NL9301828A - Process and apparatus for burning solid fuel - Google Patents

Abstract

Described is a process for burning solid fuel, in which nitrogen in the form of NH3 is released from said fuel, for example by gasification, said NH3 being excluded from the combustion process but being admixed, together with CO likewise released, to the gases released in the combustion process proper. This ensures, by relatively simple means, that the emission of noxious substances such as SO2, NOx and N2O can be reduced to a minimum. Also described is an apparatus 1, 2 for carrying out the process. In a simple and compact embodiment, the apparatus can consist of a main combustor 21 and a gasifier 11 linked thereto. <IMAGE>

Description

Title: Method and device for burning solid fuel

The invention relates to a method and a compact device for burning solid fuel.

Solid fuel, such as coal, peat, petroleum coke and biomass, generally contains sulfur and nitrogen compounds. When this fuel is burned, for example to produce thermal energy or electricity, harmful substances such as SO2, N0X (a collective term for NO and NO2), N2O, etc. can therefore be released. N0X can also be formed when burned at a high temperature. The said harmful substances will be released as pollution into the environment.

The main combustion products of sulfur-containing substances are SO2 and SO3 (referred to as S0X). The main combustion products of nitrogen-containing substances are N0X and N2O, while molecular nitrogen (N2) can be released when the fuel contains nitrogen gas. All the substances mentioned, with the exception of the latter, are harmful and therefore unacceptable for the environment.

For environmental and health reasons, it is therefore desirable to minimize the emission of said harmful substances, and measures have already been proposed in the art for this purpose, such as treating the exhaust gases or modifications to the combustion process.

For example, it is known to allow the combustion of a solid fuel to take place in a fluidized bed combustor (FBC) in order to reduce the amount of impurities released during combustion such as SO2, NOX and the like. Due to the low combustion temperature (typically on the order of 800 - 900 ° C), the formation of NOx due to thermal oxidation of nitrogen present in the combustion air is negligible, while the high loading of the solid phase in the incinerator reduces the amount of NOx formed from nitrogen-containing fuels.

However, it has been found that in an FBC the N20 emission is higher than in another combustion mode due to the lower combustion temperature.

It is known to use a so-called "staged combustion" for a further reduction of the NOx emission in an FBC. In this technique, combustion takes place in two steps. In the first step, a sub-stoichiometric amount of air is supplied to the bottom of the bed, so that reducing conditions are established whereby the NOx production can be largely suppressed. In the second step, additional air is supplied some distance from the bottom of the bed to set oxidizing conditions where the remaining fuel can be completely burned, while the N2 formed in the first stage will not be oxidized again to NOx thanks to the low combustion temperature.

With regard to sulfur, it is common to add limestone or dolomite as a sulfur scavenger to the fuel to retain sulfur in situ. Under normal combustion conditions, sulfur capture takes place according to the following reaction equations:

calcination: (1) sulfur capture: (2) (3)

In the above formulas, (s) denotes the solid phase and (g) denotes the gas phase.

In the following, for the sake of simplicity, the term "coal" will be used to denote any suitable solid fuel, such as, for example, coal, peat, petroleum coke, and biomass. Likewise, for the sake of simplicity, the term "oxidizer" will be used to designate any suitable oxidizing gas, such as, for example, steam, air, oxygen, or a mixture thereof. Furthermore, for the sake of simplicity, the term "sorbent" will be used to designate any suitable sulfur scavenger.

A general object of the present invention is to achieve a further reduction in the amount of harmful substances in the exhaust gases from the combustion process. A problem here is that the emissions of SO2 and N0X are related. First, it has been found that the introduction of sulfur sorbents has a negative impact on NOx emissions. After adding cabbage to the bed, volatiles will be released quickly. These volatiles contain nitrogenous intermediates such as NH3 and HCN, which are known precursors for NO and N2O. The sorbents are active catalysts for converting these volatile nitrogenous substances to NO.

The present invention therefore aims in particular to provide a combustion process in which the sulfur capture does not adversely affect the reduction of NOx emissions.

In order to further reduce the ΝΟχ-emission from an FBC, it has already been proposed to inject NH3 and / or urea into the top part of the bed or cyclone of an FBC. A drawback of this is, however, that extra NH3 and / or urea is required as raw material, and that the necessary equipment must be provided.

