Coal is the most commonly used fuel, while solids, liquids and gases can all be used as fuels. Key parameters of coal include elemental analytical composition (carbon, hydrogen, oxygen, nitrogen, and sulfur), industrial analytical composition (fixed carbon, volatile matter, moisture, and ash), and low-level heat generation. Different fuels or different compositions of the same fuel can have a significant impact on the boiler's structure, combustion profile, and amount of control. Reminder: each fuel type has a different effect on boiler performance.

Differences in combustion of solid, liquid, and gaseous fuels

Solid fuels, such as coal and municipal waste, are dried and heated so that the volatile components are precipitated out. The combustible portion of these reacts with oxygen in the air to produce combustion, and the combustible material in the remaining solid residue reacts with oxygen to burn, eventually leaving only non-combustible solid material, such as inorganic minerals. Combustion of solid fuels is a diffusion combustion process that can occur even at lower temperatures. During this process, oxygen diffuses to the surface of the solid and the products of combustion diffuse into the surrounding space.

Liquid fuels, such as various oils, form streams of oil vapor when heated and reduced in viscosity. These droplets come into contact with air and burn; the combustion process of liquid fuels is achieved by diffusion.

Combustion of gaseous fuels can be achieved by artificially controlling the mixing time of fuel and air. Premixed combustion, in which the fuel and air are mixed at the desired air-fuel ratio prior to combustion, results in the shortest, almost colorless and highest temperature flame. A semi premixed flame, on the other hand, has a medium flame length and temperature due to the fuel being mixed with a small amount of air prior to combustion. If the fuel and air are not mixed during combustion but only burn in contact on the flame surface, the flame is called a diffusion flame and has the longest flame length and the lowest flame temperature.

Effect of Different Coal Types

Boiler Body Structure

1.Decrease in heat and volatile matter of coal

Coal combustion heat, volatile fraction reduction will increase the heat of ignition, ignition difficulty, pulverized coal boiler furnace cross-sectional area should be reduced; will prolong the combustion time of pulverized coal gas flow, pulverized coal boiler furnace height should be increased; require hot air temperature of 380-400 ° C, so that the flue gas outlet temperature of the economizer has increased in the conditions of the feed water temperature remains unchanged, in order to Under the condition that the feed water temperature remains unchanged, in order to ensure that the heat absorption of the economizer remains unchanged, it is necessary to increase the heating area of the economizer; it will affect the effect of the swing burner nozzle on the regulation of the reheat steam temperature, and it should be changed to separate the tail flue, baffle temperature regulation.

2. Coal powder particle size change

If the particle size of pulverized coal is reduced to 1-100nm, pulverized coal particles can be divided into combustible and non-combustible particles. The specific surface area of combustible particles increases a lot, and the combustion reaction speed is very fast, then the height of the furnace will be greatly reduced, and the structure of the boiler with the change of coal type will be reduced. The specific surface area of non-combustible particles is also very large, and most of the particles will be agglomerated and melted to form fly ash particles with larger particle size. Therefore the share of bottom slag formed by the combustion process of nano-coal dust particles is very low. However, due to the rapid reaction properties of nano-coal dust, the possibility of explosion and combustion in the primary air in the coal mill, pulverized coal silo and pulverized coal pipeline will be greatly increased, so the primary air nozzle of the combustor of the nano-coal dust airflow should be equipped with additional devices to prevent backfire.

Hot Surface

1. Reduced volatility

Coal combustion volatile fraction is reduced, the boiler chamber volume remains unchanged, the cross-sectional area of the chamber is reduced, the height of the chamber is increased, the water-cooled wall area is increased; the temperature of the hot air is increased, the area of the air preheater is increased; the temperature of the hot air is increased, the temperature of the inlet flue gas of the economizer is increased less than the temperature of the outlet flue gas, in order to achieve the rated ratio of heat absorption, the area of the economizer is increased; the coefficient of excess air is increased, and the temperature of the flue gas is reduced, and the area of the superheater and reheater is increased. The area of superheater and reheater increases.

2. Economizer

(1) Coal economizer area versus coal quality parameters (boiler capacity certain).

For boilers burning high-quality bituminous coal, low-quality bituminous coal, poor coal, lignite, and non-bituminous coal, the temperature of the hot air rises sequentially, and the temperature of the flue gas outlet from the flue where the economizer is located also rises sequentially.

The temperature difference between the flue gas and the mass of the economizer decreases as the area of the economizer increases.

(2) Decrease in volatile matter, decrease in heat content and increase in ash content of coal combustion.

