I. Main structure types of pulverized coal steam boiler

At present, pulverized coal boiler mainly has four structures: WNS horizontal internal combustion shell boiler, DHS single-drum transverse water tube boiler and SZS double-drum longitudinal water tube boiler.

WNS horizontal internal combustion shell boiler: the capacity range is 4~20t/h (steam boiler), 2.8~14 MW (hot water boiler). Due to the limitation of furnace size, overall transportation size and shell wall thickness, the capacity and parameter of WNS pulverized coal fired boiler is low.

SZS double-drum longitudinal water tube boiler: the capacity range is 10~50t/h. However, SZS pulverized coal steam boiler has the problem of ash deposition in the bottom of the furnace and convection heat transfer zone.

DHS single-drum transverse water tube boiler: vertical structure is suitable for large capacity. DHS overhead burner relies on ground support, structure is compact, and the burner is mounted at the top of the furnace. Meanwhile, vertical top-blowing structure avoids ash deposition and coking in the furnace, ensuring the operation stability and continuity.

II. Design of SZS35-1.25-AIII pulverized coal steam boiler

1. Pulverized coal steam boiler design parameter

Rated capacity: 35t/h

Rated steam pressure: 1.25MPa

Rated feed water temperature: 104℃

Rated steam temperature: 193℃

Flue gas temperature: 136℃

Design efficiency: 90%

Design fuel: AIII soft coal

LHV of fuel: 25080 kJ/kg

Fuel consumption: 3460kg/h

A pulverized coal burner is arranged in the front wall of furnace. The pulverized coal is injected into the furnace through the burner, and burnt out in the furnace. High temperature flame transfer heat to the radiation heating area, then the flue gas enters the convection area through the flue duct in the tail, flows through the convection tube bundle and economizer, and is finally exhausted to the atmosphere through the chimney. The boiler is composed of furnace radiation heating area module, furnace connecting flue duct, convection heating area module, economizer connecting flue duct and economizer. 

2. Introduction of main parts

2.1 Furnace radiant heating area

Furnace radiant heating area is left and right membrane wall (tube Ф60×5) arranged between the upper and lower header (Ф377×20). The upper and lower header (Ф219×10) on the front and rear walls are connected with the upper and lower headers of the furnace, forming a fully sealed furnace structure, achieving micro-negative pressure combustion.

SNCR pipe (Ф38x 3) is placed in the middle of the furnace top. Soot blowing pipe (Ф32×4) is in the lower of front water wall. Soot blowing pipe (Ф159×6 & Ф57×5) is in the bottom of the furnace. Ash dropping port is in the rear part of the furnace.

2.2 Convection heating area

Convection heating area water cooling system consists of upper drum of Ф1200×25, lower drum of Ф800×20 and convection tube bundle of Ф51. The internal of upper and lower drum adopts a dash plate to ensure that the flow rate in the pipe is no less than 0.3 m/s, and the water circulation is reliable. Convection tube bundle is arranged between upper and lower drum, the left and right side of the convection tube bundle is fully sealed membrane wall (tube Ф51×4), forming a flue gas passage; the convection heating area tubes are welded to the drum.

Acoustic soot blower is arranged at the center of the front wall of convection heating area, and soot blowing pipe (Ф32×3) is arranged at the bottom of the convection area.

2.3 Economizer

The heat pipe economizer is arranged at the outlet of the boiler, adopting HT150 cast iron pipe and elbow and greatly improving the service life. The economizer has an ash cleaning port at the bottom and a temperature pressure measuring hole at the inlet and outlet.