8.2　Pulverized Coal Preparation

8.2.1　The type of mill and coal pulverizing system shall be selected through techno-economic analysis based upon the characteristics of coal type,possible variation of coal categories,load characteristics,suitable conditions for mills in combination with combustion mode of boiler,furnace structure and burner configuration,following the principles of allowing safe operation,improving combustion efficiency and reducing NOxemission.In addition,the type selection of mill and coal pulverizing system shall comply with the following requirements:

1　The type of mills shall comply with the following requirements:

1）Large-capacity units,if burning appropriate coal type,should be equipped with medium speed mills.

2）When the units burn lignite with high moisture content and less abrasiveness,fan mills should be adopted.

Medium speed mills may be adopted instead if the drying capacity of coal pulverizing system meets the requirements and the use of it is justified.

3）When the units burn low volatile lean coal,anthracite,or highly abrasive coal,ball mills or double inlet and outlet tube mills should be adopted.

2　The type selection of coal pulverizing system shall comply with the following requirements:

1）When medium speed mills,fan mills,or double inlet and outlet tube mills are used,direct firing pulverizing system should be adopted.

2）When non-flammable and non-explosive coal type is burned and conventional ball mills are used,bin storage type pulverizing system should be adopted.

8.2.2　The quantity and output of mills for direct firing pulverizing system shall comply with the following requirements:

1　When medium speed mills and fan mills are adopted,standby mills shall be provided.The quantity of mills shall comply with the following requirements:

1）Boilers with a capacity of 200MW and above should be equipped with no less than four medium speed mills,including one for standby；boilers with a capacity of below 200MW should be equipped with no less than three medium speed mills,including one for standby.

2）When medium speed mills are equipped for boilers burning lignite,their number shall be determined taking into consideration the boiler structure,number and arrangement of burners as well as output of mills.

3）The number of fan mills equipped for each boiler should be no less than four,including one for standby.If each boiler is equipped with six and above fan mills for normal operation,there may be one standby for operating and one standby for maintenance.

2　When double inlet and outlet tube mills are adopted,no standby mill should be provided.The number of mills shall comply with the following requirements:

1）The number of mills for each boiler should not be less than two and shall be determined based on the boiler structure,number and arrangement of burners.

2）When W-flame boiler is adopted,four or three double inlet and outlet tube mills should be provided for each boiler of 300MW units,and six for each boiler of 600MW units.

3　Some margin shall be given for the calculated output of coal mills and should comply with the following requirements:

1）For fan mills and medium speed mills,when pulverizing design coal,the total calculated output of mills,except for the standby mill（s）,shall not be less than 110%of the coal consumption of boilers at BMCR condition；and when pulverizing check coal,the total calculated output of all the mills shall not be less than the coal consumption of boiler at BMCR condition.

2）For double inlet and outlet tube mills,when pulverizing design coal,the total calculated output of the mills shall not be less than 115%of the coal consumption of boiler at BMCR condition；and when pulverizing check coal,the total calculated output of the mills shall not be less than the coal consumption of boiler at BMCR condition.

3）For fan mills and medium speed mills,the output shall be calculated taking into account the medium and late period of abrasion；for double inlet and outlet tube mills,the output shall be calculated based on the ball loading amount suggested by manufacturer.

8.2.3　The number and calculated output of ball mills for Bin storage pulverized coal system shall comply with the following requirements:

1　Each boiler should be equipped with at least two mills,and no standby mill shall be provided.

2　The total calculated output of mills provided for each boiler（at optimum ball loading amount）shall not be less than 115%of the coal consumption of boilers when operating on design coal at MCR condition,and shall not be less than the coal consumption of boilers when operating on check coal at MCR condition.

3　When one of the mills is out of service,the output of the remainder calculated for design coal shall enable the boiler to operate safely and stably with no supporting fuel oil.If necessary,pulverized coal may be borrowed from the neighboring boiler（s）through pulverized coal feeder.

8.2.4　The type,number and output of coal feeders shall comply with the following requirements:

1　Coal feeders shall be selected based on the arrangement of pulverizing system,load requirements of boiler,regulation performance of coal feed flow,operation reliability in conjunction with the measuring requirements.Coal feeders shall have good tightness,and the coal feeders for positive pressure direct firing pulverizing system shall be capable of withstanding the pressure appropriately.

The type of coal feeder should comply with the following requirements:

1）When fan mills with direct firing pulverizing system are adopted,the scraper coal feeders complete with measuring device should be adopted.

2）When medium-speed mills and double inlet and outlet tube mills with direct firing pulverizing system are adopted,belt coal feeders equipped with weighing devices that can withstand certain pressure should be adopted.

3）For ball mills with bin storage pulverized coal system are adopted,scraper or belt type coal feeders should be adopted.

2　The number of coal feeders shall match with that of mills.For the units equipped with double inlet and outlet mills,two coal feeders shall be provided for one mill.

3　The calculated output of coal feeders shall comply with the following requirements:

1）The calculated output should not be less than 110%of the maximum output of the mill corresponding to design coal and design pulverized coal fineness.

2）For coal feeders equipped with double inlet and outlet tube mills,the calculated output of a single feeder shall not be less than the maximum coal feeding demand when operating in single-side feeding mode.

8.2.5　The number and calculated output of pulverized coal feeders shall comply with the following requirements:

1　The number of pulverized coal feeders shall be equal to that of primary air connections of burners.One pulverized coal feeder shall correspond to one primary air duct.

