EP0995067A1

EP0995067A1 - Method and installation for producing heat, in a reactor with fluidised bed, by combustion of fuels with low calorific value

Info

Publication number: EP0995067A1
Application number: EP98935090A
Authority: EP
Inventors: Paul Ruble; Jean De Bloois
Original assignee: BLOOIS JEAN DE
Priority date: 1997-07-04
Filing date: 1998-07-03
Publication date: 2000-04-26
Also published as: FR2765670A1; FR2765670B1; WO1999001699A1

Abstract

The invention concerns a method for producing heat, in a reactor (1) with fluidised bed (7), by combustion of fuels with low calorific value, whereby a fuel with low degree of moisture, and therefore with low calorific value and a secondary fluid are simultaneously introduced in the reactor. The initial combustion conditions are adjusted to ensure autothermal combustion, i.e. in conditions of combustion temperature and retention time of the combustion gases at this temperature observing current regulations, for a reference fuel which is the fuel having the highest degree of moisture that can be used. As secondary fluid, an endothermal fluid is used and the respective flow rates (Q1, Q2) of the exothermal fuel and the secondary endothermal fluid are then adjusted so as to vary their proportions to maintain the combustion temperature constant and the oxygen content in the fumes as low as possible.

Description

METHOD AND INSTALLATION FOR THE PRODUCTION OF HEAT IN A FLUIDIZED BED REACTOR BY COMBUSTION OF FUELS

LOW HEAT POWER

The present invention relates to a method and an installation for producing heat, in a fluidized bed reactor, by combustion of fuels with low calorific value such as various common combustible waste and fuels from biomass. The evolution of the regulations on the limitation of solid discharges and gaseous pollutants, on the one hand, and the economic interest generated by the replacement of noble fuels, such as coal, fuel oil, gas, by fuels of recovery for energy production, on the other hand, have led to an interest in the recovery of various combustible waste with low calorific value. Among these combustible wastes are ordinary combustible industrial wastes such as cardboard, paper, unsullied plastic, etc., and fuels from biomass such as unsulled wood chips, fruit pods, almonds, woody fibers, straw, bagasse, etc. Fuels from biomass have very interesting properties because of their low or even possibly zero cost of production and their renewable nature. The use of biomass fuels for the production of heat energy is often limited by the heterogeneity of their appearance and the variability of their humidity which affect the performance of the generators of heat with regard to efficiency and power, as well as polluting discharges, and by the dispersion of the sources of these fuels, the cost of their transport and their inter-seasonal storage which make them uncompetitive compared to conventional energy sources such as coal, fuel oil or gas.

Methods and installations for producing heat in a fluidized bed reactor are already known.

Patent JP-53148165A discloses an incineration process in which, to increase the efficiency of incineration, the oxygen content of the exhaust gases is detected in order to control the supply of solid waste to the reactor and the fluidized bed and the temperature is detected in the reactor or in the fluidized bed in order to control the quantity of a coolant dispersed under the control of the reactor.

US-A-3,818,846 discloses a method and apparatus for the combustion of liquid wastes with high water content, such as sludge from treatment plants, using solid, liquid or gaseous waste as combustible. The solid waste used as fuel maintains the temperature of the fluidized bed and the flow rate of this solid fuel is regulated as a function of the oxygen content of the fumes. Patent JP-A-06-326510 discloses a combustion process in a fluidized bed reactor in which the temperature of the fluidized bed is maintained within a range of temperature specified by recycling a greater or lesser quantity of the fumes.

The object of the present invention is to provide a method and an installation for producing heat which make it possible to overcome variations in the humidity of a fuel with low calorific value, and to significantly reduce the cost of access to energy. by the combustion of low-cost, zero or even negative plant or industrial waste.

To this end, this process for producing heat, in a fluidized bed reactor, by combustion of fuels with low calorific value, such as ordinary combustible waste and fuels from biomass, into which is introduced into the reactor at the same time a fuel with a moisture content and therefore with variable calorific value and a make-up fluid, is characterized in that the initial combustion conditions are adjusted so as to ensure autothermal combustion, that is to say under conditions of combustion temperature and residence time of the combustion gases at this temperature respecting the regulations in force, for a reference fuel which is the fuel with the highest moisture content that can be used, the fuel used at any time having, because its humidity is lower than that of the reference fuel, a variable calorific value higher than that corresponding to autothermal combustion and therefore being exothermic, an endothermic fluid, that is to say which tends to lower the combustion temperature, is used as make-up fluid, and then it is adjusted the respective flow rates of the exothermic fuel and the endothermic make-up fluid so as to vary their proportions in order to keep the combustion temperature constant with the lowest possible oxygen content in the flue gases.

