WO1992001194A1

WO1992001194A1 - Method for reducing emissions of oxides of nitrogen in combustion of various kinds of fuels

Info

Publication number: WO1992001194A1
Application number: PCT/FI1991/000215
Authority: WO
Inventors: Pentti Salmelin; Martti Äijälä
Original assignee: Imatran Voima Oy
Current assignee: Imatran Voima Oy
Priority date: 1990-07-13
Filing date: 1991-07-08
Publication date: 1992-01-23
Anticipated expiration: 1993-01-13
Also published as: PL291046A1; FI87949B; CN1059021A; FI903548A0; FI87949C; AU8093091A; FI903548L

Abstract

Described herein is a method for reduction of emissions of oxides of nitrogen in combustion processes burning a solid, liquid or gaseous fuel. The invention is based on an extremely staged combustion of the fuel. The fuel is first fed in an air-deficient form in order to attain reducing conditions into a flame of a plasma torch (1), where the fuel is gasified and forced-ignited. Auxiliary air is fed in at least one stage to the partially gasified fuel flow for the purpose of further gasifying of said fuel, after which it is routed to the actual combustion chamber, e.g., a furnace, burner or similar space (6), where its combustion is completed. By virtue of the abrupt staging, the flame is subjected to reducing conditions, whereby oxides of nitrogen formed in the flame are reduced before they have a chance to exit with the flue gases.

Description

Method for reducing emissions of oxides of nitrogen in combustion of various kinds of fuels

The present invention relates to a method in accordance with the preamble of claim 1 for reducing the emissions of harmful oxides of nitrogen formed in the combustion of various kinds of fuels.

Formation of oxides of nitrogen in combustion currently presents a serious problem in plants fired with different types of fuels, such as electric power plants and district heating plants. Maximum allowable emission limits set for new facilities are already strict today and will be rapidly tightened in the future.

Lowered content of nitrogen oxides (NO_x) in flue gases released into the environment are currently attempted to be achieved by pyrotechnical methods, different additive injections and catalytic methods. The various pyrotechnical methods to reduce NO_x emissions include recirculation of flue gases, water or steam injection, two-stage firing, use of low-NO_x burners and staged fuel feed. These methods reduce emission rates by approx. 5...50 %. For instance, an initial level of 450 mg/MJ can be reduced to a level of 200...400 mg/MJ. The costs of these methods cost are 20...35 % of those of the catalytic category. Ammonia or urea -is most generally used as the additive for lowering emissions of oxides of nitrogen. Results obtained by additives can be more effective than those achievable by pyrotechnical methods; and moreover, additives can be used to complement pyrotechnical methods. Injection methods are expensive to implement, and they may cause corrosion problems in the furnace; further, nitrous oxide may be formed, at least when urea is used.

Catalytic methods offer a reduction of NO_x compounds up to 80 %, thereby making it possible to achieve a level even less than 100 mg/MJ. When aiming at such a low emission level, this technique is at its best in conjunction with a firing technology developed for low NO_x emissions. Investments and operating costs of catalytic methods are high.

Pyrotechnical methods offer only a limited success which means that mere resorting to pyrolytic techniques in new installations is not sufficient for meeting set maximum allowable emission limits. Though more efficient than pyrolytic methods, the catalytic methods are very expensive, so their use increases the production costs of energy. The different injection methods available are partly relatively inefficient, cause corrosion and, with increased injection rates, can cause odour problems in the vicinity of the power plant.

When using inherently advantageous low-NO_x firing techniques or staged firing of fuel, the efficiency of these methods is curtailed by the fact that a sufficiently abrupt staging of air feed cannot be implemented without compromising reliable fuel ignition sufficiently close to the burner, which would result in increased content of unburned fuel components in flue gases and ash, as well as flame stability problems. Increased content of unburned components reduces firing efficiency, creates environmental problems and causes problems in the further use of the ash. Worsened flame stability increases safety risks imposed on the personnel and plant due to the higher explosion hazard.

It is an object of the present invention to achieve reduced NO_x emission levels from the combustion of solid, liquid or gaseous fuels in respect to the results from the use of conventional low-NO_x firing technology, thus obviating the need for resorting to more expensive techniques such as the catalytic methods.

The invention is based on burning the fuel in an extremely abruptly staged manner and forced ignition of the fuel under extremely air-deficient conditions with the help of a plasma torch.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides outstanding benefits.

The abruptly staged combustion concept made possible by the use of plasma torch ignition offers a way of utilizing pyrotechnical methods in a manner which attains NO_x emission levels as low as those achievable by means of conventional catalytic methods. This further makes it possible to get below the present maximum emission rate limits using low-cost pyrotechnical methods, thus offering large cost savings over catalytic methods in new installations. With the help of plasma torch ignition it is possible to use forced ignition of the fuel in extremely air-deficient combustion conditions, combined with abruptly staged combustion, whereby an appreciably improved reduction of N0_X emissions is achieved over conventional staged combustion techniques. The method according to the present invention is capable of achieving NO_x emission reductions by up to 80 %.

