Methods for Improving Energy Efficiency by HESs

Process heating and motor-driven systems collectively consume more than nine quads of end-use energy in the U.S. manufacturing sector. Continued technology maturation and improvements will drive technology uptake to reduce energy intensity and can narrow the gap between current energy use and practical minimum energy requirements, especially for major energy-intensive commodities. Transformational next-generation processes and technologies that are not bound by practical (energy and emissions) limitations of current processes, such as low-thermal-budget processes and next-generation motor-driven systems, can drive manufacturing energy reductions and expand capabilities of manufacturers [26-29].

Process Heating Systems (Including Steam for Unit Operations)

Process heating accounts for approximately 61% of manufacturing end-use energy use annually [1]. Energy for process heating is obtained from a combination of electricity, steam, and fuels such as natural gas, coal, biomass, and fuel oils. In 2010, process heating consumed approximately 330 TBtu of electricity, 2,290 TBtu of steam, and 4,590 TBtu of fuel [30]. Characteristics of Common Industrial Processes that require process heating [1] are illustrated in Table 7.1.

TABLE 7.1

Characteristics of Common Industrial Processes that Require Process Heating [1]

Process Heating Operation

Description/Example Applications

Typical Temperature Range (F)

Estimated (2010) U.S. Energy Use (TBtu)

Fluid heating, boiling, and distillation

Distillation, reforming, cracking, hydrotreating; chemicals production, food preparation

150-1.000°

3,015

Drying

Water and organic compound removal

200-700°

1,178

Metal smelting and melting

Ore smelting, steelmaking, and other metals production

800-3.000°

968

Calcining

Lime calcining

1.500-2.000°

395

Metal heat treating and reheating

Hardening, annealing, tempering

200-2.500°

203

Nonmetal melting

Glass, ceramics, and inorganics manufacturing

1.500-3.000°

199

Curing and forming

Polymer production, molding, extrusion

300-2.500°

109

Coking

Coke-making for iron and steel production

700-2.000°

88

Other

Preheating; catalysis, thermal oxidation, incineration, softening, and warming

200-3.000°

1.049

Total

7,204

Common process heating systems include equipment such as furnaces, heat exchangers, evaporators, kilns, and dryers. As shown in Table 7.1, three largest energy user processes are fluid heating, boiling, distillation; drying and metal smelting and melting [1]. Industrial process heating, which consumes more than 7,000 TBtu of energy annually [31], is used for fundamental materials transformations including heating, drying, curing, and phase change. Process heating systems are associated with significant thermal losses; nearly 36% of the total energy input to process heating is lost as waste heat [31]. The largest sources of waste heat for most industries are exhaust gases from burners, heat treating furnaces, dryers, and other equipment. Waste heat can also be released to liquids such as cooling water, heated wash water, boiler and blow-down water. Solid waste heat sources include hot products that are discharged after processing or after reactions are complete, hot by-products from processes or combustion of solid materials, and hot equipment surfaces. The quality of these heat sources varies. Industrial waste heat generally occurs in four forms: sensible heat of solids, liquids, and gases; latent heat contained in water vapor or other type of vapors and gases; radiation and convection from hot surfaces; and direct contact conduction (in a few instances). Waste heat losses are a major consideration in process heating, especially for higher temperatures process heating systems such as those used in steelmaking and glass melting. Losses can occur at walls, doors, and openings, and through the venting of hot flue and exhaust gases. Overall, energy losses from process heating systems total more than 2,500 TBtu annually [1,30]. The recovery and use of waste heat offer an opportunity to reutilize wasted heat for other purposes.

TABLE 7.2

RD&D Opportunities for Process Heating and Projected Energy Savings [1]

R&D Opportunity

Applications

Estimated Annual Energy Savings Opportunity (TBtu/y)

Estimated Annual Carbon Dioxide (C02) Emissions Savings Opportunity (Million Metric Tons [MMT]/y)

Advanced nonthermal water removal technologies

Drying and concentration

500

35

“Super boilers” (to produce steam with high efficiency, high reliability, and low footprint)

Steam production

350

20

Waste heat recovery systems

Crosscutting

260

25

Hybrid distillation

Distillation

240

20

New catalysts and reaction processes (to improve yields of conversion processes)

Catalysis and conversion

200

15

Lower-energy, high-temperature material processing (e.g., microwave heating)

Crosscutting

150

10

Advanced high-temperature materials for high-temperature processing

Crosscutting

150

10

Net-shape and near-net-shape design and manufacturing

Casting, rolling, forging, additive manufacturing, and powder metallurgy

140

10

Integrated manufacturing control systems

Crosscutting

130

10

Total

2,210

155

Novel processing techniques that involve lower temperature processing or fewer heating steps through process hybridization can also reduce energy consumption. Hybrid process heating systems that combine multiple forms of heat transfer (radiative, conductive, and/or convective methods) or multiple operations into a single piece of equipment (such as hybrid distillation systems) can reduce heating time, increase energy efficiency, and improve product quality. Key research, development, and deployment (RD&D) opportunities for energy and emissions savings in industrial process heating operations as outlined by DOE report [1] are summarized in Table 7.2. While the total energy savings opportunity (2,210 TBtu) is very large, only a portion of this opportunity is technically and economically feasible to capture, as discussed in the Waste Heat Recovery Systems Technology Assessment [1].

