While natural gas is the most frequently used fuel for kiln firing, some brick manufacturers are beginning to utilize readily available alternative solid fuels like coal or sawdust. These alternative solid fuel sources can potentially shield brick manufacturers from the price volatility of the natural gas market. As the interest in alternative fuels and their economic value continues to rise, calorimetery will become increasingly necessary for kiln operations to test for quality and emissions.

The most energy-intensive area of the brick manufacturing process is the operation of the kiln. The most common type is a tunnel kiln with a separate drying area situated before the kiln. Tunnel kilns range in size from about 300 to 500 ft (91-152 m) in length and include a preheat zone, a firing zone and a cooling zone.

The firing zone is the zone with the most heat and typically is maintained at a maximum temperature of about 2000°F (1090°C), while the drying area is often heated with waste heat from the cooling zone of the kiln. The retention time of the brick in the kiln system varies depending on the specific kiln and type of brick being manufactured, but usually takes between 20 and 50 hours.

Exploring Solid Fuels

The most common alternative solid fuel being utilized by brick kilns is coal, though in some locations the coal is being substituted or mixed with a biomass derived fuel—most commonly sawdust. Utilizing a biomass-type fuel like sawdust is beneficial because it is a renewable fuel and represents the utilization of a waste product from another industry (lumber/wood products). Brick manufacturing facilities that are geographically located in proximity to coal producing areas and/or areas with processes that generate potential renewable fuels (wood products) have the greatest potential to gain from the economics of these types of solid fuels.

Alternative solid fuels can be utilized in several ways. Finer fuels such as sawdust and pulverized coal powder can be added or blown directly into the tunnel kiln. This practice is referred to as flashing or reduction firing, where an excess of fuel is added to the kiln in one area and causes a localized reducing environment within the kiln. Flashing is primarily performed to impart color and variations to the brick. The iron present within the brick matrix turns a shade of red when fired in an oxidizing atmosphere. When the brick is exposed to a reducing atmosphere during firing, a dark and varied hue can be achieved.

Utilization of solid fuels like coal and biomass require kiln modifications and/or redesign, along with potential increased analytical testing requirements for quality and emissions from the fuels. An important quality constituent, as well as the most important factor in determining the economic value of the fuel, is the determination of the gross calorific value (energy value) of the fuel.

Natural gas is processed to a standard and delivered to users with a specific gross calorific value, which the supplier determines using a chromatographic separation fractional analysis where the gas components of natural gas are separated and quantified. The gross calorific value of the natural gas is then calculated based on the gas constituents.

Quality and Emissions Testing

Enlarge this picture Figure 1. Schematic of an isoperibol bomb calorimeter.

Solid fuels like coal, biomass or a mixture of the two are much more heterogeneous than gaseous fuels, and the calorific value is often tested by both the supplier and the consumer due to the value of the fuel as well as the efficiency of the utilization of the fuel within the kiln system. Measuring the gross calorific value of solid fuels is typically performed using an isoperibol bomb calorimeter, which measures the heat that is liberated as the fuel burns completely. The heat released from the fuel sample is proportional to the calorific value of the fuel.

The use of an isoperibol bomb calorimeter starts with a measured fuel sample placed within a vessel, which is sealed and charged with oxygen at high pressure (see Figure 1). The vessel is lowered and sealed into a water bath within the instrument, known as the bucket. The cavity surrounding the bucket, called the jacket, is also filled with water. The water temperature in the jacket is closely controlled at a precise set temperature, and an electrical current moving through the fuse ignites the fuel/oxygen mixture. The temperature of the bucket and jacket water is measured by an electrical thermometer with a resolution of 0.0001 of a degree C every second.

As the fuel burns, water in the bucket surrounding the vessel absorbs the heat (energy) liberated. Since the calorimeter remains fully insulated from the ambient environment, the increase in water temperature directly reflects the heat released during the fuel’s oxidation (burning). This measurement typically takes approximately 10 minutes using a calculation based on the maximum temperature difference between the bucket and the jacket water during the analysis.

Recent Developments

Isoperibol calorimeters can offer fast and accurate calorific results in coals, cokes, fuel oils and waste materials.

Isoperibol calorimeters can offer fast and accurate calorific results in coals, cokes, fuel oils and waste materials. Having a fully automated calorimeter may seem like the best solution for increasing throughput and reducing downtime. Unfortunately, it becomes difficult to maintain accuracy and instrument precision when certain functions are left out of the control of the operator. Long-term maintenance can also become a costly factor, leading to extended downtimes in order to ensure a high level of functionality.

Industry demands a calorimeter that has the automated capabilities needed to increase throughput, but the quality of the results must be maintained. As a result, a calorimeter has been developed that isn’t completely automated, but takes away many of the cleaning and sample preparation issues associated with calorimetry.* The unit offers automated control of water volumes, heating, cooling and recirculation; the raising, lowering and equilibration of the vessel; and the sealing of the bucket and jacket chambers. Its semi-automated operation obtains a rapid analysis of calorific content and increases instrument throughput without sacrificing accuracy or instrument precision.

Another recent advancement involves the use of a thermodynamic model for heat exchange within the system.** The new model takes into account the heat capacities of the system’s components, along with corrections for energy transfer within the system.

Field testing has shown that the unit’s semi-automated functionality combined with the thermodynamic model enables an analysis time of 5.5 minutes without compromising the accuracy or precision of the calorific result. In addition, an ergonomically designed, lightweight combustion vessel implements a simple 1.5 revolution vessel cap closure for improved operator comfort and ease of use.

*AC600 isoperibol calorimeter, developed by LECO Corp.
**TruSpeed^TM, developed by LECO Corp.

For more information regarding isoperibol bomb calorimeters, contact LECO Corp., 3000 Lakeview Ave., St. Joseph, MI 49085; (800) 292-6141; fax (269) 982-8977; e-mail info@leco.com; or visit www.leco.com.