Application case 25: Energy-saving control scheme of 202m refractory brick tunnel kiln

1.       Energy saving control scheme of 202m tunnel kiln

202m tunnel kiln with 72 parking spaces, including 1 ~ 26# parking spaces for the preheating zone, 27 ~ 47# parking spaces for the firing zone, 40 ~ 72# parking spaces for cooling zone.

On the top of the preheating zone is equipped with smoke port, by the smoke machine the high temperature flue gas of the firing zone will be drawn into the firing zone, heating the car, and by adjusting opening of the gas pressure and the air outlet on the kiln top, to realize the process temperature gradient curve in the preheating zone, on both sides of 2 ~ 72# car tunnel kiln in the preheating zone temperature measuring points are set, with points on the top of 2, 4, 6, 8, 10# car tunnel kiln, and under 4, 14, 23# tunnel kiln also are set temperature measuring points, to measure the exhaust pressure, and use the micro negative pressure control preheating the preheating zone.

In the firing zone at the top of 27 ~ 47# tunnel kiln points are set, both sides are equipped with a combustion burner, 41# measures the temperature under the kiln, detects combustion air temperature, gas pressure, combustion air pressure, the firing zone works in the micro negative pressure atmosphere.

In the cooling zone the temperature on the top of 62 (63), 69 (70) # tunnel kiln is measured, the temperature on both sides of 52, 54, 56, 58 tunnel kiln is measured. In the cooling zone which is immediate vicinity of the firing zone, is equipped with suction blower, a part of the heat is sent back to both sides of the firing zone, used for the burner air distribution, a part of the heat hot to profiling section for drying Adobe. The air cooler is equipped with in the middle-lower segment of the cooling zone to cooling products.

In the preheating zone, the firing zone, the cooling zone, the pressure detections in the kiln are equipped with;

With blower and exhaust fan under the kiln, for cooling the trolley wheels and establishing a pressure balance of tunnel kiln.

Based on the technological characteristics of tunnel kiln and the preheating curve, firing curve, cooling curve of production, the energy saving control of tunnel kiln should be from the following several aspects:

1) To strengthen the sealing of tunnel kiln, reduce the infiltration of cold air and the heat radiation outside of the kiln;

2) To improve the fuel combustion efficiency, and the control precision of temperature;

3) Based on ensuring the balance of the temperature and the pressure in the tunnel kiln, maximum reducing exhaust temperature;

4) To improve the quality of the products.

The existing control method of kiln temperature system mainly is the traditional PID control. Although this method is able to achieve good control effect in the small range of vicinity of the operating point, but when there are great fluctuations in the system operating conditions, due to the effects caused by pure lag and changes of parameters, the control effect becomes worse, there are limitations. Therefore, people have been trying to improve the control of this kind of systems. For the control problem of the system with large delay, since 1957, Smith put forward the compensation control, a variety of control methods have emerged, but has not yet been solved completely. After the review of the control methods for large lag process and their improvements in recent years, based on the intelligent control theory to design the intelligent glass furnace control scheme.

Tunnel kiln is a complex multivariable nonlinear control of a time-varying coupling, its structure and its control technology will greatly affect the quality of products and the production cost. The tunnel kiln temperature system is a variable parameter system with large inertia, large delay and nonlinear characteristics, it is difficult to establish a precise number, and during operation the furnace are affected by a variety of disturbance factors. The furnace temperature is controlled by regulating the amount of gasoline injected by burner both sides of the firing zone. Thermocouple is arranged on the corresponding top of each pair of kiln burner, to detect the point temperature. The temperature of some measuring point is also affected by other burner, pressure in the kiln, blank shape, humidity, temperature of fuel and external climate environment etc, so it has the characteristics of large inertia, uncertainty, nonlinear, multivariable coupling.

The dynamic characteristics analysis of temperature system of kiln shows that, under normal circumstances the temperature system of kiln is a large inertia, large delay and variable parameter system with nonlinear characteristics. But the kiln during operation may also be affected by many disturbance factors such as gas pressure, heat value fluctuation, combustion air pressure, temperature fluctuations, feed quality fluctuation, furnace pressure, combustion atmosphere, changes of heat preservation performance, the change of work environment temperature etc.

In view of the characteristics of the tunnel kiln, the current control method is difficult to meet the requirements of high performance of furnace temperature control. Such as the control effect of simple PID control on the kiln with large delay is poor, it is difficult to adapt to the changes of object parameters, and the contradiction between overshoot and rapidity exists.

At present, the domestic and international control of tunnel kiln mainly adopts manual adjustment, or uses the decentralized controller (PID) to adjust, the parameters of the controller are adjusted by engineering method, but the coupling between variables in the utility makes the parameter adjustment become too complex, if the operating environment changes, the parameters need to be readjusted, and the effect is not very ideal.

