The gasification of communal waste, sewage sludge and animal manure results in a mixture of gases (methane, carbon-dioxide, nitrogen) of low calorific value; respectively natural gas from oil wells contains high inert-gas volume. Burning of these gas-mixtures with air from the environment results in lower flame-speed and flame-temperature, consequently, the shaft-output of the heat engine working with smoke-gas (internal combustion engine, gas-turbine) will be lower, than with burning of natural gas with air from the environment.

In the state of the art there are several solutions for utilizing of lean gases.

Hungarian patent No. P 89 01758 published on 28 May 1991 makes known a gas- torch suitable for burning gases of especially high inert content.

Hungarian patent No. HU 220 736 makes known a torch suitable for burning of lean, high inert-gas content waste gases, possibly containing poisonous components arising mainly in oil-and chemical industry. The torch made known in this patent is provided with nozzles to blow the gas into the burner head, is provided with injectors to blow the mixture of primary air and waste gas to the burner head located in the axle of the nozzles, an open burning area joined the burning head, as well as an igniting burner working with well combustible gas as well as a control unit operating them, furthermore there are intake ports and nozzles on the injectors and there is a burning stone of fireproof material and/or lining joining the burner head, which has an opening on the burning side and has a flue-gas opening as well, and the injectors are embedded in the burner head of fireproof material directed to the burning area, furthermore under the intake ports of the injectors nozzles are located in pairs on one axel and in the burning stone there is a flame-stabilizing insert.

Hungarian Patent No. P 01 01129 published on 28 August 2001 makes known a method and equipment for burning gases in a firebox, which is mainly suitable for burning lean gases in a furnace operated basically by a different type of primary fuel. The essence of the invention is, that a gas produced in a gas-developer, or any other combustible gas is introduced to the lower part of the fire-box under conditions with air-shortage, then the gas is burnt on a higher level in the fire-box, on the required level of the air-intake or on several air-intake levels.

The disadvantage of the present solutions is that, that the lean gases are burnt with environmental air, the shaft output of heat engines and the temperature of the flame leaving the burners will be lower. It results in the fact, that in gas burners gases below 10 MJ/Nm3 do not even ignite with environmental air; therefore natural gas is mixed to the lean gases in order to improve heat value. The present solutions could not advantageously realize utilization of lean gases either from technical or from commercial point of view.

When working out the invention we aimed to realize a method, with the help of which gases of low calorific value could be utilized, burned more advantageously than before, similarly to natural gases.

When working out the invention we realized, that in case the burning of the lean gas mixture takes place with higher oxygen content than burning with environmental air (20.95 % oxygen content), then the required aim can be achieved.

The invention is a method for environmentally-friendly utilization of lean gases, so that lean gases are burnt in a burning area with the addition of air, which is characterized by that a lean (7-24 MJ/Nm3) gas mixture is burnt with air with higher oxygen content than environmental air (20.95%).

In a preferable application of the method according to the invention in case of a gas-engine, or gas-turbine or gas-burner a gas-mixture of low calorific value is burnt with air of 22-36 % oxygen content.

In an other preferable application of the method according to the invention the oxygen content of air is increased to such an extent, that the temperature of flame and speed of flame-spreading became the same as that of burning natural gas with environmental air.

In a further preferable application of the method according to the invention in case of the gas-engine or gas-turbine or gas-burner the oxygen content of burning air is determined by the stoechiometric ratio of the composition (combustible and inert) and heat-value of the gas.

In a further preferable application of the method according to the invention in case of gas-engine or gas-turbine the inert content of air is decreased to such extent, respectively the oxygen content is increased within the given limits so, that corresponds to the inert substance content of the lean gas and so the same mass- stream of flue-gas that is shaft-output of gas-engine or gas-turbine is ensured as in case of burning natural gas with environmental air.

In a further preferable application of the method according to the invention in case of gas-burner the inert content of air is decreased in such extent respectively the oxygen content is increased within the given limits so, that corresponds to the inert substance content of the lean gas and this way the same flue-gas mass-stream that is gas-burner heat power is ensured as in case of burning natural gas with environmental air.

The method according to the invention is set forth by the following examples of application: Example 1 Burning of biogas : It is possible to produce biogas by fermentation from the waste produced on livestock farms. The heat value of the gas-mixture is 20-25 MJ/Nm3 which contains 56-70% methane and 44-30 % carbon dioxide (inert gas). In order to be able to achieve a flame-spreading speed and flame-temperature same as that of natural gas (94.58 % methane and 5.42 % carbon dioxide, and heat value 34 MJ/Nm3) burnt with environmental air, air of 22.8-21. 8 % oxygen content should be used, which means an improvement of 1.85-0. 85 % compared to the oxygen content of environmental air (20.95%). It means, that in order to achieve a thermal power of 1 MWth burning of a gas mixture of 0.019 kg/s methane mass stream the mass stream of carbon dioxide (inert gas) is 0.0141-0. 0077 kg/s (rate of CH4 : C02 is 1: 0,74-0, 40).