2　The calculated output of each pulverized coal feeder shall not be less than 130%of the maximum design output of the connected burners.

8.2.6　Bin storage pulverized coal system may be provided with pulverized coal conveying facilities whose configuration principle and capacity shall comply with the following requirements:

1　When each boiler is equipped with two coal mills,the pulverized coal bunker between two neighboring boilers may be interconnected through pulverized coal conveyor.

2　When each boiler is equipped with four coal mills and two pulverized coal bunkers,pulverized coal conveyors may be used to connect two neighboring pulverized coal bunkers of the one boiler,or two neighboring pulverized coal bunkers between two boilers.

3　The capacity of pulverized coal conveyor shall not be less than the calculated output of the largest coal mill connected.

4　When the length of pulverized coal conveyor is more than 40 m,double-end driving mode should be adopted.

5　Pulverized coal conveyor shall have good tightness.

6　If the pulverized coal chutes are arranged appropriately to directly supply pulverized coal to two neighboring coal bunkers of one boiler or two neighboring pulverized coal bunkers between two boilers,the pulverized coal conveyor may be eliminated.

7　Pulverized coal conveyor should not be provided for bituminous coal and lignite with high content of volatile matter and high spontaneous ignition tendency.

8.2.7　The anti-explosion and fire extinguishing facilities of pulverizing system shall comply with the relevant provisions in current national standards GB 50229 Code for Design of Fire Protection for Fossil Fuel Power Plants and Substations and DL/T 5203 Technical Code for Explosion Prevention Design of Coal and Pulverized Coal Preparation System of Fossil Fuel Power Plant.

8.2.8　The types,number,air flows,and pressure heads of primary air（PA）fans should be determined in accordance with the requirements below:

1　For the positive pressure direct-fired pulverizing system or bin storage type pulverizing system with hot air used as primary air,if trisector air preheaters are used,the cold primary air fans may be of moving vanes adjustable axial flow or speed regulation centrifugal type.Provision shall be made to prevent the axial-flow primary fan from surging and stalling.

2　The number of primary air fans should be two sets,and no standby fan shall be provided.

3　If trisector air preheater is used in conjunction with positive pressure direct-fired pulverizing system,the cold primary air fans shall be selected in accordance with the requirements below:

1）The basic air flow of cold primary air fan is calculated based on the design coal type,and shall include the primary air flow required by the boiler when operating under the BMCR condition,the primary air leakage of air preheater after operation for one year as guaranteed by the manufacturer,and the required sealing air flow supplied to the pulverizing system by primary air fans（calculated based on the total number of mills）.The basic pressure head of fans shall be calculated assuming that the boilers operate on design coal under the BMCR condition and corresponding number of mills are in service,while taking into account the resistance of mills and separators as guaranteed by the manufacturer,resistance of boiler proper at primary air side（including stack draft）,system resistance and the static pressure（negative）of furnace at burners.

2）The cold primary air fan should be selected with an air flow margin of 20%-30%plus a temperature margin（which can be determined based on"outdoor air temperature used for ventilation calculation during summer"）；the pressure head margin of the fan should be 20%-30%.

4　If trisector air preheater is used in conjunction with bin storage type pulverizing system,the cold primary air fans should be selected in accordance with the requirements below:

1）The basic air flow of cold primary air fan is calculated based on the design coal type,and shall include the primary air flow required by the boiler when operating under the BMCR condition and the primary air leakage of air preheater after operation for one year as guaranteed by the manufacturer.

2）The cold primary air fan should be selected with an air flow margin of 20%plus a temperature margin.The pressure head margin of the fan should be 25%.

8.2.9　The number,air flow and pressure head of pulverized coal exhausters shall comply with the following requirements:

1　The number of pulverized coal exhausters shall match with that of mills.

2　The basic air flow of pulverized coal exhauster shall be determined according to the thermal calculation of coal pulverizing system based on design coal type.

3　The pulverized coal exhauster should have an air flow margin of no less than 5%and a pressure head margin of no less than 10%.The maximum air flow of the fan should be designed to provide the air flow required by the mill with the maximum ball loading.

8.2.10　The number,air flow and pressure head of the sealing fans provided for the medium speed mill and double inlet and outlet mills in the positive pressure,direct firing pulverizing coal system,shall be selected according to the following requirements:

1　Each boiler shall be provided with at least two sealing fans,including one as standby.When each mill is provided with its own sealing fan,standby sealing fan may not be required.

2　The parameters of sealing fans shall be selected according to the requirements of mill manufacturers.The basic air flow of sealing fans shall be calculated based on the sealing air flow required by all mills and pulverizing system.The sealing air fan should have an air flow margin of no less than 10%plus a temperature margin（which can be determined based on"outdoor air temperature used for ventilation calculation during summer"）.When installed in tandem with the primary air fan,the temperature rise of the primary air fan shall be considered.The sealing air fan should have a pressure head margin of no less than 20%.

8.2.11　When the boilers with oil-saving ignition devices adopts cold start-up mode,the hot air required by cold start-up pulverizing system may be supplied as follows:

1　For direct firing pulverizing system,heating devices may be installed on the bypass air duct of the hot air duct at mill inlet corresponding to the ignition device.

2　Hot air from neighboring boilers may be used for cold start-up pulverizing system if justified through technical demonstration.