System and method for burning low-calorie and ultra-low-calorie fuels

Abstract

The invention relates to a system and method for the combustion / disposal of low calorific value and ultra-low calorific value fuels with a heating value below 2.5 MJ / Nm3, and in particular for the combustion / disposal of waste gases with a high inert non-combustible gas content, with a heating value below 2.1 MJ / Nm3, with no need to use additional high calorific value fuel to stabilise the gas combustion / disposal process, due to the system being provided with a semi-adiabatic combustion chamber (10) connected to a flue gas (7) duct (27), in which there are alternately mounted at least two flue gas heaters (31, 32) for the low calorific value fuel (8) and at least two flue gas heaters (41, 42) for an oxidant (9). The semi-adiabatic combustion chamber (10) is heated up from an ambient temperature to a temperature of 750°C by means of a starting fluid (6), whereas the low calorific value fuel (8) and the oxidant (9) are heated up in the flue gas heaters (31, 32, 41 and 42) to a target temperature of up to 800°C. Optionally, the system can also be used for heating process media by being retrofitted with a flue gas heater (24) or / and at least one steam heater (21, 22, 23) for the process medium (2), and for the generation of steam or for the generation of steam and electricity, by being retrofitted with a flue gas heater (51) or / and a steam heater (52) for water (5), and a turbine system (11) with an electric generator (4), driven by steam (3).

Description

[0001] The invention refers to a system and method for the combustion of low calorific value and ultra-low calorific value waste fuels, and in particular for the combustion of waste gas with a high inert non-combustible gas content.

[0002] Low calorific value waste fuels, and in particular low calorific value waste gases, are generated by various industrial processes, such as processes of the chemical, metallurgical and cement industries, of the energy sector (gasification process of biological fractions), agri-food sector (e.g. various types of fermentation processes), etc. One type of a low calorific value gas is waste gas with a high nitrogen content.

[0003] For example, Polish patent application P.438329 discloses a method and device for the combustion of low calorific value gas, in which fuel gas burned in a separate burner is used for the combustion of low calorific value waste gas by means of a flare burner. The fuel gas is mainly propane and / or butane converted from the form of a liquid gas referred to as LPG to the gaseous phase by supplying thermal energy via a set of suitable exchangers.

[0004] For example, Polish patent PL2533B1 discloses a system and method for low-emission combustion of waste gases, especially of a low calorific value, in combustion chambers of industrial energy conversion devices, in which a mixture of waste gas, a full-value fuel and air is subjected to combustion. Wherein an air chamber is fed with a continuous stream of air, which is then fed into a first combustion segment, where the air velocity is increased, and the air is mixed with the full-value fuel. This mixture is then ignited by a pilot burner which initiates and sustains the flame and, in turn, the resulting hot flue gases heat the consecutive segments of the combustion system. Once the appropriate temperature has been reached in the third segment of the combustion system, waste gas is fed into the first combustion segment via a nozzle. Similarly, once the appropriate temperature has been reached in the consecutive segments of the system, waste gas is introduced sequentially thereinto.

[0005] The above solutions are disadvantageous in that, the combustion of low calorific value waste gas requires a continuous supply of a full-value, i.e. high calorific value, gas.

[0006] It is advisable to develop a system and method for the combustion of low calorific value and ultra-low calorific value waste fuel, in which the combustion of the low calorific value and ultra-low calorific value waste fuel takes place with no need to supply a high calorific value fuel.

