2013/05/22 by Eric Masanet, Michael E. Walker · 1 citation
Chemistry · Engineering · Environmental Science · #Boiler (water heating) #Chemistry #Economics #Efficient energy use #Energy and Environment Impacts #Engineering #Environmental Impact and Sustainability #Environmental science #Incentive #Oil refinery #Primary energy #Renewable energy #Steam reforming #Waste management #Water-Energy-Food Nexus Studies
paper · doi:10.1002/aic.14148
openalex publication_date 2013/05/22 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/26
Steam systems are a ubiquitous element in nearly every type of manufacturing plant. In the United States, steam systems are the single largest consumer of energy in the industrial sector, where they account for 37% of annual onsite energy use.1 Steam use is particularly prominent in the chemicals, paper, petroleum refining, and food and beverage industries, where it is used in a wide range of processes, including reforming, distillation, concentration, cooking, and drying.2 Together, these four industries comprise nearly 90% of U.S. industrial steam demand, with chemicals manufacturing (30%) and paper manufacturing (30%) holding the largest shares.1, 3 At the national level, industrial steam systems account for around 6% of U.S. total primary energy use, or 5,900 trillion British thermal units (TBtu).1, 4 As such, much attention has been paid to steam system energy efficiency improvements as part of corporate, utility, and government energy and air pollution initiatives. Key incentives include local utility rebates, tax incentives, and low- or no-cost steam system energy efficiency audits.5 Steam system energy efficiency not only makes sense from an environmental perspective, but also from an economic perspective. As of 2006, U.S. manufacturers spent 21 billion on externally purchased boiler fuels. The actual price tag of industrial steam is likely much higher; nearly one-half of U.S. boiler fuels are self-generated within plants in the form of waste gas, black liquor, wood wastes, and other byproducts.1, 3 These byproduct fuels are not free, as they are generated from purchased materials and typically require further processing for efficient combustion. Reducing demand for boiler fuels can, therefore, help reduce operating costs and improve profit margins. While clearly justified, the historical focus on reducing energy use has overlooked an increasingly compelling benefit of steam system efficiency: namely, reduced water use. Compared to the many public and private incentives for industrial energy efficiency, there are surprisingly few external incentives for industrial water efficiency. One key barrier to such incentives is the lack of credible data on industrial water use, which, unlike data on energy use, are not compiled at the manufacturing industry or process level in regular national surveys.6 This dearth of data contributes to a general lack of awareness of the sources and scale of industrial water use within the engineering and policy communities, which limits broader attention to water efficiency beyond the plant floor. Another barrier to steam system water efficiency is that the cost of boiler water—and the associated chemicals required for its treatment—typically only represents a small fraction of boiler operating costs, which are dominated by the costs of fuel.7 However, as we discuss in this Perspective, U.S. industrial steam systems consume copious amount of water. It follows that steam systems are worth a closer look as a manufacturing water efficiency target. Several current trends suggest that water efficiency will play an increasingly prominent role in the financial and sustainability plans of U.S. manufacturers. Recent water stress due to droughts and rising water infrastructure costs have led to increased public water rates around the country.8 These conditions may worsen with a changing climate.9 An increasing number of manufacturers are reporting water use as an important environmental indicator in annual corporate sustainability reports, which raises both public awareness of and accountability for water efficiency. Many manufacturers are also being asked by their corporate customers for environmental “footprint” data as part of large-scale sustainable supply chain initiatives.10 In light of these trends, the combined reductions in energy and water use that come with improved steam system efficiency should appear more and more attractive to most manufacturers. In the remainder of this Perspective, we explore the water use of U.S. industrial steam systems, discuss their potential for energy-water efficiency in U.S. plants, and describe pathways toward greater steam system water efficiency moving forward. A boiler system is the heart of many manufacturing facilities.11 Its energy filled arteries spread out in a dizzying array of pipes to supply steam heat to processes throughout the plant. Steam is particularly well suited as an energy transfer medium in the plant environment for several reasons (1) steam and condensed water can easily be transported throughout a facility, (2) steam pressure can be tuned to supply heat to plant processes at the appropriate temperature, and (3) condensing and evaporating steam has favorable heat-transfer characteristics.11, 12 As chemical engineers, we all learn about the thermodynamics of steam in the classroom, but many of us have never seen an industrial steam generation and distribution system in operation. The practical nature of these systems is not covered in much detail unless one chooses to specialize in energy engineering. Even then, there is little focus on steam system water use. Hence, we provide here a brief review of the energy and water flows associated with a typical industrial steam system. As depicted in Figure 1, water enters the steam system as makeup water (point 1), the purpose of which is to replace water mass leaving the system in the form of process steam and/or water and steam losses. Makeup water