Gas combustion process. Complete and incomplete combustion of gas Burns natural gas

Natural gas is the most common fuel today. Natural gas is called natural gas because it is extracted from the very depths of the Earth.

The gas combustion process is chemical reaction, at which interaction occurs natural gas with oxygen contained in the air.

In gaseous fuel there is a combustible part and a non-combustible part.

The main flammable component of natural gas is methane - CH4. Its content in natural gas reaches 98%. Methane is odorless, tasteless and non-toxic. Its flammability limit is from 5 to 15%. It is these qualities that have made it possible to use natural gas as one of the main types of fuel. A methane concentration of more than 10% is life-threatening; suffocation can occur due to lack of oxygen.

To detect gas leaks, the gas is odorized, in other words, a strong-smelling substance (ethyl mercaptan) is added. In this case, the gas can be detected already at a concentration of 1%.

In addition to methane, natural gas may contain flammable gases - propane, butane and ethane.

To ensure high-quality combustion of gas, it is necessary to supply sufficient air to the combustion zone and ensure good mixing of gas with air. The optimal ratio is 1: 10. That is, for one part of gas there are ten parts of air. In addition, it is necessary to create the necessary temperature regime. In order for a gas to ignite, it must be heated to its ignition temperature and in the future the temperature should not fall below the ignition temperature.

It is necessary to organize the removal of combustion products into the atmosphere.

Full combustion is achieved if there are no flammable substances in the combustion products released into the atmosphere. In this case, carbon and hydrogen combine together and form carbon dioxide and water vapor.

Visually when complete combustion the flame is light blue or bluish-violet.

Complete combustion of gas.

methane + oxygen = carbon dioxide + water

CH 4 + 2O 2 = CO 2 + 2H 2 O

In addition to these gases, nitrogen and remaining oxygen are released into the atmosphere with flammable gases. N2+O2

If gas combustion does not occur completely, flammable substances are released into the atmosphere - carbon monoxide, hydrogen, soot.

Incomplete combustion of gas occurs due to insufficient air. At the same time, tongues of soot visually appear in the flame.

The danger of incomplete combustion of gas is that carbon monoxide can cause poisoning of boiler room personnel. A CO content in the air of 0.01-0.02% can cause mild poisoning. Higher concentrations can cause severe poisoning and death.

The resulting soot settles on the walls of the boiler, thereby impairing the transfer of heat to the coolant and reducing the efficiency of the boiler room. Soot conducts heat 200 times worse than methane.

Theoretically, 9m3 of air is needed to burn 1m3 of gas. In real conditions, more air is required.

That is, an excess amount of air is needed. This value, designated alpha, shows how many times more air is consumed than is theoretically necessary.

The alpha coefficient depends on the type of specific burner and is usually specified in the burner passport or in accordance with the recommendations for organizing the commissioning work being carried out.

As the amount of excess air increases above the recommended level, heat loss increases. With a significant increase in the amount of air, the flame may break off, creating an emergency situation. If the amount of air is less than recommended, combustion will be incomplete, thereby creating a risk of poisoning for boiler room personnel.

For more accurate control of the quality of fuel combustion, there are devices - gas analyzers, which measure the content of certain substances in the composition of exhaust gases.

Gas analyzers can be supplied complete with boilers. If they are not available, the corresponding measurements are carried out by the commissioning organization using portable gas analyzers. A regime map is drawn up in which the necessary control parameters are prescribed. By adhering to them, you can ensure normal complete combustion of the fuel.

The main parameters for regulating fuel combustion are:

• the ratio of gas and air supplied to the burners.

• excess air coefficient.

• vacuum in the furnace.

• coefficient useful action boiler

In this case, the efficiency of the boiler means the ratio of useful heat to the amount of total heat expended.

Air composition

Odorization

Combustible gases have no odor. To timely determine their presence in the air, quickly and accurately detect leakage points, the gas is odorized (gives a smell). For odorization, ethyl mercaptan (C 2 H 5 SH) is used. The odorization rate is 16 g of ethyl mercaptan per 1000 m3 of gas, 8 g of ethyl mercaptan sulfur per 1000 m³. Odorization is carried out at gas distribution stations (GDS). If there is 1% natural gas in the air, you should smell it.

