WO2016020559A1

WO2016020559A1 - System for optimizing the combustion for pulverized solid fuel boilers and boiler incorporating such system

Info

Publication number: WO2016020559A1
Application number: PCT/ES2014/070647
Authority: WO
Inventors: Francisco RODRÍGUEZ BAREA; Enrique Tova Holgado; Luis CAÑADAS SERRANO; Miguel Ángel DELGADO LOZANO; Sixto LÓPEZ ASENCIO; Ignacio ARGÜELLES HERNÁNDEZ; Mariano Reyes Valle; Manuel ACOSTA CONTRERAS; Miguel Ángel PORTILLA DE LA CONCHA COBANO; Miguel MORALES RODRÍGUEZ
Original assignee: Inerco Ingeniería, Tecnología Y Consultoría, S.A.
Priority date: 2014-08-07
Filing date: 2014-08-07
Publication date: 2016-02-11

Abstract

The invention relates to a system for optimizing combustion for pulverized solid fuel boilers, comprising: groups of main burners (2D, 2C, 2B); main mills (3D, 3C, 3B) corresponding to the groups of main burners (2D, 2C, 2B); main transport lines (22) connecting the main mills (3D, 3C, 3B) with the groups of main burners (2D, 2C, 2B); an auxiliary mill (3A) in parallel with each group of main burners (2D, 2C, 2B); and an auxiliary transport line (23) comprising: an ascending section (16) connected to the auxiliary mill (3A); a descending section (18); a transitional section (17) connecting the ascending section (16) and the descending section (18); a distributor (4) at the end of the descending section (18); and descending distribution lines (19, 20, 21) from the distributor (4) to each corresponding main transport line (22). The invention provides differential fuel input patterns and improved solid fuel transport, with versatility and a simplified structure.

Description

COMBUSTION OPTIMIZATION SYSTEM FOR SPRAYED SOLID FUEL BOILERS. AND BOILER THAT INCORPORATES SUCH

SYSTEM

D E S C R I P C I Ó N

OBJECT OF THE INVENTION

The present invention can be included within the technological field of industrial boilers. In particular, the invention relates both to a solid fuel combustion system (for example coal or biomass) sprayed, and to a boiler incorporating said system. The objectives of this invention are both the optimization of the process for the reduction of nitrogen oxide emissions, as well as the optimization of the performance and the increase of the boiler's operating flexibility.

BACKGROUND OF THE INVENTION

Much of the development of recent years for the optimization of industrial boilers (for example, in power generation units) has focused on reducing emissions of polluting gases, among which nitrogen oxides (NO _x ), generated in the combustion of fossil fuels, such as coal, fuel oil or natural gas in industrial boilers. The NO _x mainly comprise NO and N0 ₂ and are among the gaseous pollutants most harmful to health and the environment. Nitrogen oxides are precursors of photochemical smog and acid rain, phenomena with direct effects on the health of animals, vegetation and humans.

The technologies applied to reduce NO _x emissions in industrial boilers can be classified mainly into two groups: primary measures, which include modifications and adjustments to the combustion process; and secondary measures, which are based on post-combustion abatement.

Within the primary measures, one of the strategies applied is that based on the stratification of air and fuel contributions to the boiler. In this sense, the lines of action in existing units range from the adjustment of combustion parameters to the implementation of boiler modifications, such as the installation of low NO _x burners, OFA registers (air on grill, from English "Over Fire Air" ) or UFA (air under grill, English "Under Fire Air"), etc.

Said stratification seeks to develop combustion in two or more stages, of which a first stage, also called an initial stage, is richer in fuel, while a second stage (and subsequent stages) are gradually poorer in fuel. It is about reducing the available oxygen in those areas where it is critical for the formation of NO _x , as well as reducing the amount of fuel that burns at the maximum flame temperature. This procedure acts on the so-called thermal NO _x (disadvantaged by rich mixtures) and on the NO _x of the fuel (promoting its transformation to N ₂ in the fraction from the combustion of volatiles).

