Two technical directions of transformation and transformation of coal power units

1. High density (1.1 ~ 1.4g /cm3, 5 ~ 10 times larger than bulk material);

2. Easy to transport and store;

3. Easy to organize the processing of biomass fuel and mixed combustion with coal, suitable for a variety of combustion equipment, especially pulverized coal furnaces and circulating fluidized bed boilers of biomass and mixed combustion with coal.

Agricultural and forestry waste biomass fuel and coal have different combustion characteristics, mainly:

1.Volatile up to 70%, low ignition temperature;

2. Low fixed carbon content;

3. Low total carbon content (<50%), so the calorific value is low;

4. High oxygen content (>30%), small combustion theoretical air volume;

5. The general sulfur content, ash content and mercury content are low, and the nitrogen content of woody biomass is also low, which is conducive to the treatment of pollutants such as SO2, NOx, soot and mercury in the flue gas.

6. The ash contains metal elements K, Na and non-metallic elements Cl, resulting in low ash melting point of biomass fuel and easy corrosion of heating surface.

The above characteristics of biomass fuel, resulting in coal and biomass fuel combustion characteristics are also different, mainly:

1. The pyrolysis process of biomass occurs earlier than that of coal;

2. The calorific value of volatiles in biomass (kJ/kg) is lower than that of coal;

3. The calorific value of volatiles in biomass accounts for 70%, while that of volatiles in general coal (bituminous coal) accounts for 36%;

4. The oxygen content of biomass coke is higher than that of coal;

5. Biomass coke pyrolysis precipitation is mostly CO, CO2, H2O;

6. The content of alkali and chlorine in biomass ash is high, which is easy to cause ash accumulation, slagging and corrosion problems.

These combustion characteristics of biomass fuel make biomass and coal coupled mixed combustion power generation, in general, can play its advantages and overcome its disadvantages. For example, high volatile content and low ignition point are conducive to the stability of the furnace flame; When the biomass/coal mixture ratio is not too high (generally not more than 30%), the adverse impact of the ash melting point of biomass and the corrosion of ash on the safety of boiler operation will be greatly reduced; The adsorption of biomass ash is strong, which is conducive to removing heavy metals (mercury) released from coal combustion. Biomass fuel calorific value is low, oxygen content is high, under the same fuel input heat, the input biomass fuel amount will increase, so the design of fuel pipeline and burner put forward special requirements.

The basic requirements for biomass co-firing transformation of coal-fired boilers in large power stations are:

1. Keep boiler efficiency basically unchanged;

2. Keep boiler output unchanged;

3. Meet emission requirements;

4. Ensure the safety requirements of biomass fuel combustion;

5. Meet the requirements of operating life after biomass conversion.

According to foreign experience, for coal-fired units with a single capacity of 300MW and above, the most important coupling mixed firing method is now used, that is, the transmission, crushing, powder feeding pipelines and burners are completely separated from the coal powder, and the arrangement of the biomass burner and in the furnace adopts a special design. This scheme has high investment cost but minimal interference to the normal operation of the boiler. In the mixed combustion method of different grinding and different burners, the most advantageous is to use biomass powder as a secondary fuel for reburning, and send it to the biomass burner arranged in the appropriate position in the upper part of the pulverized coal furnace in the way of fuel graded combustion to control the generation and emission of NOx.

Therefore, the existing large coal-fired boiler and biomass mixed combustion, mainly for the power plant fuel storage and transportation grinding system, biomass burner, etc., the furnace heating surface and environmental protection equipment design, although fully consider the biomass combustion characteristics, but in general little change. It should be said that biomass and coal coupled mixed combustion power generation, there is no big problem in technology, as long as attention to the different characteristics of different biomass fuels, in the fuel transport, storage, grinding and combustion system to take into account its characteristics, pay attention to its high volatile ignition point low fire characteristics, pay attention to fire prevention and explosion protection measures when transforming the fuel system, It is noted that high alkali content is easy to cause ash accumulation, slag formation and corrosion problems, accumulated experience, I believe that coal and biomass fuel mixed combustion can be popularized on a large scale.