Questions&Answers

The temperature and the amount of excess air in the combustion zone are decisive for the formation of NOx. Therefore, low NOx combustion technology is to control the temperature and air volume in the combustion zone to stop NOx generation and reduce its emission. At present, the commonly used low-NOx combustion technologies are as follows.

combustion optimization

a practical method to control NOx emissions by adjusting the combustion air distribution of the boiler. It takes measures to control NOx emissions by controlling the amount of combustion air, keeping the relative balance of the wind-powder (coal-powder) ratio of each burner and making combustion adjustments to minimize the generation of fuel-based NOx.

air classification combustion technology

is a more widely used low NOx combustion technology, its main principle is to carry out the fuel combustion process in sections. The technology is to divide the combustion air into primary and secondary air, reduce the amount of air in the pulverized coal combustion area (primary air), increase the concentration of pulverized coal in the combustion area, and delay the mixing time of primary and secondary air, so that a fuel-rich zone is formed when the pulverized coal enters the furnace, so that the fuel is burned in the fuel-rich zone without oxygen to reduce the generation of fuel-based NOx. The flue gas generated by anoxic combustion is then mixed with the secondary air to make the fuel burn completely.

The burners of large units put into operation in recent years, especially the supercritical and ultra-supercritical units basically adopt low-NOx combustion technology, which better control the NOx emission concentration.

The NOx in the burner flue gas mainly comes from the nitrogen in the fuel, and in general the higher the nitrogen content of the fuel, the higher the NOx emissions. There are also many factors that affect the amount of NOx in boiler flue gas, including the fuel type, operating conditions, and boiler load.

(1) Fuel characteristics influence

The ratio of various elements in the volatile components of coal affects the NOx generation during combustion, and the larger the oxygen/nitrogen (O/N) ratio in coal, the higher the NOx emissions; even under the same O/N ratio conditions, the conversion rate is also related to the excess air coefficient, and a large excess air coefficient results in a high conversion rate, which increases NOx emissions. In addition, the sulfur/nitrogen (S/N) ratio in coal also affects the emission levels of SO2 and NOx. S and N will compete with each other when they oxidize, therefore, as SO2 emissions rise in flue gas, NOx emissions will decrease accordingly.

(2) Excess air coefficient influence

When the air is not graded into the furnace, reducing the excess air coefficient will, to a certain extent, play a role in limiting the oxygen concentration in the reaction zone, and thus has a significant control effect on the generation of NOx, using this method can reduce the generation of NOx by 15%-20%, but the CO increases and the combustion efficiency decreases. When the air is graded into, it can effectively reduce NOx emissions, and as the primary air volume decreases and the secondary air volume increases, the rate of N being oxidized decreases, and the NOx emissions also decrease accordingly.

(3)Effect of combustion temperature

The effect of combustion temperature on NOx emissions has achieved consensus, that is, as the combustion temperature increases, NOx emissions rise.

(4) Influence of load rate

Usually, when the load rate is increased and the coal feed is increased, the smoke temperature in the combustion chamber and at the tail heat surface increases, and the NOx generated increases.

Heat load is the amount of heat released per unit time when the fuel is burned in the burner. In a burner, the formula is: heat load = fuel consumption * low calorific value of fuel. The size of the heat load is determined by factors such as the size of the main burner fuel consumption.

1.Definition

Heat load contains two meanings, one is the temperature of the heated parts, and the other is the temperature difference of the parts.

The heat flow is usually used to characterize the heat load of the part, which can be expressed by different parameters in different cases. Under the action of heat flow, q=α(Tg-Tw), depending on the heat transfer coefficient and heat conduction conditions, a certain temperature field is formed on the part. Each point on the part can be characterized by the temperature of the point and its surrounding temperature gradient, the greater the heat flow, the higher the temperature on the heated wall of the part, the greater the thermal stress.

Through the temperature of the heated parts, temperature distribution and heat flow measurement, as well as experiments to obtain the operating factors and some structural factors on the temperature of the parts, the impact of heat flow, you can find out the basic laws between them, pre-estimate the heat load of the parts, to determine the potential for strengthening, but also for the design and improvement of parts, control the heat load of the parts to provide a certain basis. 

2.The calculation formula

When the amount of internal energy, kinetic energy, potential energy changes can be ignored and no axial work, the heat input to the system and the heat leaving the system should be balanced, which can be derived from the heat transfer equipment heat balance equation: Q1 + Q2 + Q3 = Q4 + Q5 + Q6

Where Q1 - the heat brought into the equipment by the material, kJ; Q2 - the heat transferred from the heater or coolant to the equipment and the material handled, kJ; Q3 - the thermal effect of the process, kJ; Q4 - the heat brought out of the equipment by the material, kJ; Q5 - the heat or cold consumed by the heating or cooling equipment, kJ; Q6 - the heat dissipated by the equipment to the environment, kJ.

