RU2095397C1 - Method of processing solid fuel - Google Patents

Abstract

FIELD: heat-power engineering and environmental protection. SUBSTANCE: invention relates to preventing pollution of effluent gases by nitrogen oxides and reducing carbon dioxide content in them when burning solid fuels in combustion chambers of boilers at heat and power stations, industrial and district heating plants. Method consists in four-step processing of solid fuel. First, feeding blowing gas with minimum oxygen content (α = 0.05-0.15) in conjunction with heating fuel-gas mixture to 600- 650 C ensure reaction of amines, predominantly NH, NH₂, and NH₃ with each other to form elementary nitrogen and its simultaneous oxidation into oxide and dioxide as intermediates which successively react with hydrocarbon pyrolysis products to form elementary nitrogen. In the second step, burning of coke residue is conducted under oxygen deficiency conditions (α = 0.85-0.90) at minimum CO₂ yield and temperature as high as 800 C providing conversion of CN and HCN into elementary nitrogen through formation of nitrogen oxides and passage over two kinetic barriers with participation of free radicals. In the third step, effluent gases are cooled to temperature below 140 C and ozone is fed to provide conversion of residual oxides into nitrogen dioxide and tetroxide and fast reaction with supplied water to yield nitric acid or with alkali solution to yield nitrates. Finally, pure high-energy gas is burned at temperature not higher than 800 C to exclude probability of arising secondary nitrogen oxides or is used to produce methanol. Using oxygen mixed with steam when conducting burning under pressure provides essentially complete purification of effluent gases to safe purity degree and decrease of burning chamber dimensions. EFFECT: enhanced efficiency of process. 3 cl

Description

 The invention relates to a power system, in particular to the combustion of solid fuels in the furnaces of boilers of thermal power plants, in industrial and district heating boiler rooms.

Methods for burning organic solid fuels are known in the heat power industry for the production of thermal energy, which include the reduction of flue gas pollution by nitrogen oxides by technological methods in the combustion chamber and the post-treatment by chemical and physicochemical methods outside it, which require large capital costs for the construction of complex plants and the costs of their operation [1, 2, 3] or only by technological methods, including three-stage combustion with the supply of natural gas to the second stage [4] It is known and large the number of purely technological methods for reducing the concentration of nitrogen oxides directly in the combustion chamber without the use of additional substances [5, 6] that provide a reduction in the concentration of NO _{x by} 2-2.5 times by each of the methods. The generalized results of all known combustion methods, including pre-treatment by gasification in a fluidized bed, show that it is possible to achieve a degree of purification of up to 99.5% and a decrease in the concentration of NO to 70-190 mg / m ^-3 [2] which, however, exceeds 100 and more than once permissible for human life.

There is also known a method of producing steam without nitrogen oxides using fossil fuels, including a two-stage combustion system consisting of a first stage of gas formation with partial oxidation with air at atmospheric pressure to produce hot gas at a temperature above 1400 ^o C and removal of liquid ash, and a second stage of complete combustion almost without the formation of nitric oxide at temperatures below 1300 ^o C [3]
The reasons that impede the achievement of the required technical result when using the known method include the fact that in the known method there are no technological methods and modes that ensure the use of modern ideas about the forms of nitrogen bonding in fuel (coals), their change during heating, pyrolysis and combustion [7 ] consisting in the fact that:
1) in the region of low temperatures up to 600 ^o C amines and partially cyans are formed, the reactions of which are carried out among themselves by a nonequilibrium chain mechanism with the formation of free radicals and are completed by conversion to molecular nitrogen in hundredths of a second;
2) in the presence of oxygen, they partially react to the formation of NO and NO ₂ and free radicals, which, having overcome two kinetic barriers, are completed by conversion to N ₂ ;
3) in parallel, the processes of interaction, mainly of amines with hydrocarbons of solid fuel pyrolysis products, also ending with conversion to N ₂ in hundredths and thousandths of a second [9]
4) at a temperature of 600-800 ^o C there are both homogeneous and heterogeneous reactions between those remaining in the coke residue, mainly cyanides with oxygen, before the appearance of NO as an intermediate product that actively reacts with CO and C fuel only to N ₂ , and when at a temperature of 800 ^o C thermal nitrogen oxides also occur with a concentration below 0.5-1 ppm, which is the limit for human life, and at 1300 ^o C only the equilibrium composition of thermal nitrogen oxides is more than 1000 ppm, which guarantees the required degree of purification of waste gases.

 There are no technological methods for the conversion of nitric oxide into more complex oxide compounds that readily chemically interact with water, which would guarantee absolute release of exhaust gases from nitrogen oxides.