An embodiment of the concept of stratified combustion in stages described above is that detailed in the international patent application PCT / ES2010 / 070039, the object of which is characterized by constituting a system consisting of a boiler equipped with a plurality of burners distributed in several groups arranged at different levels or in areas, differentiating between groups of main and auxiliary burners depending on whether the fuel injection into the boiler through the burners is carried out preferentially or only if necessary. Additionally, the system comprises a group of solid fuel mills that exceeds by at least one the number of mills needed to generate the maximum boiler load, just as the system also incorporates solid fuel transport means that communicate the mills With the burners.

The main feature that introduces the system described in PCT / ES2010 / 070039 is that it allows to establish patterns of differential contribution of solid fuel between each group of burners, associated to the achievement of a certain operational objective (reduction of NO _x , improvement of performance, reduction of unburned ones, etc.), so that said patterns are not modified by the punctual unavailability of one of the mills, nor do they need to remove the operating conditions of the mills from their design conditions. To achieve this objective, the mills comprise two Auxiliary mills, which in turn comprise a replacement mill and a support mill, both connected to the burners of each of the main burner groups and, optionally, to a group of auxiliary burners. While the replacement mill would only operate at the stop of one of the main mills, the support mill would always be in operation generating a solid fuel flow that would be added to the main burner groups.

The major drawback of this combustion system is that, based on the design considered, it is necessary to have two auxiliary mills (one replacement and one support), together with numerous auxiliary elements (new fuel transport ducts, groups of distributors, groups of flow dividers, as well as numerous baffles and shut-off valves) that provide the system with great operational complexity and a high demand for associated maintenance.

Related to the invention described in the aforementioned document PCT / ES2010 / 070039, although with a different purpose, German patent DE 833 099 refers to a home of solid pulverized fuel equipped with several main mills, each connected with groups of burners through a series of pipes through the interconnection of combined distributor heads. This patent incorporates in a novel way a reserve mill whose ducts flow, also through a distributor head, into the main mill ducts, between them and their distributor heads. Theoretically, said reserve mill has two modes of operation, being able to replace a main mill in case of failure (replacement mode), or evenly supporting the operation of the main mills in case of load peaks and always and when there is no mill out of service (support mode).

In contrast to what is explained for PCT / ES2010 / 070039, DE 833 099 has the sole objective of providing the boiler with greater flexibility of operation, leaving out of its reach the ability to adjust the fuel distribution between the different groups of burners without the need to remove the operating conditions of the mills from their design conditions, so that the reduction of nitrogen oxide emissions and the optimization of performance are not direct objectives of the DE 833 099 patent. Additionally, the design described in DE 833 099 does not solve the operational challenge of how the same conduits that connect the support mill with the rest of the mills can be sized to vehicle fuel flows so different that they are generated based on the two operating modes available (replacement and support). In that sense, a design of the diameter of these ducts based on the operation in substitution mode, would produce that, under operating conditions in support mode, the speeds would be reduced based on the number of currents in which the production of the reserve mill, such that problems of deposition of fuel dust in the ducts could occur. If, on the other hand, the diameter is sized for vehicular fuel flows according to the support mode, an increase in the speed of the mixture would occur, in case of operation in substitution mode, generating a great erosion of the pipes and a considerable increase in the loss of load in the circuit. This greater loss of load would produce an increase in the mill pressure and, as a consequence, a reduction in its production capacity.

DESCRIPTION OF THE INVENTION

The present invention relates, in accordance with a first object, to an optimization system for pulverized solid fuel boilers, with a view to reducing nitrogen oxides emissions, as well as improving boiler performance and flexibility of operation. industrial, such as those existing in power generation units. According to a second object, the invention relates to a combustion boiler incorporating said optimization system.