In the above equation, it should be noted that except for Q1 and Q4, all other Q values have both positive and negative cases. For example, when the reaction is exothermic, Q3 takes the "+" sign; conversely, when the reaction absorbs heat, Q3 takes the "-" sign, which is the opposite of what is stipulated in thermodynamics.

From the heat balance equation, we can find Q2, which is the heat load of the equipment. If Q2 is positive, it indicates that heat needs to be supplied to the equipment and the material handled, i.e., heating is required; conversely, it indicates that heat needs to be removed from the equipment and the material handled, i.e., cooling is required.

In addition, for intermittent operation, since the operation may be different in different time periods, the value of Q2 should be calculated separately according to different time periods, and the maximum value should be taken as the basis for designing the heat load of the equipment.

1.Fuel quality

Heavy oil fuel composition is complex, the calorific value parameter is to ensure that the burner can provide rated heat prerequisites, kinematic viscosity, mechanical impurities and water content parameters directly affect the combustion adequacy; kinematic viscosity, mechanical impurity content and water content exceed the standard will cause the heavy oil in the burner nozzle atomization effect is poor, oil mist can not be fully mixed with air, combustion adequacy can not be guaranteed, to ensure the adequacy of combustion, in the selection of heavy oil must meet the above parameters requirements.

2.The effect of atomization on the adequacy of combustion

Fuel oil in the oil pump pressure or oil pump pressure and high-pressure air under the joint action of the atomization nozzle through the oil gun is fog spray, the size of the oil fog particles depends on the effect of atomization, poor atomization, fog particles, and air mixing contact area is small, the combustion adequacy is poor.

3.The effect of wind-oil ratio on the adequacy of combustion

The combustion of heavy oil is inseparable from the participation of air, and the air supply required by the oil burner is provided by the fan, and the cooperation between the air supply and the oil supply is called the air-oil ratio. The air supply should be adjusted in proportion to the size of the burner when adjusting the oil supply, in order to ensure the proper amount of air required for normal combustion, the proportion is out of proportion will inevitably bring about a decline in the adequacy of combustion.

4.Maintain the appropriate pressure

Maintain the appropriate pressure is conducive to shorten the fuel ignition delay time, extend the heating time, improve heating efficiency, and can ensure full combustion of fuel. The steady flame plate is generally made into a radial dispersion ramp outlet, which can form a strong rotating vortex when the air flows through, strengthening the fuel particles to continue to spread and mix along the axial and radial directions.

5.The influence of altitude on the adequacy of combustion

The design working environment of the conventional heavy oil burner is below 500 meters above sea level, when the actual use environment exceeds 500 meters above sea level, the density of air will drop with the rise of altitude, and the total amount of oxygen in the air provided by the same burner at work will be less than the amount of oxygen required for normal combustion, resulting in inadequate combustion.

Complete premixing means that the air required for combustion is obtained entirely by gas injection, i.e., the gas and air are completely mixed before combustion.

Advantages:

 (1) complete combustion, chemical incomplete combustion loss is less;

 (2) less excess air, when used for direct heating of industrial furnaces will not cause excessive oxidation;

 (3) high combustion temperature, easy to meet the high temperature process requirements;

 (4) Flameless burner with fire escape has high combustion intensity, high volumetric heat load, and can reduce the volume of the combustion chamber;

(5) no fire channel, easy to burn low calorific value of gas.

Disadvantages:

(1) the possibility of backfire, the adjustment range is small, in order to prevent backfire, the head structure is more complex and bulky;

(2) large heat load burner, the structure is huge and bulky.

(3) The noise is large, especially when the high pressure and high load is more so.

A good performance of fully automatic oil (gas) burner installed on a boiler, whether it still has the same good combustion performance, depends largely on whether the gas dynamic characteristics of the two match. Only a good match can bring out the burner performance, ensure the stable combustion of the furnace, achieve the expected heat output and obtain a good thermal efficiency of the boiler.

1、Power

The power of the burner refers to the full combustion, how much mass (kg) or volume of fuel it can burn per hour, while also giving the corresponding heat output. And the boiler is calibrated for steam production, but also marked fuel consumption, the choice of the two must match.

2、Matching of gas power characteristics

A single automatic burner is like a flame thrower, the flame is sprayed into the furnace (combustion chamber) to achieve complete combustion and heat output in the furnace; burner manufacturers to determine the complete combustion of the product is carried out in a specific standard combustion chamber, so the conditions of the standard experiment is generally used as the burner and boiler selection conditions. These conditions are summarized as: (1) power; (2) gas flow pressure in the chamber; (3) space size and geometry of the chamber (diameter and length). The so-called gas power characteristics match, which means the degree of meeting these three conditions.

