Research on coupled biomass direct combustion power generation technology of coal-fired units



0 Introduction

Biomass energy accounts for 14% of the world's primary energy consumption and is the fourth largest energy source after coal, oil and natural gas. According to the "China Renewable Energy Industry Development Report 2019", the total amount of biomass resources that can be utilized for energy in China every year is equivalent to about 460 million tons of standard coal. Among them: the amount of agricultural waste resources is about 400 million tons, which is converted into about 200 million tons of standard coal; The amount of forestry waste resources is about 350 million tons, which is converted into about 200 million tons of standard coal. Other organic waste about 0.6 million tons of standard coal.

China's agricultural and forestry biomass power generation technology has been relatively mature, as of 2019, China's agricultural and forestry biomass power generation accumulated installed capacity of 10.8 million kW, annual power generation of 46.8 billion kWh. In September 2020, the National Development and Reform Commission, the Ministry of Finance and the Energy Bureau stipulated in the "Implementation Plan for Improving the construction and operation of biomass power generation Projects" that since January 1, 2021, new biomass power generation projects will be bidding online, and the subsidy funds will be jointly undertaken by the central and local governments, and the central part will be adjusted year by year and withdrawn in an orderly manner. This marks the future of China's biomass power generation will gradually shift from fixed electricity prices to bidding mode.

With the "30·60" carbon peak carbon neutral goal proposed, China's thermal power industry carbon emission reduction is imperative. The operation cost of flue gas decarbonization technology is high, and there is no good way to use the captured CO2, so it is difficult to promote large-scale decarbonization of coal-fired units at this stage. Biomass does not produce carbon emissions in the process of combustion and power generation utilization, so blending biomass can significantly reduce carbon emissions. The coupled biomass direct combustion power generation technology of coal-fired units has been widely studied and successfully applied in Europe and North America. Ferribridge C power plant, Drax Power Plant, Fiddler's Ferry Power Plant and Amer Power Plant in the Netherlands have all carried out successful biomass coupling transformation. Drax power plant 660 MW unit has achieved 100% pure biomass transformation. Domestic scholars have also carried out relevant research on the coupled biomass power generation technology of coal-fired units, and some power plants have carried out biomass coupling transformation, but for various reasons, most of the projects have been suspended.

The coupled biomass direct combustion power generation technology of coal-fired units has a significant effect on carbon emission reduction, and has many advantages over pure biomass burning units, which is suitable for the transformation of existing coal-fired units and for new coal-fired units. In this paper, the engineering application system design, technology and economy are studied.

1 Biomass direct combustion coupling technology route

The technical route of biomass coupled power generation mainly includes direct combustion coupling, gasification coupling and steam coupling. At present, more than 150 biomass coupling projects in Europe, the vast majority of the direct combustion coupling technology route.

Direct combustion coupling technology has low initial investment and maintenance costs and high technical maturity. According to the different coupling positions of biomass and coal, direct combustion coupling technology is mainly divided into coal mill coupling, powder pipeline coupling, coal burner coupling, independent biomass burner furnace coupling and so on.

Based on the successful practical experience in Europe, the coupling scheme of the pre-ground biomass direct injection into the powder delivery pipeline has the advantages of mature and reliable technology, simple transformation scheme, easy to quickly realize a high proportion of localization, less modification of power plant facilities, short transformation cycle, low unit cost, minimal impact on the operation and maintenance of existing power plant facilities, and good compatibility with the existing operation and maintenance system of power plant. Therefore, the coupling scheme of powder delivery pipeline can be preferred for the low proportion of biomass coupling transformation.

Different direct-combustion coupling schemes apply different biomass blending ratio. When the blending ratio is higher, the corresponding transformation cost and operation cost of coal-fired units will increase. In addition, the proportion of blended combustion is also limited by China's biomass fuel collection system, combined with the biomass fuel collection capacity of a single biomass power generation project equivalent to about 9 to 100,000 t of standard coal heat, the proportion of coupled biomass power generation of large and medium-sized coal-fired units in the short and medium term is generally within 20%, and can be increased to a higher proportion on this basis in the long term. Therefore, the choice of powder pipe coupling scheme is more appropriate in most cases. In this paper, the engineering scheme of the coupling of the powder feeding pipeline will be studied.