It is therefore particularly an object of the present invention to provide a combustion process whose exhaust gases contain a significantly reduced amount of NOx. In addition, the invention aims to achieve this object with relatively simple means, in particular without adding additional substances. This object is achieved according to the invention by making useful use of NH3 generated in the combustion bed.

Second, in the known stepped combustion, the sorbent particles experience alternating oxidizing and reducing conditions. Under oxidizing conditions, the sulfur capture will take place according to the reaction equations (2) and (3). Under reducing conditions, on the other hand, the sulfur-containing substances in the carbon will produce H2S or COS, and the sulfur capture will take place according to the reaction equations

(4) (5)

Sulfur capture per se is relatively efficient according to the said reactions. However, when the reaction conditions change, more reactions take place. For example, when the sorbent particles move from oxidizing to reducing conditions, (semi-) sulfated sorbent can be decomposed according to the reaction equation

(6)

When the sorbent particles move from reducing to oxidizing conditions, the following may occur

reactions ηη1 · τρΗρη? (7) (8)

Sulfur capture is therefore less efficient under varying reaction conditions. The known method of staged combustion has therefore suffered from the disadvantage of a less efficient sulfur capture, which is particularly the case at relatively high temperatures (higher than 850 ° C) because at a high temperature the reactions (6) and (7) predominate.

It is therefore a particular object of the present invention to provide a combustion process in which the emissions of NOx and N2O are reduced without the efficiency of the sulfur capture being reduced. More specifically, it is an object of the present invention to use a sulfur sorbent in such a way that it does not have a negative but rather a positive influence on the NOx emission.

The present invention is based on the insight that it is possible to convert at least part of the amount of nitrogen present in the solid fuel into NH3, and that it is furthermore possible to usefully use this NH3 formed from the fuel itself for a reduction of the amount of harmful substances, especially NOx, after the subsequent combustion of the fuel.

According to the invention, NH3 is therefore released in situ from the fuel in a first step, preferably by at least partly gasifying the fuel; in a second step, the remaining solids are burned, preferably under oxidizing conditions, in a combustion process which may be a conventional combustion process; and in a third step the NH3 released in the first step is reacted with the gas products released in the second step.

The release of NH3 in the first step can be done, for example, by heating under strongly reducing conditions, such as in a gasifier. More than half of the nitrogen released from the carbon will be provided in the form of N2, while the other nitrogen-containing components will be mainly HCN and NH3; only very little N0X will be formed. The method according to the invention offers a particular advantage in the case where a sorbent such as limestone is added to the fuel for capturing sulfur released from the fuel, more particularly when the sorbent is supplied in the first step, because CaO is an effective is a catalyst for the conversion of HCN to NH3 under reducing conditions. According to the invention, the sorbent then not only acts as a sulfur scavenger but also to promote the formation of NH3.

Since, prior to the actual combustion, nitrogen-containing components are mainly removed from the fuel in the form of N2 and NH3, and these volatile nitrogen-containing components are not fed into the actual combustion process, this already achieves a reduction in the amount of nitrogen that can potentially contribute to the formation of N0X. By reacting the NH3 released from the fuel with the gases released during the combustion process, a further reduction of the amount of NOx formed, if any, is achieved. By using the sorbent in the manner described above, this sorbent no longer has a negative influence on the NOx emission, but rather a positive influence.

Since the combustion of said residual solids in the process of the invention can be carried out under oxidizing conditions in an atmosphere with a reduced amount of nitrogen-containing components, the sulfur-trapping action of the sorbent will be improved, resulting in the combustion gases contain a reduced amount of sulfur components.

In addition, COS and H2S are also formed in the first step, which, according to reaction equations (4) and (5), is captured by CaO to form CaS. It should be noted here that the efficiency of CaO for the capture of COS and H2S is greater than for the capture of SO2. This CaS is converted into CaSO 4, CaO and SO 2 in the second step of the process according to the invention according to reaction equations (7) and (8), which SO 2 is trapped and reacted according to reaction equation (2) with the remaining residual solids (char). in CaSO 4. Thus, the sulfur components present in the coal have been converted into the normal end product of the sulfur catch in a very efficient manner.

