The application of impulse burning in ceramic combustion began in 1958. The combustion was mainly done with light or heavy oil. The functional construction comprised a fuel loop with a central fuel pump to directly supply the burners (see Figure 1). Combustion air was added to the firing process via burner lances. The fuel metering was done with electromagnetic activation of upstream magnetic valves via cyclical impulses from 0-250 pulses/min, whereby the fuel in the burner lances was mixed with the combustion air and, with exit at the lances in the combustion chamber, ignited at a temperature of less than 650°F.

Due to changes in impulse frequency level, the burner controlled both the fuel feed and burner output. The structure of impulse firing was changed with the availability of natural gas. The impulse-controlled fuel feed was supplied via a central impulse valve per firing zone, with six to eight burners over one fuel distributor line.

Temperature control was realized with the impulse frequency level, as with the older oil firing systems. The fuel feed on impulse basis created small explosions (the size of which depend on frequency level or impulse length) inside the combustion chamber atmosphere. As a result, the firing material was exposed to strong turbulence.

The turbulence rise inside the combustion chamber caused an ideal temperature circulation of the firing material and a homogeneous firing atmosphere. Thyristor modules were used to activate the frequent impulse valves, providing power consumption. This impulse firing technology resulted in higher quality fired materials and energy savings at around 20%.

The fuel feed of the burners was another disadvantage when zoning. Because of the conditional supply assembly of the impulse zone valve, one firing group was powered by up to 10 burners. Due to different pipeline lengths to the burners, pressure decreases could alter among each of the burner lances, and disadvantageous flow behavior could occur. Those unwanted pressure and flow behaviors needed to be compensated with different burner adjustments, which often did not lead to satisfying results . These traditional impulse firing systems were costly and time consuming, and were therefore reserved mainly for the production of technical ceramic products.

A Technology Renaissance

In1992, impulse combustion on an SPS basis with a Simatic S5 was used for the first time in Minden, Germany, by Dachziegelwerke Heisterholz. The ceiling combustion was grouped in firing zones with 10 injector burners each and equipped with an impulse zone valve capable of 300 pulses/min (see Figure 2).

Complete data logging was realized with the Simatic PLC control system. The control software evaluated the temperature situation and calculated the necessary impulse frequency. Energy savings of 25% resulted from the change to this system.

Despite the successes up to that point, a problem of pressure loss was caused by the zone valve. The fuel path to each burner in a firing zone was still varied; this led to the development of an electromagnetic micro-valve with a high switching frequency. The impulse valve is the electromagnetic heart of the impulse firing technology and is exposed to extreme mechanical stress. To eliminate the previously mentioned disadvantages of the zone valves, a cooperation with the company Danfoss took place in 1993 to develop a micro-valve for high-frequency operations. The technical features were identified as follows:

•  Small structural engineering

•  Wear-free material components

•  High-frequency operating conditions (0-1,000 pulses/min)

•  Minimal power take of < 0.4 amps

•  Operating voltage of 24 V DC    

After a short development time, the special valve went into production and complied with all the necessary requirements. The first pilot combustion with the new special valve was first used and tested at the tunnel kiln in Minden for Dachziegelwerke Heisterholz.

Converting from a firing zone with 10 burners to the new, single-impulse operation per firing point resulted in a nearly 50% investment savings compared to the impulse zone valve. The small power consumption of the impulse valve of maximum 0.37 A ensures that the electronic control can take place directly via the SPS output side. Therefore, further savings can be achieved because of the omission of coupler components and less wiring expenditure.

Single-impulse valves allow direct control of the fuel valve from the PLC digital component through the small power consumption of max 0.37 A. If a failure on one fuel valve occurs, the remaining valves in the firing zone aren’t affected. Independent adjustment settings of the fuel throughputs exist for each firing point. They offer unrestricted flexibility of the controlling possibilities of single burners in a firing zone (stokehole loading). Single-impulse valves also allow parallel operation, cyclical control loop sequences, asymmetric control and control with various impulse lengths.

