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During the past few years, many of us have had to reevaluate our lives, priorities and futures due to the drastic economic downturn. The building industry, with its serious impact on much of the structural clay and support industries, was one of the hardest-hit. I am one of those who has been affected, and the experience has given me new insight on what has to be done to survive. Likewise, this is a time for all of us in the structural clay industry to step back and reevaluate where we are and where our futures will take us. After all, it's not likely that we'll be able to just pick back up where we were when this downturn began.
Our industry is being presented with many challenges as we look forward to its recovery. New regulations from the government regarding emissions, effluent and energy consumption, as well as having to train new employees, are all factors to keep in mind. Since we have this time on our hands while activity is low, we need to take a hard look at our operations and what needs to be done so that we can survive and provide stability for the future. While there may not be a surplus of capital available now to spend on improvements, we have the time and resources at hand to plan ahead and avoid becoming casualties.
Tunnel Kilns vs. Batch (Shuttle) KilnsThere is no doubt that tunnel kilns are more thermally efficient than batch kilns for large production runs, but they are "hungry" and need to be constantly fed with product in order to maintain their efficiency. Frequent slow-downs or interruptions due to special runs (e.g., unusual shapes or specific firing needs) require radical changes in a tunnel kiln's feed rates, temperatures or loading density (cross-section as well as weight), robbing them of their efficiency.
If this is a frequent occurrence, consideration should be given to using a batch kiln for special runs so the tunnel kiln can be operated at its optimum schedules. With the lightweight insulating materials that are available today for kiln and kiln car construction, heat losses and storage can be minimized. In addition, proper burners and controls can provide good heat transfer to the products and even eliminate the need for exhaust fans.
When designing a batch kiln, it is important to properly size and place the burners for optimum heat transfer and product uniformity. Burners that are too large will have to be operated below optimum velocity levels or with high excess air rates-both costly. A combination of pulsed, on-ratio firing and thermal turndown (fixed air-variable fuel) provides thermal efficiency at higher temperatures, along with the necessary excess air rates at lower temperatures where additional drying may be necessary and organics must be properly oxidized.
Using a dedicated batch kiln for special runs can provide quality products at a cost approaching that achieved with a tunnel kiln while allowing the tunnel kiln to be operated at its optimum levels. Benefits include:
- Uniform special products at low cost
- No interruption (loss of production) of the tunnel kiln
- No loss of energy efficiency for the tunnel kiln
- Uniform quality and reduced breakage from not having to upset the tunnel kiln
Consistent LoadingOnce a tunnel kiln has been operating under good control and producing a quality product, any major changes cause upsets that, in turn, affect quality and costs. The worst offender in this area is drastic changes in the kiln settings (cross-sectional area as well as weight). For most purposes, automatic temperature controls compensate for moderate changes in weight variations. However, sending "a few" extra-heavy cars through the kiln can result in under-firing the leading cars of the heavily set cars and over-firing the leading cars of the normally set cars that follow. When making these kinds of production runs, it is better to make a sustained run rather than piece-mealing the special runs. The cost? Nothing, except some time to better plan the production runs.
Maintaining uniform cross-sections of the loads is another major problem. Sending an occasional low-set car through the kiln won't make any appreciable difference in its control. However, an occasional high-set car, or one with reduced flue spacing through the load, wreaks havoc with kiln control and upsets the air flow, possibly causing backdrafting and improper temperatures (see Figure 1). The results are reduced quality and increased fuel consumption, as well as increased electrical costs.
BurnersOver the years, most side-fired tunnel kilns have been converted from low- or medium-velocity burners to high-velocity burners. For many, this conversion was made years ago when natural gas was relatively cheap and the burners operated in a thermal turndown mode. Under these conditions, the burners were adequate for the job of providing a quality product with good uniformity across the load. However, with rising energy costs, excess air usage became very expensive.
Faced with rising fuel (and electric) costs, many kiln operators downsized their burners by reducing their airflow in order to run closer to on-ratio firing. Unfortunately, this creates a domino effect: reducing the burners' capacity by limiting the airflow decreases the velocity of the burner gases; the decreased velocity of the burner gases limits heat transfer through convection heating; and the lower velocity and limited convection heat transfer lead to non-uniformity issues (see Figure 2).
This, then, is the opportune time to evaluate burner performance vs. firing uniformity and total energy costs. If the burners are consistently running at less than 70% of the manufacturer's rated capacity, too much of the velocity effect is being wasted and smaller burners should be considered. While burner replacement may have to wait until the economy improves, evaluation and planning can, and should, be done now. Burner suppliers can provide manufacturers with the costs of this change, as well as details regarding what existing equipment can be reused, such as air butterfly valves, gas-limiting orifice valves, and air and gas metering orifices.