[0007] The object of the invention is a low calorific value fuel combustion system comprising a semi-adiabatic combustion chamber connected to a flue gas duct and having a starting fuel burner, a nozzle for introducing low calorific value fuel, and a nozzle for introducing an oxidant. The system is characterised in that it further comprises the following placed in the flue gas duct: a first flue gas heater for the oxidant; a second flue gas heater for the oxidant, fluidly connected to the first flue gas heater for the oxidant; a second flue gas heater for the low calorific value fuel, fluidly connected to the first flue gas heater for the low calorific value fuel. Wherein the first and second flue gas heaters for the oxidant are connected to an oxidant duct for heating the oxidant before its introduction into the combustion chamber, whereas the first and second flue gas heaters for the low calorific value fuel are connected to a low calorific value fuel duct for heating the low calorific value fuel before its introduction into the combustion chamber. The system provides multi-stage heating of the combustion process substrates before their introduction into the combustion chamber, resulting in an increase in the adiabatic combustion temperature, stabilising the combustion process. As a result, the system is characterised by very high efficiency and does not require supplying additional energy in the form of high calorific value fuel gases to conduct and sustain the combustion process.

[0008] Preferably, the system comprises a flue gas heater for water, and a turbine system with an electric generator driven by steam from the flue gas heater for water.

[0009] Preferably, the system comprises a first steam heater for the oxidant, fluidly connected to the second flue gas heater for the oxidant and located in a steam duct. Thus, it is possible to achieve a higher temperature of the oxidant.

[0010] Preferably, the system comprises a steam heater for the low calorific value fuel, fluidly connected to the second flue gas heater for the low calorific value fuel and located in the steam duct. As a result, it is possible to achieve a higher temperature of the low calorific value fuel.

[0011] Preferably, the system comprises a steam heater for water, fluidly connected to the flue gas heater for water and located in the steam duct downstream of the steam heater for the low calorific value fuel. Thus, the heat of the steam is used to preheat the water before its introduction into the flue gas heater for water.

[0012] Preferably, the system comprises a flue gas heater for the process medium, located in the flue gas duct.

[0013] Preferably, the system contains at least one steam heater for the process medium.

[0014] By locating the exchangers in the most convenient location for heating the process media in terms of temperature, the system can be adapted to the technological needs of the customer.

[0015] The object of the invention is also a low calorific value fuel combustion method in a low calorific value fuel combustion system, comprising the steps of: heating the semi-adiabatic combustion chamber by means of the starting fuel from ambient temperature up to a temperature of 750°C; a fuel with a heating value of less than 2.5 MJ / Nm 3< , and in particular with a heating value of less than 2.1 MJ / Nm 3< , being supplied as the low calorific value fuel. Wherein, before the low calorific value fuel is introduced into the chamber: the low calorific value fuel is introduced into the second flue gas heater for the low calorific value fuel; the low calorific value fuel is preheated in the second flue gas heater for the low calorific value fuel; subsequently, in the first flue gas heater for the low calorific value fuel, the preheated low calorific value fuel is heated up to a target temperature of up to 800°C; the oxidant is introduced into the second flue gas heater for the oxidant; the oxidant is preheated in the second flue gas heater for the oxidant; subsequently, in the first flue gas heater for the oxidant, the preheated oxidant is heated up to a target temperature of up to 800°C, following which the heated oxidant is introduced into the combustion chamber, and the heated low calorific value fuel is introduced into the combustion chamber.

[0016] Preferably, before the low calorific value fuel is introduced into the second flue gas heater for the low calorific value fuel, the low calorific value fuel is preheated in the first steam heater for the low calorific value fuel.

[0017] Preferably, before the oxidant is introduced into the second flue gas heater for the oxidant, the oxidant is preheated in the first steam heater for the oxidant.

[0018] Preferably, after the low calorific value fuel is preheated in the first steam heater, and before the low calorific value fuel is introduced into the second flue gas heater for the low calorific value fuel, the low calorific value fuel is heated in a third flue gas heater for the low calorific value fuel, which is located in the flue gas duct downstream of the second flue gas heater for the low calorific value fuel.

[0019] Preferably, before the oxidant is heated in the first steam heater for the oxidant, the oxidant is heated in a second steam heater, located in the steam duct downstream of the first steam heater for the oxidant.

[0020] Preferably, a fuel with a heating value of less than 2.1 MJ / Nm 3< is supplied as the low calorific value fuel.

