Field of the invention

The invention relates generally to the reduction of emissions of combustion gases in ener gy plants. The invention also relates to the production of heat in boilers, gas turbines, and diesel power plants and similar power stations.

Particularly, the present invention relates to a method, according to the preamble to Claim 1, for the catalytic cleaning of combustion gases containing nitrogen oxides and carbon monoxide, hydrocarbons and soot particles of energy plants using fuels with a hydrocarbon content.

The invention also relates to a method, according to the preamble to Claim 19, for produc- ing thermal energy from a fuel with a hydrocarbon content. According to such a method, the fuel is burned at an increased temperature, the heat obtained from the combustion is recovered, and the exhaust gases and soot particles from the combustion are cleaned using catalytic exhaust-gas combustion.

Background

World-wide, nitrogen-oxide (NOx), carbon-monoxide (CO), carbon-dioxide (C0 ₂ ), and hydrocarbon (VOC) emissions arising in the production of energy are being restricted in order to limit the greenhouse effect ln Europe, several directives aimed at this have been issued for thermal boilers, process devices, fireplaces, etc. In these, emission limits have been set for greenhouse gases, either directly or with the aid of efficiency or emissions lim its. In the USA, the EPA and CARB have set limits for nitrogen oxides and hydrocarbons and their compounds. A corresponding development is taking place in China. For example, in Beijing limit values of 30 mg/m ³ for NOx and 80 mg/m ³ for CO have be set for boilers. The limits are so tight that they cannot be achieved with existing conventional thermal combustion devices without after-treatment. The same strongly tightening trend will con tinue in other industrialized areas in China. Already before the aforementioned emission limits became established, in Beijing even tighter limits have begun to be aimed at. In the case of NOx, the aim is zero and even in the case of CO the aim is substantially below the previous limit.

Commercial manufacturers of UltraLow NOx and NoNOx burners use flue-gas recircula- tion, water emulsions, and gas preheating, in place of flue-gas after-treatment, in efficiently mixing burners. Despite the burner’s NoNOx name, not a single thermal-burner manufac- turer has achieved zero NOx emissions. The lowest reported value is 6 ppm with an 0 ₂ content of 3 %.

Another alternative is the cleaning of flue gases. Generally selective catalytic and non- catalytic reducing agents are used to remove nitrogen oxides (Selective Catalytic Reduc- tion, in the following, abbreviated to“SCR”, and Selective Non-Catalytic Reduction, “SNCR”). At its best, catalytic SCR achieves a 90-% cleaning level at a temperature of about 350 °Ce. In its reports, the EPA states SCR’s average NOx conversion to be 85 %. Using non-catalytic SNCR equipment, a cleaning effect about 20 % lower is achieved. Their use has been aimed at diesel vehicles.

However, SCR apparatuses requires a separate reducing agent, urea or ammonia, and its dosing equipment, which leads to significant investment and operating costs. Urea disinte grates in a catalyser to form ammonia (NH ₃ ) and carbon monoxide (CO). Ammonia and urea are transported and stored in water solutions. The ammonia content is 27 % and urea 32 %. Ammonia is a highly toxic gas. Using an SCR plant, an NOx emission level of about 7-30 ppm is achieved. In addition, the CO emission limits demand a separate oxidation catalyser. The use of ammonia as a reducing agent is based on its ability to selectively re duce NOxs in a lean gas mixture.

The high costs of SCR technology are caused by the ammonia (NH ₃ ) needed for selective reduction or urea in addition as a reducing agent and the expensive storage, dosage, heat transfer, and reduction apparatuses. In addition, the EPA has estimated the replacement interval for an SCR catalyser to be 3 years. In SRC catalysers the most usual active sub stance, i.e. the catalyst, is vanadium pentoxide (V2O5), which is easily the most toxic of the noble metals. An SCR catalyser is large in size, because its space velocity is low, i.e. 10 000-20 000 l/h. The space velocity of noble-metal catalysers is 5 _... l0-times greater, i.e. the size of a noble-metal catalyser is about one-fifth to one-tenth of that of an SCR catalys- er.