Example 2 Burning of sewage gas It is possible to produce sewage gas (bio-gas) from solid waste produced in the waste-water cleaning plants in cities. The heat value of the gas-mixture is 18-20 MJ/Nm3 which contains 50-44 % methane and 50-56 % carbon dioxide (inert gas). In order to be able to achieve a flame-spreading speed and flame-temperature same as that of natural gas (94.58 % methane and 5.42 % carbon dioxide, and heat value 34 MJ/Nm3) burnt with environmental air, air of 23.4-22. 6 % oxygen content should be used, which means an improvement of 2. 45-1. 65 % compared to the oxygen content of environmental air (20.95%). It means, that in order to achieve a thermal power of 1 MWth burning of a gas mixture of 0.019 kg/s methane mass stream the mass stream of carbon dioxide (inert gas) is 0.0179- 0. 0141 kg/s (rate of CH4 : C02 is 1 : 0,94-0, 74).

Example 3 Burning of pit gas and landfill gas: On the sites for depositing communal waste landfill gas is produced due to rotting of organic waste closed from air. A gas-mixture of similar contents and heat value is produced during the so-called complex utilization of communal waste, where the fraction of communal waste of improved organic contents is gasified by fermentation. The heat value of the gas-mixture is 12-15 MJ/Nm3 which contains 33-42 % methane and 67-58 % carbon dioxide (inert gas). In order to be able to achieve a flame-spreading speed and flame-temperature same as that of natural gas (94.58 % methane and 5.42 % carbon dioxide, and heat value 34 MJ/Nm') burnt with environmental air, air of 26.4-24. 5 % oxygen content should be used, which means an improvement of 5.45-3. 55 % compared to the oxygen content of environmental air (20.95%). It means, that in order to achieve a thermal output of 1 MWth burning of a gas mixture of 0.019 kg/s methane mass stream, the mass stream of carbon dioxide (inert gas) is 0.0364-0. 0248 kg/s (rate of CH4: C02 is 1: 1,91-1, 30).

Example 4 Burning of natural gas of high inert gas content In the earth-crust there is a significant quantity of natural gas containing inert gas, which is not economical to produce at present as it cannot be processed into natural gas with the extraction of inert gases-economically. The heat value of the gas-mixture is 7-10 MJ/Nm3 which contains 20-30 % methane and 80-70 % carbon dioxide (inert gas). In order to be able to achieve a flame-spreading speed and flame-temperature same as that of natural gas (94.58 % methane and 5.42 % carbon dioxide, and heat value 34 MJ/Nm3) burnt with environmental air, air of 36.0-27. 7 % oxygen content should be used, which means an improvement of 9.05-6. 75 % compared to the oxygen content of environmental air (20. 95%). It means, that in order to achieve a thermal power of 1 MWth burning of a gas mixture of 0.019 kg/s methane mass stream, the mass stream of carbon dioxide (inert gas) is 0.0718-0. 0419 kg/s (rate of CH4 : C02 is 1 : 3. 77-2.20).

The ratio of the oxygen content of the burning air and the heat value of the gas mixture according to the method of the invention is shown by Figure 1 and Table 1.

During the method according to the invention gases of low calorific value are burnt so, that the same flame-spreading speed and flame-temperature could be achieved as burning of natural gas (94.58 % methane and 5.42 % carbon-dioxide and heat value 34 MJ/Nm3) with environmental air. In order to achieve the set objective oxygen content of burning air is increased to the value near to Table 1.

As stoechiometric calculations are only approximate (gas mixture and air contain other components as well), values relating to the given gas-mixture should be determined by measuring.

In heat engines (gas-engine, gas turbine) mass stream of flue gas consists of burning products of chemical reaction of combustible products of lean gas mixture and oxygen of air, as well as inert gases of the gas mixture and air (carbon-dioxide and nitrogen). As the quantity of inert gases is significantly smaller, than in burning air, their effect to the mass-stream of flue-gas can be ignored.

In gas burners the heat-carrier mass-stream of flue-gas consists of burning products of combustible products of lean gas mixture and oxygen of air, as well as inert gases of the gas mixture and air (carbon-dioxide and nitrogen). As the quantity of inert gases is significantly smaller, than in burning air, their effect to the mass-stream of flue-gas can be ignored.

When determining volume ratios a mixture of two components (methane and carbon-dioxide) should be considered, which is found to be a good approach.

In a general preferable application of the method according to the invention increasing the oxygen content of air used for burning the excess air factor of flue- gas working remains unchanged, so a part of inert gases comes from the heating material, not from the air.

The advantages of the invention: With the help of the invention lean gases so far not utilized can be burnt more advantageously than before in an equivalent way with natural gas (34 MJ/Nm3) in gas engines, gas turbines, gas burners. The method according to the invention makes possible the use of gases so far unused in equivalent way with natural gas.

With burning of these gases their polluting effect (green-house effect gases) is decreased as well.

During the method according to the invention the flame-speed and flame temperature is the same as that of natural gas if oxygen content is higher than that of the air of the environment. Therefore equipment designed for natural gas can be equally well used, as their heat power is the same.