1 BIA MACT TASK FORCE: WORK PRACTICE STANDARDS FOR PERIODIC KILN OPERATIONS

By Dr.Garth Tayler, Acme Brick Company, August 20, 2009

BACKGROUND The use of work practices for periodic kilns, as opposed to attempting to establish emission

limit ations, is a sound principle for a number of reasons. The BIA agrees with the EPA that this subcategory would be well addressed in the new MACT rule, if a series of logical, realistic and appropriate work practice standards were identified and applied. The se work practice standards, if adopted by any periodic kiln operation, by their very nature, would provide control and therefore,

some limitation of HAP emissions.

PERIODIC KILN OPERATIONS In considering the batch kiln firing process, the following key operational parameters, that have a direct bearing on HAPs emissions, are discussed:

Temperature Firing cycle Product quality

Automatic Control

Combustion control

Kiln load

Temperature Various scientific test methods are used to study the reactions in brick clays during heating. Differential Thermal Analysis (DTA), Thermo Gravimetric Analysis (TGA) Simultaneous Thermal Analysis (STA), are techniques used to show the oxidation, de

- hydroxylation, vitrification reactions as well as the weight loss chara cteristics of the material. Knowledge of these reaction characteristics enables the brick manufacturer to design the kiln firing cycle for the optimization of the product quality and to minimize process losses.

Firing cycle Each periodic firing process in the brick industry is unique and is governed by the nature of the brick clay material being fired. Some shale materials have higher carbon and sulphur levels and therefore require a longer “dwell” at the oxidation temperature range from 1600 o F to 17 00 o F. Other clay materials are more refractory in nature and require higher final firing temperatures in order to develop the desired finished color and the physical properties to meet the ASTM standards required by the market. These factors therefore w ill influence the period of time in the oxidation stages, as well as the time required in the final “soak” stage of the firing cycle. Scientific evidence has shown that the HAPs emissions also take place in these stages of the firing cycle.2 Product qual ity The time and temperature relationships described will affect the ultimate quality and hence acceptability of the finished product. An “over fired” product would produce excessive shrinkage, color variation and process losses. This type of firing cycl e is also likely to produce higher HAPs emissions per ton of ware fired. Similarly, an under fired product would not meet durability standards required by the ASTM standards and the market, and logically, produce less HAPs emissions. However, more product would have to be fired to meet production requirements, which would lead to more HAP emissions per sellable ton of ware. Therefore, the work practice standard adopted would need to be a practice that produces the best product quality and the minimum HAPs emission. This optimized work practice is something every brick producer is able to determine when the optimum firing cycle for the particular brick clay body is developed.

Automatic control The design of the kiln firing system will influence the bric k manufacturers’ ability to achieve repeatable, maximum product quality results. Most periodic kiln operators in the brick industry have used modern PLC (programmable logic controller) technology for some time. These systems enable the brick manufacturer to program the kiln firing temperature over a well established, optimized time cycle, to achieve repeatable results. Modern high velocity burner technology is commonly employed.

Combustion control The use of PLC technology enables the rate of gas del ivery to the burner system to be accurately programmed, to ensure that each stage of the firing cycle is accurately controlled, to avoid over firing or under firing. The measuring devices that are part of the combustion equipment enable the kiln operator t o adjust the air to fuel ratios in each stage, to achieve the optimum combustion efficiency needed to produce the desired product. In this way the production of poor quality, rejects and losses is minimized. Obviously, a technology that does not achieve t his will produce higher losses and poor quality, resulting in additional production firing cycles being required to meet the quantities demanded by the market. The inevitable result of this will be

additional HAPs emissions

.

Kiln Load/Kiln Technology

For proper combustion, it is important that the periodic kiln not be overloaded, as overloading could cause improper combustion and lost product. In addition, the following items can be monitored to ensure proper firing:

• Draft controls are employed on exhaust fans to adjust exhaust volume flow.

• Measurement, monitoring and control of kiln pressure by adjustment of kiln exhaust.

• Measurement and monitoring of kiln temperatures.

• Measurement and control of air and fuel flow to the combustion system.3 SUMMARY

The following proposed work practice standards c ould be included in the MACT rule:

1. Each periodic kiln should establish and use a set firing time and temperature cycle for each product, which is reprogrammed and adhered to for every cycle.

2. Each periodic kil n should be loaded with no more than the maximum predetermined tonnage of

brick.

3. The maintenance procedures, calibration of temperature measuring devices, control of air to fuel ratios and programming of firing cycles etc, should be documented as part of work practice

standards

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