[0021] The object of the invention has been presented in an embodiment in the drawing, where: Fig. 1 presents schematically a flue gas system for a low calorific value waste fuel combustion system in a first embodiment; Fig. 2 presents schematically a second embodiment of the low calorific value waste fuel combustion system with flue gas heaters for process media; Fig. 3 presents schematically a third embodiment of the system, with a steam generation system and an electricity generation system; Fig. 4 presents schematically the system of Fig. 3 with a system for the combustion of an additional low calorific value fuel; Fig. 5 presents schematically the system of Fig. 4 with systems for the heating of additional process media; Fig. 6 presents a diagram of a particular embodiment of the system according to the invention; Fig. 7 presents an embodiment of a flue gas duct with flue gas heaters; Fig. 8 presents an embodiment of the system with a semi-adiabatic chamber connected to the flue gas duct.

[0022] Fig. 1 presents schematically a flue gas system for a low calorific value waste fuel combustion system in a first embodiment. Fig. 6 presents a diagram of a particular embodiment of the system according to the invention. Fig. 7 presents an embodiment of a flue gas duct with flue gas heaters. Fig. 8 presents an embodiment of the system with a semi-adiabatic chamber connected to the flue gas duct. The low calorific value fuel combustion system comprises a semi-adiabatic combustion chamber 10 connected to a flue gas 7 duct 27 and having a burner 26 for a starting fuel 6, a nozzle 81 for introducing low calorific value fuel 8, and a nozzle 91 for introducing an oxidant 9. The system further comprises the following placed in the flue gas 7 duct 27: a first flue gas heater 41 for the oxidant 9, a first flue gas heater 31 for the low calorific value fuel 8, a second flue gas heater 42 for the oxidant 9, fluidly connected to the first flue gas heater 41 for the oxidant 9, a second flue gas heater 32 for the low calorific value fuel 8, fluidly connected to the first flue gas heater 31 for the low calorific value fuel 8. Wherein the first and second flue gas heaters 41, 42 for the oxidant 9 are connected to an oxidant 9 duct 29 for heating the oxidant 9 before its introduction into the combustion chamber 10, whereas the first and second flue gas heaters 31, 32 for the low calorific value fuel 8 are connected to a low calorific value fuel 8 duct 28 for heating the low calorific value fuel 8 before its introduction into the combustion chamber 10.

[0023] The starting fuel 6, e.g. a high calorific value gas, is only used to pre-heat the combustion chamber 10. The rest of the process takes place without the use of the starting fuel 6.

[0024] For example, the oxidant 9 can be air, a mixture of air with an addition of flue gases, a mixture of air with an addition of oxygen, or a mixture of air with an addition of oxygen and flue gases.

[0025] The low calorific value fuel 8 is a fuel with a heating value of less than 2.5 MJ / Nm 3< , which can, for example, be a low calorific value gas, and in particular a waste gas with a high inert gas content of at least 84%.

[0026] The flue gas heaters 41, 31, 42, 32 have the form of heat exchangers that absorb heat from the flue gas 7 and transfer it to the medium-the oxidant 9-or to the low calorific value fuel 8-flowing therethrough. Preferably, the heaters (heat exchangers) are continuously operating shell or tube heat exchangers, mounted alternately (oxidant 9, low calorific value fuel 8) and in a multi-stage arrangement (at least two-stage heating of the oxidant 9, at least two-stage heating of the low calorific value fuel 8).

[0027] A series of alternately mounted flue gas heaters 41, 31, 42, 32 (that is, at least two flue gas heat exchangers 31, 32 for heating the low calorific value fuel 8 and at least two flue gas heat exchangers 41, 42 for heating the oxidant 9) located in the flue gas 7 duct 27 allow for multi-stage heating of the combustion process substrates before their introduction into the combustion chamber 10. The flue gas heaters 41, 42 for the oxidant 9 are connected via the oxidant 9 duct 29 to a source of the oxidant 9, whereas the flue gas heaters 31,32 for the low calorific value fuel 8 are connected via the low calorific value fuel 8 duct 28 to a source of low calorific value fuel. In the first embodiment, the system allows for two-stage heating of the oxidant (e.g. air) to a target temperature of 600°C and for two-stage heating of the low calorific value fuel 8 (e.g. waste gas with a high nitrogen content) to a temperature of 450°C.