The previously mentioned impulse firing systems occurred only in the injector burner sector without an igniter, so the impulse firing was limited to temperatures below the ignition temperature of 650°F. These circumstances forced another improvement, which made it possible to use high-speed burners with ignition and flame control to work in the pre-fire and heating area with turbulences underneath the ignition temperature. This created a homogeneous combustion atmosphere in all areas of the oven.

The high-speed burner couldn’t be used like the injector burner, where the fuel mix is ignited with entry in the firing zone, since the time dependent progress, ionization setup and control of the safety valve weren’t possible with impulse firing. In order to adhere with regulatory guidelines, it was necessary to start the burner in the least possible power range (~ 10%) to achieve the ionization control, or flame stability. This power state of the burner (fire load operated) requires a minimal flame power, which has no temperature-based technical impact on the firing process but still features a solid firing behavior to connect to impulse firing, whereby the control technical process takes place.

Impulse Bypass and Control

Once all of the problems for impulse operation of self-activating burners were detected and understood, the impulse bypass was developed in 1994. The impulse bypass is split into three sections: A, B and C (see Figure 3). With the activation of the integrated magnetic valve 4 and 7, different physical fuel ways emerge. The necessary fuel throughput of single firing ways is individually made by means of the fine dosing devices 5 and 6. With demand on the burner, gas safety valve 7 opens, and the fire load operation builds up via the bypass section C.

The ignition flame stability is controlled via an automatic burner control. The S7 control is signaled after flame stability and stable flame ionization. Bypass sector A initiates the impulse operation. The control calculated impulse frequency 0-500 pulses/min controls the fuel feed via the impulse valve 4. The metrological and controlled activation results via specific software modules on the basis of Simatic S5, S7 and Vipa S7 systems.

The software-based impulse control technology doesn’t need conventional components, like temperature controllers, measuring transducers, coupling elements, etc., because the complete data logging is done via the PLC control. Through the modular built-in control software, all input data is metrological and control technically evaluated and temperature technically calculated.

The control of the burners (ionization monitoring, release and impulse frequency) results directly via the output-side. Due to uncomplicated expansion possibilities of the PLC hard- and software structure, it is possible to convert any firing system economically from complete reconstruction or section-wise conversions.

From the fuel medium, the valve loop at T_i provokes a fuel gas jet expansion. Dependent on the flame length from the opening cycle, the resulting heat energy is transported to a continuous combustion chamber range for each impulse. The burner output is calculated via the impulse control module by means of evaluation of the nominal/actual temperature. The impulse frequency controls the performance from 0-100% (0-500 pulses/min) through the automated impact of the valve reaction time t_p (variable pause time).

The variable break time t_p is shortened or extended by the deviations of the temperature difference (ΔT) dependent on the PID parameter level, whereby the necessary fuel inlet results through the fireplace via the variable automated impulse frequency. The bigger the combustion chamber of a kiln, the bigger the temperature difference in the combustion chamber atmosphere at the high’s cross-section; therefore, no equal firing can take place. With only one solid valve reaction time t_i, this problem couldn’t be solved satisfactorily.

Since practical experience showed that a large number of tunnel kilns are working with a temperature difference of up to 80°C, the operator had to accept a quality decrease of his product. Because of that, the band-width modulation (BBM) was developed.

Bandwidth Modulation

To compensate for the differences in temperature at the height cross-section, the earlier described impulse firing method was further modified by inserting additional, cyclically automated opening time parameters into the software control structure (t_i1, t_i2 and t_i3). The parameter places (t_n1, t_n2 and t_n3) can be altered. Therefore, through entry of different time constants, the ring cyclical treatment produces three different fuel gas jet expansions regulated by the temperature-dependent calculation of the valve break time.

The software-based impulse control component possesses a modularly built structure (recording of temperatures, PID level, power level in pulses/min, BBM modulation level). In conjunction with the individual impulse valves at every firing point, this construction enables the selection of individual firing unit or aggregate over an independent impulse software controller by firing group constructions with several firing points. As a result, each burner can be individually adjusted according to requirements. Due to the software-based impulse control component (BBM bandwidth modulation), a temperature equalization of almost 100 % (i.e., a homogenous firing atmosphere) is achievable in the entire combustion chamber.

