The fast boiler

Biomass boiler, affect the combustion process of variables you know how much?

Time : 2021-04-21

I. Water content

 

The water content of different types of fuel varies greatly, depending on the type of fuel and how it is stored.In order to maintain the combustion stability of biomass boiler fuel, it is necessary to air and sun before use (dry shed should be added when conditions permit).The increase of water content will decrease the heat storage temperature in the furnace and increase the incomplete combustion of fuel in the combustion chamber.Too much water content is the main reason that the biomass power plant can not carry the load.The fuel with more water is difficult to catch fire, which affects the combustion speed, reduces the temperature in the furnace, and increases the heat loss of mechanical and chemical incomplete combustion. When the fuel moisture is greater than 45%, the combustion is very difficult.In the combustion process, water will consume a large amount of heat of vaporization due to evaporation and vaporization.The volume of flue gas after combustion of the fuel with large moisture content is larger (the volume of water into steam has increased by 1200 times). Because the outlet flue gas has a temperature of about 130℃, the heat loss away with the flue gas is more (this phenomenon can be observed through the smoke exhaust of the chimney, showing a large number of milky white gas), and the thermal efficiency of the boiler is lower.In addition, the volume of flue gas increases, the electric energy consumed by the induced draft fan also increases, the power of the induced draft fan increases, so that the velocity of the flue gas speeds up, the combustion moves up, and it is difficult to build a reasonable combustion condition and ensure the balance of the combustion power of the grate (can not keep the primer).

 

The acceleration of flue gas velocity increases the ash carrying capacity of flue gas and accelerates the wear of the heating surface at the end of the furnace.

 

 

Two, calorific

 

(1) Concept

 

(1) high calorific value (wet base).Refers to the total amount of heat produced by the complete combustion of 1kg of fuel per unit time (mJ /kg).

 

(2) low calorific value (dry base).Is the heat released by the latent heat of vaporization (water vapor) generated during combustion, in mJ /kg.

 

(2) the factors that affect calorific value

 

The general factors affecting calorific value are moisture, ash and impurities in the fuel, which is also a common problem in domestic biomass power plants (the problem of excessive moisture and ash in fuel is not solved, it is difficult for biomass power plants to develop).Too wet and too long fuel is blocked in the feeder and feeder, and cannot be burned through on the grate, which increases the incomplete combustion loss (a power plant cannot carry high load for a long time because the ash content of the furnace fuel is usually greater than 40%).

 

 

3. Excess air coefficient

 

The ratio of theoretical air volume to actual air volume is called the excess air coefficient.

 

The combustion excess air coefficient of biomass boiler should be greater than 1, usually greater than 1.25, to ensure the full mixing of combustion-supporting air and combustibles.

 

The excess air coefficient is followed by the data of pulverized coal boiler, and there is no authoritative demonstration now.The actual boiler combustion should be oxygen-rich combustion because of the high volatile content of biomass and fast combustion speed.

 

The excess air coefficient is mainly provided by the secondary air. It has been proved by practice that the upper secondary air should be as large as possible, and the principle is not to overheat the superheater, so as to facilitate the construction of combustion conditions and the burnout of fuel.The test proved that: the biomass boiler combustion oxygen at any time to be greater than 3%, when the furnace temperature can maintain a high, oxygen to maintain at 3% ~ 5% to complete combustion.Uneven because of biomass granule, burning area, became a biomass boiler from the oxygen-enriched combustion of pulverized coal fired boiler a striking feature (a domestic power plant on the front and back of secondary air to open to more than 40%, the oxygen content is 5%, the combustion condition is good, the superheater area by gray ash, can be thought of as domestic biomass boiler combustion model;The ratio of the primary and secondary air rate of the Danish biomass boiler is 3:7 for the yellow stalk boiler and 4:6 for the gray stalk boiler.But now most of the domestic biomass boilers do not follow this treatment, the main reason is that the quality of domestic fuel is too poor, moisture, ash content is too high.Many biomass boilers have a wind rate ratio of 5:5 or 8:2.In particular, yellow stalk boiler with less primary air can not float thick material layer, reduce the thickness of the material layer, furnace heat storage can not meet the high load.

 

 

Four, air temperature

 

The hot air of the boiler of biomass power plant uses the method of incoming water from the deaerator to heat the air. Therefore, it is necessary to improve the air temperature as far as possible to make the grate fire at the high end.

 

 

V. Types of fuel

 

Different characteristics of different fuels affect the combustion process, among which the main influencing factors are fuel composition, volatile and fixed carbon content, thermal performance, density, porosity, size and active surface, etc.The fuel composition changes continuously with the burnout degree.Compared with coal, biomass boiler fuel usually has a higher volatile content and a lower fixed carbon content, which belongs to a highly active fuel.However, the volatile content of different biomass fuels is different, which affects the thermal performance of the fuels.The small DUMP chemical structure and bonding links of various biomass fuels also affect the thermal performance of the fuels.It shows that the law of volatilization analysis is obviously different.The density of different biomass fuels also varies greatly. For example, the density of bark and rice husk varies greatly.The high fuel density affects the unit combustion chamber volume and the combustion characteristics of the fuel.Porosity affects fuel reactivity (mass loss per unit time) and affects the rate of volatilization analysis.Especially when the powdery fuel burns, the flue gas carries a large number of particles. The small particles stay in the combustion chamber for a short time. If the flame cannot be suppressed, it is bound to cause the flame to rise, the heat loss of chemical incomplete combustion increases, and the induced draft fan impeller wears.During the commissioning of a biomass power plant, rice husk was burned, and the impeller of the boiler induced draft fan was worn through within a month.

 

 

6. Combustion temperature

 

Combustion temperature is the basis of the combustion of biomass boiler. If the combustion temperature is not enough, the optimization of combustion cannot be formed and the complete combustion cannot be guaranteed.The three-strong theory of strengthening boiler combustion is as follows:

 

(1) the initial stage of intensified combustion.

 

(2) Enhance convective heat transfer of high temperature flue gas and fuel.

 

(3) Enhance the concentration of reducing atmosphere during fuel combustion.

 

No matter what fuel is burned, we must find a way to maintain the temperature of the furnace, furnace temperature below 850℃ cannot form the intensified combustion.

 

The experiments show that the furnace temperature above 400℃ boiler ignition success, furnace temperature above 600℃ boiler can form continuous combustion, furnace temperature above 850'C may form the strengthened combustion.

 

 

Seven, with the wind

 

Effective air distribution is the key to biomass boiler combustion. Primary air can blow the fuel and increase the gap ignition surface.The secondary wind suppresses the flame, forming the intensified combustion.To increase the combustion time, maintain the furnace temperature, reduce incomplete combustion heat loss (recommended air distribution parameters for a biomass power plant, 30MW load as an example, the total air volume of the boiler is 7.5kPa, the primary air high-end 50%, 60%, 30%, the front and rear wall secondary air 50%, the burnout air 20%).

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