The subject of the invention is also an installation for producing heat by combustion of fuels with low calorific value, comprising a fluidized bed reactor formed above a fluidization grid, a circuit for supplying fluidization air supplied by a fan, allowing a correct distribution of this air under the fluidization grid and passing through this grid from bottom to top for the formation of the fluidized bed, a device for supplying solid fuel of suitable particle size, opening into the reactor above the fluidization grid, a regulator controlling the fuel flow, a sensor detecting the oxygen content of the fumes produced, a device for injecting a make-up fluid into the fluidized bed, this device comprising a pipe connected to at least a source of make-up fluid and opening into the reactor at the fluidization grid or into the fluidized bed, a regul generator controlling the flow rate of the make-up fluid and a temperature sensor detecting the temperature of the fluidized bed, characterized in that it comprises an automatic control device with two inputs, connected respectively to the sensor detecting the oxygen content of the flue gases and to the sensor detecting the temperature of the fluidized bed, and with two outputs, respectively connected to the regulator controlling the fuel flow and to the regulator controlling the flow of the make-up fluid, so as to make the relative proportion of the flows of the fuel and the make-up fluid to maintain constant the temperature of the fluidized bed, the make-up fluid being endothermic, c that is to say, tending to lower the combustion temperature.

The endothermic make-up fluid injected into the fluidized bed can be non-combustible, such as water or liquid effluent resulting from the treatment by washing of the fumes produced, or else a liquid fuel with low calorific value, such as, for example , sludge from a treatment plant. If the liquid effluent resulting from washing is low in heat, the energy it contains is recovered by its combustion in the hearth. In the two preceding cases, the suspended or dissolved compounds are partially trapped by the solids constituting the bed and evacuated with clinkers.

The method and the installation according to the present invention have several advantages. First of all, the process is independent of the humidity level of the fuel, therefore of the season of use and the surrounding conditions. Furthermore, the heat flux supplied by the fluidized bed reactor is constant and the efficiency of the installation is constant whatever the quality of the fuel introduced into the reactor. The fluidization speed and therefore the fluidization air flow and the excess air are also constant, as is the residence time of the fumes at the temperature fixed by the regulations before their release to the atmosphere. Regarding energy recovery, the exchangers in the reactor work optimally regardless of the quality of the fuel, because they are swept by a constant heat flow. The response to variations in the quality of the fuel used is rapid because the adjustment of the make-up fluid flow rate is easy and a variation of this flow rate causes an immediate change in the temperature of the hearth. Finally, the regulation of the operation of the entire installation is particularly easy.

Various embodiments of the present invention will be described below, by way of non-limiting examples, with reference to the appended drawings in which: FIG. 1 is a diagram of a heat production installation implementing the process according to the present invention.

Figure 2 is a partial vertical sectional view of an embodiment of the fluidization grid of the fluidized bed reactor of the installation.

Figure 3 is a vertical sectional view of a nozzle for distributing the fluidizing air and the make-up fluid forming part of the fluidizing grid.

FIG. 4 is a view in horizontal section taken along line IV-IV of FIG. 3.

Figure 5 is a vertical sectional view of an alternative embodiment of a fluidizing air distribution nozzle and introduction of makeup fluid. The heat production installation which is represented in FIG. 1 comprises a fluidized bed reactor

1 in the lower part of which extends a horizontal fluidization grid 2 defining below it a compartment 3 for inlet of the fluidization air. This fluidizing air is introduced into compartment 3 by a pipe 4 connected to a fan 5 which also supplies, by a pipe 6, air in the combustion chamber of the reactor 1 located above the fluidization grid. 2 and in which there are heat exchangers not shown. This grid supports, in the conventional manner, a fluidized bed 7 which consists of solid particles, for example of sand, which are lifted from the grid

2 during the passage of the fluidizing air upwards through the openings of the grid 2.