[0028] Preferably, the flue gas heaters 31, 32, 41, 42 are mounted alternately, so that the first flue gas heater 41 for the oxidant 9 is located first in the flue gas 7 duct 27 and is followed sequentially by: the first flue gas heater 31 for the low calorific value fuel 8, the second flue gas heater 42 for the oxidant 9, fluidly connected to the first flue gas heater 41 for the oxidant 9, and the second flue gas heater 32 for the low calorific value fuel 8, fluidly connected to the first flue gas heater 31 for the low calorific value fuel 8. Depending on the needs and on the design of the flue gas 7 duct 27, it is possible to use more flue gas heaters for the oxidant 9 and low calorific value fuel 8. Depending on the technical and / or technological conditions, the order in which the exchangers are mounted may change, i.e. so that the first flue gas heater 31 for the low calorific value fuel 8 is positioned first in the flue gas 7 duct 27, and it is followed sequentially by the first flue gas heater 41 for the oxidant 9, the second flue gas heater 32 for the low calorific value fuel 8, and the second flue gas heater 42 for the oxidant 9.

[0029] For example, air at a temperature of 20°C is preheated to a temperature of 250°C in the second flue gas heater 42 for the oxidant 9, and subsequently it is heated in the first flue gas heater 41 for the oxidant 9 to a target temperature of 600°C. Similarly, low calorific value gas 8 is preheated to a temperature of 200°C in the second flue gas heater 32 for the low calorific value fuel 8, and subsequently it is heated in the first flue gas heater 31 for the low calorific value fuel 8 to a target temperature of 450°C.

[0030] The thermal energy from the combustion of the low calorific value waste fuel 8 is transferred into substrates supplied to the combustion process, resulting in an increase in the adiabatic combustion temperature, stabilising the combustion process. As a result, the process is characterised by very high efficiency and does not require supplying additional energy in the form of high calorific value fuel gases to conduct and sustain the combustion process.

[0031] The use of a semi-adiabatic combustion chamber 10 allows for performing zonal volumetric combustion using zonal (gradual) injection of the fuel (e.g. low calorific value gas) and the oxidant into the combustion chamber 10.

[0032] In addition, the low calorific value fuel combustion system may comprise a first flue gas heater 51 for water 5 and a turbine system 11 (preferably with a twin-screw turbine) with an electric generator 4 driven by steam 3 (wet steam-containing a mixture of steam and water) generated in the first flue gas heater 51 for water 5. Fig. 3 presents schematically a third embodiment of the system, with a steam generation system and an electricity generation system. The first flue gas heater 51 for water 5 has the form of an evaporator, which generates the steam 3 used further down in the system. For example, the first flue gas heater 51 for water may be located downstream of the last flue gas heater 32 for the low calorific value fuel 8.

[0033] Preferably, the turbine system 11 is a twin-screw turbine which, together with the electric generator 4, constitutes an electricity generation unit 4 using high-pressure wet steam 3 (low-temperature steam). However, it is possible to use a different type of turbine unit to generate electricity from steam.

[0034] In addition, the system may comprise a first steam heater 43 for the oxidant 9, fluidly connected to the second flue gas heater 42 for the oxidant 9 and located in a steam 3 duct 13.

[0035] In such a system configuration, the oxidant (e.g. air) 9 is first preheated in the first steam heater 43 for the oxidant 9, and then passes sequentially to the second and first flue gas heaters 42, 41 for the oxidant 9.

[0036] In the system, it is also possible to use more steam heaters for the oxidant 9, for example, at least two steam heaters for the oxidant 9.

[0037] In addition, the system may comprise a steam heater 33 for the low calorific value fuel 8, fluidly connected to the second flue gas heater 32 for the low calorific value fuel 8 and located in the steam 3 duct 13.