The optimization system, in accordance with the first object of the present invention, is specially designed to be implemented in a combustion boiler equipped with a combustion chamber, just as the system consists of:

- a plurality of burners, which are pulverized solid fuel burners, and which are distributed in several groups arranged in different levels or zones, where each group of burners is generally formed by a predetermined number of burners distributed according to columns in the chamber of combustion, as is known in the state of the art; - a plurality of mills, to grind the solid fuel, where the number of mills exceeds by at least one the number of mills necessary to generate the full load of the boiler; Y

- means of transport, to communicate the mills with the burners, allowing solid pulverized fuel to be transported from the mills to the burners.

The groups of burners comprise groups of main burners, which receive fuel injection preferentially. The burner groups may additionally comprise auxiliary burner groups, which receive fuel only if necessary, for example, failure of the main burners. Alternatively, as will be discussed below, auxiliary burners can be used to introduce air into the combustion chamber, in accordance with the OFA technology mentioned in the background.

For their part, the mills comprise main mills and an auxiliary mill. Each main mill is connected to all the burners of the same group of main burners. The main mills are in operation whenever the demand of load requires it. For its part, the auxiliary mill is connected to each of the main burner groups and, optionally, to one or more auxiliary burner groups. Therefore, each group of main burners is connected to its corresponding main mill, as well as all groups of main burners are connected in parallel to the auxiliary mill. The auxiliary mill feed is derived, as will be explained below, to the group of main burners and, where appropriate, of desired auxiliary burners.

The present invention is preferably intended to be used in boilers in which it is intended to reach full load only by using the main burner groups, so, preferably, it is not intended that auxiliary burners become operational. Therefore, details about whether and, where appropriate, how, the auxiliary burner groups are connected to the mills are not essential for the operation or for the understanding of the present invention.

The means of transport incorporate main transport lines, as well as auxiliary transport lines. Each main transport line communicates a main mill with a corresponding burner from its corresponding main burner group. Preferably, for each main mill there are as many main transport lines as there are columns in the combustion chamber, that is, as burners there are in the corresponding burner group.

On the other hand, auxiliary transport lines, of which, preferably, by analogy with the main transport lines, there is one for each column of each group of main burners and, where appropriate, auxiliary, communicate the auxiliary mill with each group of main burners and, where appropriate, with the group or each group of auxiliary burners. The auxiliary transport lines preferably have the same diameter as the main transport lines, as well as each of the auxiliary transport lines comprises the following components:

- an ascending section, preferably vertical, connected to the auxiliary mill, from which it receives the charge of pulverized solid fuel;

- a descending section, preferably vertical, connected to the ascending section;

- a transition section, interspersed between the ascending section and the descending section;

- a distributor, to distribute corresponding flows of solid fuel sprayed from the descending section to each of the main burner groups and, where appropriate, auxiliary, where each distributor has an input connected to the descending section of one of the lines auxiliary transport and several exits;

- distribution lines, of descending configuration, preferably vertical, located in corresponding exits of the distributor, and leading to corresponding main transport lines of each of the main burner groups and, where appropriate, auxiliary.

The main feature introduced by the invention is that it allows establishing, as just explained, by means of a single auxiliary mill, patterns of differential contribution of solid fuel between each group of burners, associated with the achievement of a certain operational objective (reduction of NO _x , performance improvement, reduction of unburned, etc.), so that said patterns are not modified by the punctual unavailability of one of the mills and that is not required for the operation away from the mills of their conditions of design. On the other hand, the fundamental particularity of auxiliary transport lines is the presence of distribution lines of descending configuration, preferably vertical. In this way, the flow of solid fuel, both in the descending sections, as well as in the distributors and in the distribution lines, is favored by gravity, not requiring a minimum speed of the transport air for the support of the particles made out of fuel. Although it is preferred that the distribution lines, as well as the descending sections, have a vertical direction, that is, that they form a right angle with the horizontal orientation, it is really enough that they form an angle, in descending direction, that is sufficient as to provide a displacement of the fuel that is free of traffic jams.