Other weaknesses of SCR catalysers are NH ₃ leaks (2-5 ppm) and the risks during han dling and transportation caused by the toxicity of NH ₃ . Particularly in the USA, demands are being made to replace the toxic SCR catalyst (V2O5) with, for example, non-toxic zeo lite.

Limits are also put to catalytic combustion by so-called catalyst poisons, which the com bustion gas must not contain. The most important of these are organic silicon, heavy-metal, and phosphoric compounds. They deactivate catalysers permanently. Sulphuric compounds do not damage a catalyser activated with platinum, but sulphuric acid arising as the result of reactions can cause corrosion, if it concentrates on the surfaces of a heat exchanger at temperatures of more than 100 °C.

US 4 118 171, US 2017/153024, and US 2009/284013 can be mentioned as publications of the prior art.

Summary of the invention

The present invention is intended to eliminate at least some of the problems relating to the prior art and create a completely new type of solution for producing thermal energy from fuels containing hydrocarbons and correspondingly for catalytically cleaning the exhaust gases, containing nitrogen oxides and carbon monoxide, hydrocarbons and soot particles, of energy plants using fuel containing hydrocarbons.

In a first embodiment of the invention, the energy plant’s exhaust gases are led to an ex haust-gas burner, in which the gases are brought to catalytic oxidation and reduction, in order to reduce the NOx, CO, VOC, and particle content of the exhaust gases and to simul taneously produce thermal energy. Typically, NOx compounds are first reduced and then the CO, VOC compounds and the particulate impurities of the gases are oxidized and at the same time recoverable thermal energy is produced. In a second embodiment of the invention, thermal energy is produced in at least two units, when thermal energy is first produced in the energy plant by burning a fuel with a hydro- carbon content. Heat is recovered, for example, in a heat exchanger. Fuel and air are fed into the exhaust gases obtained from combustion to form a gas mixture and the gas mixture which is then brought to catalytic combustion, which is performed at a high temperature.

By performing combustion in the presence of a catalyst in reducing and correspondingly oxidizing conditions at a temperature of at least 600 °C, the nitrogen oxides contained in the flue gases are reduced and the carbon monoxide, hydrocarbons, and soot particles are oxidized. The heat obtained from the catalytic combustion is also recovered.

More specifically, the solutions according to the said invention are mainly characterized by what is stated in the characterizing portions of the independent Claims.

Considerable advantages are gained using the present invention.

As already stated above, using thermal combustion or existing flue-gas cleaning methods it is not possible to produce as clean thermal energy as is now being demanded in Beijing. However, the goal can be reached using the solution according to the present invention, in which, using catalytic flue-gas combustion it is possible to reduce the NOx, VOC, CO, and particulate flue-gas emissions of even already operating boilers, diesel, and gas-turbine power plants to an almost zero level. At the same time, it is also possible to burn the small soot particles produced by oil and gas boilers, and diesel power plants.

With the aid of the invention, a method is created to clean the NOx, CO, HC, and particle emissions of thermal energy plants using a single apparatus more effectively than by using any existing production technology, and the same time produce additional energy. Using the solution according to the invention, all of the emissions itemized above are eliminated in a single device.

Thus, using the present solution, it is possible to reduce NOx compounds so that their re- sidual content is less than 1 ppm, and CO and VOC compounds can be oxidized to that their residual content is less than 2 ppm. Small soot particles can also be burned in a flue- gas burner at a temperature of 600 °C or more. The preferred temperature range for the burner is 850-1000 °C. In this range the soot particles also burn rapidly.

Simultaneous energy production permits the use of a flue gas of a thermal boiler, turbine, or diesel power plant, etc., as a cooling and heat-transfer agent in catalytic combustion.

The inert thermal mass of exhaust and flue gases is then used in combustion to control temperature and transfer heat. With the aid of a flue gas, the temperature can be kept with in desired limits, preferably in the range 850-1000 °C.

Unlike other cleaning methods, the present solution can be used to increase the thermal energy production capacity of a thermal boiler by up to 60 %.