Because the true realization of automatic control, to achieve the effect of energy saving and improving the quality of products, the mathematical model of kiln must be established, to predictive control through multivariable control technology, and establish a real-time mathematical model and can self-correct online. Tunnel kiln is an advanced kiln in the industrial production of refractory materials. For the convenience of study, the tunnel kiln is divided into the kiln structure (including the kiln wall, the top of the kiln and the rigid frame structure), preheating structure (including exhaust system), firing structure (including combustion system), cooling zone structure (including cooling system) and other structures (including kiln structure and cooling structure under car etc). The kiln body is a long tunnel built by kiln wall, kiln roof and the lining of the kiln car and other refractory bricks. The rail is laid in the tunnel kiln for kiln car running, the bricks to be burned are neatly put on the kiln car. The whole tunnel kiln along the direction of kiln car running is divided into three belts: preheating, sintering, and cooling.

 (1) The structure of the preheating zone of tunnel kiln

The preheating zone front of tunnel kiln, the bricks of the kiln car firstly are preheated in here. The kiln wall of the preheating zone is provided with a waste gas hole, inside of the kiln wall is hollow, and is provided with the exhaust pipe. The number of waste gas hole is set according to the amount of flue gas exhaust, to ensure the flue gas temperature in the preheating zone not exceed the prescribed temperature.

Tunnel kiln structure

the firing zone intermediate tunnel kiln, on the kiln wall is set a plurality of high temperature burner, products on the kiln car are baking in here, the number of the main burner is decided according to the firing products categories and how much the calcination time needed by products.

 (2) The structure of the cooling zone of tunnel kiln

At the end of the tunnel kiln cooling zone, cooling products are cooled in here. Draw into a mass of cold air from the cooling zone, the cold air itself also be preheated while cooling the product. Part of the preheated air is used as the combustion supporting secondary-air, the other as a part of the primary air to be sent to each burner for combustion of fuel, , or sent to other places for the heat source of drying products.

In order to avoid the loss of heat, at the front of tunnel kiln two kiln doors are set that outside door and inside door. The two doors use the winch transmission mechanism, with the help of terminal switch and corresponding electrical equipment to control, feeding when opening the outside door every time, discharge when the inside door opening every time. Combustion equipments comprise a combustion chamber and the nozzle, the fuel (heavy oil or natural gas) burns in the combustion devices, combustion products enter into the tunnel, transfer the heat to products. The ventilation devices comprise a smoke exhaust system, gas and air circulation device and cooling system. They are composed of the exhaust fan, chimney, blower and various flue, pipeline. Its role is to enable the kiln gas flow in a certain direction, and the flue gas exhaust, the air supply, the hot air out, and maintain the temperature, atmosphere and pressure in the kiln.

2.     The work principle of the tunnel kiln

The working principle of tunnel kiln and kiln is basically the same. It is through the use of raw materials and fuel heat, on the product and half finished drying, preheating, burning, heat preservation, cooling. General can be divided into preheating zone, firing zone, and cooling zone. Preheating zone here mainly to blank preheating, drying, dehydration, make it safely into the high temperature firing zone; firing zone is to continue heating to high temperature for the body, to make it in the high temperature zone melting, crystallization, solidification, and complete physical and chemical reaction process; cooling zone to ensure product cooling without crack. Through the organic cooperation of the above zones, to take a certain high temperature smoke heat and cooling heat to supply for drying kiln to dry brick, so as to compose of the combustion system with high efficiency and organic contact. So we can make different firing curve according to different raw materials and different products, in order to achieve high quality, high yield, and low consumption. The work system of tunnel kiln should, according to different raw materials and different products, to make different drying and firing system (including the temperature control system, air system and pressure system). If expressed in chart, we referred to that firing curve is controlled by the kiln operation, including temperature control, atmosphere control and pressure control. The pressure system is the guarantee of the temperature control and the atmosphere system.

2.1 The temperature control in each zone

According to the process requirements such as the properties of raw materials, the shape and the size of products, and the moisture of kiln, to formulate a reasonable firing temperature curve. When baking, according to the curve to ensure a certain temperature, heat preservation and cooling system. Tunnel kiln can be divided into three parts, such as preheating, firing zone and cooling zone. The following summarizes the temperature control of each zone.

2.1.1 The temperature control of preheating zone

The preheating zone is from the products into the kiln to the first combustion chamber, usually including a dozen car spaces. Temperature control is defined as the uniform heating according to the temperature curve, generally in the kiln roof is provided with a thermocouple to monitor temperature. If the kiln temperature is too high, easy to make the high moisture products entering into kiln into burst, but if the water content is less than 0.5%, the kiln temperature can be higher. If it is the quartz Chang type of transition temperature, a volume change, should maintain a stable temperature. So, not only to control the temperature on the kiln top, but also control the temperature on the surface of kiln car, so as to reduce the temperature difference.