Through this configuration, the auxiliary mill can selectively operate both in replacement mode, providing fuel to the burners of one of the main mills that may be out of service, or in support mode, generating fuel delivery currents that are added to the main burner groups and, where appropriate, auxiliary to obtain the pre-established fuel stratification patterns.

The present invention solves the problem of ensuring the absence of depositions and jams in the auxiliary transport lines, in particular, in the delivery lines, when the system is operated in support mode. In this sense, the division that occurs in the distributors gives rise to distribution currents that, although they present speeds lower than the speeds required for the fuel support in transitional sections and in ascending sections, do not imply a limit to the flow at Be this assisted by gravity itself.

The fact that, as explained above, due to the effect of gravity on the descending sections and on the distribution lines, there are no limitations on the distribution of fuel in the distributor, which could be imposed by the need to ensure a minimum transport speed on the delivery lines, a combined replacement / support mode of operation is additionally enabled. In this way, the auxiliary mill can replace a main mill that is out of service, with a high percentage of the nominal load of said auxiliary mill, for example, above 75%, and simultaneously support one or more of the main mills using a remaining portion of the nominal load.

The means available in the state of the art for establishing fuel stratification strategies in conventional boilers are normally reduced to the adjustment of mill production. In this sense, the normally narrow operating margins of the mills with respect to their nominal capacity impose an insurmountable level to the potential benefits that greater stratification can offer. In addition, and not least, this way of operating, far from the optimal design point of the mills, has a negative influence on its operation that can lead to mechanical problems (wear, vibration, solid fuel rejection, etc.) as in a worsening in the particle size of the solid fuel produced.

The present invention, on the contrary, makes it possible to ensure and adjust the fuel inputs to certain groups of main burners, allowing to establish feeding patterns that can lead to a large stratification between different groups of burners by means of the availability of a single auxiliary mill, without the need to operate the mills away from their normal design point and regardless of the mill that may eventually be out of service for maintenance or other purpose.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical implementation thereof, a set of drawings is attached as an integral part of said description. where, for illustrative and non-limiting purposes, the following has been represented:

Figure 1 shows a preferred embodiment of the invention applied to a front boiler of pulverized solid fuel (coal typically) equipped with eight burners, arranged in four levels of two columns, where for each level there is one burner per column. The burners and the means of transport associated to one of the two columns are represented, their arrangement being analogous to that of the another column

A typical pattern of stratified fuel feed by levels for the reduction of NO _x emissions in a boiler such as that described in Figure 1 but not incorporating the invention described herein is shown in Figure 2.

Figure 3 shows, for the boiler in question, a more pronounced fuel stratification pattern than in the case of Figure 2, obtained by the application of the present invention operating in the support mode for achieving a greater reduction of NO _x , but keeping the commitment to control the level of unburned fuel when operating with all mills in service (one more than in the usual case of operation described in Figure 2).

Figure 4 shows, for the boiler in question, a fuel stratification pattern similar to that of the case of Figure 2, obtained by application of the present invention operating in the replacement mode of an out-of-service mill.

Figure 5 shows, for the boiler in question, a fuel stratification pattern similar to that of the case of Figure 3, obtained by application of the present invention operating in the combined mode. In said mode of operation, by means of the auxiliary mill, a mill that is out of service is partially replaced and additionally a differential fuel supply is generated between the different groups of burners.

PREFERRED EMBODIMENT OF THE INVENTION

A description of a possible embodiment of the described invention is given below. Its application is extensive both in the case of execution in a new boiler and in the case of adaptation of an existing one.