[0038] For example, the steam heater 33 for the low calorific value fuel 8 is located downstream of the first steam heater 43 for the oxidant 9. In such a system configuration, the low calorific value fuel 8 (e.g. waste gas with a high inert gas, e.g. nitrogen, carbon dioxide, water vapour, content) is first preheated in the steam heater 33 for the low calorific value fuel 8, and then passes sequentially to the second and first flue gas heaters 32, 31 for the low calorific value fuel 8.

[0039] In the system, it is also possible to use more steam heaters for the low calorific value fuel 8, for example at least two steam heaters for the low calorific value fuel 8 arranged alternately between the other steam heaters in the steam 3 duct 13.

[0040] In addition, the system may comprise a steam heater 52 for water 5, fluidly connected to the first flue gas heater 51 for water 5 and located in the steam 3 duct 13 downstream of the steam heater 33 for the low calorific value fuel 8. The heat of the steam 3 is then used to preheat the water 5 before its introduction into the first flue gas heater 51 for water 5.

[0041] In addition, the system may comprise a flue gas heater 53 for the steam 3, fluidly connected to the first flue gas heater 51 for water 5. Thus, it is possible to achieve a higher temperature of the steam 3. For example, the flue gas heater 53 for the steam 3 is located in the flue gas 7 duct 27 upstream of the second flue gas heater 42 for the oxidant 9.

[0042] In addition, the system may comprise a second flue gas heater 54 for water 5, for preheating the water 5 before its introduction into the first flue gas heater 51 for water 5.

[0043] In addition, the system comprises a condensate cooler 15 connected to the steam heater 52 for water 5 and a feedwater 5 tank 16 connected to the condensate cooler 15 and connected to the steam heater 52 for water. In addition, the system may comprise a water 5 preparation station 20, fluidly connected to the water tank 16. Before the water 5 is introduced into the tank 16, it is purified of undesirable compounds to the required system parameters, which takes place in the water preparation station 20.

[0044] Fig. 2 presents schematically a second embodiment of the low calorific value waste fuel combustion system with flue gas heaters for process media. In the second embodiment, the system comprises at least two flue gas heaters 24, 25 for the process medium 2, located in the flue gas 7 duct 27, downstream of the flue gas heaters for the oxidant 9 and low calorific value fuel 8. An example of a process medium 2 can be process oil, process gas, hot utility water, water in a central heating system, etc. The system may also comprise at least one steam heater 21, 22, 23 for the process medium 2 (Figs. 5, 6).

[0045] In addition, the system may include systems for introducing other additional low calorific value fuels 1 into the combustion process, varying in composition and varying in the calorific value, as well as varying in flow per a time unit, which can be introduced into the combustion system during the operation of the primary waste fuel disposal system, providing an additional amount of energy. Fig. 4 presents schematically the system of Fig. 3 with a system for the combustion of an additional low calorific value fuel.

[0046] The invention also relates to a method for the combustion of low calorific value fuel in the low calorific value fuel combustion system shown above. The low calorific value fuel combustion method comprises steps in which a semi-adiabatic combustion chamber 10 is heated with the starting fuel 6 from ambient temperature up to a temperature of 750°C, wherein a fuel with a heating value of less than 2.5 MJ / Nm 3< is supplied as the low calorific value fuel. Wherein, before the low calorific value fuel is introduced into the chamber: the low calorific value fuel 8 is introduced into the second flue gas heater 32 for the low calorific value fuel 8, the low calorific value fuel 8 is preheated in the second flue gas heater 32 for the low calorific value fuel 8. Subsequently, in the first flue gas heater 31 for the low calorific value fuel 8, the low calorific value fuel 8 is heated up to a target temperature of up to 800°C. Whereas the oxidant 9 is introduced into the second flue gas heater 42 for the oxidant 9, and the oxidant 9 is preheated in the second flue gas heater 42 for the oxidant 9. The oxidant 9 is then heated in the first flue gas heater 41 for the oxidant 9 up to a target temperature of up to 800°C, following which the heated oxidant 9 is introduced into the combustion chamber 10, and the heated low calorific value fuel 8 is introduced into the combustion chamber 10. The combustion process for the low calorific value fuel is carried out at a temperature of 750°C to 850°C.

