Two technical directions of transformation and transformation of coal power units



"Depth peak balancing"

According to relevant planning, by 2030, the total installed capacity of renewable wind power and solar power generation will reach more than 1.2 billion kilowatts, from the installed capacity point of view, will become the most important power supply in China. According to the forecast, by 2050, the proportion of non-fossil energy generation will increase to 90%, and the proportion of wind and solar power installed capacity will also increase. However, due to the shortcomings of wind power and solar power generation is "uncontrollable", the installed power generation capacity is seriously limited by weather, seasons, wind and other natural meteorological conditions. According to reports, in 2019, when the proportion of non-fossil energy generation in the country was only 32.6%, wind power and optoelectronics have generally faced problems such as grid-connection difficulties, absorption difficulties, and scheduling difficulties. Under the new situation of more rapid development of wind power supply in the future, an increasing proportion of intermittent power supply is involved in the power grid, and the demand for regulating power supply capacity of the power system is very huge. Therefore, it is imperative to vigorously develop a safe and reliable power supply with deep and flexible adjustment characteristics, which is also a historical responsibility that coal power must and should be able to undertake in the future.

The peak-load capacity of coal-fired thermal power units depends on the boiler's adaptability to low load, and the so-called peak-load capacity is the ratio of the maximum and minimum stable combustion load of the unit. The so-called "depth peak regulation" is the proportion of the minimum stable combustion capacity of the boiler of the coal power unit, which is generally believed to have a peak regulation capacity of 20% under ultra-low load, so that the peak regulation depth can reach 80%, is "depth peak regulation". Therefore, the focus of deep peaking reform should be to adopt innovative technology to solve how to achieve stable ignition and combustion of pulverized coal furnace at 20% load. The key and difficulty of deep peaking transformation technology is to solve the safety and stable ignition and combustion performance of the boiler under 20% ultra-low load, and at the same time, it should also be able to achieve ultra-low emission of pollutants under 20% load. As a regulatory power supply in the future, the operation characteristics of coal power should first ensure the absorption and regulation of wind power and solar power, therefore, the operation of coal power units in the future will be in a long-term low load or deep peak load operation state, therefore, The transformation of boiler should also consider how to keep the efficiency as high as possible and the coal consumption as low as possible when the unit is under medium and low load and deep peak load. On this basis, how to ensure that the boiler unit can still achieve ultra-low emissions in the middle and low load and deep peak regulation, which is also the goal that must be considered and achieved in the deep peak regulation transformation of coal power units.

"Biomass coupled power Generation"

The low-carbon development of coal power should be able to greatly reduce the use of coal in the case of producing the same amount of electricity, which can not be achieved by improving efficiency and reducing coal consumption by the steam cycle coal power system itself, and must use low-carbon fuels for partial or all fuel replacement, that is, biomass fuel and coal coupling mixed combustion. Under possible conditions, the mixture ratio of biomass fuel is continuously increased until the complete replacement of biomass fuel is finally achieved. Biomass power generation and wind power, solar power and other renewable energy power, are (nearly) zero carbon emissions of electricity production, and the former has wind power and solar power do not have the advantage, that is, in nature, the annual cycle of agricultural and forestry solid residue resources generated is relatively stable, fuel can be transported and stored for more balanced use throughout the year. The use of large and efficient coal-fired units to burn biomass fuel for power generation is an advanced technology to achieve biomass power generation. It can not only greatly improve the efficiency of biomass power generation, save biomass resources, but also significantly reduce the carbon emissions of coal power units, improve the flexibility of coal and biomass coupled power generation, and thus strengthen the sustainability of coal-fired power generation. It is a practical and feasible path for coal power to low carbonization. Moreover, it needs to be emphasized that biomass fuel is now mixed with coal in large and efficient coal power units, and it is not an expedient or transitional technology for low-carbon development of coal power, because biomass is renewable energy, biomass mixed combustion power generation is high efficiency and low emissions and flexible thermal power generation, its essence is an advanced form of renewable energy power generation. And uncontrollable wind power and solar power generation is different, for the grid safety and reliable power supply, support and consumption of wind power plays a regulating and guaranteeing role.