20% of the gas indoors causes suffocation

5-15% explosion

0.15% carbon monoxide CO- poisoning; 0.5% CO = 30 min. breathing is fatal; 1% carbon monoxide is lethal.

Methane and other hydrocarbon gases are not poisonous, but inhaling them causes dizziness, and high levels in the air lead to suffocation due to lack of oxygen.

Complete and incomplete combustion of fuel:

To burn 1m³ of gas you need 10m³ of air.

The combustion of natural gas is a reaction that converts the chemical energy of the fuel into heat.

Combustion can be complete or incomplete. Complete combustion occurs when there is sufficient oxygen.

With complete combustion of gas, CO 2 (carbon dioxide), H 2 O is formed

(water). When gas is incompletely burned, heat loss occurs. Lack of oxygen O 2 oxidizing agent.

The products of incomplete combustion of CO are carbon monoxide, poisonous, C carbon, soot.

Incomplete combustion is an unsatisfactory mixture of gas with air, excessive cooling of the flame before the combustion reaction is completed.

Combustion reaction of the main components of natural gas:

1:10 methane CH 4 + 20 2 = CO 2 + 2H 2 O = carbon dioxide + water

incomplete combustion of gas CH 4 + 1.5O 2 = 2H 2 O + CO - carbon monoxide

Advantages and disadvantages of natural gas over other types of fuel.

Advantages:

The cost of gas production is significantly lower than coal and oil;

High calorific value;

Complete combustion and easier conditions for operating personnel are ensured;

The absence of carbon monoxide and hydrogen sulfide in natural gases prevents poisoning from gas leaks;

When burning gas, a minimum air residue in the furnace is required and there are no costs due to mechanical afterburning;

When burning gas fuel provides more precise temperature control;

When burning gas, the burners can be placed in an accessible place in the furnace, which allows for better heat transfer and the required temperature conditions;

The ability to change the shape of the flame to heat in a specific place.

Flaws:

Explosion and fire hazard;

The gas combustion process is possible only when oxygen is displaced;

Explosion effect during spontaneous combustion;

Possibility of detonation of a mixture of gas and air.

Anthropotoxins;

Products of destruction of polymeric materials;

Substances entering the room with polluted atmospheric air;

Chemical substances released from polymeric materials, even in small quantities, can cause significant disturbances in the condition of a living organism, for example, in the case of allergic exposure to polymeric materials.

The intensity of the release of volatile substances depends on the operating conditions of polymer materials - temperature, humidity, air exchange rate, operating time.

A direct dependence of the level of chemical pollution of the air on the general saturation of the premises has been established polymer materials.

A growing organism is more sensitive to the effects of volatile components from polymeric materials. Increased sensitivity of patients to the effects of chemical substances released from plastics compared to healthy ones. Studies have shown that in rooms with a high saturation of polymers, the population’s susceptibility to allergies, colds, neurasthenia, vegetative dystonia, and hypertension was higher than in rooms where polymer materials were used in smaller quantities.

To ensure the safety of using polymer materials, it is accepted that the concentrations of volatile substances released from polymers in residential and public buildings should not exceed their maximum permissible concentrations established for atmospheric air, and the total ratio of the detected concentrations of several substances to their MPC should not be higher than one. For the purpose of preventive sanitary supervision of polymeric materials and products made from them, it is proposed to limit the release of harmful substances V environment either at the manufacturing stage or shortly after their release by the manufacturing plants. Currently, permissible levels of about 100 chemicals released from polymer materials have been substantiated.

IN modern construction There is an increasingly clear trend towards chemicalization of technological processes and use as mixtures various substances, primarily concrete and reinforced concrete. From a hygienic point of view, it is important to take into account the adverse effects of chemical additives in building materials due to the release of toxic substances.

No less powerful internal sources of indoor environmental pollution are human waste products - anthropotoxins. It has been established that in the process of life, a person releases approximately 400 chemical compounds.