[0047] Preferably, before the low calorific value fuel 8 is introduced into the second flue gas heater 32 for the low calorific value fuel 8, the low calorific value fuel 8 is preheated in the steam heater 33 for the low calorific value fuel 8. By recovering some of the energy from the steam, the efficiency of the system is increased. As a result, it is possible to achieve a higher temperature of the low calorific value fuel 8.

[0048] Preferably, before the oxidant 9 is introduced into the second flue gas heater 42 for the oxidant 9, the oxidant 9 is preheated in the first steam heater 43 for the oxidant 9. Thus, it is possible to achieve a higher temperature of the oxidant 9.

[0049] Preferably, after the low calorific value fuel 8 is preheated in the steam heater 33, and before the low calorific value fuel 8 is introduced into the second flue gas heater 32 for the low calorific value fuel 8, the low calorific value fuel 8 is heated in a third flue gas heater 34 for the low calorific value fuel 8, which is located in the flue gas 7 duct 27 downstream of the second flue gas heater 32 for the low calorific value fuel 8. Wherein, between the second flue gas heater 32 for the low calorific value fuel 8 and the third flue gas heater 34 for the low calorific value fuel 8, there may be flue gas heaters for water 5 and / or the oxidant 9 and / or other process media 2.

[0050] The use of a larger number of heaters allows for maximising the use of energy from steam and flue gases, and for increasing the efficiency of the system. By locating the exchangers in the most convenient location for heating the process media in terms of temperature, the system can be adapted to the technological needs of the customer.

[0051] Preferably, before the oxidant 9 is heated in the first steam heater 43 for the oxidant 9, the oxidant 9 is heated in a second steam heater 44, located in the steam 3 duct 13 downstream of the first steam heater 43 for the oxidant 9. Wherein between the steam heaters 43, 44 for the oxidant 9 in the steam 3 duct 13 there may be additional steam heaters for heating other process media 2 and / or substrates of the combustion process.Specific embodiment

[0052] In the specific embodiment shown in Fig. 6, the low calorific value fuel combustion system comprises two flue gas heaters 41, 42 for the oxidant 9, three flue gas heaters 31, 32, 34 for the low calorific value fuel 8 arranged alternately with the flue gas heaters 41, 42 for the oxidant 9 in the flue gas 7 duct 27, one steam heater 33 for the low calorific value fuel 8, and two steam heaters 43, 44 for the oxidant 9 arranged alternately in the steam 3 duct 13 downstream of the turbine system 11 with the electric generator.

[0053] Air at a temperature of 20°C was introduced into the second steam heater 44 for the oxidant, and the air was heated up to a temperature of 85°C. Then the air was heated in the first steam heater 43 for the oxidant from a temperature of 85°C to a temperature of 130°C. Subsequently, the air was heated from a temperature of 130°C to a temperature of 235°C in the second flue gas heater 42 for the oxidant, followed by heating the air from a temperature of 235°C to a target temperature of 600°C in the first flue gas heater 41 for the oxidant 9.

[0054] At the same time, low calorific value gas 8 at a temperature of 20°C was introduced into the steam heater 33 for the low calorific value fuel 8, and the low calorific value gas 8 was heated up to a temperature of 90°C. Subsequently, in the third flue gas heater 34 for the low calorific value fuel 8, the low calorific value gas was heated up from a temperature of 90°C to a temperature of 140°C. Subsequently, in the second flue gas heater 32 for the low calorific value fuel 8, the low calorific value gas 8 was heated up from a temperature of 140°C to a temperature of 215°C, followed by heating the low calorific value gas 8 from a temperature of 215°C to a target temperature of 450°C in the first flue gas heater 31 for the low calorific value fuel 8.