A flue-gas burner can be added to all energy-production devices, in which the sulphuric emissions and particle emission are low, and in which there are are no so-called catalyser poisons. On the other hand, the amounts of NOx, CO, and VOC emissions of the energy sources are of no practical importance. In afterburning, the small soot particles produced by, e.g., oil boilers and diesel power plants also bum. Unless there is a storing POC cata lyser or filter in connection with the catalysers, it is preferable to arrange an intermediate chamber before heat recovery, in which the particles have time to burn before energy re covery.

The particles of diesel engines are in size mainly small so-called nanoparticles. More than 90 % of them have a diameter of less than 50 nm. They travel into the lungs with the breathing air and go partly through them into the short blood circulation, causing many deaths. Nanoparticles contain carbon, water, hydrocarbons, and often sulphur, as well as small amounts of other compounds. They are very porous. Once the hydrocarbon and sul phur compounds have oxidized and the water evaporated, the carbon ignites on all surfaces and bums rapidly, but the gaseous compounds more slowly. The temperature of the ex haust gas in the aforementioned range (850-1000 °C) is advantageous for these reactions, particularly the combustion of carbon. Using an exhaust-gas burner, the emissions of existing older more polluting energy- production devices can be brought to the level of new, tighter demands.

In principle, the exhaust-gas burner can be combined with a boiler or heat exchanger to clean all kinds of gases containing NOx, CO, and VOC emissions, because catalytic com bustion operates below the LEL limit. There is then not the same kind of safety risk as in thermal combustion boilers, in which the combustion of VOCs has led to fatal accidents.

Because the amount, particularly of gaseous emissions, is of little significance in terms of the operation of an exhaust-gas burner and of the cleaning result, liberties can be taken in the adjustments and device choices of the primary energy source. Boilers do not need ex pensive LowNOx or UltraLowNOx burners and the air-fuel ratio can be optimized to max imal output. In diesel engines, neither exhaust-gas recirculation (EGR) nor extremely lean mixture ratios are needed to reduce NOx emissions, etc.

The exhaust-gas burner is also suitable for operating energy plants that do not meet the emissions requirements of ever-tightening norms. Emissions-reducing investments are of ten worth making, because the plants have a long life and demand large investments. New plants form another application.

In the following, the present technology is examined in detail with the aid of a description referring to the accompanying drawings.

Figure 1 shows a process diagram of one embodiment,

Figure 2 shows a process diagram of a second embodiment,

Figure 3 shows a process diagram of a third embodiment, and

Figure 4 shows a process diagram of a fourth embodiment.

Embodiments

In the present context,“energy plant” refers mainly to a combustion plant producing ther mal energy, i.e. heat energy, in which energy is produced from a fuel with a hydrocarbon content, with the aid of boilers, diesel turbines, or gas turbines. In the first embodiment, the term“fuel with a carbon content” refers to a fuel that contains compounds comprising mainly, but not necessarily only, carbon and possibly hydrogen, such as hydrocarbons. In addition to hydrocarbons, the fuel may contain compounds with an oxygen content, such as ethers, esters, and alcohols. Examples of fuels with a carbon content according to the first embodiment are oils, petrol, diesel, and natural gas.

In the second embodiment, the term“fuel with a carbon content” also refers to a fuel that contains mainly alcohol (hydroxy) groups, ether groups, or ester groups, or combination of these comprising carbon compounds, for example, hydrocarbon compounds, which are substituted with these groups. These fuels are various biofuels, which are produced from a biomass, such as lignocellulose, vegetable oils and animal fats, cultivated plants.

The expressions“CO” and“VOC emissions” and correspondingly“NOx emissions” and “soot particle emissions” refer to the amount (as mass) of the CO, VOC, and NOx gases and correspondingly soot particles contained in the exhaust gases.

A“rich” fuel/oxygen (or fuel/air) mixture contains a greater stoichiometric amount of fuel (relative to oxygen), and“lean”, in turn, a smaller stoichiometric amount of fuel.