By virtue of ensuring reliable ignition, flame stability and controlled placing of the ignition point, the use of a plasma torch realizes a good combustion process in extremely difficult pyrotechnical conditions and creates a favourable atmosphere for perfect combustion of the fuel. The available combustion chamber volume is maximally utilized, because the ignition point of the fuel is brought close to the burner, the ignition point is controllable and a sufficiently long retention time in the combustion process can be ensured. The invention is next examined in detail with the help of the attached drawings.

Figure 1 shows diagrammatically a burner suitable for the implementation of the present invention.

Figure 2 shows the test arrangement for investigations performed in the use of the method according to the invention.

Fig. 1 shows diagrammatically a burner, operated by a plasma torch and suitable for the implementation of the present invention. The fuel fed to the burner is pulverized coal. The burner includes the plasma torch 1, a fuel feed union 2, a first auxiliary air feed union 3 and a second auxiliary air feed union 4. The fuel feed union 2 and the air feed unions 3 and 4 are joined to the burner perpendicularly to its longitudinal axis, whereby the fuel and combustion air fed to the burner are forced to a rotational motion about the longitudinal axis of the burner. The burner shown here is a staged burner, in which fuel combustion at the different stages 8, 9, 10 is adjusted by altering air feed volumes to the stages. In the first combustion stage, the fuel fired in the burner is routed to a first gasification zone 8 in front of the plasma torch 1. Air volume along with fuel feed at this stage is extremely small so that the air volume used is 5...30 % of the overall combustion air volume necessary for complete combustion. Thus, the stoichiometric coefficient at this gasification stage is approx. 0.05...0.3. Because of the extremely low air content of the fuel entering the stage 8, complete combustion of fuel at this stage 8 is avoided. However, the hot flame, which has a high energy density, of the plasma torch 1 performs an efficient gasification of the fuel, whereby carbon monoxide and hydrogen are formed simultaneously as the hot flame of the plasma torch 1 performs the forced ignition of a portion of the fuel and formed carbon monoxide. Resultingly, the burning carbon monoxide further performs an efficient gasification of the fed coal. The temperature at the zone 8 is locally 3500 °C, advantageously even above 4000 °C.

The partially burning and gasified fuel flows forward in the burner to the next zone 9, wherein it meets next with the auxiliary air fed via a first auxiliary air feed union 3. The air volume at this stage, together with the air volume from the previous stage 8, is 5...50 % of the overall air volume. At this stage 9, a major portion of the fuel is gasified into carbon monoxide, and water contained in the fuel is dissociated into hydrogen and oxygen under the plasma torch flame heat from the previous stage and the heat emitted from the subsequent stages 8, 9. The fuel mixture containing carbon monoxide and hydrogen in abundance is ignited and completely combusted in the next zone 10 by virtue of additional air fed into the mixture via a second auxiliary air feed union 4, whereby the gasified fuel ignites when the stoichiometric air/fuel ratio increases to a sufficiently high value. Alternatively, additional air can be fed via a second auxiliary air feed union 3 in order to further gasify the fuel in the zone 10, whereby ignition is performed with the help of combustion air fed into the actual combustion chamber, e.g., a power plant furnace. In this case, the overall volume of primary and secondary air fed by the first 3 and second 4 auxiliary air unions is relatively small, each air volume into the fuel mixture being maximally approx. 10 % of the required overall air volume, but preferably only approx. 1...5 % of the overall air volume. The rest of the necessary combustion air volume is fed into the combustion chamber used, e.g., a furnace.

The staged combustion technique described above offers significant reductions of NO_x emissions in combustion processes. The greatest difference between the staged techniques according to the invention and conventional staged combustion techniques is therein that the combustion method according to the present invention allows suffi¬ ciently abruptly staged combustion. In order to attain an efficient chemical reduction of nitrogen oxides, an efficient method must be available for the gasification of the fuel as a sufficiently air-deficient mixture. This is necessary to make all oxygen contained in the mixture to react with the fuel, instead of nitrogen. A sufficiently air-deficient mixture can be efficiently gasified only with the help of the flame having a high energy density delivered by a plasma torch.

The decreased quantity of oxides of nitrogen is achieved on maintaining the conditions in the hot zones of the flame, particularly the plasma torch flame, strongly reducing with the help of staged combustion. Then, oxides of nitrogen formed in the hot flame are reduced to molecular nitrogen. Similarly, with the help of staged combustion, temperatures in the other zones of the flame are maintained so low as to avoid formation of oxides of nitrogen. Because the plasma flame does not need fuel or combustion air, it is void of oxygen necessary for the formation of oxides, thus permitting a rapid reduction of oxides of nitrogen possibly formed in the extremely air-deficient conditions. The plasma gas can also be nitrogen without causing an essential increase of NO_x content, because oxides of nitrogen formed by reacting with the combustion air are rapidly reduced in the first air-deficient stages of the combustion process. Instead of nitrogen, the plasma gas can also be any other inert gas.