3, the impact of the size and geometry of the furnace chamber

For boilers, the space size of the furnace chamber, in the design of the first decision in the selection of the heat load intensity of the furnace, according to which the volume of the chamber can be initially determined. After the volume of the furnace chamber is determined, its shape and size should also be determined. The design principle is to make full use of the volume of the furnace chamber; try to avoid dead ends, to have a certain depth, reasonable flow direction, to ensure that there is enough reaction time, so that the fuel is completely burned in the furnace chamber. A boiler to be repeatedly debugged to have a good match with the burner.

4, the gas pressure in the furnace chamber

In an oil (gas) boiler, the hot air flow from the burner, through the furnace, heat exchanger, flue gas collector and exhaust pipe discharge to the atmosphere, consisting of a fluid thermal process. The furnace chamber must maintain a certain flue gas pressure, for the burner called back pressure, for boilers without induced draft device, the chamber pressure must be higher than the atmospheric pressure after taking into account the loss of pressure head along the way.

The good or bad combustion conditions of oil and gas boilers and furnaces mainly depend on the operating performance of the burner. Oil Burner atomization quality is not good or unreasonable air distribution will cause the following adverse effects.

(1) incomplete combustion, pollution of the boiler tail heating surface, the exhaust temperature rises, and even cause secondary combustion.

(2) Increasing heat loss from incomplete combustion of combustible gases.

(3) Increasing heat loss from incomplete combustion of combustible solids.

(4) Coking in the oil burner outlet or furnace chamber.

(5) There are accidents such as flameout, gunning and even furnace explosion.

1.Waste liquid types

The organic waste streams in waste stream burners are extremely diverse. Organic waste streams are divided into three categories according to their chemical composition.

① Halogen-free organic waste streams. The organic compounds in this waste stream contain only C, H, O and sometimes S. When the waste stream contains less water, it is combustible and can be used as fuel (such as waste of organic

②Halogen-containing organic waste liquids. The organic compounds in waste liquids include CC4, vinyl chloride, bromomethane, etc. The calorific value of the waste liquid depends on the content of halogens. During incineration treatment, the need for auxiliary fuel is determined according to its calorific value. After oxidation in the incinerator, the waste liquid will produce monomeric halogens or hydrogen halides (HF, HCl, HBr, etc.), which can be removed or recovered as needed.

③Containing high salinity organic degree liquid. Such waste streams containing high concentrations of inorganic or organic salts will produce molten salts after combustion, so refractory materials, selection of combustion temperature and determination of residence time will be the main considerations in the design, and since such waste streams usually have low calorific value, auxiliary fuels are needed to achieve complete combustion.

2.The incineration process

General organic waste collection incineration process includes pre-batch surface high temperature incineration, waste heat recovery and tail gas treatment and other stages.

Pre-treatment mainly includes the aisle into the collection, plantain hair according to shrinkage, adjust the viscosity, etc., the purpose of which is to provide optimal conditions for the subsequent incineration over the process. The nature of the hair liquid varies: For COD value is very different, organic waste liquid incineration treatment can be directly sent to the incinerator burner for incineration treatment, in the face of the calorific value is not very high waste high, calorific value is also very high organic waste liquid, organic waste liquid containing relatively high moisture, you can first steam the liquid, you can add auxiliary fuel to help waste liquid for incineration.

When the waste liquid does not contain harmful low boiling point organics, the method of direct concentration of high temperature flue gas can be considered, and then incineration can be carried out after concentration. However, the concentration method with indirect heating should be used for organic waste streams containing harmful low-boiling components.

Boiler burner refers to the burner used on the boiler, boiler burner is the most important equipment in the supporting auxiliary equipment of oil and gas boilers, mainly composed of fuel injector, air distributor and igniter and other components.

The role of the injector: to control the amount of fuel in the firebox; atomize the fuel to ensure the quality of combustion in the furnace.

The role of the air distributor is to control the primary air volume and secondary air volume, to create conditions for the combustion air to fully mix with the oil mist, to promote rapid vaporization and decomposition of the oil mist, in order to facilitate stable and full combustion.

Igniter is an electric spark generator, used to ignite the oil mist. In addition, there are flame receptors and artificial ignition holes, etc.

Classification

Atmospheric burner usually uses gas to induce primary air, so it belongs to the induced burner. According to the different gas pressure, it can be divided into low-pressure pilot type and high (medium) pressure pilot type two. The former is mostly used for residential appliances, and the latter is mostly used for industrial installations.

The advantages of atmospheric burner

1.than the natural draft diffuser burner flame short, strong fire, high combustion temperature.

2.can burn different nature of gas, more complete combustion, high combustion efficiency, less CO content in the flue gas.

3.It can apply low-pressure gas, and no air supply equipment is needed because the air depends on gas inhalation.

4.Strong adaptability, can meet the needs of more processes.