Studies have shown that the air environment of unventilated rooms deteriorates in proportion to the number of people and the time they spend in the room. Chemical analysis indoor air allowed us to identify a number of toxic substances in them, the distribution of which according to hazard classes seems to be in the following way: dimethylamine, hydrogen sulfide, nitrogen dioxide, ethylene oxide, benzene (second hazard class - highly hazardous substances); acetic acid, phenol, methylstyrene, toluene, methanol, vinyl acetate (third hazard class - low-hazard substances). A fifth of the identified anthropotoxins are classified as highly hazardous substances. It was found that in an unventilated room the concentrations of dimethylamine and hydrogen sulfide exceeded the maximum permissible concentration for atmospheric air. The concentrations of substances such as carbon dioxide, carbon monoxide, and ammonia exceeded or were at their level. The remaining substances, although they constituted tenths or smaller fractions of the maximum permissible concentration, taken together indicated an unfavorable air environment, since even a two to four hour stay in these conditions negatively affected the mental performance of the subjects.

A study of the air environment of gasified premises showed that during an hour-long combustion of gas in the indoor air, the concentration of substances was (mg/m 3): carbon monoxide - on average 15, formaldehyde - 0.037, nitrogen oxide - 0.62, nitrogen dioxide - 0.44, benzene - 0.07. The air temperature in the room during gas combustion increased by 3-6 °C, humidity increased by 10-15%. Moreover, high concentrations of chemical compounds were observed not only in the kitchen, but also in the living areas of the apartment. After shutdown gas appliances the content of carbon monoxide and other chemicals in the air decreased, but sometimes did not return to the original values ​​even after 1.5-2.5 hours.

Studying the effect of combustion products domestic gas on external breathing of a person revealed an increase in the load on the respiratory system and a change in the functional state of the central nervous system.

One of the most common sources of indoor air pollution is smoking. Spectrometric analysis of air polluted by tobacco smoke revealed 186 chemical compounds. In insufficiently ventilated areas, air pollution from smoking products can reach 60-90%.

When studying the effects of tobacco smoke components on non-smokers (passive smoking), the subjects observed irritation of the mucous membranes of the eyes, an increase in the content of carboxyhemoglobin in the blood, an increase in heart rate, an increase in the level of blood pressure. Thus, main sources of pollution The air environment of the room can be divided into four groups:

The significance of internal sources of pollution in different types of buildings varies. IN administrative buildings the level of total pollution most closely correlates with the saturation of premises with polymer materials (R = 0.75); in indoor sports facilities, the level of chemical pollution most closely correlates with the number of people in them (R = 0.75). For residential buildings, the closeness of the correlation between the level of chemical pollution both with the saturation of premises with polymer materials and with the number of people in the premises is approximately the same.

Chemical pollution of the air in residential and public buildings under certain conditions (poor ventilation, excessive saturation of premises with polymeric materials, large cluster people, etc.) can reach a level that has Negative influence on general state human body.

IN last years According to WHO, the number of reports of the so-called sick building syndrome has increased significantly. The described symptoms of deteriorating health of people living or working in such buildings are very diverse, but they also have a number of common features, namely: headaches, mental fatigue, increased frequency of airborne infections and colds, irritation of the mucous membranes of the eyes, nose, throat, feeling of dry mucous membranes and skin, nausea, dizziness.

First category - temporarily "sick" buildings- includes newly built or recently reconstructed buildings, in which the intensity of the manifestation of these symptoms weakens over time and in most cases, after about six months they disappear completely. A decrease in the severity of symptoms may be due to the patterns of emission of volatile components contained in building materials, paints, etc.

In buildings of the second category - constantly "sick" The described symptoms have been observed for many years, and even large-scale health measures may not be effective. An explanation for this situation is, as a rule, difficult to find, despite a thorough study of the composition of the air, the work ventilation system and building design features.

It should be noted that it is not always possible to detect a direct relationship between the state of the indoor air environment and the state of public health.

However, ensuring an optimal air environment in residential and public buildings is an important hygienic and engineering problem. The leading link in solving this problem is the air exchange of rooms, which provides the required air parameters. When designing air conditioning systems in residential and public buildings, the required air supply rate is calculated in a volume sufficient to assimilate human heat and moisture, exhaled carbon dioxide, and in rooms intended for smoking, the need to remove tobacco smoke is also taken into account.

In addition to regulating the quantity supply air and him chemical composition The electrical characteristics of the air environment are of known importance for ensuring air comfort in an enclosed space. The latter is determined by the ionic regime of the premises, i.e., the level of positive and negative air ionization. Negative Impact The body is affected by both insufficient and excessive ionization of air.

Living in areas with a content of negative air ions of the order of 1000-2000 per ml of air has a beneficial effect on the health of the population.