2 Research on coupling engineering scheme of powder feeding pipeline

Based on a 350MW cogeneration unit, this paper studies the process flow of the coupling scheme of powder feeding pipeline and the influence of coupling on the main and auxiliary machines.

2.1 Boiler type

The boiler type is π type DC furnace with supercritical variable pressure operation, tangential-round combustion mode, one intermediate reheating, single furnace balanced ventilation, solid slag discharge, tight and closed, all steel frame.

2.2 Fuel Data

In this study, straw and agricultural and forestry waste are used as fuel, and biomass is broken separately into the powder pipeline as the main scheme. The biomass molding particles are used as fuel, enter the independent biomass mill, and enter the powder feeding pipe after grinding as a comparative scheme.

Blended biomass is considered as bulk and formed particles respectively.

Relying on the project of 2×350MW unit, only one boiler is considered to burn biomass at 10% of the heat, and the biomass combustion amount is shown in Table 6. The hourly blending amount of bulk biomass is equivalent to the fuel amount of a typical 35MW pure biomass burning unit, and the proportion of 10% bulk biomass can be considered appropriate. For the convenience of comparison, the sintering ratio of the formed particle scheme is also set at 10%.

2.3 Coupling scheme of bulk crushing and powder delivery pipeline

The flow diagram of this scheme is shown in Figure 2. After entering the field, the bulk biomass is weighed by the truck scale first, and then unloaded to the dry shed or open storage yard.

The bulk material is transported to the crusher near the boiler room by belt conveyor and cut into small sections of no more than 10mm, and then enters the dryer through the chute. The dried bulk material first enters the biomass bin for temporary storage, and then enters the hammer mill by the screw feeder for further crushing, and is filtered to about 1mm before being sent into the pneumatic conveying pipeline.

The pneumatic conveying pipe is connected near the burner. The access mill is tentatively designated as the D mill connected with the upper and middle burners (hereinafter referred to as "D mill"). When 10% biomass is mixed in a single furnace and enters the four powder feeding pipes of a single mill, the mixing ratio of a single mill is 40%.

2.4 Coupling scheme of independent biomass mill feed pipeline

The flow diagram of this scheme is shown in Figure 3. In this scheme, a semi-open storage yard for biomass pellets is added, and the biomass pellet fuel is transported to the biomass pellet silo by belt conveyor.

The particles in the biomass pellet bin are fed into a special biomass mill by the weighing belt feeder to grind into small particles with a particle size of no more than 1mm. Then feed into the pneumatic conveying pipe.

2.5 Effects of direct combustion coupling of biomass

2.5.1 Impact on boiler

Due to the large difference between the characteristics of biomass fuel and coal, when the combustion ratio is 10%, the main affected gas volume, exhaust temperature and boiler efficiency are mainly affected, and other performance parameters are basically unchanged.

Under the condition that 10% of the designed coal is mixed with biomass, the relevant performance data of the boiler are shown in Table 7. It can be seen that the smoke volume of biomass direct-combustion coupling changes little. The smoke temperature increases and the boiler efficiency decreases after mixing biomass, and the boiler efficiency changes little when mixing biomass particles.

2.5.2 Impact on coal mill

The blending scheme of this project only affects the D-grinding of coupled biomass. Under the working condition of boiler maximum continuous rating (BMCR), the proportion of coupled biomass in the powder feeding pipe of D mill was 40%, which means that the grinding output and drying output of D mill only need to reach 60% of the normal output.

2.5.3 Impact on the Three Major Fans

After adding biomass into the D mill feed pipe, the output of D mill decreases to 60% and the ventilation volume decreases to 84%, which has little influence on the flow rate and pressure of the primary fan. There is basically no effect on the blower and the induced draft fan.

