Principle of circulating fluidized bed boiler
1. Principle of circulating fluidized bed
1.1 Fluidized state process
When fluid flows upward through the bed of particles, its motion state changes. When flow rate is low, particles are stationary, and fluid is passes through the gap between particles. When the flow rate increases to a certain speed, particles are no longer supported by distribution plate, but completely fluid friction. Under this condition, an individual particles no longer maintains its spatial location by relying on contact with neighboring particles. On the contrary, after losing previous mechanical support, each particle can move freely in the bed. Thus the whole bed layer has many properties of fluid. This state is called fluidization.
The minimum velocity for particle bed to change from static to fluidization is called critical fluidization velocity.
The formation of fast fluidization dynamic characteristics is very important to circulating fluidized bed.
1.2 Principle of circulating fluidized bed boiler
Under the power of high-speed airflow, fuel in high temperature furnace combusts in the state of boiling suspension (fluidization). Solid material carried out of chamber by air flow are collected by gas solid separation device, and is sent back to the chamber by material returning device. The first wind is introduced from the bottom of the bed to determine flow rate. The second wind is introduced from the upper coal entrance to ensure coal particles to combust fully in the suspension section. Heat is exchanged through the membrane water wall surrounding the suspension section.
2. Characteristics of circulating fluidized bed boiler
2.1 Working conditions of circulating fluidized bed boiler are as follows:
- Temperature (c): 850 - 950
- Bed pressure drop (kPa): 11 - 12
- Fluidization velocity (m/s): 4 - 6
- Particle concentration in the furnace (kg/m3): 150 - 600 (bottom of the furnace)
- Particle size of bed material (m): 100 - 700
- Ca/S molar ratio: 1.5 - 4
- Bed material density (kg/m3): 1800 - 2600
- Wall heat transfer coefficient [W/ (m2 / K)]: 210 - 250
- Fuel particle size (mm): <12
- Desulfurization agent particle size (mm): 1 or so.
2.2 Characteristics of circulating fluidized bed boiler are as follows:
- Wide fuel adaptability, almost all kinds of coal can be burned;
- Low pollution, desulfurization efficiency is as high as 90%;
- Large combustion heat intensity, furnace chamber is much smaller than that of the conventional boiler;
- High bed heat transfer coefficient, which reduces metal abrasion on the heated surface, and thus the heating surface is arranged compactly;
- Large load adjust range (30%-100%), stable combustion at low load;
- Ash can be widely utilized;
- Energy consumption of circulating fluidized bed boiler is 10% less than pulverized coal fired boiler;
- Coal needs only to be crushed and does not have to be pulverized, saving coal grinding cost.
Main equipment of circulating fluidized bed boiler
Circulating fluidized bed boiler is mainly composed of combustion system, gas solid separation circulation system and convection flue.
The combustion system includes air chamber, air distributor, burning chamber, furnace chamber and coal feeding system.
Gas solid separation circulation system includes material separating device and back feeding device. Convection flue comprises superheater, economizer and air preheater.
Gasification device, separating device and solid loopback device are main components that differs circulating fluidized bed boiler from other types of boilers.
Key components of circulating fluidized bed
1. Air distributor
The air supply system for the combustion of a fluidized bed boiler is composed of fan, air duct, wind chamber, air distributor, regulating baffle plate, and measuring device. Air distributor has three functions: first, supports stationary fuel layer; second, exert certain degree of resistance to air flow that pass through the air distributor, so that there is uniformed air flow velocity distribution on the air distributor, thus provide good fluidization working condition; third, take advantage of the resistance of air distributor on air flow to maintain stability of fluidized bed and avoid instability.
2. Feeding equipment
Referring to the device that send broken coal and desulfurization agent into fluidized bed. Usually feeding equipment consists of belt, chain plate, buried scraper, pneumatic conveying equipment, disc feeder and spiral feeder etc.
3. Ignition equipment
The function of ignition equipment is to heat bed material to the minimum temperature required for operation, so that the boiler combusts stably after operation.
4. High temperature slag cooling equipment
Discharged high temperature slag from fluidized bed boiler takes plenty of physical heat, which can be recycled through appropriate heat transfer devices. This can be done in cold slag device (or slag cooler). The main functions of the slag cooler are:
- Heating water, acting the role of economizer;
- Heat air, acting the role of air pre heater;
- Drying coal;
- Heat water and air simultaneously;
- Keep feeding quantity in the furnace and good fluidization;
- Fine particle sorting and loopback, thus improve combustion and desulfurization efficiency.
UNE circulating fluidized bed gasification technology and application
UNE technologies can achieve:
- Produce synthesis gas at 1.2MPa pressure;
- Produce combustion gas at 0.4MPa pressure.