The presence of people in rooms causes a decrease in the content of light air ions. In this case, the ionization of air changes more intensely, the more people there are in the room and the smaller its area.

A decrease in the number of light ions is associated with the loss of air's refreshing properties, with its lower physiological and chemical activity, which has an adverse effect on the human body and causes complaints of stuffiness and “lack of oxygen.” Therefore, the processes of deionization and artificial ionization of indoor air are of particular interest, which, naturally, must have hygienic regulation.

It must be emphasized that artificial ionization of indoor air without sufficient air supply in conditions high humidity and air dustiness leads to an inevitable increase in the number of heavy ions. In addition, in the case of ionization of dusty air, the percentage of dust retention in the respiratory tract increases sharply (dust carrying electrical charges is retained in the human respiratory tract in much greater quantities than neutral dust).

Consequently, artificial air ionization is not a universal panacea for improving the health of indoor air. Without improving all hygienic parameters of the air environment, artificial ionization not only does not improve human living conditions, but, on the contrary, can have a negative effect.

The optimal total concentrations of light ions are levels of the order of 3 x 10, and the minimum required is 5 x 10 in 1 cm 3. These recommendations formed the basis for current Russian Federation sanitary and hygienic standards of permissible levels of air ionization in industrial and public premises (Table 6.1).

Gas combustion is a reaction between the flammable components of a gas and oxygen in the air, accompanied by the release of heat. The combustion process depends on the chemical composition of the fuel. The main component of natural gas is methane; ethane, propane and butane, which are contained in small quantities, are also flammable.

Natural gas produced from Western Siberian fields almost entirely (up to 99%) consists of CH4 methane. Air consists of oxygen (21%) and nitrogen and a small amount of other non-flammable gases (79%). Simplified, the reaction of complete combustion of methane looks like this:

CH4 + 2O2 + 7.52 N2 = CO2 + 2H20 + 7.52 N2

As a result of the combustion reaction, complete combustion produces carbon dioxide CO2 and water vapor H2O, substances that do not have a harmful effect on the environment and humans. Nitrogen N does not participate in the reaction. For complete combustion of 1 m³ of methane, 9.52 m³ of air is theoretically required. For practical purposes, it is believed that for complete combustion of 1 m³ of natural gas, at least 10 m³ of air is required. However, if you apply only theoretically required amount air, then it is impossible to achieve complete combustion of the fuel: it is difficult to mix the gas with air so that the required number of oxygen molecules is supplied to each of its molecules. In practice, more air is supplied to combustion than is theoretically necessary. The amount of excess air is determined by the excess air coefficient a, which shows the ratio of the amount of air actually consumed for combustion to the theoretically required amount:

α = V actual/V theoretical

where V is the amount of air actually consumed for combustion, m³;
V is the theoretically required amount of air, m³.

The excess air coefficient is the most important indicator, characterizing the quality of gas combustion by the burner. The smaller a, the less heat will be carried away by the exhaust gases, the higher the efficiency gas-using equipment. But burning gas with insufficient excess air results in a lack of air, which can cause incomplete combustion. For modern burners with complete pre-mixing of gas and air, the excess air coefficient lies in the range of 1.05 - 1.1”, that is, air consumed for combustion is 5 - 10% more than theoretically required.

With incomplete combustion, the combustion products contain a significant amount of carbon monoxide CO, as well as unburned carbon in the form of soot. If the burner works very poorly, then the combustion products may contain hydrogen and unburned methane. Carbon monoxide CO (carbon monoxide) pollutes indoor air (when using equipment without exhausting combustion products into the atmosphere - gas stoves, speakers of low thermal power) and has a poisonous effect. Soot contaminates heat exchange surfaces, sharply reduces heat transfer and reduces the efficiency of household gas-using equipment. In addition, when using gas stoves, the dishes become contaminated with soot, which requires considerable effort to remove. In water heaters, soot contaminates the heat exchanger, in “neglected” cases, until the transfer of heat from combustion products almost completely stops: the column burns, and the water heats up by several degrees.

Incomplete combustion occurs:

• when there is insufficient air supply for combustion;
• with poor mixing of gas and air;
• when the flame cools excessively before the combustion reaction is completed.