Closest to the invention is a method of processing solid fuel to produce high-calorie gas or synthesis gas, comprising pre-oxidizing the fuel with air at elevated temperature and subsequent gasification with a mixture of air and water vapor, implemented in a plant for processing coal and generating electricity and gas [10]
For reasons that impede the achievement of the desired technical result when using the known method adopted for the prototype, is that it does not provide for the prevention of nitrogen oxides during fuel processing.

 The objective of the invention is to provide in the exhaust gases during the processing of solid fuels safe concentrations of nitrogen oxides for human life.

The problem is solved by achieving a technical result in the implementation of the invention, which consists in the conversion of nitrogen fuel to elemental nitrogen N ₂ .

The specified technical result in the implementation of the invention is achieved by the fact that the known method of stepwise processing of fuel, including preliminary oxidation of the fuel with air at elevated temperature and subsequent gasification with a mixture of air and water vapor, is carried out in four stages. At the oxidation stage, a preheated inert exhaust gas with an oxygen content in the range of 0.05-0.15 from the stoichiometric (α 0.05-0.15) provides heating to 600-650 ^o C and oxidative pyrolysis of volatile substances, and the interaction of arising amines and partially cyans by a nonequilibrium chain mechanism between themselves and the resulting nitrogen oxides and dioxides as intermediate compounds, culminating in the conversion to elemental (N ₂ ) nitrogen. At the same time, reactions with hydrocarbons of solid fuel pyrolysis products, which also result in conversion to elemental nitrogen, also take place in parallel. At the gasification stage, water vapor is supplied and the oxygen supply is increased, depending on the fuel grade, to a <1, providing a temperature increase of up to 800 ^° C, stable combustion of the coke residue and the conversion of all nitrogen compounds, including their oxides, to elemental nitrogen. In the third stage, the exhaust gases are cooled to a temperature below 140 ^o C and ozone is supplied in relation to the remaining nitric oxide O NO 1.1, followed by washing with water or an alkali solution. In this case, the remaining nitrogen oxides are first converted into dioxide and nitrogen tetroxide, which have the property to chemically react with water and alkalis, and then completely converted into useful products in the form of nitric acid or nitrates in water or alkali. In the fourth stage, the resulting high-calorie gas is burned at a temperature of up to 800 ^o C, eliminating the possibility of thermal nitrogen oxides, or sent for processing into methyl alcohol.

At the stage of gasification, the oxygen supply is carried out in a mixture with water vapor in an amount that maintains a temperature of 800 ± 30 ^o C.

In addition, gasification is carried out under a pressure of at least 5 kg / cm ^-2 .

 The above set of features ensures the achievement of the specified technical result, which determines the causal relationship between the features and the technical result and the materiality of the features of the claims.

 The analysis of the prior art, including a search by patent and scientific and technical sources of information and identification of sources containing information about analogues of the invention, allowed to establish that no analogue was found, characterized by features identical to all the essential features of the invention, and the definition from the list of identified analogues of the prototype as the analogue closest in terms of the totality of features made it possible to identify the set of distinctive prizes essential in relation to the perceived technical result nakov in the claimed subject matter set forth in the claims. Therefore, the invention meets the requirement of "novelty."

 To check the compliance of the invention with the requirement of the inventive step, an additional search was carried out for known solutions in order to identify features that match the distinctive features of the invention, the results of which show that the invention does not explicitly follow the prior art, that is, meets the requirement " inventive step ".

 Information confirming the possibility of carrying out the invention to obtain the above technical result are as follows.

Одновременно происходит реагирование с кислородом O₂ по реакциям
NH + O₂ NO + OH
NH₂ + O₂ NO + H₂O
и завершается конверсией с участием углеводородов продуктов пиролиза

Возникающие при этом свободные радикалы ускоряют процессы химического реагирования при низких температурах, которые завершаются за сотые доли секунды [9] Требуемая температура и количество подаваемого дутья регулируются автоматически с учетом устойчивости газодинамического потока для перемещения частиц требуемого размера, а также отсутствия оксидов азота на выходе из зоны. Наиболее перспективными являются топки вихревые и с кипящим слоем.At the first stage of the process, a complete conversion of nitrogen-containing compounds released in the heating and pyrolysis zone to elemental nitrogen is ensured. For this, the fuel-gas mixture is first heated to 600-650 ^o C due to the supply of oxygen in an amount of 5-15% of the stoichiometric (a 0.05-0.15) depending on the brand of fuel. This amount is sufficient for the partial oxidation of the products of pyrolysis and the formation of nitrogen oxides as intermediates and conversion to elemental nitrogen. First, the formed NH, NH ₂ and NH ₃ , which have the properties of free radicals, react with each other by a chain mechanism

At the same time there is a reaction with oxygen O ₂ reactions
NH + O ₂ NO + OH
NH ₂ + O ₂ NO + H ₂ O
and ends with a conversion involving hydrocarbons of pyrolysis products

The resulting free radicals accelerate the chemical reaction processes at low temperatures, which are completed in hundredths of a second [9] The required temperature and the amount of blast supplied are automatically controlled taking into account the stability of the gas-dynamic flow to move particles of the required size, as well as the absence of nitrogen oxides at the outlet of the zone . The most promising are swirl and fluidized-bed furnaces.