The installation also includes a device 8 for supplying fuel with low calorific value, this fuel can be constituted by ordinary industrial waste or even by fuel coming from biomass. The fuel used is naturally in the form of particles or grains of suitable size to be able to integrate easily into the fluidized bed 7 and be burned there. The flow rate Ql of the fuel supplied to the reactor 1 is determined by a regulator 9 which acts on the fuel supply device 8 and which is connected to a first output of an automatic control device 10 itself connected, to a first input, to a sensor 11 placed in the part upper part of reactor 1 and emitting a signal corresponding to the oxygen content of the fumes.

The installation according to the present invention is also provided with means which make it possible to inject into the hearth of the reactor 1, that is to say into the region of the fluidized bed 7, an endothermic make-up fluid to lower the temperature of the fluidized bed 7. This endothermic make-up fluid is supplied by a pipe 12 which opens into the reactor 1, at the level of the fluidization grid 2 or directly into the fluidized bed 7, by means of a valve 13 controlled by a regulator 14 controlling the flow of make-up fluid. This regulator 14 is connected to a second output of the automatic control device 10 to a second input of which is connected a temperature sensor 15 housed in the fluidized bed 7 and emitting a signal representing the temperature of this bed.

In Figure 1 are shown several sources capable of supplying the endothermic make-up fluid. This make-up fluid can either be water supplied by a source 16, or a liquid fuel with very low calorific value, such as, for example, a sludge from a purification station, supplied by a source 17. The fluid make-up can also be constituted by the liquid effluent resulting from the treatment of the smoke by washing. In Figure 1 is shown schematically an apparatus 19 for washing the fumes at the bottom of which is collected the washing effluent 21 which can be directed either to the line 12 for supplying make-up fluid either to another treatment or recovery facility.

The suspended or dissolved compounds found in the endothermic liquid fuel with low calorific value, such as a sludge from a purification station, supplied by the source 17 or in the washing effluent 21, are trapped by the solid particles constituting the fluidized bed 7 and evacuated with bottom ash.

The method and the installation according to the present invention take into account variations in the quality of the fuel introduced into the reactor 1 and they ensure instantaneous self-regulation of the operation. Indeed, the fuels with low calorific value that are used, namely ordinary combustible industrial waste and fuels from biomass, have a very variable humidity rate and such a variation in the humidity rate leads to a disturbance in performance installation in terms of energy efficiency and gas residence time in the home. For example, an increase in the humidity of the fuel results in a drop in the temperature of the hearth, hence the need for additional fuel and if the temperature cannot be maintained at its set value, the time gas stay at the set temperature is reduced hence also the need to use an additional fuel to meet this residence time. To solve this problem, it is planned, according to the present invention, to regulate the initial conditions of combustion, in the reactor 1, so as to ensure autothermal combustion, that is to say self-sustaining, at a predetermined set temperature, and during a residence time or holding the solid fuel and gaseous effluents at this temperature which respect the regulations in force, for a reference fuel which is the wettest fuel to be used. If the humidity of the fuel varies, that is to say to decrease compared to that of the reference fuel, tending to cause an increase in the combustion temperature, the automatic control device 10 intervenes on regulators 9 and 14 to vary the relative proportion of the flow rates Ql of the fuel and Q2 of the make-up fluid and to keep the temperature of the fluidized bed 7 constant. More particularly, if the humidity rate decreases relative to that of the reference fuel, the oxygen content of the fumes tends to decrease and this decrease is detected by the sensor 11 of the oxygen content. This sensor signals this reduction to the control device 10 which acts on the regulator 9 in order to then reduce the flow rate Ql of fuel in order to keep the oxygen content of the fumes constant. At the same time, the temperature of the fluidized bed 7 tends to increase. This increase in temperature is detected by the sensor 15 and is transmitted to the control device 10 which acts on the regulator 14 to increase the flow rate Q2 of the endothermic make-up fluid introduced into the fluidized bed 7, in order to lower the temperature. of this bed to keep it constant. The installation according to the present invention may also include, as is known in the prior art, heat exchangers which are more or less immersed in the fluidized bed 7 or with variable immersion in this bed, by varying the height of the fluidized bed or by a mechanical system acting on the position of the exchangers in the fluidized bed, in order to regulate the temperature of the fluidized bed 7.

A more detailed description will now be given, with reference to FIGS. 2 to 5, of various embodiments of the fluidization grid 2 adapted for implementing the method according to the present invention.

