Body fuels, i. e., combustible substances added to the brick feed that are then combusted upon firing the brick, have been in use since the Egyptians added straw to the brick feed. Owing to the remaining organic content, biomass combus­tion ashes might be considered a low calorific value body fuel.

A tunnel kiln (Fig. 9.2) works as a counterflow heat exchanger. In the tunnel kiln packs of bricks set on a car train on rails move through the kiln one after the other. During their journey, the cars move toward, through, and past the stationary firing section at the center of the structure. During its travel, the brick set on the kiln car is slowly, ideally uniformly, heated up to the required firing temperature and then cooled down again (Fig. 9.3). Heat transfer within a tunnel kiln and within the

usually densely packed bricks on the kiln car takes place primarily by forced convection of the turbulent kiln gas flow, and, limited to the firing zone, by radiance of the burner flame to the brick. Further heat transfer by conduction from brick to brick occurs at the contact surfaces of the tightly stacked bricks. At a temperature range of 150-350°C the volatile proportion of any organic addition made to the brick feed is released as low-temperature-carbonization gases. These gases are usually conveyed to the chimney or to an appropriate postcombustion system.

As heating of the bricks set on the kiln car does not take place uniformly across the section of the brick, release of low-temperature-combustion gases at different points of the travel through the kiln can cause problems. The firing curve pictured in Fig. 9.4 shows the delayed reaction of the energy rich additions to the clay body:

Vertically stacked brick Brick stacked in Brick stacked in direction of flow and

direction of air flow

Fig. 9.3 Methods of stacking bricks on kiln cars

The combustion of the energy content in the core of the pile occurs later than that on the perimeter.

Only at a temperature of about 550-600°C do the temperature differences between the lower and the upper layers of the brick stacked on the kiln car diminish considerably.

The problems of using body fuels featuring a high percentage of volatiles is explained here with the example of paper sludge: At a temperature of about 150°C, all water that is still present in the cellulose is evaporated. This evaporation process is concurrent to driving out water still present in the clay body and extends into the first phases of release of crystalline water. At temperatures between 100 and 200°C, volatile substances are dissociated and evaporate, releasing carbon monoxide, hydrogen, and hydrocarbons.

Recent research by the author has shown the effects of concurrent combustion (a substance with an otherwise higher ignition temperature is ignited by the combustion of a substance with a lower ignition temperature) and the impact on the energy balance of a tunnel kiln. Such an example is pictured in Fig. 9.5 for a brick feed with 20% of paper sludge added to the clay in addition to a 1.5% of a bituminous coal featuring a high volatile content.

The above-mentioned differential thermal analysis (DTA) is of particular inter­est when it is compared with the DTA for the same basic mix but with 1.5% of anthracite and less than 2.5% of volatiles instead of the bituminous coal in the previous example. The DTA in Fig. 9.6 shows release of energy at the same temperatures as in the previous case but a second, smaller peak is observed at higher temperatures as well. This second peak has a positive impact on the thermal balance of the kiln.

An effect similar to that observed for the anthracite/paper sludge mix was observed in the tunnel kiln when a biomass ash/paper sludge mix was fired. This effect has to be confirmed in further and longer-lasting tests to prove that, biomass combustion ashes contribute positively to the energy balance of a brick tunnel kiln.