At the second stage (gasification), all nitrogen remaining in the coke residue is converted into molecular nitrogen, for which additional oxygen is supplied in an amount of α <1 and water vapor (taking into account the moisture contained in the fuel), the amount of which is automatically controlled by the content of NO, O in the exhaust gases ₂ , CO ₂ , CO, H ₂ and CH ₄ . At the same time, a minimum concentration of CO ₂ is achieved to reduce the effect on the greenhouse effect, as well as a minimum concentration of CH ₄ and the H ₂ CO 2 ratio in order to more efficiently use exhaust gases for the production of methyl alcohol. The gas temperature is automatically maintained within 800 ± 20 ^o C by changing the ratio between the supply of oxygen and water vapor, the numerical value of which depends on the type of fuel (the yield of volatile substances, moisture and ash content) and the amount of heat removed from the combustion chamber through screens and other cooling surfaces fireboxes. In the absence of the latter, the yield of CO ₂ decreases by 30-40% and the yield of combustible components (CO + H ₂ + CH ₄ ) increases and, accordingly, the calorific value of the exhaust gases increases. In this case, the total volume of exhaust gases compared with burning in air, and therefore the volume of the furnace, is reduced by approximately 3 times.

Первым кинетическим барьером конверсии NO до N₂ является реагирование с углеродом и углеводородами топлива

Вторым кинетическим барьером являются промежуточные реакции с радикалами типа

Образующиеся радикалы NH и NH₂ легко рекомбинируют между собой до образования N₂ за доли секунды [9] Экспериментально доказано, что при подаче кислорода при отношении O C 0,85 ± 0,95 (в зависимости от марки угля) концентрация NO в отходящих газах снижается более чем в 1000 раз и достигает значения менее 1 ppm [7, 8]
Процесс газификации ведут под давлением выше 5 кг/см^-2, что также дополнительно гарантирует снижение концентрации до менее 0,5 ppm за время менее 1 с за счет увеличения частоты соударений реагирующих молекул между собой [7]
На третьей стадии ведут доочистку отходящих газов от остатков оксида азота путем доокисления его озоном до диоксида и четырехокиси азота
3NO+O₃ _→ N₂O₄+NO₂
и реагирования их с водой до азотной кислоты или со щелочью до нитратов, являющихся промышленным сырьем.At this stage of coke residue combustion, predominantly cyans (CN, HCN) arise, and for individual fuels and NH ₃ with the formation of free radicals by reactions with oxygen

The first kinetic barrier to the conversion of NO to N ₂ is the reaction with carbon and hydrocarbon fuels

The second kinetic barrier is intermediate reactions with radicals of the type

The formed NH and NH ₂ radicals easily recombine with each other to form N ₂ in a fraction of a second [9] It has been experimentally proved that when oxygen is supplied at an ratio of OC of 0.85 ± 0.95 (depending on the grade of coal), the NO concentration in the exhaust gases decreases more than 1000 times and reaches a value of less than 1 ppm [7, 8]
The gasification process is carried out at a pressure above 5 kg / cm ^-2 , which also additionally guarantees a decrease in concentration to less than 0.5 ppm in less than 1 s due to an increase in the frequency of collisions of the reacting molecules with each other [7]
In the third stage, the exhaust gas is treated after residual nitric oxide by oxidizing it with ozone to dioxide and nitrogen tetroxide.
3NO + O ₃ _ → N ₂ O ₄ + NO ₂
and reacting them with water to nitric acid or with alkali to nitrates, which are industrial raw materials.

In the fourth stage, off-gases are burned at a temperature not exceeding 800 ^o C, which eliminates the possibility of thermal nitrogen oxides occurring when air blast is supplied by known methods (using flameless burners in a fluidized bed with an inert coolant, etc.) or sent for processing to methyl alcohol.

An example implementation of the method. In the first stage, pre-crushed fuel along with hot combustion products is fed into a special chamber where they maintain a temperature of 600-650 ^o C and add oxygen to α 0.05-0.15, depending on the type of fuel. The fuel is kept in this chamber until the bulk of the volatile matter leaves it. Here, oxidative pyrolysis of volatiles and the interaction of arising amines, cyans and hydrocarbons occur, and the resulting nitrogen oxides are converted by a chain mechanism to elemental nitrogen N ₂ .

