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

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