FR2823291A1 - Condensation heater using fuel from biomass has gas outlet from reactor linked directly to humidifiers - Google Patents

Abstract

The heater consists of a fluidised bed reactor (1), a source (7) of an exothermic fuel with a variable humidity level, and a movable and immersed air/water heat exchanger (5). The outlet of the combustion gases from the reactor is connected directly to humidifiers (20, 42), and has a first condensation stage able to transmit the heat from vaporisation to a fluid circulating in the primary circuit of a water/water heat exchanger (30). The latter's secondary circuit is connected in series with and upstream of the immersed air/water heat exchanger, and is fed by return water (R) to be reheated.

Description

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The present invention relates to an installation for producing high temperature hot water from the combustion of solid fuels and in particular of fuels from biomass.

 We know that the evolution of 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 recovery fuels 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 recoverable wastes are fuels from biomass or biofuel, such as unsullied wood shavings, fruit pods, almonds, wood fibers, straw, bagasse, etc. Biofuels have very attractive properties due to their low or even possibly zero production cost and their renewable nature. However, the use of these biofuels for the production of heat energy is often limited by the variability of their humidity which strongly depends not only on the level of their evolution but also on their period of collection, atmospheric conditions during this operation and storage conditions until their return to stock before their combustion. This large variation in humidity leads to tolerating significant excess air in the case of dry fuels or to injecting additional fuel

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 (gas, fuel oil or coal) in the case of very humid fuels, this in order to control combustion in the fireplace.

 The applicant has proposed to solve this problem, in patent FR-A-2 765 670, to size the combustion unit constituted by a fluidized bed reactor, for the wettest biofuel allowing autothermal combustion, that is to say that is to say maintaining the combustion temperature at a set value, and compensating for the variation in humidity of this biofuel by injecting, into the fluidized bed, a variable quantity of an auxiliary fluid endothermic, such as water or a fuel having a calorific value lower than that corresponding to the autothermicity of the combustion unit.

The endothermic make-up fluid can also consist of the liquid washing effluent collected at the bottom of a smoke washing apparatus leaving the fluidized bed reactor. The above installation allows the production of heat energy at a single temperature level.

 In a second patent FR-A-2 795 487 the applicant also proposed a heat production installation automatically taking into account variations in the humidity of the basic biofuel while guaranteeing both a constant total power supplied and a control of the distribution of this power between high temperature energy (high level energy) and lower temperature energy (low level energy), at the lowest installation cost.

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 Note, however, that the use of low temperature heat produced in such installations is often only possible in limited and very specific fields, such as in particular those of the wood industry and greenhouse crops. Furthermore, given the low level of temperatures obtained, the cost in comparison with that of the heating bodies then becomes prohibitive most of the time.

 The object of the present invention is to propose an installation of the aforementioned type, the construction of which is not only simplified compared with those of the prior art, but which, moreover, proves to be particularly advantageous in terms of energy balance.

 The subject of the present invention is therefore an installation for producing heat by combustion of fuels from biomass, of the type comprising a fluidized bed reactor, an exothermic fuel source with variable humidity, an immersed air heat exchanger circuit. / water capable of occupying different positions in the reactor hearth depending on the amount of heat to be removed by this exchanger, characterized in that: - the upper part for discharging the combustion gases from the reactor is in direct communication with suitable means to humidify these gases and transform their sensible heat into heat of vaporization,

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départ étant délivrée en sortie de l'échangeur eau/eau. - it includes a first condensation stage capable of recovering this heat of vaporization and of transmitting it to a fluid circulating in the primary circuit of a water / water exchanger, - the secondary circuit of the water / water exchanger is arranged in series and upstream of the submerged air / water exchanger and is supplied by the return water to be heated, the water from

flow being delivered at the outlet of the water / water exchanger.

 The means capable of humidifying the combustion gases may consist of a washer or, in an alternative embodiment of the invention, by a dryer, in particular a rotary wood dryer.

 The installation may include a second condensation stage and means capable of recovering the heat from it and transmitting it to the combustion air of the reactor. Preferably, the means for reheating the combustion air will be coupled with means for humidifying it.

 In one application of the invention, in particular an application operating as an incinerator, the installation will include means capable of short-circuiting the passage of water in the primary circuit of the water / water exchanger.

 Preferably, the secondary circuit of the water / water exchanger will include a variable flow pump and control means capable, by controlling this flow, of maintaining a constant temperature difference between the temperature of the return water which enters the circuit

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 secondary of the water / water exchanger and the temperature of the water (D) delivered.

 The present invention makes it possible to modulate, as required, the thermal efficiency of the installation between 0% (operation in incinerator) and 94% on PCS, ie the Higher Calorific Power (heating operation).