Consider the front boiler represented in the figures, in particular in Figure 1, which is equipped with a combustion chamber (1), and with eight burners grouped into four groups of burners (2A, 2B, 2C, 2D) located at different levels or heights, where each group of burners (2A, 2B, 2C, 2D) comprises two columns, where in each of the two columns there is a burner located in the front wall of the combustion chamber (1), so that in each group of burners (2A, 2B, 2C, 2D) there are two burners, one For each column. Among the groups of burners (2A, 2B, 2C, 2D), those corresponding to the three lower levels are called groups of main burners (2D, 2C, 2B), while the group of burners of the upper level is called the group of auxiliary burners (2A). The figures show a case in which there is only one group of auxiliary burners (2A).

The pulverized solid fuel feed (coal typically) to the combustion chamber (1) comes from four mills (3A, 3B, 3C, 3D), from which a two-phase air-fuel mixture is distributed to the burners (2A, 2B, 2C, 2D) through transport means comprising a pneumatic transport network formed by transport lines (22, 23). The mills (3A, 3B, 3C, 3D), comprise main mills (3D, 3C, 3B), which respectively feed the main burner groups (2D, 2C, 2B), through corresponding main transport lines (22 ).

The full load of the boiler can be achieved with the contribution, operating according to its nominal load, of only three of the mills (3A, 3B, 3C, 3D), in particular of the main mills (3D, 3C, 3B). In figure 1, only the transport lines (22, 23) to one of the two columns of the burners of the burner groups (2A, 2B, 2C, 2D) have been represented for simplicity, the distribution for the other column being totally analogous

The present invention is preferably intended to be used in boilers in which it is intended to reach full load only using the main burner groups (2D, 2C, 2B), so, preferably, it is not intended that the burner group auxiliary (2A) come into operation. Therefore, the details on whether or not they are connected and, where appropriate, how, the auxiliary burner group (2A) to the mills (3A, 3B, 3C, 3D) are not represented in the attached figures.

In this case, as indicated in the previous paragraph, it was decided to represent a scenario in which the auxiliary burner group (2A) would remain out of service at all times, so the auxiliary mill (3A) is only shown connected to the burners of each of the main burner groups (2D, 2C, 2B), although, preferably, the auxiliary mill (3A) can also be connected to the group or, where appropriate, auxiliary burner groups (2A), analogously to what is explained for the main burner groups (2D, 2C, 2B). In particular, the distribution shown has a preferred application in which the auxiliary burner groups (2A) are dedicated to introducing air into the combustion chamber (1), in accordance with the OFA technology mentioned in the background section.

Similarly to the main transport lines (22), the transport lines (22, 23) additionally comprise auxiliary transport lines (23), to connect the auxiliary mill (3A) with the main burner groups (2D, 2C , 2B) and, optionally, although not represented, with the auxiliary burner group or groups (2A). Each auxiliary transport line (23) comprises an ascending section (16), preferably vertical, that ascends to an elevation such that, after a transition section (17), it faces a descending section (18), preferably vertical, until it meets a corresponding descending distributor (4), of which there are two, one per column of the main burner groups (2D, 2C, 2B), where each distributor (4) has several outputs each of which is connected to a burner of each of the three groups of main burners (2D, 2C, 2B) and, where appropriate, auxiliary (2A).

The transition sections (17) can adopt varied configurations. According to the example represented in the figures, the transition sections (17) are horizontal. However, other configurations are also possible, such as diagonal transition sections (17) or elbow-shaped transition sections (17), for example at 180 °, which are not shown in the figures.

Each distributor (4), corresponding to a column, can distribute the solid fuel flow from the auxiliary mill (3A) in three adjustable flow streams directed towards the burners of said column belonging to the main burner groups (2D, 2C, 2B) and, where appropriate, auxiliary (2A). The distribution of the biphasic mixture from the auxiliary mill (3A) between the main burner groups (2D, 2C, 2B) and, where appropriate, auxiliary, is regulated by means of movable baffles (5, 6) and delivery valves (7, 8, 9) that isolate each of the exit routes of the distributors (4). Said baffles (5, 6) are provided with mechanisms for their displacement in a suitable position depending on the required distribution in flows and granulometry of the solid fuel, allowing the operation of the system in the three desired modes: replacement mode, support mode and combined mode (ie, simultaneously replacement mode and support mode).