In the second stage, the gas-fuel mixture is sent to the gasifier (vortex or fluidized bed) without heating surfaces, oxygen is added to the mixture with water vapor or water to a <1. The temperature of 800 ± 20 ^o C is maintained in the boiler furnace by regulating the supply of oxygen and water, providing stable combustion of the coke residue and the conversion of nitrogen compounds to elemental nitrogen N ₂ .

In the third stage, the combustion products are cooled in the boiler flues by heating surfaces and other known methods, for example, in gas-water mixing heat exchangers, to a temperature of 100 ^o C, the gases are dried to an economically feasible level, ozone is mixed into the gases in an amount determined by the gas humidity and fuel grade, providing the ratio to the remaining nitric oxide O ₃ NO 1.1, which is necessary for the conversion of NO into nitrogen dioxide and nitrogen dioxide actively reacting with aqueous solutions of alkalis. Then all the gas is washed with water or an alkali solution in a known manner, for example in foam machines, by binding nitrogen oxides and obtaining a marketable product, for example nitrates.

In the fourth stage, the resulting high-calorific combustible gas without nitrogen oxides is sent for processing to methyl alcohol or burned to produce heat at a temperature not exceeding 800 ^o C to avoid the formation of thermal nitrogen oxides, or sent to consumers of gas fuel.

Thus, the above information indicates that when using the invention the following combination of conditions:
the means embodying the invention in its implementation is intended for use in industry, namely in the combustion chambers of the furnaces of thermal power plants, industrial and heating boilers;
for the invention, in the way it is characterized in the independent claim, the possibility of its implementation using the above means and methods is confirmed;
means embodying the invention in its implementation, is able to ensure the achievement of the perceived technical result.

 Therefore, the invention meets the requirement of "industrial applicability".

Information sources:
1. Kotler V.V. Grachev S.P. Combined method for reducing emissions of SO _x and NO from thermal power plants. Energy economy abroad. 1991, 6, p. 23-25.

 2. Gorin V.I. Dyakov A.F. Nechaev V.V. Olkhovsky G.G. Electricity from fossil fuels. Thermal Engineering, 1993, 6, p. 12-22.

 3. PCT Patent WO 86/03821, CL F 23 C 6/04, 1986.

 4. Kotler VV Reznichenko Yu. Experience of EER company (USA) in reducing emissions of nitric oxide and sulfur dioxide in pulverized coal boilers. Thermal Engineering, 1993, 8, p. 69-72.

 5. Enikin Yu.P. Kotler V.R. Babiy V.M. Shtalman S.G. Scherbachenko S.I. VTM works to reduce nitrogen oxide emissions by technological methods. Heat Power Engineering, 1991, 6, p. 33-38.

 6. US patent N 7742787, CL. F 23 D 1/00, 1986.

 7. Yavorsky I. A. On ways to prevent emissions of nitrogen oxides by technological methods in the combustion of solid fuels. Heat power engineering. 1995, N 2, p. 19-23.

 8. Burd T.E. Tillman F.R. Wen-jin chen atot. Parfitoning of Nitrogenous. Speies in the Fuel-Rich stage of Reburning. Energy-Fuel, 1991.5, N 2, p. 231-243.

 9. I. Boyle et al. Nitrogen oxide reducfiin from Post-Combustion gases. Fuel, 1993. 72, N 10, p. 407-427.

 10. Patent of Russia N 1058509, cl. C 10 J 3/00, 1983.

Claims (3)

1. A method of processing solid fuel to produce high-calorie gas or synthesis gas, comprising pre-oxidizing the fuel with air at elevated temperature and subsequent gasification with a mixture of air and water vapor, characterized in that the fuel is oxidized at 600 - 650 ^o C with the supply of hot blast with an oxygen content of α = 0.05-0.15, and at the gasification stage, the oxygen consumption is increased to α = 0.85-0.90, which provides coke residue combustion and oxidation of the remaining nitrogen compounds to nitrogen oxides and the conversion of the latter of elemental nitrogen, and the resulting gas is cooled to a temperature below 140 ^o C, it is mixed with ozone in an amount providing a ratio of O ₃ / N 1,1 followed by washing the gas mixture with water or alkali solution, and in a fourth step the formed burn rich gas at temperatures up to 800 ^o C or sent for processing in methyl alcohol. 2. The method according to claim 1, characterized in that the gasification of fuel is carried out under pressure above 5 kg / cm ² .  3. The method according to PP. 1 and 2, characterized in that the temperature at the stage of gasification of fuel and the minimum content of carbon dioxide in the produced gas is controlled by changing the ratio between the supplied oxygen and water vapor, depending on the type of fuel, its moisture content and the presence of screen or other cooling surfaces in the gasification chamber.