Application of Primary Air On-line Monitor System in Direct-fired Boiler
[Abstract] This text presents the character of big-volume direct-fired boiler unit milling system which is widely used at present, and the analysis of importance of application of primary air on-line monitor system in direct-fired boiler. The practice shows that it s very important that primary air on-line monitor system to raise auto-control and regulation level and raise boiler combustion efficiency during the process of boiler combustion running control.
[Key Word] Direct-fired System, Combustion Optimization, Load Regulation, Primary Air On-line Monitor
1.Summary: Comparing with the traditional pulverized coal-storage system, the Direct-fired System is simple, has no powder separator, bunker, adjustable powder machine equipment, and has obvious advantage in the respect of ignition, stable combustion and burn-off of meagre coal. And in this big-scale thermal power plant construction, the boiler of combusting meagre coal is equipped with pulverizing mill Direct-fired System.
In order to raise auto-regulation level, load response speed and combustion efficiency and burn-off rate and avoid local slag-bonding, the direct-fired boiler has higher requirement to pulverized coal concentration, the uniformity of fineness, air and powder distribution, powder fineness, ratio of air to the coal and the uniformity and adjustability of each winding pipe parameters supplied by the powdered coal system.
This test presents the importance and necessity of monitoring the primary air flow and speed of each winding pipe of direct-fired boiler, and the introduction of a solution of measuring primary air flow and speed as the relative concentrated air-solid two-phase and the successful application in practice.
2.
The demand of Direct-fired Boiler to Primary Air On-line Monitor
2.1 Demand of Combustion Adjustment
The boiler which is equipped with direct-fired system, the uniformity and equalization of primary air and powder flow shall decide the boiler’s combustion uniformity and stability, the different overall resistance coefficient, which caused by different length of coal powder handling pipe, different number of pipe knee bends etc., shall cause un-uniformed air-powder distribution as parallel running at the same pressure difference.
The resistance leveling of each primary air pipe of the boiler powdered fuel system is done by cold pure air resistance leveling method, i.e. under cold condition, as the pure air flows the primary air pipes in the same level, add throttle orifice with suitable openness in several primary air pipes which the resistance is relative small, to make them equal to these primary air pipes which the resistance is relative big. In fact, this resistance balancing method can only keep the balance of the same leveled pipes’ resistance under cold condition. And for that the pure air resistance characteristic is different from the one of coal powder air – solid two phase flow in the powder handling pipe, it can’t keep the real balance of resistance during actual operation as the air-powder mixture, this causes the un-uniformity of flow, speed and concentration of air-powder to each primary air pipe in actual operation. Even hot powdered resistance leveling, it shall lead to bad uniformity of primary air for un-balanced wearing of each throttle orifice.
One 600MW boiler often has abnormal condition of that the temperature of one side of reheat steam is relative low after running for certain period, this impairs the economic and safe running of the unit. This boiler adopts double-in double-out cold primary air fan pressurized direct-fired system, lay 24 cyclone DS burners in three levels along the furnace fore and back wall, the powder and air is fed from the fore and back wall, and opposed firing in the furnace. The inlet of each powder handling pipe has throttle orifice (restriction jet), and balance the resistance and primary air flow of each pipe by adjusting the openness of the restriction jet. And the resistance is adjusted with the powder as debugging this unit, but after long-term operation, the throttle orifice in each coal handling pipe has different wearing, this causes bad uniformity of air and powder, which impairs the uniformity of combustion in the boiler, making the temperature of each side of main steam and reheat steam has relative big difference. If adopt primary air on-line monitor, may find this abnormal condition and adjust it in time.
2.2
Demand of adjustment of powder fineness and air-powder ratio
As the coal burned is changed, the boiler shall require relative change of economic powder fineness. Should adjust the fineness of the powder at any time to meet the change of boiler load and fuel. And China, the power plant generally adjusts well the fineness of the powder as unit debugging, and does not pay some attention to the adjustment during running, but it can’t keep economic powder fineness during actual running, which impairs the operational efficiency of the power plant.
As for the direct-fired boiler which the auto-control level is relative high, the signal of boiler load change shall adjust the feed of coal feeder first, and adjust the flow of the relative primary air fan. As the pulverizing mill runs in the scope of 40%-100% rated load, the relation between the ventilation volume and load-factor of the pulverizing mill is linear relation, but the mini ventilation volume of powdered fuel system requires that is should beat 70% of the rated value. In general, as the pulverizing mill runs at rated output and relative rated ventilation volume, it may obtain a good air-coal ratio which is suitable for combustion. Nut with the dropping of output, the air-coal ratio rises, the coal powder concentration decreases. As running at low load, the furnace temperature level has decreased, and the air-powder ratio is too big additionally, this is very bad for powder ignition and stable combustion. Especially that the unit manufactured in China has relative low auto-control level, many power plants do not adjust the ventilation volume as the mill load changing, this causes the air-powder ratio at low load running deviates the suitable value obviously.
The on-line monitor of primary air pipe air volume and flow and speed and powder concentration may provide the basis for the adjustment of adjustable powder distributor, or adjusting the powder distributor continuously by closed-control system during running. This may keep the best powder fineness and air-powder ratio of the air and the powder which are fed into the furnace.