The disadvantages of atmospheric burner

1.Because only part of the air is premixed, not all the air required for combustion, so the thermal intensity of the fire hole and combustion temperature is higher than that of the natural draft diffusion burner, but it is still limited and still cannot meet the needs of certain processes.

2.When the heat load is large, the structure of the multi-hole burner is relatively bulky.

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The specific advantages of the biogas burner lie in.

1, stretching the combustion area, weakening part of the combustion intensity, in a certain period of time, inhibit the formation of NOx.　

2, low load combustion smoothly. Because the lower air volume is reduced, the fuel is burned very smoothly even at low concentration. Even 40% load stable combustion can be achieved.

3、When low load, the furnace flame filling degree is better, and the water-cooled wall absorbs heat evenly.

Under the requirement of environmental protection policy, industry now also attaches great importance to energy saving and emission reduction measures. Low-NOx combustion technology has certain advantages in environmental protection, but at the same time, it also has certain influence on the operation of boilers, so it is necessary to further strengthen the measures to deal with the problems occurring in new environmental protection technology, and to strive for the maximum environmental protection mechanism for sustainable industrial development. This paper discusses the relevant content, which has certain practical significance.

(1) the impact of combustion stability, the stability of the boiler is reflected in many aspects, its most important manifestation is in the stability of the temperature and the stability of the operation process. Low nitrogen burner in the primary jet port set thick and light combination, the use of thermal reflux relay combustion and other technologies, in the combustion process according to the asymmetric principle of heat and power design, pyrolysis jet port of coal powder and the boiler center of the composite jet vortex connection, high reflux rate of hot reflux toner to extend the residence time, improve the heat generated by combustion within the vortex, boiler low nitrogen transformation and the number of oxygen control, in the combustion process Oxygen is needed to meet the combustion conditions to generate heat, and as the oxygen number is controlled the heat generation is inhibited, thus affecting the operation of the boiler. These two operating methods have an impact on the stable operation of the boiler.

(2) The internal environment of the boiler has less influence on the low-NOx burner nozzle than the conventional burner, so the flame produced by the fuel moves. The reduction in combustion area reduces the boiler's reception of temperature, while the pressure state generated inside the boiler changes, leading to incongruity. The modification of the low-NOx burner equipment changes the boiler components and the operation is different. The modification of the low-NOx burner affects the oxygen quantity inside the boiler to a certain extent. Usually there is a maximum operating oxygen quantity and a minimum operating oxygen quantity, and the adjustment of the unit cannot keep up with the adjustment of the oxygen quantity and produces negative oxygen. The change of the oxygen quantity and the change of the combustion degree change the pressure inside and outside the boiler and change the internal environment, and the personnel concerned need to adjust the oxygen quantity and carry out the air supply operation directly.

(3) The boiler reheating temperature is affected, and the original burner elevation is lowered after the low-NOx burner transformation, which has a greater impact on the reheating temperature. After the burner transformation, the unit coordination is slow, and the boiler pressure tracking adjustment cannot keep up, so it is easy to have overshoot and temperature change. The modified low-NOx burner has a set of swinging nozzles, when the nozzle swings up the steam temperature will rise, the temperature will drop when the nozzle swings down, but because there is only one set of swinging nozzles so the speed of temperature adjustment is limited, the adjustment time is too long plus the wind sub-ratio is controlled by the low oxygen conditions, affecting the efficiency of the machine. In order to ensure that the boiler outlet temperature is within the low load range, a powder manufacturing system must be used, which will lead to high temperatures on the heating surface, making it difficult to overtemperature. At the same time, the outlet temperature should be controlled within a certain range, which affects the normal operation of the boiler.

(4) the impact on the coking of the furnace chamber, low nitrogen burner in the use of the process, although the burner in the operation process will appear coking phenomenon, especially when starting the lower powder making system, will have an impact on the coking of the furnace chamber, but in the operation process, the burner nozzle is still coking phenomenon, especially when starting the lower powder making system, it will obviously affect the negative pressure, indicating that the combustion zone condition is not good, because the main combustion zone for oxygen deficiency combustion, so the cold wall coking phenomenon will appear near the burner, especially when starting the lower powder making system, which will obviously affect the boiler combustion condition, indicating that the combustion zone condition is not good because the main combustion zone carries out oxygen deficiency and oxygen deprivation, so the cold wall coking phenomenon will appear near the burner, which has an impact on the furnace coking.

(5) The low nitrogen burner increases the combustibility of the slag while reducing the NO yield. Low nitrogen combustion technology uses low temperature and low oxygen conditions for combustion, the more the temperature in the combustion zone drops, the greater the impact of pulverized coal ignition, the oxygen in the combustion zone drops, the combustion capacity of pulverized coal decreases, the combustion process also increases, the combustibility of slag increases, the burner nozzle area changes, the mixing wind is delayed, and the pulverized coal airflow in the boiler.