In a conventional combustion process, fresh sorbent is introduced into the combustion bed along with the coal. A problem here, however, is that CaO has a catalytic effect on the conversion of nitrogen components from the carbon to NOX. In the process according to the invention, on the other hand, in the first step at least a part of the sorbent is reacted to CaS, which, as already mentioned, is converted at least partly in CaSO 4 / CaS in the second step and CaSO 4 do not have the said catalytic influence on the conversion of nitrogen components from the coal to N0X, so that this too contributes to the reduction of the formation and emissions of N0X.

The CaO which is fed to the actual combustion process with the aforementioned residual solids is utilized according to the invention, because CaO has a strong catalytic influence on the conversion of N2O to N2 and 02 / which are of course "clean" products. .

An additional advantage of the invention is that CO is formed in the first step, which in the third step has a stimulating effect on the reduction of NO by NH3. In an incinerator, according to the invention, the introduction of NH3 and CO at the top of the bed creates an effective post-combustion zone for reducing NO.

Further aspects, features and advantages of the invention will be further elucidated by the following description of preferred embodiments of the method and device according to the invention, with reference to the drawing. Herein: figure 1 schematically shows a first preferred embodiment of a combustion device according to the invention; and figure 2 schematically a second preferred embodiment of a combustion device according to the invention.

In Figure 2, the same or comparable parts as in Figure 1 are indicated with the same reference numerals.

The combustion device generally referred to by reference numeral 1 in Figure 1 comprises a relatively small gasifier 11 and a relatively large main burner 21.

The gasifier 11 is provided near its underside with an inlet 12 for supplying the coal to be burned and the sorbent, a gas inlet 13 for supplying air, and a first outlet 14 for discharging solid particles to the main burner 21 Furthermore, the gasifier 11 is provided near its top side with a second outlet 15 for discharging the gases formed.

The main burner 21 is provided near its bottom side with a first inlet 24 for receiving the solid particles from the first outlet 14 of the gasifier 11, and a primary air inlet 23 for supplying the primary oxidizer. Furthermore, at a higher position, in a low portion of the freeboard, the main burner 21 is provided with a second inlet 25 for receiving the gases formed in the gasifier 11, and at approximately the same height, or slightly higher position there is a secondary air inlet 22 for supplying secondary oxidizer. In the exemplary embodiment shown, the gasifier 11 and the main burner 21 are mounted against each other, so that the first outlet 14 of the gasifier 11 and the first inlet 24 of the main burner 21 are combined into a first passage opening, in which, if desired, a valve may be included. Likewise, the second outlet 15 of the gasifier 11 and the second inlet 25 of the main burner 21 are combined into a second passage opening.

Fresh cabbage, together with a sorbent known per se, such as limestone for capturing sulfur components, is supplied via the inlet 12 to the gasifier 11, which is preferably operated in a bubbling fluidized bed mode. Air is supplied to the bottom of the bed through the gas inlet 13 to fluidize the solid phase. In the gasifier 11, the carbon is pyrolysed and partially gasified. CO will be formed in this process. The further released H 2 S and COS will be trapped by the sorbent according to the reaction equations (4) and (5). The released HCN will be converted to NH3 under reducing, at least non-oxidizing, conditions, the presence of sorbent having a catalytic effect. The gaseous reaction products therefore mainly consist of CO, CO2 and NH3, which gases will go to the second outlet 15 of the gasifier 11,

Suitable parameters for operating the gasifier 11 are, for example: the air to be supplied at 13: 30-60% of the required stoichiometric amount of air for the coal supplied to the gasifier 11; temperature: 850-900 ° C; pressure: generally about 1 atm, but the gasifier 11 can also be operated at higher pressures, e.g. 10 atm;

The solids remaining after the gasification process, such as char and sulfided sorbent, are transported from the gasifier 11 via the first outlet 14 to the main burner 21.