The combustion technique in dense fluidized bed 7 in particular presupposes a good distribution of the air under the grate in order to put the whole mass of the solid particles constituting the bed 7 in suspension in the air. The materials constituting the fluidization grid 2 must be able to withstand the high temperature prevailing in the hearth. These materials are generally either refractory compounds (shaped parts or refractory concrete, refractory steel, etc.) if the grid is not cooled, or more common steels if the grid is cooled. In the latter case, this assumes that the fuel characteristics vary relatively little around an average value, which is rarely the case when burning fuels from biomass. For these latter fuels, it is important to dimension the fluidization grid 2 for a composition of the biomass corresponding to the characteristics most often encountered. When the calorific value of the fuel coming from the biomass comes to vary suddenly, it is necessary, as the case may be, to inject an additional fuel, in the case of a wetter and / or more ashy biomass, or to tolerate an excess of more air or fuel spraying, in the case of drier and / or less ashy biomass. This presupposes independent fluid supply circuits associated with a complex regulation system. The fluidization grid shown in Figures 2 to 5 overcomes these constraints.

As can be seen in FIG. 2, the fluidization grid 2 consists of a set of main air distribution nozzles 22 each comprising a vertical duct 23 delimited by a vertical side wall 24, cylindrical or prismatic, capped at its upper end by a horizontal cap 25 which is distant from the upper end of the wall 24 by delimiting between them radial orifices 26. The fluidizing air coming from the lower compartment 3 flows vertically upwards in each conduit 23 , it is deflected horizontally and radially by each horizontal cap 25 and it again flows vertically upwards, in each interval 27 between the neighboring caps 25, to enter the fluidized bed 7. The main air distribution nozzles 22 are distributed so as to allow a homogeneous fluidization of the entire bed 7 over the entire surface of the hearth. The main nozzles 22 are associated with secondary injectors 28 which extend vertically in each conduit 23, along its side wall 24, and these secondary injectors 28 are connected, via the valve 13 and the pipe 12, to any one of the sources of make-up fluid shown on Figure 1.

In the embodiment shown in Figures 2 to 4, the injection port 28a of each secondary injector

28 is located at the top end of the duct

23 for the passage of the fluidizing air and the internal end of a radial orifice 26 opening into the fluidized bed

7. The make-up fluid leaving the orifice 28a, located at the upper end of each secondary injector 28, is thus entrained in the fluidized bed 7 by the stream of horizontal fluidizing air. In the case of the use, as make-up fluid, of a liquid fluid or of a mud suspended in water, the main distribution nozzle 22 described above has the advantage of not requiring a special sprayer. Indeed, the speed of the fluidizing air flowing opposite the outlet orifice 28a of each secondary injector 28 is sufficient to cause a

"bursting" of the make-up fluid and entrainment of the droplets formed.

According to a variant, each main distribution nozzle 22 can be equipped with several secondary injectors 28 which are respectively connected to circuits for supplying fluids with different additions, which makes it possible to inject different fluids into the fluidizing air. 'extra as needed. According to another variant, each main distribution nozzle 22 can have only one secondary injector

28 connected to a make-up fluid supply circuit equipped with a regulation device making it possible to switch from one make-up fluid to another according to the development of the quality of the fuel coming from the biomass used.

When stopping the installation or during a thermostatic shutdown or even with restarting the installation, it is preferably to sweep the make-up fluid circuit terminating at the secondary injectors 28 with air in order to avoid any risk of incident due to an accumulation of gas in the fireplace.

In the variation of execution shown in the figure

5, each secondary injector 28 is closed at its upper end and the orifice 28a for injecting the make-up fluid is provided in the lower part of the secondary injector 28 so that the make-up fluid is injected into the zone where the fluidizing air enters the central duct 23.

The fluidization grid 2, of which non-limiting embodiments have been described previously with reference to FIGS. 2 to 5, has several advantages. It is capable of burning various and varied fuels, alone or in mixture; at start-up, it provides thermal support for warming up the fluidized bed, by bringing the mass of inert particles constituting the bed, from the self-ignition temperature of the make-up fuel to the temperature so that the power of the starter burner is reduced and, when stopped, it allows to complete the combustion of the organic parts which are in the fluidized bed 7 and thus avoiding the risks of solidification of the bed and the incomplete and uncontrolled combustions which can cause risks of explosion when the installation is restarted.

