Pulse Firing: An Optimal Solution for Firing Ceramics
Pulse firing combustion and control systems enable ceramic manufacturers to alleviate the need for large amounts of excess air during firing.
Ceramic manufacturers all over the world must consider two important factors when selecting the optimum equipment for their firing needs: running costs and the ability to make products with consistent quality. When we refer to the operating costs to run a kiln, we’re normally talking about energy usage (fuel and electricity) and maintenance. On the other side, quality from a kiln is normally achieved by having perfect control of the following parameters: heating and cooling rates, controlling the different atmosphere levels inside the kiln chamber, and achieving very tight temperature uniformity throughout the kiln length and height.
Compared to alloy or metal heat treatment, firing ceramics is much more challenging. In many cases, the concepts of quality and energy efficiency fight against each other, complicating the life of ceramic manufacturers in very difficult market conditions. Having the best kiln design and using the best process practices are both key to helping ceramic producers balance the constant struggle between quality and energy efficiency.
Ceramic manufacturers can ensure that the properties of their products are going to be dependably homogeneous by maintaining temperature uniformity throughout the kiln volume. Kiln design has evolved over the years, and two main techniques are used to promote temperature uniformity: firing with high volumes of excess air, and using a pulse firing combustion and control system.
Using excess air means using more air than what is actually required for proper combustion. For example, while firing a burner with natural gas, it is said that 10 volumes of air are required for each volume of gas going through the burner in order to achieve a stoichiometric ratio. Excess air increases uniformity by increasing the amount of heated mass (the extra air) in a kiln and thus increasing convection.
With this in mind, if we want to achieve ideal temperature uniformity, firing the kiln with high volumes of excess air is an option. However, the problem is that excess air comes with a cost: the extra air going into the kiln has to be heated, which dramatically increases the energy usage for the firing process.
One of the best available options to fire ceramics efficiently without the need for large amounts of excess air is a pulse firing combustion and control system. A pulse firing system uses a time/proportion algorithm to turn burners from high-fire to low-fire as the means for temperature control. In some cases, and depending on the application, the burners can also be cycled from high-fire to an off mode. The selection between these two possible control schemes varies depending on what you are trying to achieve.
The system uses high-velocity burners, and it takes advantage of the recirculating action and increased rates of heat transfer that occurs as an effect of cycling the burners from high-fire to low-fire in a very fast and perfectly synchronized way. In addition, pulse firing uses the burners only in two operating outputs: high-fire and low-fire. This system allows the burners to always perform at their optimum operating condition; when the burner is in high-fire, we have higher flame velocities, more recirculation and entrainment (plus a larger flame radiation area). All of this increases the heat transfer rate and promotes better temperature uniformity.
Because we have increased recirculation action and a higher convection effect created by pulse firing, it is possible to reduce the amount of excess air required while maintaining or even improving the temperature uniformity inside the kiln. With pulse firing, excess air is only used when the process requires it and not to promote temperature uniformity. By using a pulse firing system, savings of up to 30% can be achieved when comparing fuel usage with other traditional systems.
A smart pulse firing system should have the ability to work in different operating modes to cope with the different needs required by the process. It should vary the conditions inside the kiln depending on what the product and each firing segment requires. With this combustion and control scheme, it is possible to vary the carbon monoxide (CO) and oxygen levels throughout the different segments of the firing cycle.
Pulse firing can be very demanding to the kiln combustion hardware—mainly the burners and valves, which are cycled up and down from 3-6 times a minute, 365 days a year. Having specifically designed, heavy-duty hardware and a smart control system is a necessary requirement to a good pulse firing system. Some of the benefits of the system could still be achieved without these elements, but maintenance costs and kiln downtime would increase and possibly neutralize the added benefits through increased operational costs. The following is required in order to have a reliable pulse-firing system:
• A specially designed, high-velocity burner capable of maintaining its stability throughout a very wide operating range (from about 30% excess fuel to about 6,000% excess air)
• Air and gas valves with fast opening times capable of maintaining their adjustment and designed to sustain many millions of open/close cycles
The pulse firing combustion and control scheme is very flexible and works well with added heat recovery technologies. Over the years, pulse firing has been successfully integrated with energy recovery equipment such as heat exchangers to pre-heat combustion air at temperature ranges from 800-1,000°F (426-537°C). It has also been used successfully in conjunction with self-recuperative burner technology capable of pre-heating combustion air at temperatures up to 1,470°F (798°C). Combining a pulse firing system with energy recovery technology results in a kiln with one of the most efficient systems available.
Using pulse firing on an existing kiln installation is an available option for most manufacturers firing ceramics. Over the years, many kilns have been retrofitted with state-of-the-art combustion and control systems, making it possible for companies to achieve the efficiencies and quality required by the market today with a small capital investment. With the increased level of quality and the reduction in energy consumption, the typical return on investment for these types of projects is between one and two years.
Pulse Firing and Ceramics
The pulse firing system has been used on many ceramic firing processes, including:
• Electro-porcelain insulators
• Technical ceramics
• Fiber processing
• Clay pipe