The main burner 21, which is preferably operated in the bubbling fluidized bed combustor (BFBC), has three combustion stages or zones 26, 27 and 28. In the first combustion stage 26, which is located at the bottom of the main burner 21 and bounded by the bed surface 18, the said solids are burned under preferably oxidizing conditions, whereby NOx and CO2 will be generated. The (partially) sulfided sorbent experiences a transition from reducing conditions to oxidizing conditions at the transition from the gasifier 11 to the main burner 21, and will therefore react according to reaction equations (7) and (8). The SO2 released in the reaction (7) will again be trapped by CaO according to the reaction equation (2).

Therefore, virtually all of the sulfur components present in the coal are converted into CaSO 4, the normal end product of the sulfur catch.

According to the invention, the temperature in the first combustion stage 26 is kept relatively low, because it achieves a relatively high efficiency for the sulfur capture and a relatively low NOx production. Although this may increase the co-production somewhat, the N2O formed is broken down very efficiently in a later combustion step, as will be explained further below.

Suitable operating parameters for the first combustion stage 26 are, for example: the primary oxidizer to be fed at 23: about 1.1 times the stoichiometric amount of the oxidizer for the coal fed from the gasifier 11 to the main burner 21; temperature: 800-825 ° C; pressure: generally about 1 atm, but the main burner 21 can also be operated at higher pressures, e.g.

10 atm;

Above the bed surface 18 is the post-combustion zone or second combustion stage 27, which is formed by the introduction of the gas stream from the gasifier 11. Basically, the conditions are in that part of the second combustion stage 27 located below the second entrance 25 similar to the conditions in the first combustion stage 26, except that the solids concentration is lower. In the second combustion stage 27, the NOx formed in the first combustion stage 26 reacts with the NH 3 received from the gasifier 11 at the second inlet 25 to be further reduced, the presence of CO also formed in the first combustion stage 26 having a stimulating effect on that reduction so that the reduction will be more efficient. For example, already in the absence of CO, a reduction of about 50% in the amount of NO can be achieved when NH3 is introduced with a molar ratio NH3 / NO of about 3. This molar ratio can easily be realized when carrying out the method according to the invention turn into. In practice, a further reduction than the aforementioned 50% will be achieved because of the high CO concentration.

Above the second combustion stage 27 is the third combustion stage 28. Here, the reduction of NO with NH3 continues, and CO is further burned with the secondary oxidizer introduced via the secondary air inlet 22 without compromising sulfur capture and / or degradation of N2O affected. The gas flow from the gasifier 11 can contain about 20-40% of the calorific value of the coal, depending on the type of coal. Therefore, during the combustion of CO and other combustible materials, the temperature in the third combustion stage 28 will increase (e.g., by 50 ° C), which will significantly reduce the amount of N2O formed in the first combustion stage 26 as this higher temperature is beneficial for the breakdown of N2O. The amount of secondary oxidizer is such that the total stoichiometric oxidator ratio for burning the coal is about 1.1-1.2.

It is noted that in practice the second combustion stage 27 and the third combustion stage 28 can form a single, combined combustion stage.

The transport of the solid particles from the gasifier 11 to the main burner 21 can take place according to the principle of interconnected fluidized beds, in which two or more fluidized beds with different fluidization gas velocities are interconnected. The fluidization takes place in the gasifier 11 at a lower gas velocity than in the main burner 21, so that the bulk densities in the two beds are different. Thus, at equal bed heights, which is illustrated in Figure 1 by schematically indicating the bed surface 17 in the gasifier 11 and the bed surface 18 in the main burner 21, a pressure difference can be built up between the two beds, and the bottom of the beds a continuous flow of solid particles from the gasifier 11 through the first passage 14, 24 to the main burner 21. The flow rate of this flow can be controlled by adjusting the difference in the fluidization rates and by adjusting of the bed height in the gasifier 11.

In this way, the NH3 released from the coal can be fully utilized in situ to reduce the NOx amount, and the negative effect of the sorbents on NOx emission can be minimized and even turned into a positive effect. Sulfur capture efficiency can be kept at a high level, and NOx emissions can be reduced. All this is achieved according to the invention with a relatively simple and compact combustion system, in which no extra and expensive equipment is required for the injection of auxiliary substances such as NH3.

Thus, the combustion gases, which finally leave the main burner 21 at an outlet 29 to be supplied to a conventional heat transfer section, according to the invention are particularly low in harmful substances such as SC> 2f NOx, and N2O. The solid combustion products (ash and CaSC> 4) are continuously discharged through a discharge 30.