The temperature control in the preheating zone is that mainly through regulating the frequency converters of air brake and the exhaust fan to control the gas flow. if the air brake open large, in preheating zone the negative pressure will be high, easy to cold air, increasing gas layer, increase the temperature difference; if air brake is opened small, draw strength will be insufficient, the flue gas volume small, temperature rise slowly. High-grade baking kiln adopts frequency conversion cabinet to control the product temperature. the air brake after adjusted to be locked, if the air brake at the end of the preheating zone open large, a large gas premature will be discharged, low heat utilization rate, low temperature of the kiln head, and the temperature of products rise slowly. If the air brake in the preheating zone does not open, a lot of smoke will flow to the kiln head, so that the kiln head temperature is too high, is not conducive to product preheating. The air brake of kiln head cannot open too large, so as to avoid the negative pressure is large, a lot of cold air enter into from the kiln door.

In short, to ensure that the product is preheated in a certain temperature, and to ensure that the temperature difference is small, the joint of kiln car must be tight and not leak wind, the sand sealing plate joints must be tight, and the sand sealing plate should be buried into sand. In addition, the reasonable muddle- stacking can also reduce the product temperature difference, according to the different situation of internal and external combustion, the billet stack must be reasonable stacking, the space between billet stack and the kiln wall cannot be too large, should have enough gas channels and smooth, increase air resistance, reduce the air flow upper and surrounding, to make the air be evenly distributed in the billet stack, to achieve uniform temperature inside and outside.

2.1.2 The temperature control of firing zone

The temperature control in the firing zone is to control the actual combustion temperature and high temperature. General flame temperature shall be higher than the sintering temperature, the flame temperature control is achieved by adjusting in the unit time the mixture ratio of the fuel consumption and air, in unit time the combustion fuel combusts thoroughly and air quantity is appropriate, then flame temperature will be high. The combustion products to control the fuel and oxygen demand according to the products hot value. The control of the highest temperature point is very important, generally controlled in the last 2 parking spaces of the firing zone. Forward of the highest temperature point makes the holding time become too long, easy to make the product burnt deformation; and the shift makes the holding time become insufficient, and then format soft brick.

2.1.3 The temperature control of the cooling zone

Products get into the cooling stage after firing, the cold air directly enters into the kiln end and cooling, that’s slow cooling stage, depends on the fan distributed in that stage to let the hot air out, make the product cooling to the right temperature and then out of the kiln. Before products can quench, must decide the position and the quantity of the high temperate rapid cooling wind according to the product properties, loading conditions and trolley speed.

2.2 The atmosphere control of the firing zone

Burning zone can be divided into two kinds of atmosphere, the oxidation atmosphere and the reduction atmosphere. The oxidizing atmosphere is easy to control, to control the excess air coefficient greater than 1, but it is better to not too large, so as to save fuel, increase the temperature. In the oxidation process there are full airs to fully burn the residual CO, but also a certain temperature (self) must be maintained. Atmosphere control and temperature control are closely related, for example: in the oxidizing atmosphere, because the original air is too much, if maintain material constant and reduce excess air, the flame temperature will increase; when the air is reduced, until the excess air coefficient is close to 1, the temperature will be highest. If the air is lacking, the temperature will drop into the incomplete combustion atmosphere. On the contrary, in the reducing atmosphere, due to the lack of air, the temperature lowered into the state of incomplete combustion of fuel, if maintain fuel unchanged with increasing air, due to the combustion is more complete, the flame temperature will increases, and continue to increase air to the theoretical requirement, the combustion temperature will be highest, if increase air again, the temperature will decrease into the state of complete combustion, and then the oxidation atmosphere of air will be too much, so from the changes of temperature the changes of atmosphere can also determined.

2.3 The pressure control of each zone

The pressure system is to guarantee to realize the temperature system and atmosphere system. The most important is to control pressure stable at both ends of the firing zone in the kiln. If the furnace negative pressure is too large, the cold air leakage into will be too much, on one hand, reducing the temperature in the kiln, the gas layer is serious, the temperature difference is great; on the other hand, in the firing zone the calcination atmosphere is difficult to be maintained. If the kiln positive pressure is too large, a lot of warm air will be out, heat loss, deterioration of working conditions, the thermal string into the kiln will burn the kiln car, causing the accident. The ideal operate is to maintain zero pressure, but the whole kiln to maintain the zero pressure is impossible, only in the firing zone to maintain the zero pressure and micro positive pressure. Because the preheating zone need to pull the flue gas, and then into a negative pressure, and the cooling zone will be blowing cold air to cooling products, and then must be at the positive pressure. Because the negative pressure drop to positive pressure need pass by zero pressure, from the cooling zone to the preheating zone need pass by the firing zone, so in the firing zone operation is at zero pressure near, control the zero pressure surface is very important, the general control is near the interface of preheating zone and firing zone, so that the burning zone in the micro positive pressure, because the firing zone will leak into the cold air, especially in preheating the negative pressure is large, leakage into the cold air is much, the temperature difference will be great.