In the present technology, a solution is generally created for treating exhaust gases and producing energy using a catalytic exhaust-gas burner. The method can be applied to both producing heat and cleaning exhaust gases, as described below in greater detail. In the method, additional air and fuel is fed to an exhaust gas arising in thermal combus- tion, to form the amount of a gas mixture required in catalytic combustion, after which the gas mixture is led to a catalytic combustion zone for burning. The heat obtained from com bustion is recovered. As a result of combustion, CO, VOC, NOx, and soot-particle emis- sions diminish significantly in the catalytic combustion exhaust gases.

In one embodiment, exhaust gases, fuel, and air are mixed together evenly to produce a homogenous gas mixture. In this embodiment, additional air and fuel can be fed to the exhaust-gas burner and their feed can be controlled by the temperature after the catalyser and a linear oxygen sensor according to the air/fuel mixture ratio needed by each catalyser.

For the reactions described below to take place, as much fuel is added most suitably to the exhaust gas that a rich or stoichiometric ratio is achieved. In the former only the reduction of the NOxs into nitrogen (N ₂ ) and oxygen (0 ₂ ) takes place and in the latter, in addition also the oxidation of the CO and VOCs into carbon dioxide (C0 ₂ ) and water (H ₂ 0).

When operating with a rich mixture, a second catalyser is most suitably used, when an ad- ditional air feed required by a lean mixture is arranged for it. The best result is obtained using separate oxidation and reduction stages.

If it is wished to produce the maximum amount of clean energy, then air in addition to fuel must be sprayed into the flue gas. So that nitrogen oxides will not arise in afterburning, the temperature must be limited suitably to about 1000 °C. Using this solution differing from other cleaning methods it is possible to increase the thermal-energy production capacity of a thermal boiler by as much as 60 %, as described in greater detail below.

In one embodiment, fuel and air are mixed in a nested perforated feed pipe and a static mixer, to form an evenly mixed gas mixture.

The static mixer can be used to ensure the homogeneity of the gas mixture, which is par ticularly preferred to ensure even combustion.

In one embodiment, catalytic combustion is performed in one or more stages in reducing and correspondingly oxidizing conditions.

In one embodiment, catalytic combustion is performed in at least two stages, to reduce par ticularly nitrogen oxides and to oxidize carbon monoxide, hydrocarbons, and soot particles.

In one embodiment, is brought to catalytic combustion in a three-way catalyser in an oxi dation and reduction catalyser. The gas mixture can then be burned, for example, in the three-way catalyser using a stoichiometric oxygen/additional-fuel ratio in the combustion plant to oxidize the CO and VOC compounds that have remained unburned in the combus- tion plant and to reduce NOx emissions, and oxidize soot particles.

If combustion takes place at a temperature of above 600 °C, soot particles too will bum.

Alternatively, in the reduction portion of a two-part catalyser, a rich mixture is used to re- duce NOx emissions into nitrogen (N ₂ ) and oxygen (0 ₂ ) and to oxidize most of the CO and VOC emissions into carbon dioxide (C0 ₂ ) and water (H ₂ 0).

After this, additional air is added to the gas mixture to make it lean and then the mixture travels through an oxidation catalyser. In it, the remaining CO and VOC emissions are oxi- dized. In the following heat-exchange stage, the thermal energy that arises is taken into use, for example in water with the aid of welded finned-pipe radiators, after which the ex- haust gas leaves the boiler to the chimney, aided if necessary with an aspirating fan.

In one embodiment, the gas mixture is burned in an oxidation and reduction catalyser, first with a rich additional-fuel/oxygen mixture to reduce the nitrogen oxides and then with a lean additional-fuel/oxygen mixture to oxidize the CO and VOC compounds and soot par ticles.

In reducing noble-metal catalysers, the reaction chain runs mainly through vapour- reformation and water-gas transfer reactions:

H ₂ 0 + HC -> ¾ + CO and H ₂ 0 + CO -> ¾ + C0 ₂ and then

¾ + NO _x -> N ₂ + H ₂ 0.

Some of the reactions are direct oxidation and reduction reactions.