Depending on the product charge that needs to be burned and product formats, kiln facilities that are often loaded with changing fuels, combined fuels or different fuel weights need different parameter standards to achieve an ideal burn result. These product-specific standards are automatically transferred to the burn zone controllers.

Most of the tunnel kilns have an integrated tunnel kiln vehicular trace. Because of this, all positions of the kiln vehicles (and therefore also the respective fuel information in the burning conduit of the PLC control) are known. This tunnel kiln vehicular information is evaluated in the PLC control and the necessary control parameters (BBM modulation times) are automatically transferred to each individual burning aggregate. Because of the fully automated assimilation of the burning parameters, the entire firing facility automatically converts in all areas of the burning zones (pre-fire and main fire) based on the saved formulation data in combination with the vehicle positions.

The temperature-dependent bandwidth modulation works independently from the necessary parameter standards that are manually or automatically transferred to the software controllers to influence the length of the burning gas rays. The alteration or assimilation of the impulse length parameter (ti1, ti2 and ti3) from the impulse valve occurs, depending on the firing zone through three temperatures (firing, hold, cooling), which are determined from the height cross-section. The sensor, which is arranged in the middle, provides the control component with the actual value in order to implement temperature regulation with the power outlay (impulse frequency).

In the PLC control unit, the temperature information of the upper and lower combustion chamber cross-section are compared to deviations with the middle cross-section temperature. In the case of deviations of the middle reference temperature, the impulse length parameters are altered depending on the calculated temperature deviations by an intelligent functional component, which communicates with the modulation level. Consequently, a targeted energy dislocation through burning gas ray alterations in the combustion chamber (BGS1, BGS2, and BGS3) takes place.

New Developments

These impulse firing technologies have been established by means of very good operating characteristics, economization and quality optimization, and are still being used nowadays in the ceramic industry. Conditional on increasing environmental guidelines, further innovative developments will likely be necessary to optimize the combustion process. The following listed points are the basis for innovative and cost-effective impulse firing technologies, which can be applied in all areas of the industry:

•  Optimization of energy efficiency

•  Reduction of emission values (due to optimization of the firing process)

•  Reduction of the necessary primary energy

•  Savings of cost-intensive control units

The two-stage fan burner has an independent combustion air supply. Due to two integrated, motor-driven throttles, it can be applied to gas and combustion air modulating (see Figure 4). Throttle 1 and 2 can be controlled electrically and independently of each other. The activation of the gas and air throttle takes place (process dependent) in the working area 0-100% via the PLC control unit, where the necessary gas and air volume is added in the perfect rate at any stage into the combustion process—and therefore creates an ideal combustion.

By integrating the impulse bypass periphery, the gas impulse burner can be used with symmetrical impulse operation or with bandwidth modulation. Different burner tube materials and lengths are available for different mounting situations. During conventional impulse operation, the flame output is controlled by regulating the impulse frequency.

The required burner output is achieved by changing the gas volume. So far, the necessary air volume was consistently configured. Combustion air in the range of 90-100% is consistently set to eliminate oxidizing firing. Therefore, a relatively complete combustion takes place, and acceptable emission values are achieved (λ 1.0-1.4). When the air volume is set to a consistent level, the following disadvantages result:

• By reaching the nominal temperature, the impulse frequency is reduced. However, the air volume remains constant, which results in an unwanted cooling effect via air overflow in the combustion chamber. The impulse controller tries to compensate for this temperature drop, so the control receives a swinging performance, which negatively influences the temperature technical efficiency.

• During the combustion with high air overflow, negative or illegitimate emission values result. As a result, only in the top power range of the burner does an optimal energy and emission technical combustion process occur.

The energy technical efficiency in connection with minimal emission load on the environment via combustion exhaust is in direct relation to the combustion air ratio and is dependent on it. The combustion air ratio places the actual, for the combustion available air mass (mL,tats) in relation to the least necessary stoichiometric air mass (mL,st), which is necessary for a complete combustion. To reach a complete combustion with an impulse burner, in the power range 0-600 pulses/min, it is necessary to automatically adjust the burner fuel ratio with the combustion air ratio to the continuously changing impulse frequency.