[0055] The air and low calorific value gas thus heated were introduced into a semi-adiabatic combustion chamber 10. Wherein the semi-adiabatic combustion chamber 10 was heated in advance to a temperature of 750°C using starting fuel 6. The combustion process for the low calorific value fuel was then carried out at a temperature of 750°C to 850°C.

[0056] In the specific case of performing the combustion method described above, using the system according to the invention, it allows for waste fuels with a heating value of 0.6 MJ / Nm 3< to 0.7 MJ / Nm 3< to be combusted with no need to use additional high calorific value fuel to sustain the combustion process.

[0057] The presented system and method allow for the utilisation of low calorific value and ultra-low calorific value fuel, e.g. waste gas with a significant content of inert non-combustible components, including, e.g. nitrogen, through its efficient low-emission volumetric combustion, with no need to consume additional high-calorific value afterburning fuel (e.g. natural gas, LPG, etc.) to sustain the combustion process.

[0058] In addition, the presented system and method do not require the use of additional complex catalytic systems necessary to lower the activation energy in order to sustain a stable combustion process at a lower temperature, the use of which has the following main disadvantages: a complex thermal energy recovery system, sensitivity of the catalyst bed to contaminants and substances carried by waste fuel, the need for periodic regeneration or replacement of the catalyst bed, the need to consume high-calorific value fuel in the process, especially for ultra-low calorific value gases, and a lower temperature of the process.

[0059] In addition, the presented system and method do not require the use of complex regenerative heating systems for heating the substrates in order to conduct a stable combustion process, the use of which has the following main disadvantages: a complex system of regenerative substrate heaters, sensitivity of regenerators to contaminants and substances carried by waste fuel, the need for periodic replacement of regenerators, the risk of underfiring of the combustible parts due to the regenerative / switching nature of the substrate heating system, and high investment costs.

[0060] In particular, the presented system and method ensure high heating of the substrates fed into the combustion chamber, which results in an increase in the adiabatic combustion temperature, and consequently contributes to stabilisation of the combustion process. As a result, as mentioned above, there is no need to use high-calorific value fuel to sustain the combustion process.

[0061] In particular, the presented system and method allow for the combustion of waste fuels with a heating value below 2.5 MJ / Nm 3< with no need to use additional high calorific value fuel to sustain the combustion process, and with no need to use complex regenerative heating systems or complex catalytic systems to lower the activation energy in order to sustain a stable combustion process at a lower temperature.

[0062] The multi-stage and alternating heating system for the substrates of the combustion process also allows for the installation of additional exchangers for heating other process media required for any on-site technological process. It is also possible to use systems generating electricity based on low-temperature steam circuits.

[0063] The compact, component-based design of the system allows for its application at the place where the waste fuel (of a low calorific value and ultra-low calorific value) is present (generated).

[0064] Moreover, simultaneous combustion of additional other low calorific value fuels, with a variable calorific value and a variable flow rate at different time intervals, as well as availability, is possible. Wherein the combustion of the above fuels proceeds with high process efficiency while maintaining low emissions of hazardous substances, with no need to install additional flue gas treatment systems.

Claims

1. A low calorific value fuel combustion system comprising: - a semi-adiabatic combustion chamber (10) connected to a flue gas (7) duct (27) and having a burner (26) for a starting fuel (6), a nozzle (81) for introducing low calorific value fuel (8), and a nozzle (91) for introducing an oxidant (9); characterised in that it also comprises: - the following placed in the flue gas (7) duct (27): - a first flue gas heater (41) for the oxidant (9); - a first flue gas heater (31) for the low calorific value fuel (8); - a second flue gas heater (42) for the oxidant (9), fluidly connected to the first flue gas heater (41) for the oxidant (9); - a second flue gas heater (32) for the low calorific value fuel (8), fluidly connected to the first flue gas heater (31) for the low calorific value fuel (8); wherein the first and second flue gas heaters (41, 42) for the oxidant (9) are connected to an oxidant (9) duct (29) for heating the oxidant (9) before its introduction into the combustion chamber (10), whereas the first and second flue gas heaters (31, 32) for the low calorific value fuel (8) are connected to a low calorific value fuel (8) duct (28) for heating the low calorific value fuel (8) before its introduction into the combustion chamber (10).

