For ceramic manufacturers, many of today’s technically proven heat recovery technologies can provide significant opportunities for improving industrial energy efficiency through waste heat. Before beginning a waste heat recovery project, it is essential to identify all potential sources of accessible waste heat, the types of recovery technologies available for these sources and, most importantly, how the recovered waste heat energy can be used.
After conducting a survey of the plant equipment that uses large quantities of electricity and/or natural gas, a preliminary plan for the capture of waste heat energy should be developed. Through this process, the essential heat transfer projects can be identified and a cost analysis performed to identify the feasibility of potential projects.
Reused or Transferred
Captured industrial waste can either be “reused” within the same process or transferred to another process. For example, thermal oxidizers, ovens and furnaces all exhaust hot products as a result of combustion. The hot exhaust products can then be fed to air-to-air heat exchangers to provide preheated air to combustion equipment such as boilers, furnaces, and ovens, or used for space heating and drying/curing room heating.
Alternately, regenerative thermal oxidizers (RTO) can also be used by air-to-liquid heat exchangers to heat water, oil or other process fluids. Improving equipment efficiency through preventive maintenance and tuning must also be considered.
Next, steam boiler and hot water boiler efficiency can be improved by capturing waste heat through the use of economizers, condensing economizers, and blowdown heat exchangers. By preheating the boiler feed water, the amount of energy required to heat the water to operating temperature can be reduced significantly. Finally, heat exchangers can also be used to transfer “heat of combustion” from thermal oxidizers and furnaces to drying ovens and curing rooms, and for space heating. By doing this, the recovered heat can replace fossil fuel energy that would have otherwise been used. Such methods for recovering waste heat can help facilities significantly reduce their fossil fuel consumption and greenhouse gas emissions, as well as reduce associated operating costs.
According to the European Commission, the layout of the plant is also very important, since many ceramic dryers now use hot air recovered from the cooling zones of tunnel kilns, usually supplemented with hot air from gas burners.1 In particular, low-temperature excess heat can only be managed usefully if the length of the pipes (i.e., the distance between excess heat generation and use) is very limited. In any case, suitable heat insulation of the pipes is required; significant energy savings have been achieved in this way.
Finally, heat recovery should be considered only after the kiln has been optimized. This minimizes the heat available for recovery, but is the most energy-efficient route.
Additional tips from Tangram Technology Ltd. can help maximize energy savings:
• Kiln operation should never be compromised to supply a heat recovery system. Continuous kilns should use most of the heat from the cooling product to pre-heat the product before the main firing. A minimum amount of heat should be available for recovery when kilns are operated at their design capacity. It can be difficult to recover heat from intermittent kilns due to the continuous changes in the amount available.
• The first priority should be to reuse waste heat in the same kiln. After the recovery options at the kiln have been exhausted, then waste heat should be used for drying. However, this should never dictate kiln operation.
• The best opportunities for maximizing heat recovery are in kiln design, hot air supply to the burners, and using self-recuperative and regenerative burners.2
Why Save Energy?
According to the British Ceramic Confederation, ceramic manufacturers should be concerned about energy savings for several reasons, including:
• Ceramic manufacturing is energy intensive (30% of total cost)
• Lowers rising energy costs (e.g., wholesale, distribution, environmental taxes and charges)
• Meets EU and UK legislation/legal obligations
• Reduces environmental impacts of manufacturing
• Protects the world’s resources
• Reduces carbon footprint
• Makes good business sense by saving money and improving profitability3
A recent project completed for a coating operation (one that is applicable to ceramic manufacturing) included process exhaust recirculation to reduce the heating costs of the customer’s wash system. The project decreased the temperature of the volatile organic compound (VOC)-laden air stream handled by a regenerative thermal oxidizer through the installation of multiple air-to-water heat exchangers.
A 400°F oven exhaust was fed to an air-to-water heat exchanger, for two purposes: to heat water for a plant parts cleaning operation; and to reduce outlet temperature of the oven exhaust so that common spiral-wound galvanized duct could be used for transfer of the solvent-laden air stream to the regenerative thermal oxidizer, which was required to meet air pollution abatement permit conditions. A significant cost savings resulted from the use of spiral-wound galvanized ducting instead of welded solid construction carbon steel ducting.
A local utility incentive/rebate program was used for an energy-saving process improvement measure; the project was pre-qualified for a $145,000 rebate for the heat exchanger project and energy-efficient RTO. In addition, a $95,000 cost savings was realized due to the change in material selection for the ducting installation. Specifics include:
• Recirculation heat exchanger—Therm kW savings of approximately 374,057 at $0.07 per kWh saved
• Make-up water heat exchanger—Therm kW savings of ~ 299,143 at $0.07 per kWh saved
• RTO—Therm kW savings of
~ 309,585 at $0.32 per Therm saved
Assess to Save
In the end, every ceramic manufacturing facility has unique requirements that must be carefully considered. Many facilities can benefit from an energy consumption assessment, which includes recommendations for heat recovery or other energy reducing measures such as equipment tune-ups and repairs to steam lines, steam traps, compressor systems, HVAC systems, and other high energy consuming equipment. The potential savings and impact from improving industrial energy efficiency through waste heat is a worthwhile endeavor.
For more information, visit www.shipandshore.com.
1. “Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry,” European Commission, August 2007.
2. “Energy Efficiency in Ceramics Processing: Practical Worksheets for Industry,” Tangram Technology Ltd., www.tangram.co.uk.
3. McDermott, Andrew, Ph.D., “Ceramic Industry: Current/Future Energy Efficiency Solutions,” British Ceramic Confederation, www.ceramfed.co.uk.
This article was originally posted on www.ceramicindustry.com.
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