2017 Volume 57 Issue 12 Pages 2266-2268
This work introduces a programmable pulse combustion controller. It has two main parts, namely burner flame switch timing sequence and burner startup sequence; and the sequences being controlled by three groups of programmable timers. Burner output capacities for high and low flames are adjustable online, and the corresponding working periods for different flame sizes can be computed automatically using the proposed equations tested in practice. Proper cooperation between the burner flame size and its working period can achieve the required control objective well. Comparison results in several projects of reheat furnaces show the proposed methodology can optimize chamber temperature distribution and improve control precision effectively.
Combustion technology in the area of metallurgy has advanced greatly in recent years, which in part is due to the demand for clean and efficient combustion techniques. Some achievements have been made in high temperature air combustion (HiTAC)1,2) and pulse combustion technique. Pulse combustion has attracted more attention because of its high combustion efficiency and low pollutant emission etc. The first pulse combustor is recorded in the literature in 1906, with Putnam et al.3) giving a review of work up to the early 1980s. Lockwood4) and Yamaguchi et al.5) applied for patents with a pulse combustion device and energy system in the late 1980s. Keller et al.6) analyzed the importance of characteristic times in pulse combustion in 1989. In the early 1990s, Zinn7) presented a review report concerning the research issues for existing pulse combustors and applications. Recently, Meng et al.8) give a detailed review of pulse combustion fundamentals, model comparisons and both experimental findings for the latest advances.
Despite the prominent advances made in pulse combustion, intelligent control strategies can play an important role in improving combustion performance further. Particularly worth mentioning is that programmable control schemes can be implemented using industrial software, which makes combustion control flexible and economical. A number of control technologies have been used for different types of furnaces. Elster Krom-Schroder is famous for its outstanding combustion control products including pulse combustor, pulse controller and actuator etc. Its MPT700 can control up to eight burners to work simultaneously in a heating process.9) The Engineers of Beijing Shenwu Co. developed a pulse combustion system with a pilot ignition combustor and supporting software.10) There have also been some other synthetic schemes developed in recent years. However, little work has been deeply researched on the practical timing sequences for pulse combustion control. For conventional pulse combustion, a burner usually works in two burning states, namely RUN and STOP. When a burner starts, it works at full load, or at an optimum value depending on the gas-air mixing ratio and flame jet velocity. A Stop state is sometimes substituted by a small flame which makes a burner work in a minimum capacity and prevents it from extinguishing to avoid igniting burners repeatedly. The chamber temperature is controlled by adjusting the duty cycle, or frequency between the two flame states. However, using the conventional combustion mode, when burners run, the heating supply is generally strong. Reports from the site engineers show this usually leads to local overheating in billets, especially for parts close to flames during a heating process. Specialized hardware controllers for pulse combustion will increase the system cost. In view of the above problems, a programmable pulse combustion controller is proposed in the present work.
The PPCC is a combination of continuous combustion and conventional pulse combustion. The main concept is to adjust flame size and the corresponding working periods to meet the control objective. It’s a function module downloaded to a programmable logic controller (PLC) which directs the electrical combustion equipment to implement a specific pulse sequence. It mainly consists of two parts, one is burner switch timing sequence between high flame and low flame, and the other is the startup sequence for N burners in the combustion process. The main scheme is analyzed as follows.
2.1. Burner Switch Timing SequenceGiven N burners in a chamber, then there will be 2N delay-off timers defined for the timing sequence control. They are divided into two groups, group one with timers t1, t2, ···, tN−1, tN and group two with tN+1, tN+2, ···, t2N−1, t2N. As shown in Fig. 1, group one is used for setting the pulse period Tb in high flame, while group two is used for controlling the low flame period Ts. Tp is known as a full pulse period.
Pulse combustion sequence for N burners.
The switch timing sequence for a burner between high flame and low flame can be programmed as follows. First, timer t1 starts up 1# burner to work in high flame. When t1 stops, 1# burner switches to low flame with the duration time being controlled by timer tN+1. Next, 1# burner will switch to high flame again when tN+1 finishes. Similarly, the switching timing sequence for 2#-N# burners are controlled by timers t2, tN+2, ···, tN, t2N. The complete sequence keeps in step with 1# burner. However, the working periods need to be determined to implement this sequence. For the period computation, we present two equations based on engineering experience.
Period in a high flame
| (1) |
Period in a low flame
| (2) |
Segment of control output OUTp with corresponding flame sizes.