It thus makes it possible, with the same installation, to satisfy needs for supply of heating which vary over time and for the elimination of D.I.B (Common Industrial Waste) which are also variable.

 The present invention is also advantageous in that it implements a water / water exchanger, in contrast to the aforementioned devices of the prior art, which use air / water exchangers. However, water / water exchangers have a heat transfer ratio which is four to five times higher than that of air / water exchangers. The present invention thus makes it possible, with an equal heat transfer ratio, to lower the construction costs of this element in a particularly substantial manner.

 Furthermore, it is known that in fluidized bed reactors, the ashes produced during combustion are entrained with the combustion gases and have a fouling and abrasion effect on the exchanger with which they come into contact, with for consequences of requiring repeated maintenance and causing wear of this element. The present invention avoids this drawback insofar as the ashes are removed in the washer before they pass through the exchanger.

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Embodiments of the present invention will be described below, by way of non-limiting example, with reference to the appended drawing in which:
Figure 1 is a schematic representation of a fluidized bed heat production installation according to the invention.

 FIG. 2 is a schematic representation of an alternative embodiment of a fluidized bed heat production installation according to the invention, in which the installation supplies a dryer.

 The heat production installation which is shown in FIG. 1 comprises a fluidized bed reactor 1 of a type known from the prior art.

This reactor thus comprises at its lower part a grid 3 which supports a fluidized bed 5, consisting for example of solid particles, in particular of sand, which are lifted by an air flow which is pulsed from below the grid 3.

 The installation includes a fuel source 7 which feeds the hearth through a line 9, the flow rate of which is controlled by a solenoid valve II controlled by a regulator 13. This regulator is connected to a temperature sensor 16 which measures the temperature of the fluidized bed.

 The upper part of the reactor 1 comprises a submerged exchanger 15 which is mounted vertically movable inside the fluidized bed 5 under the action of control means 17 controlled by regulation means 19 in relation to the regulator 13.

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 The upper part of the reactor 1 is in direct communication with the upper part of a washer 20. This washer consists of a tubular element which has at its upper part a watering ramp 21 and at its lower part a tank 23 allowing to collect the irrigation water. The washer also includes means for filtering and removing ash not shown in the drawing. A pump 27 makes it possible to return this water, via a pipe 28, to the spraying booms 21, so as to reuse it to wash the fumes.

 The washer 20 is in communication, via a gas conduit 24, with a double condenser / heater 25. In known manner, the latter consists of a spraying ramp 31 which opens out above a bed of elements column packing solids 32 on which water flows, and which make it possible to increase the exchange surface, these packing elements being held by a grid 33 below which opens the conduit 24. Below this there is a tank 34 intended to receive the condensation water, which is connected by a pipe 29 to the inlet 30a of the primary circuit (or hot source) of a heat exchanger 30, the outlet 30b of which supplies the watering boom 31 .

 The return water R from the installation arrives at the inlet 30c of the secondary circuit (or cold source) of the exchanger 30 and, after having crossed it, leaves it by its outlet 30d to cross then the immersed exchanger 15 and return to D to the installation with warmed water.

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 The installation comprises a second stage of condensation which is coupled with a heater for the combustion supply air. This second stage thus consists, as before, of a spraying boom 36 which opens out above a bed of packing elements 38, held by a grid 40 below which is disposed a recuperator 42 connected to a pipe 44 which brings the recovered and heated water into a spray bar 46 disposed above a bed of packing elements 48 held by a grid 50. The water is recovered at the bottom of the condenser / heater 25, in a tank 52 from which it is brought back by a pipe 54, by means of a pump 56, to the watering boom 36.

 A fan 58 draws in the outside air A and pulses it under the grid 50, so that it passes through the bed of packing elements 48 where it heats up and humidifies and is brought, by a pipe 60, into the reactor 1 where it supplies the air to the fluidized bed 5 (combustion air).

 The operation of the present installation will now be described.

 As is known from the installation described in patent FR-A-2 795 487, the exchanger 15 is used to keep the temperature of the fluidized bed 5 constant. Thus, when an increase in temperature of the hearth is detected by the temperature sensor 16, the regulating means 19 act on the control means 17 to lower the immersed exchanger 15 into the fluidized bed 5 where it absorbs a greater quantity of heat, thereby balancing

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 the temperature of the hearth. Conversely, a drop in temperature of the hearth causes the exchanger 15 to rise.

 At the outlet of the reactor 1, the combustion gases penetrate into the washer 20 where they are subjected to the action of the water delivered by the ramp 21, under the action of which they cool by evaporating part of the water from washing, so that their temperature goes from about 850 C to the saturation temperature of the mixture formed by these gases and the evaporated water vapor. This saturation temperature is known to be inversely proportional to the excess combustion air. Thus, in the present embodiment of the invention, in the case of a dense fluidized bed, with an excess of air of 30% and a wood fuel at 17% on raw wood, this saturation temperature of the mixture is 85, 2. C.