While every effort should be made to reduce waste heat losses (for example, by integrating advanced insulation techniques and selective heating technologies into process heating equipment), some heat losses are unavoidable. The recovery and reduction of waste heat generated in manufacturing systems offer an opportunity to reduce manufacturing energy use and associated emissions. Waste heat can be recycled either by redirecting the waste stream for use in other thermal processes (e.g., flue gases from a furnace could be used to pre-heat a lower-temperature drying oven) or by converting the waste heat to electricity in a process called waste heat-to-power. In some cases, the technologies needed to economically recover waste heat from hot gases, liquids, or solids are already available. However, industrial facilities often do not implement these technologies, based in part on technology issues (e.g., fouling, corrosion, and high maintenance requirements). According to U.S. EIA Manufacturing Energy Consumption Survey (MECS) data, approximately 6% of U.S. manufacturing facilities were using some type of waste heat recovery as of 2010 [1,32].

Industrial users demand equipment lifetimes of several years, low maintenance and cleaning requirements, and consistent and reliable performance over acceptable life. For low-temperature waste heat streams (i.e., less than 400°F), low heat transfer rates and large recovery equipment footprints are major barriers. For high- temperature waste heat streams (i.e., above 1,200° F), materials are needed that can withstand high-temperature gases that may be contaminated with particulate matter (PM) or corrosive chemicals [1,33]. DOE report [1] points out that in order to address these challenges, research and development in the following areas will require:

  • 1. Antifouling technologies that can remove contaminants from waste heat streams or mitigate build-up of debris on heat exchanger surfaces, promoting long-term operation of heat recovery equipment, and avoiding service interruptions for cleaning
  • 2. Advanced materials that can withstand high-temperature waste heat sources
  • 3. Compact, low-cost heat exchangers to reduce the size or footprint of heat recovery equipment
  • 4. Secondary heat recovery technologies to supplement and enhance the performance of primary waste heat recovery equipment
  • 5. Heat recovery chillers that capture waste heat from chilled water systems
  • 6. Integrated heat recovery technologies that combine heating elements with heat recovery equipment, eliminating the need for hot-air piping and external heat recovery equipment
  • 7. Innovative condensing heat exchangers for gases containing high moisture levels and particulates, such as the waste streams discharged from paper and food production equipment
  • 8. Liquid-to-liquid heat exchangers for heat recovery from wastewater that contains contaminants
  • 9. Solid-state (e.g., thermoelectric) generators for electricity production from otherwise unusable waste heat streams, thermophotovoltaic (TPV) system, or piezoelectric system
  • 10. Industrial heat pumps, including chemical heat pumps (e.g., adsorption/ desorption and chemical looping reactions).

Motor-Driven Systems

Industrial machine and motor-driven systems include pumps, fans, compressors, air conditioners, refrigerators, forming and machining tools, robots, and materials processing and handling equipment. These systems account for 68% of manufacturing electricity consumption [1,32]. The majority of this energy is consumed in just three manufacturing sectors: chemicals, forest products, and food and beverage manufacturing. While electric motors have high efficiencies, end-use motor-driven systems have much lower system efficiencies, particularly for pumps, fans, compressed air, and materials processing equipment. As a result, overall machine-driven system losses total 1,470 TBtu annually [1,32]. The total energy uses for major categories of machine-driven systems in U.S. manufacturing are as follows: pumps (614), fans (291), compressed air (333), materials handling (175), materials processing (497), process cooling (212), and facility heating (241). The numbers in brackets are in TBtu units for 2010 [1]. Key energy-saving opportunities can be identified by focusing on opportunities to improve the motor system, rather than focusing solely on the motor. A 2004 study estimated the electricity savings opportunities from the use of available technologies on motor-driven systems. Only 13% of these opportunities were from the motors, while variable speed drive adoption accounted for an additional 25%, and improvements to applications would account for the remaining 62% [1].

Process Intensification

DOE report [1] points out that process intensification (PI) targets dramatic improvements in manufacturing and processing by rethinking existing operation schemes into ones that are both more precise and efficient. PI frequently involves combining separate unit operations such as reaction and separation into a single piece of equipment, resulting in a more efficient, cleaner, and economical manufacturing process. At the molecular level, PI technologies can significantly enhance mixing, which improves mass and heat transfer, reaction kinetics, yields, and specificity. These improvements translate into reductions in energy use, waste generation, environmental impact, and amount of equipment, and thereby minimize cost and risk in chemical manufacturing facilities. Table 7.3 shows energy reduction opportunities in eleven chemicals using PI technologies [1].

 
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