In Fig. 2, OUTp is divided into three segments such as SEG1, SEG2 and SEG3. OUTp1 and OUTp2 are the demarcation points. Three segments correspond to three groups of burner flame sizes, namely,
For the startup sequence of N burners, another group of timers will be defined, namely t2N+1, t2N+2, ···, t3N−1, t3N. They are also delay-off timers. Working periods for them are all set as Tw which denotes a delay time to start a next burner. The start-up process needs to cooperate with the timers in group one as shown in Fig. 1. When timer t1 begins to work, it will also start t2N+1 up. Accordingly 1# burner is ignited to work. Next, when timer t2N+1 stops, it will start timer t2 up, namely 2# burner begins to work. At the same time the t2N+1 falling edge will start t2N+2. t2N+2 is used for starting the next burner up. Similarly, the rest of the burners will be started up in succession. The delay time Tw is calculated by
| (3) |
Therefore N burners will be started up within a whole pulse combustion period Tp. The recurrently alternate working mode can ensure the furnace temperature distribution uniformity. Then as long as we assign one of Tp, Tb and Ts on SCADA system, such a pulse combustion process can be implemented.
Taking a heating process with four burners in a chamber for example, then twelve timers t1, t2, ···, t11, t12 will be defined in three groups. Suppose that a dead band δ% is defined as 5% and a return difference σ% is defined as 3%. Segments of OUTp and burner flame sizes are defined as
When current OUTp is 60%, then
Pulse sequence with 80% and 20% burner capacities for high and low flames.
Other burner flame sizes suitable to the control output can also be configured. The corresponding pulse period adjustment can make them get to the same control objective. When OUTp is within δ%, all burners work either in full power or extinction. When OUTp is close to demarcation points within σ%, flame sizes are chosen basing on the current temperature trend. Comparing results in the following projects using different combustion modes show the proposed method is effective.
Average temperature errors at the control points in Tables 1 and 2 are calculated using practical data coming from the master thermocouples in different production cycles for each project. Results indicate temperature precision can reach 0.34% using a PPCC scheme while it’s about 0.85% using a traditional Run/Stop combustion mode. The control accuracy is significantly improved. Furthermore, the statistical temperature errors in the whole chamber by auxiliary thermocouples can be controlled within 0–12°C with the new PPCC, whereas it’s up to 20°C using a conventional method as shown in the tables. Thus, the temperature uniformity in a chamber is distinctly better than before.
| Plants | Projects and zones | Setting value | Control point error | Chamber error |
|---|---|---|---|---|
| Russia OAO Lipetsk Iron Works | 58 m ductile iron pipe annealing furnace, heating zone | 1080°C | 3.52°C | 9.25°C |
| India Jindal Saw Co. Ltd. | 36 m ductile iron pipe annealing furnace, holding zone | 1020°C | 3.83°C | 10.82°C |
| China First Heavy Industries | Car type forging heating furnace | 860°C | 2.92°C | 7.70°C |
| Ningxia Nonferrous Metals Co. Ltd. of China | 42 m beam type reheat furnace, holding zone | 850°C | 3.65°C | 11.61°C |
| Plants | Projects and zones | Setting value | Control point error | Chamber error |
|---|---|---|---|---|
| Turkey Samsun Makina Sanayi A.S. | 42 m ductile iron pipe annealing furnace, Heating zone | 980°C | 6.68°C | 17.32°C |
| China First Heavy Industries | Car type forging heating furnace | 860°C | 6.81°C | 13.49°C |
| Ningxia Nonferrous Metals Co. Ltd. of China | 42 m beam type reheat furnace, Holding zone | 850°C | 9.25°C | 19.51°C |
| Taiwan Xinguan Iron and Steel Group Company | Car type heating furnace | 960°C | 7.26°C | 14.22°C |
We have presented an improved pulse combustion methodology named PPCC for industrial reheating furnaces. Technical advantages of the proposed methodology can be summarized as follows:
(1) The PPCC is implemented using an instruction list in the form of a function downloaded to a programmable logic controller. The design costs for a project can be reduced in comparison with a specialized hardware pulse controller.
(2) Burner output capacity is adjustable and the corresponding pulse period can be calculated online. Cooperation between the flame sizes and pulse periods with a specific combustion sequence achieves the control objective and makes the temperature rise more smoothly. Implementation is convenient and flexible. Billet local overheating problem can be effectively avoided.
(3) The adjustable range of the controller output value OUTp can be enlarged in comparison with former combustion method as the high or low flame size is changeable. Especially when OUTp is low, proper configuration for the pulse combustion states can improve temperature accuracy and make the fuel burn more sufficiently to reduce pollutant emissions.