On the other hand, the exit routes of the distributors (4) flow into the main transport lines (22), corresponding to the main mills (3D, 3C, 3B), by means of corresponding distribution lines (19, 20, 21) in a downward direction, preferably vertical, such that the transport of solids without deposition is ensured regardless of the fluid velocity. Each distribution line (19, 20, 21) flows into a corresponding junction (10, 1, 1, 12) located downstream of a corresponding main valve (13, 14, 15) located on each main transport line, to selectively isolate the corresponding main mill (3D, 3C, 3B) with respect to the distribution line (19, 20, 21).

The configuration described for the means of transport allows the application of operating methodologies that are not feasible or maintained over time in the conventional combustion units, represented in Figure 2, in which each group of burners (2A, 2B, 2C , 2D) is fed exclusively by a single mill (3A, 3B, 3C, 3D). As an example of these methodologies with a single mill (3A, 3B, 3C, 3D) per group of burners (2A, 2B, 2C, 2D), the strategy of fuel stratification for the reduction of NO _x emissions is highlighted. For a conventional front boiler, such as that shown in Figure 2, with four mills (3A, 3B, 3C, 3D), normally capable of generating full load with three of them in operation at nominal load, a significant reduction has been found of the NO _x when the upper burner group (2A) is stopped and a feed pattern characterized by a higher contribution by the lower burner group (2D) is forced, where said contribution is gradually reduced towards the upper burner groups (2C, 2B, 2A), as shown in Figure 2. In said figure 2 the amount of solid fuel contributed by each group of burners (2A, 2B, 2C, 2D) and that produced by each mill (3A, 3B, 3C, 3D) has been represented by percentages, taking as reference a production 100% nominal for each of them.

This way of operating, described in the previous paragraph for the example of the state of the art represented in Figure 2, is limited by the maximum production capacity of the mills (3A, 3B, 3C, 3D) and, in any case, conditioned by the availability of said mills (3A, 3B, 3C, 3D). In this sense, a stop of one of the mills (3B, 3C, 3D) associated with the lower burner groups (2B, 2C, 2D) would force the start of the upper mill (3A) to generate the maximum boiler load , which would be associated with the entry of fuel into the combustion chamber (1) through the upper group of burners (2A), whereby the balance of NO _x would be detrimentally displaced towards a greater generation of NO _x .

In comparison with the conventional system just mentioned, the system of the present invention advantageously allows establishing more accentuated stratification patterns from the point of view of the reduction of NO _x , as shown in Figure 3. In said figure shown in percentage: the production of each mill (3A, 3B, 3C, 3D); the distribution of auxiliary mill production (3A) between each group of main burners (2D, 2C, 2B) operating in addition mode; and the total flow rate for each group of burners (2D, 2C, 2B). The position of the main valves (13, 14, 15) and, approximately, of the baffles (5, 6), which allow the desired stratification pattern to be achieved, is also represented graphically. As can be seen in Figure 3, the three main burner groups (2D, 2C, 2B) are fed through their respective main mills (3D, 3C, 3B) operating slightly below their nominal load, as a consequence of the start of the auxiliary mill (3A). The auxiliary mill (3A) operates at the same load as the main mills (3D, 3C, 3B), distributing its production, based on the position of the baffles (5, 6), to the three lower levels. The distribution in question would be 7% for the upper burner group (2B), 33% for the intermediate burner group (2C) and 60% for the lower burner group (2D). The previous pattern of solid fuel contribution is very practical in cases where co-combustion of coal with some other alternative solid fuel (for example, biomass) is considered. In this case the alternative solid fuel (for example, biomass) can be pulverized in the auxiliary mill (3A) and from it, by virtue of the defined transport means, be fed to the main burner groups (2D, 2C, 2B ), together with the respective streams of pulverized coal from the other mills.