2.3
Demand of Quick Load Adjustment
As boiler load instruction changes, the coal feeding volume changes, and adjust the openness of the primary air damper regulator or the primary air fan speed, changing the primary air flow feeding the mill, the ventilation output changes, the change of primary air ventilation is relative quick for that the change speed of air flow is far quicker than the one of drying volume. Adopt the method of ahead changing ventilation volume, adjust the coal feeding volume and change the ventilation volume at the same time, although the milling output has no time to change, the powder taking by the ventilation has changed, so it can lighten the impair of powdered fuel system lagging to the boiler load adjustment that keeping the accuracy and flexibility of air flow adjustment of the mill.
It may conclude that it is very important and useful for direct-fired system that adopting primary air on-line monitor.
For that gas-solid two-phase flow is very complex, Direct-fired boiler primary air on-line monitor shall face many problems.
(1) The measurement device can’t meet the requirement anti-blockage as long-term running in the environment of air-solid two-phase flow, must adopt continuous or periodical back flushing method.
(2) The measurement device is not wearing-proof, this device shall deform and wear as long-term running, can’t keep measurement accuracy, can’t provide useful operational parameter for the combustion of the boiler.
(3) Can’t keep the measurement device the low-degree wearing, add the pipe drag force. Each primary air pipe of the outlet of the mill has be balanced according to relative drag force, and after installing this device, the additional draught loss shall impair original operational state.
(4) The piping in the flow shall be limited by the space and position, can’t meet the requirement of length of the straight pipe section needed by the flow-rate and flow meter.
(5) The accuracy of the flow and flow-rate meter can’t be certificated, this limits the study, development and spreading and application of the new type flow and flow-rate meter.
(6) These unfavorable factors, such as low speed, big change of temperature and pressure, impair the accuracy and reliability of flow and flow-rate meter, the performance of the meter is hard to be improved, and can’t meet the demands of the user.
LUCULENT CO. LUSE Type Primary-Second Air On-line Monitor System is composite of Speed Detecting Device, Transmitter and Man-machine Interface which is developed by the principle of Bernoulli's Equation. This speed detecting device adopts the special technology in view of the detecting condition of high-speed air-solid two-phase flow, to resolve the blockage, wearing, big pressure loss and low accuracy etc. which Direct-fired Boiler Primary Air On-line Monitor is nagged for long term.
LUSE Type Primary-Second Air On-line Monitor System has following features:
(1) Pressure Port is specially designed in respect of high pressure effectiveness, anti-blockage and protecting static measuring pipe.
(2) The special designed anti-blockage component can real-time clear the ash for the whole wall of the pressure pipe by helping the kinetic energy of the measuring medium.
(3) Adopt multi-curve labyrinth pipe to avoid the ash from leading into the pressure-in tube.
(4) Adopt high-temperature ceramic alloy with outstanding anti-wearing performance.
(5) Adopt pendulum principle to absorb the kinetic energy of the measuring medium fully.
(6) Integrate the anti-abrasive thermal resistance to provide thermo-compensation.
| Name of Parameter | Unit |
Numerical Value |
Drum Pressure |
MPa |
17.8 |
Water Supply Flow |
t/h |
1045 |
Water Supply Pressure |
MPa |
18.3 |
Water Supply Temperature |
℃ |
265 |
Superheated Steam Flow |
t/h |
943 |
Superheated Steam Outlet Pressure |
MPa |
16.6 |
Superheated Steam Outlet Temperature |
℃ |
538 |
Superheater Desuperheating Water Flow |
t/h |
75 |
Reheater Inlet / Outlet Pressure |
MPa |
3.7/3.5 |
Reheater Inlet / Outlet Temperature |
t/h |
278/539 |
Reheater Desuperheating Water Flow |
t/h |
0 |
Primary Air Pressure |
KPa |
11.9 |
Second Air Pressure |
KPa |
1.0 |
Air Chest Pressure |
KPa |
0.8 |
Primary Air Flow |
t/h |
134 |
Second Air Flow |
t/h |
924 |
Primary Air Temperature |
℃ |
322 |
Second Air Temperature |
℃ |
312 |
Coal Economizer Outlet O2 Content |
% |
3.1 |
Exhaust Smoke Temperature |
℃ |
135 |
After putting into operation, the unit runs badly as at high load, such as difficult adding air flow, un-stable combustion and bad combustion efficiency, and insufficient temperature of steam, and these problems can’t be solved although the manufacturer has carries out times of combustion adjustments. And install one set of Primary Air On-line Monitor System during the period of overhaul in 2001, carrying out scientific organization for air-powder matching and primary and second air flow proportioning for each burner to keep each burner’s coal powder concentration and suitable primary-second air flow rate, making the ignition quick and the combustion stable and complete, the carbon content of the fly ash at the full load of the boiler reduces to 13% (9%-10% at the lowest) from original 30%, and carbon content of slag reduces to about 15% from original 30%, and the problem of insufficient steam temperature has be solved completely, raising the safety degree and efficiency of boiler combustion by big margin, as for two boilers in the first stage, AnShun Power Plant can save 67,392 tons coal every year, amounting to RMB 7,413,000yuan.
5. Conclusion