One option is to run with fewer of the burners actually firing so that the rest are operated at (or close to) their rated capacities. The drawback to this practice, however, is that too much heat can be directed at the setting and localized overheating could occur. This is especially the case when the burners fire across a narrow hearth directly into the load. Keep in mind that kiln width can adversely affect the success of this practice if the burners are very large and the kiln is narrow. This system works well with under-deck firing since it promotes improved circulation of the kiln gases (see Figure 3).
Whether the kiln is direct- or under-deck-fired, now is the time to look at reducing the weight of the kiln cars. This is especially important for under-deck-fired kilns. Lighter-weight and insulating refractories reduce heat storage and losses that are carried out of the kiln. If the burners are firing under the deck, consider reducing pier heights to reduce weight, but also to possibly allow an additional course of brick to be added.
If firing under-deck, also consider the effect of relocating the burners and/or lowering the deck height to direct-fire the product. This is much more efficient than under-deck firing since the products of combustion are directed through the load, which improves convective heat transfer and product uniformity. This is a longer-term project, but testing can be done in advance, especially since such a change is difficult to implement during kiln operation. The potential benefits include:
- Fuel savings
- Less refractory storage
- Increased productivity (more product per car)
- Improved heat transfer
- Reduced electric consumption
- Reduced refractory cost
Some newer burners have the capability to dynamically change the area of heat release along the vertical plane of the setting to provide a more-uniform energy release to improve temperature uniformity and reduce fuel usage. The burner maintains the desired air-to-fuel ratios, as well as a constant energy release along the vertical plane. These can be used on new installations as well as retrofitted. All the adjustments to the burners are external and can be customized for areas of heat release through a dedicated programmer or a small (or existing) PLC. In addition, the burner air-to-fuel ratios can be changed externally by adjusting the valves to each burner.
Some roof-fired kilns have had the injector burners replaced with high-velocity burners with pulse-fired control. This is the time to pull the burners and check the integrity of the tiles and the internals of the burners.
Unfired Preheat SectionsThe preheat section of most tunnel kilns either have recirculating fans to promote temperature uniformity or nothing. The result of the latter is great temperature gradients-at times approaching 900°F from the top to the bottom of the load. The spread is less when fans are used, but it can still be significant-possibly 200-300°F. Fans, however, use electrical power and require maintenance, especially when used in the latter portion of the unfired preheat section.
One option is to utilize small high-velocity burners (normally in an over/under arrangement) to provide improved temperature uniformity in this section of the tunnel kiln (see Figure 4). We are not looking to add temperature in this section, but to use the high-temperature gases below the roof to heat the bottom of the load rather than overheating the top of the load before being exhausted, thus wasting the energy contained in the products of combustion.
InsulationWe all are aware that insulating the kiln is a good way to reduce heat losses through the walls and roof. With many kilns in idle mode, this would be the time to consider adding insulation. But where? The obvious choice is on the inside surfaces of the kiln. However, care must be taken not to reduce the flue spacing too much between the load and the walls, which could restrict the necessary flow of gases through the kiln.
If the burner section(s) includes a wide hearth, adding insulation to the inner walls is not a problem. Otherwise, insulating the outside could help, but is not normally feasible except over the roof of the kiln. Care must be taken to prevent the roof refractory beneath the added insulation from being exposed to temperatures above their recommended values.
EmissionsBurners have some degree of control on two areas of emissions-CO and NOx. The former is a function of how well the air and fuel mix before the products of combustion drop below the ignition temperature of CO (1128°F, in normal air). If the kiln temperature is above the minimum ignition point and there is sufficient oxygen present to mix with the CO, it can be oxidized and become a part of the system fuel supply. Controls can be added to tunnel kilns to take advantage of this by using oxygen sensors and high-velocity burners.
In contrast, NOx generation, though also a product of combustion, is not readily disposed of within the kiln by normal processes. The best cure is prevention. All burner manufacturers' high-velocity burners are inherently low-NOx generation units. However, even high-velocity burners in tunnel kilns and at high temperatures generate NOx due to the amount of preheated oxygen present, which increases the hot mix temperature of the flame.
Some burner manufacturers have lean-premix burners that utilize optimum air/fuel ratios to minimize NOx production. These burners are currently above 700,000 BTU/hour input rates. There is also a patented method of a special application of high-velocity burners that may be applicable on some tunnel kilns, especially those with under-deck firing and wide hearth areas.
Plan for the FutureThe concepts discussed in this article are tried-and-true methods for reducing costs and improving operations in the structural clay industry. Some actions can be implemented at very little, if any, cost and in a short time. Though requiring more time and capital, others should be considered now, while the normal pressures of operation are reduced.
Many resources-combustion manufacturers, controls people and consultants, to name a few-are available to assist in planning and evaluation. Your own people can also provide many new concepts if given the opportunity to express their ideas and needs. Whatever approach is taken, now is the time to plan for the future.