The quality of gas combustion can be controlled by the color of the flame. Poor gas combustion is characterized by a yellow, smoky flame. When the gas is completely burned, the flame is a short torch of bluish-violet color with a high temperature. Used to control the operation of industrial burners special devices, analyzing the composition flue gases and temperature of combustion products. Currently, when setting up certain types of household gas-using equipment, it is also possible to regulate the combustion process by temperature and analysis of exhaust gases.

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Characteristics of methane

§ Colorless;

§ Non-toxic (non-poisonous);

§ Odorless and tasteless.

§ Methane consists of 75% carbon, 25% hydrogen.

§ Specific gravity is 0.717 kg/m 3 (2 times lighter than air).

§ Flash point is the minimum initial temperature at which combustion begins. For methane it is 645 o.

§ Combustion temperature- this is the maximum temperature that can be achieved during complete combustion of gas if the amount of air required for combustion exactly corresponds to the chemical formulas of combustion. For methane it is 1100-1400 o and depends on the combustion conditions.

§ Heat of combustion– this is the amount of heat that is released during the complete combustion of 1 m 3 of gas and it is equal to 8500 kcal/m 3.

§ Flame propagation speed equal to 0.67 m/sec.

Gas-air mixture

Which gas contains:

Up to 5% does not burn;

From 5 to 15% explodes;

Over 15% burns when additional air is supplied (all this depends on the ratio of the volume of gas in the air and is called explosive limits)

Combustible gases are odorless; in order to timely detect them in the air and quickly and accurately detect leaks, the gas is odorized, i.e. give off a smell. For this purpose, ETHYLMERCOPTAN is used. The odorization rate is 16 g per 1000 m 3. If there is 1% natural gas in the air, you should smell it.

Gas used as fuel must comply with GOST requirements and contain harmful impurities per 100m 3 no more than:

Hydrogen sulfide 0.0 2 G /m.cube

Ammonia 2 gr.

Hydrocyanic acid 5 g.

Resin and dust 0.001 g/m3

Naphthalene 10 gr.

Oxygen 1%.

Using natural gas has a number of advantages:

· absence of ash and dust and removal of solid particles into the atmosphere;

· high heat of combustion;

· ease of transportation and combustion;

· the work of service personnel is facilitated;

· sanitary and hygienic conditions in boiler houses and surrounding areas are improved;

· wide range of automatic control.

When using natural gas, special precautions are required because... leakage is possible through leaks at the junction of the gas pipeline and fittings. The presence of more than 20% of gas in a room causes suffocation; its accumulation in a closed volume of more than 5% to 15% leads to an explosion of the gas-air mixture. Incomplete combustion releases carbon monoxide, which is poisonous even at low concentrations (0.15%).

Natural gas combustion

Burning called the rapid chemical combination of combustible parts of the fuel with oxygen in the air, occurs when high temperature, is accompanied by the release of heat with the formation of flame and combustion products. Combustion happens complete and incomplete.

Full combustion– Occurs when there is sufficient oxygen. Lack of oxygen causes incomplete combustion, in which less heat is released than at full carbon monoxide (has a poisonous effect on service staff), soot is formed on the surface of the boiler and heat loss increases, which leads to excessive fuel consumption, a decrease in boiler efficiency, and air pollution.

The products of natural gas combustion are– carbon dioxide, water vapor, some excess oxygen and nitrogen. Excess oxygen is contained in combustion products only in cases where combustion occurs with excess air, and nitrogen is always contained in combustion products, because is integral part air and does not take part in combustion.

Products of incomplete combustion of gas can be carbon monoxide, unburned hydrogen and methane, heavy hydrocarbons, soot.

Methane reaction:

CH 4 + 2O 2 = CO 2 + 2H 2 O

According to the formula For the combustion of 1 m 3 of methane, 10 m 3 of air is required, which contains 2 m 3 of oxygen. In practice, to burn 1 m 3 of methane, more air is needed, taking into account all kinds of losses; for this, a coefficient is used TO excess air, which = 1.05-1.1.

Theoretical air volume = 10 m3

Practical air volume = 10*1.05=10.5 or 10*1.1=11

Completeness of combustion fuel can be determined visually by the color and nature of the flame, as well as using a gas analyzer.

Transparent blue flame - complete combustion of gas;

Red or yellow with smoky streaks – combustion is incomplete.