entering a steam system joins with returned condensate and is first pumped to the deaerator, which removes oxygen and noncondensable gases. Upon leaving the deaerator, the boiler feedwater is brought up to system pressure via the feedwater pump and is introduced to the boiler (point 2).2 Within the boiler unit, feedwater is converted to steam with heat supplied by the combustion of fuel (point 3). The steam generated in the boiler can be utilized for a number of purposes, including power production, direct steam injection, and indirect process heating (point 4). Note that the system depicted in Figure 1 is a simplified representation of an industrial steam system; items such as valves, boiler heat recovery apparatuses, and holding tanks are not included. In many plants, the majority of water use in the steam system is attributable to direct injection of steam into production processes. However, as shown in Figure 1, there are also several other sources of steam and water loss that are present in a typical system. These other losses can be particularly pronounced at inefficient plants. Certain losses are inherent to the operation of the steam system; the most significant example of this is boiler water blowdown. As steam is generated in a boiler unit, nonvolatile components in the boiler water become more concentrated and may ultimately degrade the performance of boiler equipment. To prevent this, a portion of the boiler water is periodically released or “blown down.” Blowdown losses have been reported to constitute up to 10% of boiler water flow in many plants; however, properly maintained systems and high-quality makeup water treatment can reduce these losses to below 5%.13 Another source of inherent losses is the deaerator vent, where a small fraction of steam is released as part of deaerator operation. A deaerator is a tank that is heated to the saturation point of the feedwater for the purpose of removing oxygen and other dissolved gases.2 Oxygen present in the feedwater is problematic because it is corrosive at high temperatures and can, therefore, deteriorate process equipment. Other system losses include steam venting, steam leaks, and steam trap losses. Venting of steam may occur at steam headers or from unit processes when steam generation exceeds process needs. Proper process and boiler control and optimization can help to minimize these venting losses. Steam leaks can occur when high-pressure steam passes through piping, joints, and valves (particularly relief valves).14 While leaks may exist due to faulty installation, they can also develop over time through corrosion or erosion. Therefore, proper maintenance and monitoring is important to prevent these losses. Steam traps are devices utilized to remove steam condensate from a unit process such as a heat exchanger. This condensate is typically returned to the boiler to reduce the need for makeup water. When functioning properly, these devices allow the discharge of condensate with only minor loss of steam. However, over the course of operation erosion can cause the trap to wear down and leak or fail, thus, releasing steam unnecessarily and leading to energy and water losses. Steam leaks and steam trap losses are particularly variable between plants and depend heavily on plant maintenance routines and monitoring. On the other hand, deaerator vent losses are needed for proper operation, but monitoring and control can help to minimize deaerator steam requirements. Deaerator losses can be estimated to be on the order of 0.5% of the system steam flow.15 Figure 1 also highlights the fact a number of industrial steam systems incorporate a combined heat and power (CHP) generation strategy. The particular CHP configuration shown above (boiler/steam turbine) is only one of several CHP strategies. Other examples include combustion turbines with heat recovery steam generators and combined cycles adapted for CHP. The use of CHP can be advantageous because it can achieve higher thermal efficiency than is possible through the production of steam and electricity through separate processes.16 While makeup water use can be minimized through efficient operations, energy inefficiencies that are common at many U.S. plants result in unnecessary and avoidable makeup water demand. Even at well managed plants, audits routinely reveal numerous opportunities for energy use reductions through the adoption of proven, cost-effective energy efficient technologies and operations strategies. Energy engineers often refer to such opportunities as “low-hanging fruit,” and further know that such fruit can “grow back” over time due to equipment degradation, suboptimal maintenance practices, changing priorities on energy efficiency, and technology evolution. For steam systems in particular, energy audits frequently identify significant savings potentials if plants were to adopt more efficient boiler operations, improved heat containment and recovery, improved condensate return, better maintenance programs, and improved process designs. For example, a 2002 U.S. Dept. of Energy (DOE) study of steam system efficiency opportunities in the U.S. chemicals, paper, and petroleum refining industries concluded that each industry might reduce its boiler fuel use by as much as 12% through the adoption of efficiency best practices.17 Moreover, the simple payback periods for the identified improvements were generally less than 2 years, indicating quick returns on investment. Just how pervasive are these opportunities in U.S. industry today? To help answer this question, we turn to data from recent energy audits at over 1,000 major U.S. manufacturing plants, which were conducted through the U.S. DOE's Save Energy Now (SEN) program between 2006 and 2011.18 The SEN program targeted the nation's largest energy using plants—those with energy use exceeding 1 trillion TBtu per year—with strategic audits of steam, compressed air, process heating, fan, and pump systems. As of this writing, summary data