The embodiment variant generally referred to in Figure 2 by reference numeral 2 is of the "circulating fluidized bed combustor" (CFBC) type. A CFBC mainly comprises three functional parts: a riser section formed by the main burner 21, one or more separators 42 (shown as a cyclone in the figure for example), and a return circuit 40. The riser section 21 is operated at a high fluidization gas velocity, typically of the order of 4-8 m / s, whereby the solid particles will be rapidly captured by the cyclones 42. The trapped particles are returned to the ascending section 21 via the return chain 40.

Thus, a CFBC consists of a loop with continuously circulating particles. Normally, the combustion of the solid fuel takes place in the riser section 21.

In principle, a CFBC is more flexible in the use of the stepped combustion, and more flexible in the control of the recirculation speed of the solids. It therefore offers additional advantages to apply the present invention to a CFBC system.

The basic principle is the same as with the BFBC system. Normally, in the return section 40, there is a cooler for the hot particles in order to control the bed temperature or the burner load. This cooler operates in the bubbling fluidized bed mode, and can be used directly as a gasifier 11.

The main burner 21 has a secondary air inlet 32 located at a higher level than the first inlet 24 but at a slightly lower level than the second inlet 25, and a tertiary air inlet 33 located at a higher level than the second inlet 25. Furthermore, a main particle return circuit 40 is connected to the main burner 21. The return circuit 40 comprises a separator 42 for separating exhaust gases and solid particles entrained therein, which separator 42 in the example shown is a cyclone separator. Since the nature and construction of the separator 42 is not the subject of the present invention, and knowledge of it is not necessary for a person skilled in the art to understand the present invention, they will not be described further.

An inlet 41 of the cyclone separator 42 is coupled to the outlet 29 of the main burner 21 for receiving the exhaust gases from the main burner 21. The cyclone separator 42 has a first outlet 43 for discharging the exhaust gases after the solid particles have been discharged therefrom. have been removed. According to the invention, these exhaust gases contain only particularly low concentrations of SO2, NOx and N2O. Said solid particles can be returned to the main burner 21 via a second outlet 44, which is coupled to a third inlet 34 of the main burner 21, which third inlet 34 is located at a higher level than the tertiary air inlet 33.

The cyclone separator 42 further has a third outlet 45 which is coupled via a line 46 to the gasifier 11. A valve 47 is included in the line 46.

Fresh coal and sorbent is supplied through the inlet 12 to the gasifier 11. The gasifier 11 also, when the valve 47 is at least partially opened, continuously receives the solid particles trapped by the cyclone separator 42, which behave as heating media. The bed in the gasifier 11 can therefore be operated at an even lower oxidation ratio, typically on the order of 0.25-0.5. The valve 47 serves to control the amount of solid particles to be supplied to the gasifier 11, and to prevent the gas sealing of the gases produced in the gasifier 11 from reaching the cyclone 42 as a gas seal. The solid particles that are not supplied to the gasifier 11 are fed directly to the main burner 21 via the second outlet 44.

The remaining solids from the gasifier 11 are conveyed via the outlet 14 to the inlet 24 near the bottom of the main burner 21. The particle flow rate can optionally be controlled by means of an arrangement arranged between the outlet 14 and the inlet 24 of the main burner 21, not shown for the sake of simplicity, preferably non-mechanical valve (such as, for example, a V-shaped valve).

The operating parameters discussed above for the BFBC shown in Figure 1 are also suitable for the CFBC shown in Figure 2. Combustion preferably takes place under oxidizing conditions, and the same reactions will take place as with the BFBC.

The gas stream from the bed in the gasifier 11, the composition of which corresponds to that of the preferred embodiment described with reference to Figure 1, and which, depending on the type of fuel, can contain 20 to 40% of the calorific value of the fuel, is fed through the outlet 15 to a relatively high-lying particulate-lean zone of the main burner 21 to form a post-combustion zone. Since a CFBC is smaller than a BFBC of comparable capacity, this feeding can be done radially or in a swirl, depending on the size of the device, to improve mixing.