2. The system according to claim 1, characterised in that it comprises a first flue gas heater (51) for water (5), and a turbine system (11) with an electric generator (4) driven by steam (3) from the first flue gas heater (51) for water (5).

3. The system according to claim 2, characterised in that it comprises a first steam heater (43) for the oxidant (9), fluidly connected to the second flue gas heater (42) for the oxidant (9) and located in a steam (3) duct (13).

4. The system according to claim 2 or 3, characterised in that it comprises a steam heater (33) for the low calorific value fuel (8), fluidly connected to the second flue gas heater (32) for the low calorific value fuel (8) and located in the steam (3) duct (13).

5. The system according to claim 4, characterised in that it comprises a steam heater (52) for water (5), fluidly connected to the first flue gas heater (51) for water (5) and located in the steam (3) duct (13) downstream of the steam heater (33) for the low calorific value fuel (8).

6. The system according to any one of claims 1 to 5, characterised in that it comprises a flue gas heater (24) for the process medium (2), located in the flue gas (7) duct (27).

7. The system according to any one of claims 2 to 5, characterised in that it comprises at least one steam heater (21, 22, 23) for the process medium (2).

8. A low calorific value fuel combustion method in a system according to any one of claims 1 to 7, characterised in that it comprises the steps of: - heating the semi-adiabatic combustion chamber (10) by means of the starting fuel (6) from ambient temperature up to a temperature of 750°C; - supplying fuel with a heating value of less than 2.5 MJ / Nm3 as the low calorific value fuel, wherein, before the low calorific value fuel is introduced into the chamber: - the low calorific value fuel (8) is introduced into the second flue gas heater (32) for the low calorific value fuel (8); - the low calorific value fuel (8) is preheated in the second flue gas heater (32) for the low calorific value fuel (8); then - in the first flue gas heater (31) for the low calorific value fuel (8), the preheated low calorific value fuel (8) is heated up to a target temperature of up to 800°C; - introducing the oxidant (9) into the second flue gas heater (42) for the oxidant (9); - preheating the oxidant (9) in the second flue gas heater (42) for the oxidant (9); then - heating the preheated oxidant (9) up to a target temperature of up to 800°C in the first flue gas heater (41) for the oxidant (9); - introducing the heated oxidant (9) into the combustion chamber (10); - introducing the heated low calorific value fuel (8) into the combustion chamber (10).

9. The method according to claim 8, characterised in that, before the low calorific value fuel (8) is introduced into the second flue gas heater (32) for the low calorific value fuel (8), the low calorific value fuel (8) is preheated in the steam heater (33) for the low calorific value fuel (8).

10. The method according to claim 8 or 9, characterised in that, before the oxidant (9) is introduced into the second flue gas heater (42) for the oxidant (9), the oxidant (9) is preheated in the first steam heater (43) for the oxidant (9).

11. The method according to any one of claims 9 to 10, characterised in that after the low calorific value fuel (8) is preheated in the first steam heater (33), and before the low calorific value fuel (8) is introduced into the second flue gas heater (32) for the low calorific value fuel (8), the low calorific value fuel (8) is heated in a third flue gas heater (34) for the low calorific value fuel (8), which is located in the flue gas (7) duct (27) downstream of the second flue gas heater (32) for the low calorific value fuel (8).

12. The method according to any one of claims 10 to 11, characterised in that, before the oxidant (9) is heated in the first steam heater (43) for the oxidant (9), the oxidant (9) is heated in a second steam heater (44), located in the steam (3) duct (13) downstream of the first steam heater (43) for the oxidant (9).

13. The method according to any one of claims 8 to 12, characterised in that a fuel with a heating value of less than 2.1 MJ / Nm3 is supplied as the low calorific value fuel.

Images