have been provided for 346 SEN steam system audits at plants whose combined annual energy use totaled 2,650 TBtu, or 17% of the direct energy use of U.S. industry. These audits resulted in over 2,200 identified steam system improvement opportunities. In total, these opportunities could save the audited plants over 660 million in energy costs each year and reduce their annual energy demand by 82 TBtu. These data are summarized in Table 1, which indicates that energy savings opportunities were particularly significant among the audited chemicals, paper, and primary metals plants. Thus, for a sizable segment of U.S. industry—which includes some of the nation's largest and best managed plants—the SEN data suggest that low-hanging fruit for steam system efficiency improvements are still plentiful. Not all steam system energy efficiency improvements will reduce water use. For example, both improved boiler combustion controls and better distribution system insulation can save energy, but neither will affect the mass throughput of water. However, a number improvements—including reducing steam leaks and vented steam, improving condensate return, reducing blowdown rates, and lowering process steam—can directly reduce makeup water demand while also saving energy. The SEN audit results for these specific opportunities are summarized in Table 2, which indicates that 887 of the 2,220 identified opportunities (40%) across all industries held strong potential for combined energy-water savings. Table 2 contains SEN data of two types: those which refer to identified opportunities and those which refer to implemented opportunities. An implemented opportunity is one that was reported by plant staff as installed/pursued in post-audit follow-up communications to the SEN program. Comparison of identified and implemented results sheds light on the characteristics of opportunities that were ultimately pursued by the audited plants. Table 2 also provides a summary of results for chemicals plant audits alone, which showed particularly high potential for combined energy-water savings. Across all audited plants, less than 40% of the identified energy savings and less than 30% of the identified cost savings in Table 2 were actually implemented at the time of this writing. Comparison of identified and implemented data reveals that, in general, audited plants chose to implement a subset of opportunities with faster-than-average payback periods and greater-than-average energy savings, indicating a rational preference for the most compelling investments. The greatest savings were realized through reductions in process steam demand, with the vast majority of these realized savings (85%) occurring in the audited chemicals manufacturing plants. Across the board, the audited chemicals plants realized much higher fractions of total identified energy and cost savings than the industry average, and did so through a focus on opportunities with energy savings much higher than the industry average. These data suggest a strong commitment to action at the audited chemicals plants, as well as a willingness for (sometimes difficult) reengineering of key processes to reduce overall plant steam demand. Still, the data in Table 2 suggest that many low cost opportunities for combined energy-water savings are not being pursued at the audited plants. As of this writing, these plants had still left around 175 million of the available energy cost savings on the table. Furthermore, 47 million of that amount remained unrealized at the audited chemicals manufacturing plants. Despite the low-cost, high-return nature of opportunities identified through SEN audits, the history of the program suggests that many opportunities are ultimately rejected by participating plants. Of the 2,200 opportunities listed in Table 1, one-half were rejected by the time of this writing. Another 260 were designated as “in progress of and were designated as “in Thus, while there is still potential for energy savings as a result of the SEN audits, it is that many opportunities will be left including many of the energy-water savings opportunities listed in Table U.S. manufacturers so many compelling energy savings opportunities on the The SEN data that of all rejected opportunities across industries and system steam, process and compressed nearly were rejected due to and flow or on investment. While such may the nature of SEN such are pervasive across U.S. industry and have likely been in recent due to the economic reasons for priorities on generation over energy cost increasingly for energy and engineering staff time to energy efficiency These can be particularly pronounced at small and whose and staff are typically less than those of the plants covered by the SEN Another of rejected opportunities were due to or opportunities which can include plant rejected for these reasons can often be a However, that are to energy savings at the of their are often to the engineering and operations associated with process and equipment As manufacturers become increasingly of water use, however, the makeup water savings that come with many steam energy efficiency opportunities might improve their as sustainability investments. As industrial water use data are not compiled in a regular in U.S. national The few that have to this have on of the total water use of specific manufacturing Moreover, these have treatment of boiler makeup water use. As engineers, however, we need not be in the we can use to the of industrial steam system water demand. national data on boiler fuel by manufacturing industry and fuel type are available from the U.S. DOE's Energy These data the total thermal on the boiler of the nation's steam systems. data from process energy demand and of specific industries light on the and of steam in U.S. A of a steam generation and distribution system can the between total fuel and total process demand to annual makeup water requirements. Figure 2 makeup water demand on this for U.S. industrial