Catalytic combustion takes place the whole time below the lower explosion limit (LEL). The fuel can often be the same as in a primary-energy production device. If the flue-gas temperature before catalytic afterburning drops below 250 °C, and the fuel is natural gas or some other fuel igniting at a high temperature, then the structure of the catalyser should be a recuperative or regenerative heat exchanger, for example, a metallic cross-flow catalyser, or, to maintain combustion, a fuel igniting at a lower temperature, such as methanol or ethanol, should be fed to the fuel mixture as a support fuel.

The catalysers used in combustion are most suitably surfaced with stable metal oxides, es- pecially oxides, the cation of which is Al, Ce, Zr, L, or Ba, and to which noble metals, such as Pd, Pt, Rh, or their mixed oxides with base metals are attached.

These noble-metal catalysts are not toxic, nor do toxic compounds arise in reactions, as takes place in traditional SCR catalysers. The temperature in a catalyser is at least 600 °C, particularly 850-1000 °C in reducing conditions, or in both reducing and oxidizing conditions.

In a three-way catalyser, the space velocity is kept at a value of 50 000-150 000 l/h, for example about 60 000-100 000 l/h, while in a reducing and oxidizing catalyser the space velocities are, for example, about 60 000-200 000 l/h, preferably 70 000-150 000 l/h.

The present technology is applicable particularly in situations, in which the fuel is burned or has been burned in a combustion plant, which is an oil or gas boiler, a gas turbine, a die- sel power plant, or a similar energy plant.

In one embodiment a one or two-stage only catalytically reducing and oxidizing exhaust or flue gas-cleaning and clean energy producing solution is created. In such a solution, the flue gas has also a heat binding and transfer role. As catalytic combustion is considerably faster than thermal, it is preferable to use a flue or exhaust gas that is essentially inert, for binding energy. In this way, an excessive rise in temperature can be avoided.

As stated above, in one embodiment the present technology can be applied to produce thermal energy from a fuel containing hydrocarbons by performing combustion in at least two stages. In such a solution, part of the fuel is burned in the first combustion stage in combustion plant, to produce heat and an exhaust gas with a nitrogen and oxygen-oxide content. After this, the heat and exhaust gas obtained from the first combustion stage are recovered. In the second combustion stage, the second part of the fuel is fed into the ex- haust gas obtained form the first combustion stage. Air too is fed to form a combustible gas mixture. The gas mixture thus obtained is burned to produce heat and break down the ni trogen and oxygen oxides, As described above, in at least one catalyser zone reducing con ditions are maintained and combustion is performed in these conditions at a temperature of more than 600 °C.

The heat obtained from the second combustion stage is recovered.

In one embodiment, in the second combustion stage 10 %, most suitably 15-80 mol-% of the total amount of fuel with a hydrocarbon content is burned. With the aid of the solution, a significant part, which is about 60 %, of thermal energy additional to the primary energy source can be produced in the second combustion stage.

In one embodiment, flue gas from a boiler, turbine, or diesel power plant is used as a cool ing and heat-transfer agent in catalytic combustion. Without a cooling inert additive in stoichiometric catalytic combustion, modelling shows that the temperature will rise to more than 2500 °C. This is due to the fact that catalytic combustion is about twenty times faster than thermal. In the aforementioned embodiment, in which the temperature is raised to at least 600 °C, but preferably to at most 1000 °C, in catalytic combustion the flue gases of a thermal energy plant are most suitably used as an inert heat storage and transfer agent to keep the temperature of the catalytic combustion within the preselected temperature range. It has been shown that the unbumed gases contained in the flue gases, such as nitro gen and carbon dioxide, do not react in the conditions described, but as inert components even the heat and prevent an uncontrolled rise in temperature.

In the embodiment described above, the thermal energy contained in the gases obtained from combustion is recovered. Recovery can be made in at least one heat-transfer stage, when the thermal energy is most suitably transferred to water, air, or some other liquid or gaseous medium. In a second embodiment, the present technology is applied to catalytically cleaning, in re- ducing and oxidizing conditions, the exhaust gases, containing nitrogen oxides and carbon monoxide, hydrocarbons, and soot particles, of energy plants using fuels containing hydro- carbons. In this solution, fuel and air are fed to the exhaust gases to form a gas mixture and the gas mixture is brought to one or two-stage catalytic combustion, to be performed at a temperature of more than 600 °C, in order to reduce the nitrogen oxides and to oxidize the carbon monoxide, hydrocarbons, and soot particles. In this way, the emission level of the NOxs of the gas obtained from catalytic combustion is 1 ppm or less, and the level of CO and VOC emissions is at most 2 ppm. The small soot particles also burn at the preferred operating temperature of the exhaust-gas burner of 850- 1000 °C, because soot ignites at about 600 °C and burns with increasing speed above it. According to one embodiment, the present methods are implemented as continuously oper ating processes.

