April 28th 2007 filter firing Yemen GTZ

Notes on the Sana’a, Yemen filter firing April 28th, 2007
The firing of the ceramic filters followed a firing of tanoor ovens by two days insuring that the kiln was well dried.
In firing the filters I was particularly concerned that all of the sawdust would be completely burned out so that trapped carbon would not cause a problem with the filters. This is necessary so that the finished filter be as strong as possible and so that flow rates not be effected by carbon in the filter body. Care must be taken in the stacking of the filters in the kiln to separate the filters using spacers so that heat can infiltrate throughout the setting. Filters should not stacked bottom to bottom without spacers as well. The filters staked on the floor of the kiln should be stacked on top of brick to elevate them. Another thermocouple was installed near the floor of the door to monitor the difference in temperature between the floor and the upper part of the kiln. It is recommended that monitoring of the temperature advance is done with a combination pyrometer, Orton temperature cones and draw trials.

A warm up period with a target temperature of 300 Celsius in the first 4 hours proved adequate to insure both moisture and carbon was not trapped.
After the warm up, an advance of 100 C per hour was desired up until a target temperature of 900 C. In practice we took longer to advance in temperature due to the amount of smoke which was generated by the burning out of the sawdust . At target temperature the firing was soaked for one hour. I had intended to soak for a longer period as per advice by other filter technicians, but after discussion with Bernd Pfannkuchen the kiln designer, and through evaluation of the draw trials* at the target temperature I decided that one hour soaking was adequate. An examination of the finished filters showed that there was no trapped carbon. The original recommendation of a longer soaking period is based on the use of the Mani kiln which is using solid fuels (wood) whereas the Sana’a catenary kiln fires with Propane under oxidizing conditions. Having the designer of the kiln to assist in the filter firing was a real advantage. Bernd gave advice and the kiln design proved very versatile. It is noteworthy that the Orton cones and the pyrometers showed less then one cone differential in temperature throughout the kiln.
*The draw trials were made from the same mix of clay and sawdust and formed into rings. These trials were positioned in the kiln on two levels near the floor and withdrawn through the ports in the door. These rings were then examined for residual carbon as well as for strength.