The combustion process in accordance the present invention was investigated measuring concentrations of oxides of nitrogen achievable in tests performed according to the invention. A burner shown in Fig. 1 was connected to a test boiler 6' in the manner illustrated in Fig. 2. The fuel and air connection unions of the burner shown in Fig. 2 are designated with the same reference numbers as those shown in Fig. l. In addition to these unions, the boiler 6 is further equipped with a third air feed union 5 and a flue gas scrubber 7. The purpose of the test runs was to study the effect of different parameters on NO_x emission levels.

The test runs were performed in order to investigate the effect of the following factors on the NO_x emission levels in flue gases from the burner.

Test run no.Variable investigated

Test series #1

1 Level of NO_x emissions without staging

2 Effect of plasma gun power output (no staging applied)

3 Effect of combustion chamber temperature (no staging applied) 4...7

Effect of auxiliary tertiary air

Test series #2

8

Effect of nitrogen replacing air as plasma gas of the plasma gun 9

Effect of plasma gun power output 10

Effect of argon replacing nitrogen

11 Effect of auxiliary tertiary air

12...15

Level of N0_X emissions in controlled optimum conditions of test runs 8...11

The air rate settings and results obtained are given in Tables 1...4 below.

1) Test run part 11a was performed using a smaller tertiary air flow rate than in test run part lib.

2) Entire duration of test run 11.

Temperature in combustion chamber front end

(set value of coal feed)

Fuel transport air flow rate

Primary air flow rate

Secondary air flow rate

Tertiary air flow rate

o Overall air flow rate 10

Table 3 ,

no. [%1 [mg Jl [%1 [mg/MJl

1 2 3 4 5 6 7

Table 4.

Test run CO_a CO N0₂ no. [%] [mg/MJ] [%] [mg/MJ]

8a 8b,

2) 9a^' 9b 10a 3)

10b

11a 4) lib

12

13

14

15

1 ) Test run part 8a was performed using air as the plasma torch gas, while nitrogen was

2) used as plasma gas in test run part 8b.

In test run part 9a the plasma torch output

3) was higher than in test run part 9b.

In test run part lOa the plasma gas was argon, while nitrogen was used as plasma gas

4) in test run part 10b.

In test run part lla the tertiary air flow rate was smaller than in test run part lib. The above tables clearly show that abrupt staging achieves a substantial reduction in NO_x emissions. Lowest emission levels are attained by feeding less than 30 % of the combustion air volume into the burner as the transport gas of fuel transfer, while the primary and secondary air volumes together form less than approx. 10 % of the overall combustion air volume.

In addition to firing with pulverized coal, the combustion method according to the present invention can be applied to firing with other kinds of gaseous, liquid or solid fuels. The method can be adapted to all types of fired power plants such as boilers, combustion chambers of gas turbines and different kinds of furnaces. Fuel and air can be fed into the burner along its longitudinal axis, as well. It is also possible to have a greater number of air and fuel feed unions than those discussed in the text above.

Claims

WHAT IS CLAIMED IS:

1. A method for reduction of emissions of oxides of nitrogen in combustion processes burning a solid, liquid or gaseous fuel, said method being based on staged combustion of the fuel, c h a r a c t e r i z e d in that

- the fuel is first fed in an extremely air- deficient form in order to attain reducing conditions into a flame of a plasma torch (1), into a first gasification zone (8) , where the fuel is gasified and partially forced-ignited,

- auxiliary air is fed in at least one stage to the partially gasified fuel flow in the subsequent second gasification zone (9) for the purpose of further gasifying and partially burning said fuel,

- the gasified fuel flow is routed to an actual combustion chamber, e.g., a furnace, burner or similar space (6), where its combustion is completed.

2. A method as defined in claim 1, c h a r a c t e r - i z e d in that a fuel flow fed to the flame of the plasma torch (1) contains air not more than 30 % of the overall combustion air volume necessary for the complete combustion of the fed fuel volume.

3. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that into the partially gasified fuel flow is mixed an auxiliary air flow having, together with the air volume routed to the plasma torch flame, maximally a volume not greater than 50 % of the overall combustion air volume necessary for the complete combustion of the fed fuel volume.

4. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the stoichiometric coefficient in the first ignition and gasification zone (8) in front of the plasma torch (1) is maintained in the range 0.05...0.3.

5. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the fuel is fed in front of the plasma torch (1) with the help of an air flow.

6. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the combustion air is fed into the fuel flow in four stages: mixed with the fuel inlet flow, as a primary and secondary air in order to control the degree of fuel gasification, and finally as the actual combustion air to attain completed combustion of the fuel.

7. A method as defined in claim 6, c h a r a c t e r ¬ i z e d in that more than half of the overall combustion air is fed to the fuel flow in the last stage.