is conveyed to the actuators (55, 56) in lower parts of the cooled return channels (20, 21) for operating the same, is used, as it comes out of the actuators (55, 56), for the purpose of flushing the fluidized material (2) flowing in the cooled return channels (20, 21).

12. A method as set forth in any of claims 1-11, characterized in that the transfer of heat from the uncooled return channel (17) and from the riser channel (7) to the cooled return channels (20, 21) is essentially disabled.

13. A circulating bed reactor (1), in which at least some of the heat, possessed by combustion gases evolving in the circulating bed reactor (1), transfers into a fluidized material (2) adapted to circulate within the circulating bed reactor (1), and which circulating bed reactor (1) comprises air distribution means (23), fuel supply means (26), a fluidization chamber (4), a riser chamber (7), a separator means (8, 9) for separating the fluidized material (2) from combustion gases, a set of return channels (17, 20, 21) for returning the fluidized material (2) back into the fluidization chamber (4), as well as heat transfer elements (11, 13) for the vaporization of water and/or for the superheating of vaporized water, characterized in that the set of return channels (17, 20, 21) is made up of at least one cooled return channel (20, 21), which is provided with a recovery of the thermal energy possessed by the fluidized material (2) passing therethrough, as well as of at least one uncooled return channel (17), in which a flow of the fluidized material (2) passing therethrough is adaptable to become substantially adiabatic, that the heat transfer elements (11, 13) for the vaporization and/or the superheating of water are disposed in the cooled return channel (20, 21) for recovering thermal energy from the fluidized material (2), and that the fluidized material (2) is capable of being conveyed into the set of return channels (17, 20, 21), such that the difference between a total flow of the fluidized material (2) passing through the riser channel (7) and a flow of the fluidized material returning through the cooled return channels (20, 21) is capable of being guided into the uncooled return channel (17).

14. A circulating bed reactor (1) as set forth in claim 13, characterized in that the cooled return channels (20, 21) have lower parts thereof supplied with a flushing gas, and that the flushing gas pressure in lower parts (15, 16) is higher than in an outlet (6) of the cooled return channels (20, 21) and/or higher than in inlets (18) of the cooled return channels (20, 21), and that the flushing gas pressure in the lower parts (15, 16) of the cooled return channels (20, 21) is lower than a fluidizing gas pressure required for the minimum fluidization of a solid matter present in the return channels (20, 21).

15. A circulating bed reactor (1) as set forth in claim 13 or 14, characterized in that a pressure difference required to obtain a minimum fluidized condition of the cooled return channels (20, 21) is each time set to exceed a total pressure difference between the riser channel (7) and the separator means (8), preferably to be at least double with respect to the mentioned pressure difference.

16. A circulating bed reactor (1) as set forth in any of claims 13-15, characterized in that a flow of the fluidized material (2), which proceeds through the return channel (20) housing a vaporizing heat transfer element (11), is adapted to be adjusted on the basis of a temperature (Tl) of the riser channel's (7) upper part (71) and/or a flow of the fluidized material (2), which proceeds through the return channel (21) housing a superheating heat transfer element (13), is adapted to be adjusted on the basis of a temperature (T2) of superheated steam.

17. A circulating bed reactor (1) as set forth in claim 16, characterized in that the adjustment regarding a flow of the fluidized material (2) has been conducted as a set point control.

18. A circulating bed reactor (1) as set forth in claim 16 or 17, characterized in that said temperature adjustments of the cooled return channels (20, 21) are implemented in such a way that the instantaneous flows of the fluidized material (2) proceeding through the cooled return channels (20, 21) are governed in a substantially discontinuous manner between minimum and maximum flows, such that the duration and/or the frequency of maximum flow cycles are varied as set point controls of the cooled return channels (20, 21).

19. A circulating bed reactor (1) as set forth in any of claims 12-18, characterized in that the fluidized material (2) is adapted to flow in the cooled return channel (20, 21) in a substantially packed condition for a heat transfer between the fluidized material (2) and the heat transfer elements (11, 13).

20. A circulating bed reactor (1) as set forth in any of claims 13-19, characterized in that for the cooled return channels (20, 21) are provided actuators (55, 56) for regulating a flow of the fluidized material (2) through each relevant return channel (20, 21).

21. A circulating bed reactor (1) as set forth in any of claims 13-20, characterized in that the portion of a total flow of the fluidized material (2) discharged from the riser channel (7), which is not guided into the cooled return channels (20, 21), is deflected to return into the fluidization chamber (4) by way of the uncooled return channel (17).

22. A circulating bed reactor (1) as set forth in any of claims 13-21, characterized in that the gas, which is conveyed to the actuators (55, 56) in lower parts of the cooled return channels (20, 21), is deflected, as it comes out of the actuators (55, 56), into the cooled return channels (20, 21) for flushing the fluidized material (2).

23. A circulating bed reactor (1) as set forth in any of claims 13-22, characterized in that the transfer of heat from the uncooled return channel (17) and from the riser channel (7) to the cooled return channels (20, 21) is essentially disabled.

24. A circulating bed reactor (1) as set forth in any of claims 13-23, characterized in that the return channels (17, 20, 21) are arranged in at least two rings around the riser channel (7), the uncooled return channel (17) being located in an inner position.