In the continuously operating processes, a hydrocarbon is used as the reducer and the ener gy source. The production of additional energy is another property of the process.

In one embodiment, the oxidation of particles and the production of clean energy are im plemented at a high temperature (at least 1000 degrees). For both of these a temperature is required, which at the same time improves conversion efficiency, as a particle oxidizer and energy producer, and is boosted by a higher temperature.

An exhaust-gas burner using the same fuel with a thermal boiler has several advantages compared to selective (SCR) or non-selective (SNCR) NOx emission reducing apparatus es:

- It is a more efficient eliminator of NOx, CO, and VOC emissions. It can be used to achieve NOx emissions at a level of about 1 and, depending on the catalyser solu tion, CO and VOC emissions at a level of 2 ppm.

- It can be used to bum small soot particles.

- It can be used to increase the thermal-energy production of a boiler by about 60 %. - It does not require separate additional fuel nor its separate storage and dosing sys- tem.

- The noble-metal catalysers in it have a longer life than SCR catalysers, in which the thermal and chemical durability of the catalyst (V2O5) used are poorer than those of noble metals. The new replacement SCR catalysts are various zeolites, which are sensitive to sulphur poisoning.

- SCR catalysers are 5-10-times greater in size than noble-metal catalysers. There is no significant difference in their prices, because the difference in size compensates for the difference in unit price. Noble-metal catalysers cost about 60 - 70€/dm ³ and SCR catalysers about 10€/dm ³ .

- The EPA has demonstrated an SCR catalyser’s NOx removal costs to be about 1400-2000 USD/t NOx. The costs of a catalytic flue-gas burner are substantially lower. The plant is smaller and simpler. The reducing agents using in an SCR plant, ammonia (NH ₃ ) or urea, are about the same price as fuels, but do not produce uti- lizable thermal energy.

- The greatest difference arises when an exhaust-gas burner can be used to produce additional energy at a competitive cost. The elimination of NOx, CO, and VOC emissions then takes place as a by-product, without separate cleaning costs.

- Ammonia is transported and stored as a 27-% water solution, as it is highly toxic.

- In SCR reduction, a 2-5-ppm ammonia leak arises (EPA), which must be catalyti- cally oxidized.

In the following, embodiments of the present technology are examined on the basis of the accompanying drawings.

Figures 1 and 2 show two embodiments, with Figure 1 showing a solution, in which both thermal and electrical energy are produced from fuel in an energy plant (a power plant), when the energy plant’s exhaust gas is cleaned primarily using a catalytic combustion pro cess. For its part, Figure 2 shows a solution, in which heat is produced, on the one hand in a thermal power plant and, on the other, by a catalytic combustion process.

As can be seen from the drawings, reference numbers 10; 20; 30; and 50 show a thermical- ly burning boiler, diesel power plant, gas turbine, or other such energy plant or power plant, which uses a gaseous or liquid fuel. In the case of Figure 1, the fuel is fed mainly to the energy plant, in which thermal energy is produced, in addition to which electricity is produced from at least part of the thermal energy thus produced.

In the case of both figures, the energy plant’s exhaust gas is guided from the exhaust duct to a mixing chamber 12; 22, into which additional air is blown and fuel is sprayed. The mixing chambers can comprise a distribution network.

In one embodiment a mixing honeycomb structure is used. Examples of this are the solu- tions that are disclosed, e.g., in utility models 10627 or CN205001032. Thus the distribu- tion network can consist of diagonally corrugated steel folio sheets, which are stacked or folded on top of each other with the corrugations crosswise. The folio sheets can be at- tached to each other at the crossing points, for example, using resistance welding or braz ing. The flow channels formed in each layer of the honeycomb cross over each other, which causes mixing and turbulence in the flow at the higher flow velocities.