Biomass fuel market

A prerequisite for the realization of biomass mixed combustion power generation of large coal power units is that there must be sufficient and relatively stable biomass fuel supply. China is a big agricultural country with abundant resources of crop straw and agricultural product processing residues. Although the natural forestry resources are relatively few, there are many fruit trees, artificial afforestation and fuelwood forests. It is estimated that China's existing crop straw and agricultural product processing residual resources and forest fruit wood processing industry residual resources are equivalent to about 1 billion tons of standard coal/year, and these biomass resources provide a guarantee of biomass fuel sources for coal power unit mixed combustion. However, having biomass resources does not mean having a market for biomass fuels. The above resources are distributed in the vast rural fields, mountain forest and urban green space, it is not easy to establish its collection, transportation, processing, storage and logistics system. Due to the characteristics of biomass fuel resource dispersion and different forms, the production supply chain of biomass fuel has not yet formed in China. In particular, because biomass is a fuel with low energy density, irregular shape and high water content in the initial state, its collection, transportation and fuel treatment are difficult. Therefore, the production, collection, treatment, transportation and storage of biomass fuel, as well as the combustion process and the impact on combustion equipment are different from coal. Therefore, the end use of biomass fuel costs (in terms of heat) must be higher than fossil fuels. In order to develop coal and biomass coupled mixed combustion power generation, the formation of biomass fuel market, so for the development of biomass fuel market, the state needs to support food production, agricultural machinery sales, fresh agricultural products transportation, the introduction of consistent appropriate financial subsidies and tax incentives. At the same time, in the establishment of biomass pellet fuel supply and demand market, China should also implement the policy of two markets double cycle, in the domestic cycle at the same time, learn from the advanced experience of foreign countries, rational use of foreign resources, international cooperation, a certain degree of external cycle, in order to meet the needs of China's coal power low-carbon development.

It is mainly used in large and efficient coal-fired power plants

The biomass coupled mixed combustion transformation should be mainly used in large and efficient coal-fired power plants. For the pulverized coal furnace of a large coal-fired power plant, due to the large boiler capacity, the biomass fuel heat output is low, even if the use of a small biomass blending ratio (according to the fuel heat), its annual consumption of biomass fuel is quite considerable. Taking a 660MW ultra-supercritical unit boiler as an example, its power supply coal consumption is about 290g (standard coal) /kWh, if 10% biomass combustion ratio to achieve mixed-combustion power generation, biomass fuel application base heat value of 14.7MJ/kg (grass and woody mixed fuel), the annual operation of 4500 hours, The annual electricity supply (about 300 billion KWH) requires about 172,000 tons of biomass fuel. Because the 660MW ultra-supercritical unit has high power supply efficiency and low coal consumption for power supply, if this fuel amount is used for existing small biomass generators, its power supply is only about half, which shows the energy efficiency superiority of large high-efficiency coal power units with mixed biomass power generation. At present, China has a total of 29.52 million kilowatts of installed capacity of small biomass fuel power generation, and the newly added biomass installed capacity will be 2.43 million kilowatts in 2020, with a growth rate of 22.6%. Because of the low efficiency and high coal consumption of small units, the biomass fuel power generation of small units is compared with the mixed combustion power generation of large advanced and efficient coal-fired units. Priority should be given to the development of large coal-fired units and biomass mixed combustion power generation.

For the biomass co-burning of large coal-fired power plants, such a large amount of biomass fuel cannot be directly transported to compact and highly automated coal-fired power plants in the form of bulk materials, and must be pre-pressed into granular fuel at the source of biomass raw materials and then supplied to the power plant. The density of biomass pellet fuel can reach close to thermal coal (1.1g/cm3 or more), the shape is regular, so it is easy to transport, and the storage, transportation and grinding into powder in the power plant (for the pulverized coal boiler) will be all mechanized and automated operation. Depending on the way biomass and pulverized coal are mixed and the burner is different, the biomass particles to the plant are ground into fine powder, and the average particle size can be from about 0.1 mm fine particles to more than 10 mm coarse particles. In developed countries, solid biomass molding fuel manufacturing and use, has more than 30 years of development history, the main characteristics of biomass pellet fuel are:

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.