Claims

1.- A process for producing heat, in a fluidized bed reactor (1) (7), by combustion of fuels with low calorific value, such as ordinary combustible waste and fuels from biomass, into which is introduced into the reactor both a fuel with a humidity rate and therefore with variable calorific value and a make-up fluid, characterized in that the initial combustion conditions are adjusted so as to ensure an autothermal combustion, it is that is to say under conditions of combustion temperature and residence time of the combustion gases at this temperature complying with the regulations in force, for a reference fuel which is the fuel with the highest moisture content that can be used , the fuel used at all times having, because its humidity is lower than that of the reference fuel, a higher variable calorific value ur to that corresponding to autothermal combustion and therefore being exothermic, an endothermic fluid, that is to say which tends to lower the combustion temperature, is used as make-up fluid, and then the respective flow rates of the exothermic fuel and of the endothermic make-up fluid so as to vary their proportions in order to keep the combustion temperature constant with the lowest possible oxygen content in the flue gases.

2.- Method according to claim 1 characterized in that one injects, as endothermic make-up fluid, a non-combustible fluid such as water.

3.- Method according to claim 1 characterized in that one injects, as endothermic make-up fluid, a weakly combustible liquid effluent, a fuel with very low calorific value such as a sludge from a purification station or else a liquid effluent resulting from the treatment by washing of the fumes produced.

4.- Method according to any one of the claims

1 to 3 characterized in that the fluidized bed is injected

(7), an endothermic make-up fluid selected from several make-up fluids of different natures.

5.- Installation for producing heat by combustion of fuels with low calorific value, comprising a reactor (1) with a fluidized bed (7) formed above a fluidization grid (2), a circuit (4,6) supply of fluidizing air supplied by a fan

(5), allowing a correct distribution of this air under the fluidization grid (2) and passing through this grid from bottom to top for the formation of the fluidized bed (7), a device (8) for feeding solid fuel with particle size adapted, opening into the reactor (1) above the fluidization grid (2), a regulator (9) controlling the fuel flow (Ql), a sensor (11) detecting the oxygen content of the fumes produced, a device for injecting a make-up fluid into the fluidized bed (7), this device comprising a pipe (12) connected to at least one source (16,17,19) of make-up fluid and emerging in the reactor (1) at the level of the fluidization grid (2) or in the fluidized bed (7), a regulator (14) controlling the flow of the fluid d 'top-up (Q2) and a temperature sensor (15) detecting the temperature of the fluidized bed (7), characterized in that it comprises an automatic control device (10) with two inputs, connected respectively to the sensor (11) detecting the oxygen content of the fumes and the sensor (15) detecting the temperature of the fluidized bed (7), and two outputs, respectively connected to the regulator (9) controlling the fuel flow (Ql) and to the regulator (14) controlling the flow rate of make-up fluid (Q2), so as to make the relative proportion of the flow rates (Ql) of fuel and (Q2) of make-up fluid to maintain constant the temperature of the fluidized bed (7), make-up fluid being endothermic, that is to say tending to lower the combustion temperature.

6.- Installation according to claim 5 wherein the fluidization grid (2) comprises a plurality of main air distribution nozzles (22), extending vertically, each comprising a central duct (23) traversed by an ascending current fluidizing air which is then diverted horizontally before entering the fluidized bed (7), characterized in that each main air distribution nozzle (22) comprises at least one secondary injector (28) connected to a source of make-up fluid (16-19) for injecting make-up fluid into the fluidizing air stream before it enters the fluidized bed (7).

7.- Installation according to claim 6 characterized in that the secondary injector (28) has an injection orifice (28a) which is located at the upper end of the injector (28) where the current vertical fluidizing air is deflected horizontally before entering the fluidized bed (7).

8.- Installation according to claim 6 characterized in that the secondary injector (28) has an injection orifice (28a) which is located on the side wall of the vertical secondary injector (28) and which is directed towards the center of the main nozzle (22) so as to inject the make-up fluid perpendicular to the vertical fluidizing air stream.

9.- Installation according to any one of claims 6 to 8 characterized in that each main air distribution nozzle (22) comprises several secondary injectors (28) which are respectively connected to the different sources of makeup fluid (16 -19) in order to be able to selectively inject, into the fluidizing air, a makeup fluid from among several makeup fluids of different natures.