steam systems as of 2006, which is the year data are These were generated using typical for industrial steam system blowdown rates, venting boiler and leak rates as on the can be in The data in Figure 2 are as a in which the total makeup water use of each listed manufacturing industry is by a with a is into four to sources of steam and water loss Figure In total, an estimated million per of makeup water are by U.S. industry. This amount is to the water use of million U.S. which is the of the nation's The paper, chemicals, petroleum refining, and food and beverage industries account for the vast majority of estimated makeup water demand. These results are not that these four industries also account for the of industrial steam demand. Within each direct injection of steam by the largest source of makeup water use, which suggests that energy efficiency that reduce process steam demand might also significant water savings. While the data in Figure 2 are engineering their that of water are required for steam in U.S. plants. in that steam systems may be a for water efficiency at many and within the paper, chemicals, petroleum refining, and food and beverage industries in This point is by Figure which the estimated makeup water savings associated with the SEN audit opportunities listed in Table These were generated using the and are for the industry depicted in Figure water savings associated with both identified and implemented SEN opportunities are listed with to water savings The estimated makeup water savings through the 887 identified energy-water saving opportunities in Table 2 to around or an amount to the water use of around U.S. Of this total water savings however, only an estimated were realized the implemented opportunities. Of all audited plants, those in the chemicals industry realized the greatest fraction of their estimated water savings potential by process steam demand opportunities as seen in Table The other four industry realized or less of their estimated water savings It is from Figure 3 that, in to leaving many compelling steam system energy savings opportunities on the many of the audited plants may have makeup water savings as The combined energy-water savings associated with steam system efficiency improvements should attractive to manufacturers to reduce their environmental in light of the of water efficiency. The summarized here suggest that the makeup water of U.S. industrial steam is and that this can be reduced through improved steam system efficiency. In particular, reductions in process steam demand in the paper, chemicals, petroleum refining, and food and beverage industries are While the of water efficiency may from plant to plant on local water and water a greater focus on steam systems is in general on the nation's water and are several opportunities both within and external to the engineering that can this moving forward. while the engineering summarized here an important better water use data reported by manufacturing plants on a regular a more of the water of U.S. industry. use data reported at the process level within specific manufacturing industries be particularly for awareness of the sources and scale of industrial water use at the and national While a number of manufacturers provide corporate water use data as part of annual sustainability reports, these data are reported at the plant or process Moreover, many manufacturers not on water use, and many have to sustainability A greater commitment to water efficiency at the of manufacturing might the needed among plants to and water use and national government might industrial water use data with greater detail and as for For example, the DOE's might be easily to annual boiler makeup water use data with annual boiler fuel use data many plants have to both of data via their boiler control systems. improved public and private incentives for industrial water efficiency can with combined energy-water savings more attractive to plants that might opportunities on the of energy savings incentives are often for technology for small and medium which often have for efficiency plant water use data will be for the for such incentives, which can include equipment from local utility or tax at the or incentives are available for energy efficiency and several of incentives exist for water and water to at the plant level, manufacturers can adopt a strategic water that for and water use performance plans are for improvement across the including the of steam demand and inherent system losses. A strategic water can be with strategic energy examples of which are in U.S. and of energy-water savings opportunities. One of such plans can be to the of corporate by The of the water be to out water savings opportunities and awareness of the of water efficiency among plant for such to a commitment to water efficiency at the of plant is often we have a opportunity for improving on the nature and of energy and water use in steam systems as part of the engineering While nearly every engineering is introduced to steam as part of on few in the practical of steam generation and distribution systems. it is these practical that have the most on energy-water efficiency the performance of steam condensate systems, and heat As such, many engineers are to the costs of the steam traps and and valves that us in the typical plant to local manufacturing plants to the operation of plant systems be toward engineering to equipment of the energy-water is particularly for nation's chemical engineers, are typically on the of process for major processes in most manufacturing As it is these processes that the greatest potential for combined energy-water savings through more efficient process and increased awareness among all engineers in manufacturing among chemical engineers in be the most important opportunity for increasing the efficiency of U.S. industrial steam systems moving forward. The of provided the SEN audit summary data as well as much to help their proper