In a straight-channel honeycomb the flow is laminar. The dimensionless Sherwood (Sh) number depicting the mass transfer is then about 3, the flow velocity being 10 m/s. In a mixing metal-bodied honeycomb it is about 10-12.

From the mixing chamber the gas travels through a static mixer 13; 23 to a catalyser 14;

24, 25. Behind the catalyser (in the direction of flow of the gas mixture) is a linear lambda sensor (not shown), which is arranged to measure and, for its part, to adjust the air/fuel ra tio, as well a temperature sensor controlling the temperature.

After one or two catalysers 14; 24, 25, the gas travels to a connection made, for example, of a welded ribbed pipe, or to several heat-exchanges 15; 27, in which heat is transferred to water, or some other useful purpose. The heat exchangers 15; 27, can be, for example, of welded pipes, such as preferably manufactured of ribbed pipes.

Figures 3 and 4 show the structure of the catalytic combustion system in greater detail. In the figures, the primary energy production, for example, using a diesel engine, a gas tur bine, or a combustion boiler, is marked with reference numbers 30 and 50, fuel being fed into the exhaust gas obtained from which along a feed duct 31; 51. To form a gas mixture, air is fed by fans 37; 57. The mixture is is mixed before the catalyst zone by leading through the static mixer 38; 58. The mixture is preferably rich before being led to the cata- lyst zone.

In the case of Figure 3, the catalyst zone comprises a cross-flow catalyser 33. In the case of Figure 4, the catalyst zone comprises a recuperative heat-exchanger-catalyser.

The gas mixture obtained from the first, typically reducing, catalyst zone 33; 53 is led to the second catalyst zone 35; 55, which typically comprises an oxidation catalyser. Addi- tional air is then fed to the gas mixture by a secondary-air-feed fan 39; 59.

Before starting, the catalyser or catalysers are typically pre-heated, e.g., using a hot-air fan, a gas burner, or some other heater, to above the reaction temperature of the catalyser.

The first catalyser of the exhaust-gas burner can be a conventional straight-duct catalyser, if the temperature difference between exhaust gas and the ignition temperature of the fuel is small (< 150 °C), if the exhaust gas’s carbon monoxide (CO) and nitrogen oxide (N0 ₂ ) contents are high. Carbon monoxide will ignite in a catalyser already at about 150 °C and the second oxygen of nitrogen oxide detaches easily and reacts aggressively.

A cross-flow or rotating honeycomb recuperative heat-exchanger-catalyser 53 is needed when the temperature of the incoming gas is substantially lower (> 150 °C) than the igni tion temperature of the fuel used in the catalyser.

In the device according to the invention, a three-way catalyser’ s space velocity is, depend ing on the fuel, 50 000-150 000 l/h, preferably 60 000-100 000 l/h. In reducing and oxi dizing catalysers the space velocity is 70 000-200 000 l/h, preferably 60 000-150 000 l/h. In the second catalyst zone, the thermal energy of the hot gas obtained is recovered in a heat exchanger 36; 56, in which it is transferred, for example, to water. The heat exchang ers 36; 56, can be manufactured from, for example, welded pipes, preferably ribbed pipes.

After the heat transfer, there can be an aspirating fan 40; 60, unless the outputs of the pri mary energy plant’s and additional-air fans are sufficient to transfer sufficient gas through the apparatus. Outlet gas, i.e. cleaned exhaust gas, is led from the apparatus to an outlet pipe e.g. 41; 61.

In one embodiment, the present apparatus for burning a flowing fuel with a hydrocarbon content in the presence of oxygen or air comprises, in the order of flow of the substance flows being treated

- a mixing zone, a catalytic combustion zone, a heat recovery zone, and a gas remov al zone, in which

- the mixing zone is equipped with a feed connection for the gas to be cleaned, and a feed connection for fuel, and a static mixer for mixing the gas and fuel evenly,

- the catalytic combustion zone contains, in the flow direction, at least two consecu tive combustion zones, in the first of which reducing conditions are created and in the second oxidizing conditions, and

- the heat recovery zone contains a heat-transfer element, which is connected to the catalyst zone to recover the heat released in it.