Combustion is regulated by increasing the air supply to the firebox or decreasing the gas supply. This process uses primary and secondary air.

Secondary air– 40-50% (mixed with gas in the boiler furnace during combustion)

Primary air– 50-60% (mixed with gas in the burner before combustion) a gas-air mixture is used for combustion

Combustion characterizes flame distribution speed is the speed at which the flame front element distributed by relatively fresh stream of gas-air mixture.

The rate of combustion and flame propagation depends on:

· on the composition of the mixture;

· on temperature;

· from pressure;

· on the ratio of gas and air.

The burning rate determines one of the main conditions for the reliable operation of the boiler room and characterizes it flame separation and breakthrough.

Flame break– occurs if the speed of the gas-air mixture at the burner outlet is greater than the combustion speed.

Reasons for separation: excessive increase in gas supply or excessive vacuum in the firebox (draft). Flame separation is observed during ignition and when the burners are turned on. The separation of the flame leads to gas contamination of the furnace and gas ducts of the boiler and to an explosion.

Flame breakthrough– occurs if the speed of flame propagation (burning speed) is greater than the speed of outflow of the gas-air mixture from the burner. The breakthrough is accompanied by combustion of the gas-air mixture inside the burner, the burner becomes hot and fails. Sometimes a breakthrough is accompanied by a pop or explosion inside the burner. In this case, not only the burner, but also the front wall of the boiler can be destroyed. Overshoot occurs when there is a sharp decrease in gas supply.

If the flame comes off and breaks through, the maintenance personnel must stop supplying fuel, find out and eliminate the cause, ventilate the firebox and flue ducts for 10-15 minutes and re-ignite the fire.

The combustion process of gaseous fuel can be divided into 4 stages:

1. Gas leaking from the burner nozzle into the burner device under pressure at an increased speed.

2. Formation of a mixture of gas and air.

3. Ignition of the resulting combustible mixture.

4. Combustion of a flammable mixture.

Gas pipelines

Gas is supplied to the consumer through gas pipelines - external and internal– to gas distribution stations located outside the city, and from them through gas pipelines to gas regulatory points hydraulic fracturing or gas control device GRU industrial enterprises.

Gas pipelines are:

· high pressure first category over 0.6 MPa up to 1.2 MPa inclusive;

· high pressure of the second category over 0.3 MPa to 0.6 MPa;

· average pressure of the third category over 0.005 MPa to 0.3 MPa;

· low pressure fourth category up to 0.005 MPa inclusive.

MPa - means Mega Pascal

Only medium and low pressure gas pipelines are laid in the boiler room. The section from the network gas distribution pipeline (city) to the premises together with the disconnecting device is called input.

The inlet gas pipeline is considered to be the section from the disconnecting device at the inlet if it is installed outside the room to the internal gas pipeline.

There should be a valve at the gas inlet into the boiler room in a lighted and convenient place for maintenance. There must be an insulating flange in front of the valve to protect against stray currents. At each branch from the gas distribution pipeline to the boiler, at least 2 shut-off devices are provided, one of which is installed directly in front of the burner. In addition to fittings and instrumentation on the gas pipeline, in front of each boiler, an automatic device must be installed to ensure safe work boiler To prevent gases from entering the boiler furnace in the event of faulty shut-off devices, purge candles and safety gas pipelines with shut-off devices are required, which must be open when the boilers are idle. Low pressure gas pipelines are painted in boiler rooms in yellow, and medium pressure in yellow with red rings.

Gas-burners

Gas-burners- a gas burner device designed to supply to the combustion site, depending on the technological requirements, a prepared gas-air mixture or separated gas and air, as well as to ensure stable combustion of gaseous fuel and control the combustion process.

The following requirements apply to burners:

· the main types of burners must be mass-produced in factories;

· burners must ensure the passage of a given amount of gas and the completeness of its combustion;

· ensure a minimum amount of harmful emissions into the atmosphere;

· must operate without noise, flame separation or breakthrough;

· must be easy to maintain, convenient for inspection and repair;

· if necessary, could be used for reserve fuel;

· samples of newly created and existing burners are subject to GOST testing;

The main characteristic of burners is its thermal power, which is understood as the amount of heat that can be released during complete combustion of the fuel supplied through the burner. All these characteristics can be found in the burner data sheet.