The main burner 21 is supplied with a secondary oxidizer and a tertiary oxidator through the secondary air inlet 32 and the tertiary air inlet 33, respectively, which are located near the height position of the inlet 25, in order to prevent reducing conditions from occurring, which would result that SO2 can be released from the sulphated sorbent particles. Just as with a BFBC, a temperature rise will also occur with a CFBC, which leads to very small N20 emissions.

The solids consumed in the plant (ash and sulfated sorbent) can be discharged through outlet 30.

By designing the gasifier and the main burner separately, the entire system has a compact construction.

It will be clear to a person skilled in the art that it is possible to change or modify the illustrated embodiments of the device according to the invention, without leaving the inventive idea or the scope of protection. By way of example, it is possible that the solid combustion products drain 30 has been replaced by a drain 30 'included in the return chain 40.

It is also possible, for example, to implement the invention with an existing main burner by coupling a gasifier to it.

Claims (17)

A method of burning solid fuel, wherein NH3 is released from the fuel in a first step; wherein in a second step the solids remaining in the first step are burned; and wherein in a third step the NH3 released in the first step reacts with the gas products released in the second step. Method according to claim 1, characterized in that the liberation of NH3 is effected by heating under reducing conditions, preferably in a fluidized bed. Method according to claim 1 or 2, characterized in that the liberated NH 3 is kept separate from the remaining solid components. A method according to at least one of the preceding claims, characterized in that a sorbent such as limestone is added to the fuel to trap sulfur released from the fuel, which sorbent is preferably chosen to promote the conversion in the first step of liberated nitrogenous substances such as HCN to NH3 and N2. Method according to at least one of the preceding claims, characterized in that in the second step said solid components are burned in a bubbling fluidized bed or a circulating fluidized bed. A method according to at least one of the preceding claims, characterized in that in the second step said residual solid components are burned under oxidizing conditions. Method according to at least one of the preceding claims, characterized in that in the second step the combustion takes place at a relatively low temperature in order to ensure that the (partially) sulphided sorbent particles oxidize directly to CaSO4 instead of releasing SO2, and the production of NOx during combustion is suppressed. Process according to at least one of the preceding claims, characterized in that the third step is fed with an oxidizer to complete the combustion of combustible substances. Method according to at least one of the preceding claims, characterized in that said residual solids are moved from the first to the second step on the basis of different bed densities. Solid fuel burning apparatus according to any of the preceding claims, comprising a gasifier (11); an inlet (12) of the gasifier (11) for receiving the solid fuel; a first outlet (14) from the gasifier (11) for discharging solids; a second outlet (15) from the gasifier (11) for discharging gases released from the solid fuel; a main burner (21); a first inlet (24) of the main burner (21), which is coupled to the first outlet (14) of the gasifier (11); and a second inlet (25) of the main burner (21), which is coupled to the second outlet (15) of the gasifier (11). Device according to claim 10, characterized in that the main burner (21) has at least two combustion zones for carrying out the second and third steps, respectively. Device according to claim 10 or 11, characterized in that the main burner (21) has a secondary air inlet (22), which is preferably positioned at about the same height, or slightly higher, than the second inlet (25). Device according to claim 10 or 11, characterized in that the main burner (21) has a secondary air inlet (32) located at a lower level than the second inlet (25), and a tertiary air inlet (33) located higher than the second input (25). Device according to claim 13, characterized in that the outlet (29) of the main burner (21) is coupled to an inlet (41) of a separator (42), and that an outlet (44) of the separator (42) ) is coupled to a third inlet (34) of the main burner (21), which third inlet (34) is at a higher level than the tertiary air inlet (33). Device according to claim 14, characterized in that a further outlet (45) of the separator (42) is coupled to the gasifier (11) via a conduit (46) in which a valve (47) is included. Device according to any one of claims 10-15, characterized in that the second inlet (25) and / or the secondary air inlet (22; 32) and / or the tertiary air inlet (33) are directed radially in order to radially introduce the gases introduce the main burner (21). Device according to any one of claims 10-15, characterized in that the second inlet (25) and / or the secondary air inlet (22; 32) and / or the tertiary air inlet (33) are arranged to form the gases on a swirling way into the main burner (21).