The entering cold wind in the cooling zone must be equilibrium with the hot wind out of the zone, it will not have cold air enter into the firing zone, make the firing zone can control the highest firing temperature and calcination atmosphere, if need to quench, products before can be quenched, blowing cold air into the kiln and the side of kiln, to form a air curtain. If at the quench place the positive pressure is too large, considerably more than positive pressure at the end of the firing zone, this shows that the drum into cold air is too much and is not pumped out, lots of low temperature air enter into the firing zone, may cause insulation defects, if the quench at positive pressure and negative pressure appears insufficient, because the hot air out too much, then the flue gas at the firing zone flow back to the heat preservation zone and the cooling zone, which makes the smoked product to be waste, so the quench gas curtain is difficult to control.

In addition to the pressure control system of the kiln, but also the pressure under the control car, there are best to maintained close to the equilibrium with the pressure in the kiln, in the cooling zone under the car maintain positive pressure, in the preheating zone under car maintain negative pressure, in the firing zone near the zero pressure, so in the preheating zone there is no cold air leaking into the kiln, in the cooling zone no hot air is took into the tunnel, if there is difficult to control, it is better that the pressure under car in the preheating zone and firing zone with is less than the pressure in the kiln, to avoid leakage into the cold air, increasing the kiln temperature difference, the pressure under car in the cooling zone should be greater than the pressure in the kiln, to avoid burning kiln.

The plan of saving energy and increasing the rate of qualified products only detect and control the firing section, the corresponding mathematical model is established, the preheating section and the cooling section and ancillary equipment will not enter the system temporarily, but the system hardware reserved interface which enter the system, after the transformation, the effects achieved, and then this part is fit into control system, only need to increase the expansion chassis and I/O temperature transmitter module, signal isolator, relay, the cable etc.

3 System configurations:

The host computer and lower computer PLC two levels control

The host computer uses Advantech IPC, mainly to establish the mathematical model, control the output of algorithm, display, alarm, record the images of process parameters, formulate baking curve, and to set pressure values.

The lower computer adopts Siemens S7-300 series PLC, mainly for various process parameters and the device state acquisition, safety interlock and control output.

The list of the modification of 202 meters tunnel kiln combustion control system
serial number	Name	Manufacturers	specifications and models	Units	quantities
1	Control cabinet	Beijing	Imitation Rittal 2000×800×600		1
2	The PLC power module	SIEMENS	6ES7 307-1EA01-0AA0	Block	2
3	CPU	SIEMENS	6ES7 314-1AG14-0AB0	Block	1
4	Memory card	SIEMENS	6ES7 953-8LG11-0AA0	Block	1
5	The Ethernet communication module	SIEMENS	6ES7 343-1EX30-0XE0	Block	1
6	The interface module	SIEMENS	6ES7 360-3AA01-0AA0	Block	1
7	The interface module	SIEMENS	6ES7 360-3CA01-0AA0	Block	1
8	Interface cable	SIEMENS		Block	1
9	Output switch module	SIEMENS	6ES7 322-1BL00-0AA0	Block	3
10	Analog input module	SIEMENS	6ES7 331-7KF02-0AB0	Block	9
11	The front connector	SIEMENS	6ES7 392-1BM01-0AA0	Block	3
12	The front connector	SIEMENS	6ES7 392-1BJ00-0AA0	Block	9
13	Module frame	SIEMENS	6ES7 390-1AF30-0XE0		2
14	Ladder diagram software package	SIEMENS	STEP7 V5.4 SP5	set	1
15	Temperature transmitter	New power	One in - two out（4～20mA）	Block	24
16	Signal isolator	New power	（4～20mA）	Block	48
17	Isolated relays	OMRON	HP52P DC24V	Block	96
18	DC power supply	SIEMENS	6EP1 334-3BA00	Block	1
19	PLC cabinet for electrical components	Schneider		set	1
20	Shielding silicone rubber cable	Anhui	5×1.5	M	2200
21	Shielding silicone rubber cable	Anhui	2×1.5	M	200
22	Cable trough		200×80	M	150
23	Threading pipe		DG20	M	460
24	Regulating valve	Taiwan	701B		42
25	Optimal control software	CIMAC		set	1
26	IPC	Advantech	610L		1
27	LCD display	Samsung	22"		1
28	Special burner		CMT-G119	set	42