2.5.4 Impact on pollutant control

After adding biomass, the initial emission concentration of NOx decreases, the concentration of soot decreases, the concentration of SO2 at the inlet of the desulfurization unit decreases significantly, and the change of smoke volume is small, which will not adversely affect the smoke emission of the power plant in general.

3. Technical and economic analysis

Relying on the project to carry out the biomass direct combustion coupling transformation, the static investment of bulk material coupling project is about 66 million yuan, and the static investment of particle coupling project is 42 million yuan.

The economy of the renovation plan is measured below.

When the carbon tax inverse tariff is not taken into account (the biomass direct combustion coupling of the newly added single unit and the original 2×350MW unit are uniformly considered), the bulk material blending leads to the increase of the tax-included on-grid tariff

10 yuan /MWh(including tax), particle combustion led to an increase in tax-included on-grid electricity price of 11.41 yuan /MWh(including tax), and the tax-included on-grid electricity price of the two schemes was about 0.32 yuan /kWh.

According to the benchmark price of coal 364.4 yuan /MWh(including tax), the carbon tax subsidy of bulk material scheme reaches 80 yuan /t, and the carbon tax subsidy of particle scheme needs to reach 93.5 yuan /t to maintain the internal rate of return of capital.

Due to the high cost of biomass fuel and low calorific value, the biomass direct combustion coupling will lead to an increase in on-grid electricity price, and the cost of the power plant will increase, and it is recommended that the construction unit actively strive for electricity price subsidies, electricity subsidies or carbon tax subsidies.

4 Advantages of biomass direct combustion coupling scheme

The carbon emission reduction effect of the 10% biomass direct-burning coupling scheme is equivalent to that of the pure biomass burning scheme, and its index analysis is carried out below.

1) Unit efficiency

The power generation of the 35MW pure biomass burning unit is equivalent to that of the 350MW coal-fired unit with 10% biomass burning, as shown in Figure 4. The unit efficiency of the 35MW pure biomass burning unit, 350MW coal-fired unit and 350MW coal-fired unit with 10% biomass burning bulk material with high temperature and high pressure parameters is compared. Unit efficiency corresponding to 10% biomass blending is the unit efficiency when the biomass part is considered independently, that is, the boiler efficiency corresponding to coal burning is assumed to remain unchanged, and the reduction of boiler efficiency after mixing 10% biomass bulk is fully reflected in the boiler efficiency corresponding to biomass bulk. It can be seen that the efficiency of the biomass direct-combustion coupling unit is significantly better than that of the high-temperature and high-pressure pure combustion biomass unit.

2) Initial investment

The initial investment of the 35MW pure burning biomass unit is about 350 million yuan, the initial investment of the direct-burning coupled 10% biomass bulk coupling program of the 350MW unit is about 66 million yuan, and the initial investment of the particle coupling program is about 42 million yuan. The investment of coal-fired coupled biomass direct combustion power generation scheme is significantly lower than that of pure combustion units.

3) Feed-in tariff

The on-grid price of pure combustion biomass unit is 0.75 yuan /kWh, and the on-grid price of 10% direct combustion coupling unit is about 0.32 yuan /kWh.

It can be seen that under the premise of the same carbon emission reduction effect, the biomass direct-combustion coupling scheme has obvious advantages compared with the pure combustion biomass unit in unit efficiency, initial investment, on-grid electricity price and other aspects.

If the use of flue gas post-decarbonization technology to achieve carbon emission reduction of coal-fired units, not only will increase the initial investment, but also will increase the operating cost, the same 350MW coal-fired unit 10% of the flue gas post-decarbonization as an example, the initial investment of about 150 million yuan, the operating cost will cause the on-grid electricity price increase of about 0.12 yuan /kWh. Compared with the flue gas post-decarbonization scheme, the economic benefits of biomass direct-combustion coupling power generation scheme are also very significant.

Therefore, the coupled biomass direct combustion power generation technology is currently a more suitable carbon emission reduction technology route and biomass energy utilization mode for coal-fired units.