The volatile fraction of coal combustion will make the combustion time of coal dust prolonged, the water-cooled wall heat absorption is relatively insufficient, the boiler evaporation is reduced, and the output is insufficient. In order to maintain the boiler output, more coal must be put into the flue gas temperature rises, flue gas volume increases, increasing the heat absorption of the economizer, but also reduces the wear and tear of fly ash particles on the economizer. The lower heating capacity of coal combustion is generally accompanied by a decrease in the volatile fraction of coal and an increase in the ash content, the excess air coefficient will increase, the flue gas volume increases, the flue gas flow rate is accelerated, the concentration of fly ash contained in the flue gas is increased, and the wear and tear of the coal economizer will be more serious.

3. Water-cooled wall, superheater heat absorption ratio

In order to stabilize combustion, if the volatile matter and heat content of the coal are reduced, the excess air factor must be increased, then the temperature of the flue gas in the furnace chamber will certainly be reduced, and the evaporation of the water-cooled wall will be reduced.

The amount of decline in the furnace chamber by the radiation heat transfer formula can be seen, the reduction of the furnace chamber flue gas temperature on the radiation heat transfer is the effect of the power of 4 relationship. In order to maintain the boiler's evaporation, need to increase fuel and air according to the same air-fuel ratio, the flue gas flow increased, because the flue flow area is unchanged, the superheater in the flue flue gas flow rate is also increased, according to the code equation of the steady-state forced convection, the increase in gas flow rate on the flue side of the convection heat transfer coefficient of the flue gas flow rate of the n power, n = 0.6-0.65.

In summary, the volatile matter and heat content of the coal combustion is reduced, more reduce the furnace radiant heat transfer, and less increase the convective heat transfer of the superheater, that is, the water-cooled wall of the proportion of heat absorption decreased, the proportion of heat absorption of the superheater rises.

4. The relationship between the heat generation of coal combustion and the pollution coefficient of water-cooled wall

If the heating capacity of the coal used is significantly lower than that of the designed coal, the burning time of pulverized particles will be prolonged, the excess air coefficient will be larger than the design value, the theoretical combustion temperature will be lowered, and the flue gas temperature in the furnace will be lowered, which will make less ash melting and evaporation, and the water-cooled wall will be less polluted, and the pollution coefficient will be increased. Conversely, the pollution coefficient decreases.

Temperature

1.Sensitivity of rate constants of different coal types to temperature

According to Arrhenius' law, the universal gas constant and frequency factor are unchanged, the activation energy is related to the coal type, and the numerical order of the activation energy is anthracite＞poor coal＞bituminous coal＞lignite, so the sensitivity of the combustion rate of pulverized coal to temperature is anthracite＞poor coal＞bituminous coal＞lignite.

2. Effect on vapor temperature

Moisture, ash increase, or volatile matter, heat content, coal fineness decrease, will cause delayed coal ignition, flame center position upward, water-cooled wall heat absorption capacity is reduced, superheater, reheater heat absorption capacity is relatively increased, superheated steam and reheat steam temperature increases.

Low NOx Combustion Technology for Pulverized Coal Boilers

The heating capacity and volatile fraction of coal combustion are lowered, which makes the ignition unstable, prolongs the burning time of pulverized coal, and moves the combustion center position in the furnace vertically upward. Low NOx combustion technology requires that the excess air coefficient a <1.0 in the main combustion zone, which is unfavorable to the ignition stability, and will increase the CO concentration in the flue gas formed by the coke in the combustion process, resulting in more incomplete combustion loss of gas. The temperature in the burner area can be increased by increasing the area of the guard burner zone, which in turn will increase the temperature in the burner area ensure stable combustion.

Pulverized Coal Boiler Water Cycle Power

Lower heat generation, lower volatile matter, higher water content, or higher ash content of coal combustion will lead to higher excess air coefficient and higher flue gas volume, and the numerical ranking of furnace flue gas temperature will be anthracite＞poor coal＞bituminous coal＞lignite coal, so the sensitivity of the burning rate of pulverized coal to the temperature will be anthracite＞poor coal＞bituminous coal＞lignite coal.

Thermal Deterioration

The heat load distribution in the furnace chamber of pulverized coal furnace is not uniform. For coals with high volatility and fast combustion reaction speed, such as bituminous coals, the excess air coefficient is schooled, and the furnace flue gasHigh temperature, in the vertical and horizontal direction, high temperature area is more concentrated, more likely to occur heat transfer deterioration. On the contrary, such as poor coal, the high temperature region is more dispersed, moreNot prone to heat transfer deterioration.