 This heat of vaporization of the vapor produced in the washer 20 is supplied to the first stage of condensation. In this case, this heat is supplied to the primary circuit of the exchanger 30 via the water collected in the tank 34, the circulation being ensured by a pump 35. This thus ensures the heating of the return water R circulating in the secondary of the exchanger 30. According to the invention, this water is then brought into the submerged exchanger 15 which is arranged downstream of the latter and raises the temperature of the heat transfer fluid above the saturation temperature of the combustion gases. at the outlet of the washer 20.

 The second condensing stage, which works like the first, performs two functions, on the one hand

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 recovers additional heat which it supplies to the combustion air and on the other hand it humidifies it, so that its contributions of sensible heat and water vapor to this combustion air have the effect of rising the vapor content of the gases leaving the washer 20 as well as their saturation temperature which thus goes from 85.2 C to 88.6 C.

 With such an installation using a dense fluidized bed with an excess of air of 30% and a fuel consisting of wood at 17% humidity on gross weight, we were able to raise from 75 C to 95 C l return water R to be heated arriving in the installation (At = 20 C).

 For a given installation this At must be kept at a constant value. To do this, a circulation pump 64 will be used, the flow of which will be controlled as a function of the operating conditions of the installation, so as to maintain this value of At at a given constant value.

 Of course, in the case of biomass combustion, when the humidity thereof exceeds 50% of the total weight, the heating / humidification of the combustion air is gradually eliminated. The system will then produce low temperature heat, which in certain modes of use can prove to be interesting.

 There is shown in Figure 2 an alternative embodiment of the invention in which it is used to operate a drying installation.

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 In this application of the invention, the outlet of the reactor 1 is directly connected to the inlet 70 of a wood drier 72, so that the combustion gases pass through it before arriving in the washer 20. One realizes thus, according to the invention, the conversion of the sensible heat at the outlet of the reactor into heat of vaporization by drying a wet product.

 In another implementation variant of the present invention, the installation can be used by operating it in an incinerator.

 In this embodiment, it is desired to burn a fuel without recovering heat.

 To do this, the installation comprises a bypass 73 fitted with a valve 75 which makes it possible to short-circuit the primary circuit, or hot source, of the water / water exchanger 30. Under these conditions the water which enters and leaves the condenser 25 is at the same temperature. Furthermore, in order to remove from the system the heat input from the submerged exchanger 15, water is admitted into the fluidized bed 5. To do this, a valve 77 is opened which allows the fluidized bed 5 to be placed in communication with the pipe 28 which connects the tank 23 to the spraying boom 21. A vaporization is thus produced which lowers the temperature of the fluidized bed 5, so that the regulating means 19 control the ascent of the submerged exchanger 15.

Claims (1)

CLAIMS 1.-Installation for producing heat by combustion of fuels from biomass, of the type comprising a reactor (1) with a fluidized bed, an exothermic fuel source (7) with variable humidity, a heat exchanger circuit submerged air / water (15) capable of occupying, in the hearth of the reactor (1), different positions depending on the amount of heat to be removed by this exchanger, characterized in that: - the upper part for evacuating the combustion gases of the reactor (1) is in direct communication with means (20, 42) capable of ensuring humidification of these gases so as to transform their sensible heat into heat of vaporization, - it comprises a first stage of condensation capable of recovering this heat vaporization and to transmit it to a fluid circulating in the primary circuit of a water / water exchanger (30),

 - the secondary circuit of the water / water exchanger (30) is arranged in series and upstream of the submerged air / water exchanger (15) and is supplied by the return water (R) to be heated, the water flow (D) being delivered at the outlet of the water / water exchanger (15).  2. Installation according to claim 1, characterized in that the means capable of humidifying the combustion gases consist of a washer (20).  3. -Installation according to claim 1, characterized in that the means capable of humidifying the gases <Desc / Clms Page number 13>  combustion systems consist of a dryer (72), in particular a wood dryer.  4. Installation according to one of the preceding claims, characterized in that it comprises a second condensation stage and means capable of recovering the heat from it and transmitting it to the combustion air supplying the reactor (1 ).  5. Installation according to claim 4, characterized in that the means for reheating the combustion air are coupled with means for humidifying the latter.  6. -Installation according to one of the preceding claims, characterized in that it comprises means capable of short-circuiting the passage of water in the primary circuit of the water / water exchanger (30).  7. Installation according to one of the preceding claims, characterized in that the secondary circuit of the water / water exchanger (30) comprises a variable flow pump (64) and suitable control means, by controlling this flow. to maintain a constant temperature difference (At) between the temperature of the return water (R) which enters the secondary circuit of the water / water exchanger (30) and the temperature of the water (D) delivered .