By including the physical parameter gas pressure (PE) at the impulse valve, in dependency of the valve opening time (t_i) and the temperature-dependent impulse frequency (0-600 pulses/min), a software impulse-control-calculated gas flow rate results at the burner. The impulse control constantly analyzes the gas volume and changes the airflow via the speed control of the air fan for optimal combustion at the burner. As a result, full combustion is given over the total impulse frequency range.

The energy savings with impulse firing in chamber kilns, shuttle kilns, tunnel kilns, etc., are about 18-25%, provided that the oven is solidly constructed and operational characteristics are normal. With modifications of the existing impulse fired systems with Lambda-automatic, further energy savings can be achieved through complete combustion. Furthermore, better emission values that meet the legal requirements result.

With software-only integration, energy efficiency increases from 25% to 40% in a modernized chamber kiln. Different tests with varied kiln loads proved that the energy savings remained constant.

All of the media distribution components (such as pipeline construction, ventilators, measuring devices, etc.) are not applicable when reconstructing firing facilities with impulse fan firing technology. This can save up to 15% of the firing facilities price.

The rapid cooling takes place via the fan-firing unit in a temperature-dependent operation, whereby the peripheral cooling unit is not applicable. Failure of a decentralized fan, which supplies several burners, would result in a complete operation stoppage and possibly expensive product damage. However, failure of one impulse fan burner (for systems with perhaps six burners) would have no impact on the product. The firing process could still be completed, and the malfunctioned fan burner could easily be replaced during the process.

Drying plants (e.g., rotary kiln dryer, chamber dryer, etc.) mostly have complex air heating systems that are equipped with indirect air heating to avoid affecting the material to be dried with waste gas. Decentralized air heaters from 600 kw are very expensive energy sources. In addition, they demand complex air supply systems (like warm air channel, including complete periphery, etc.).

When using impulse fan burners with Lambda-automatic drying technology, however, the emission values are so low that the dry atmosphere can be directly fired. Gas exhaust takes place indirectly over the exhaust air and wet-air duct. The performance of two continuous flow chamber dryers for the polyurethane industry showed optimal values in all relevant areas. Through the direct heating in the drying chamber, the usual combustor for indirect heating is not needed. In the drying chamber, the exhaust gas is collected via the flow circulation of the drying atmosphere. The exhaust gas evacuation is done via the cyclical exchange of the saturated wet air. Through optimal combustion with the Lambda-automatic, minimal emission values result, and drying systems can be run in a temperature area of 50-180°C.

Another example for the use of impulse fan burners was the reconstruction of a rotary dryer for the production of ceramic micropearls. Reconstruction objectives included:

•  lowering the energy costs

•  conversion with BBM-impulse fan burners for temperature distribution in axial rotary tube depth via variable bandwidth modulation

•  conversion of liquid gas to natural gas

The modernization led to following results:

•  variable temperature control in axial direction

•  reduction of energy use (before: 6.70 euro, approximately $9.07, per operating hour; after: 1.55 euro, approximately $2.10, per operating hour

•  fully automated

At most combustion plants, existing burners are still in good, usable shape. Thus, the last question was how to achieve the complete system’s efficiency while using the existing burners of a kiln and avoid the cost factors for a modernization.

The PLV box evolved from this question. With this development, it is now possible to retrofit all types of existing burners with the impulse combustion technology with a centralized air fan, which is very cost effective and provides enormous energy savings.

Conclusion

Many different problems with impulse firing methods have occurred over the years, which necessitated the development of new methods and technologies. This steady modernization led to a modular, structured system in which specific software components can be used for individual problems.

The latest development, the BBM-impulse fan burner including Lambda-automatic, is based on automated impulses inside the combustion chamber. Benefits to this development include optimum product quality, energy efficiency and the smallest possible amount of pollution. The recent modernization of a tunnel kiln firing technical ceramics with 20 burners resulted in 21% less gas consumption and a 40% reduction of electrical power, as well as the proportional reduction of emissions and amortization of around 1.5 years.