Example

Additional energy production

Thermal boiler operating with natural gas, with an output of 60 MW

Exhaust-gas input 61.000 Nm ³ /h

Additional-air input 35.000 Nm ³ /h

Additional natural-gas input 24 g/Nm ³ in 96.000 Nm ³

Additional energy created in combustion 35,2 MW i.e. 59 % (heat value 55 MJ/k) Temperature after combustion 920 °C, if input temperature is 150 °C Reduction of NOxs

NOx emission 500 mg/Nm ³

Total amount of NOx 61.000 Nm ³ /h x 500 mg/Nm ³ = 30,5 kg/h Comparison with SCR plant

The reducing cost of an SCR plant according to the lowest cost calculated by the EPA = 1400 USD/ton x 30,5 kg/h x 0,98 = 41,8 USD/h Annual cost using SCR technology = 8000 h/a x 41,8 USD/h = 344.400 USD/a

If the energy produced by catalytic afterburning is no more expensive than comparable en ergy production, then the removal of a thermal boiler’s NOxs does not cost anything, i.e. the annual saving would be 344.400 USD. At the same time, NOx emissions can be brought to a level of 1 ppm and CO and VOC emissions can be cut to a level of less than 2ppm using two-stage combustion (Figure 4).

To achieve the targets, the invention has characterizing aspects that are presented in the independent Claims.

Industrial applicability

The present solution is suitable as the simultaneous power cleaner of the, NOx. VOC, and CO emissions of, for instance, boilers, diesel power plants, gas turbines, and similar. The solution according to the invention, without any additional devices or additional costs, is also suitable for burning the particles of, for example, solid-fuel boilers and diesel power plants. It is also suitable for producing additional energy in boilers, diesel power plants, gas turbines, and similar. Using the method according to the invention, nitrogen oxides (NOx) can be reduced so that their residual content is less than 1 ppm, and carbon monoxide (CO) and hydrocarbons (VOC) can be oxidized so that their residual content is less than 2 ppm. These values can be achieved even though the primary energy source emissions are high. Small soot parti- cles too can be burned in an exhaust-gas burner, so that using the method according to the invention the particle filters of boilers and diesel engines can be replaced.

Thanks to the solution according to the invention, a primary energy plant does not require Low NOX or Ultra Low NOX burners, nor does a diesel engine require EGR or very low mixing ratios to reduce NOX emissions. The output of the primary energy plant can then be maximized. In addition to this, using the method according to the invention as much as about 60 % additional thermal energy can be produced for the primary energy source. This is so especially when exhaust gas produced in first combustion is used as a cooling and heat-exchange agent in catalytic combustion, and when fuel is fed to exhaust gases to per form second-stage catalytic combustion.

Reference numbers:

10 Primary energy production

11 Generator

12 Distribution networks

13 Static mixer

14 3 -way catalyser

15 Heat exchanger

20 Primary energy production

22 Distribution networks

23 Static mixer

24 Reducing catalyser

25 Air distribution network

26 Oxidizing catalyser

27 Heat exchanger

30 Primary energy production

31 Fuel feed ducts

32 Rich mixture

33 Cross-flow catalyser

34 Lean mixture

35 Oxidizing catalyser

36 Heat exchanger

37 Fan

38 Static mixer

39 Secondary-air feed fan

40 Vacuum fan (if required)

41 Outlet pipe (exhaust-gas pipe)

50 Primary energy production

51 Fuel feed ducts

52 Rich mixture

53 Recuperative heat-exchanger-catalyser

54 Lean mixture 55 Oxidizing catalyser

56 Heat exchanger

57 Fan

58 Static mixer

59 Secondary-air feed fan

60 Vacuum fan (if required)

61 Outlet pipe (exhaust-gas pipe)

Reference publications

Patent literature

US 4 118 171

US 2017/153024

US 2009/284013.