Figure 4 shows, with the same symbology as Figure 3, a configuration that allows to establish an operating pattern in case of stopping of one of the main mills (3D, 3C, 3B), for example the main mill (3B) that It feeds the third level of burner groups (2B), and a differential contribution of solid fuel is not required between each burner group (2D, 2C, 2B) by means of the auxiliary mill (3A). In this case, the main mills (3D, 3C, 3B) in service generate a production similar to that presented in Figure 2, while the auxiliary mill (3A) works at a production similar to that of the main mill (2B) outside of service also presented in this figure 2. Regarding the situation presented in the previous figure (figure 3), the distribution valves (8, 9) corresponding to the distribution lines (20, 21) of the two groups of lower burners (2D, 2C), as well as the main valve (13) corresponding to the upper main burner group (2B), and the flow deflectors (5, 6) of the distributors (4) would be activated so that the entire supply of the auxiliary mill (3A) to the upper main burner group (2B).

Finally, Figure 5 shows, with the same symbology as Figures 3 and 4, the configuration that allows to establish an operating pattern in case of shutdown of one of the main mills, (3B, 3C, 3D), for example the mill main (3B), which feeds the third level of groups of burners (2B), and if a differential contribution of solid fuel between each group of burners (2B, 2C, 2D) through the auxiliary mill (2A) is additionally required. In this case, all the mills (3A, 3C, 3D) in service work at their nominal load. Regarding the situation presented in the previous figure (figure 4), the distribution valves (8, 9) corresponding to the distribution lines (20, 21) of the two lower burner groups (2D, 2C), and be they would activate the flow deflectors (5, 6) of the distributors (4) for more accentuated stratification presented in Figure 3.

Claims

1. - Combustion optimization system for solid fuel boilers equipped with a combustion chamber (1), where the optimization system comprises:

- a plurality of groups of main burners (2D, 2C, 2B), located at different levels or zones, to supply solid fuel sprayed in the combustion chamber (1);

- a plurality of main mills (3D, 3C, 3B), each connected to all the burners of a corresponding group of main burners (2D, 2C, 2B), to feed the burners with pulverized solid fuel;

- main transport lines (22) to communicate the main mills (3D, 3C, 3B) with the main burner groups (2D, 2C, 2B);

- an auxiliary mill (3A), connected in parallel to each of the main burner groups (2D, 2C, 2B); Y

- auxiliary transport lines (23) to communicate the auxiliary mill (3A) with the main burner groups (2D, 2C, 2B);

characterized in that the auxiliary transport lines (23) comprise:

- an ascending section (16) connected to the auxiliary mill (3A);

- a descending section (18), connected to the ascending section (16);

- a transition section (17) that communicates the ascending section (16) and the descending section (18);

- a distributor (4) at the end of the descending section (18), in the direction of flow, to distribute the solid fuel flow; Y

- distribution lines (19, 20, 21), descending, departing from the distributor (4) and each flow into its corresponding main transport line (22).

2. - Optimization system according to claim 1, characterized in that the transition sections (17) are horizontal.

3. - Optimization system according to claim 1, characterized in that the transition sections (17) are diagonal.

4. - Optimization system according to claim 1, characterized in that the transition sections (17) have an elbow shape.

5. - Optimization system according to claim 1, characterized in that the descending section (18) and / or the distribution lines (19, 20, 21) are vertical.

6. - Optimization system according to claim 1, characterized in that the distributors comprise movable baffles (5, 6) equipped with displacement mechanisms to divide the flow rate according to the required distribution.

7. - Optimization system according to claim 1, characterized in that the burners are divided into columns;

where each column is formed by a burner of each group of main burners (2D, 2C, 2B);

where the auxiliary transport lines (23) comprise an auxiliary transport line (23) for each of the columns.

8. - Powdered solid fuel boiler, comprising a combustion chamber (1);

characterized in that it further comprises the optimization system described in any one of the preceding claims.