The industrial sector accounts for about 34% of total U.S. energy consumption. This energy is consumed as electricity, which is purchased or self-generated, and as fossil fuels (e.g., natural gas, propane, fuel oils and coal). Understanding these energy sources and their associated uses, equipment, efficiencies, costs, availabilities and waste streams is critical to developing a sustainable energy-efficiency program.
Every manufacturing (or heat treat) plant has raw materials that come into the receiving dock and finished products that are sent out from the shipping dock. Between the receiving and shipping docks, transformation occurs. Transformation adds value to the materials in a step-by-step process, and energy is required. Evaluating the process transformation steps and energy inputs provides clues about where to look for energy savings.
Energy Efficiency and Intensity
The total energy into a system is Ein, which is the amount that appears on your utility bill. The total energy out of the system is Eout, which represents the useful energy that adds value to the product during the process. The difference between Ein and Eout is the loss.
Loss is wasted energy that is not useful to the process and degrades efficiency. For sustainable energy efficiency, energy losses must be identified, documented, tracked, corrected and prevented from recurring. If the loss was zero, the system would be 100% efficient, which does not occur in the real world.
A simple way to envision energy efficiency is to think about water flowing through a pipe – the water represents the energy, and the pipe is the process. Ein would be the water into the system in gallons per minute (gpm), and a loss could be a leak in the pipe that reduces the amount of water available to add value to the product (Fig. 1).
Using the water-pipe example, we can calculate energy efficiency: Ein is 100 gpm, and the loss due to leaks is 10 gpm. Thus, Eout is 90 gpm, and the energy efficiency of the system is 90% (Eout/Ein).
While water leaking from a pipe is a useful visualization, a process heated by natural gas is more realistic. In this example, Ein is 1,000,000 BTU/hour of natural gas, and the fuel combustion loss is 100,000 BTU/hour, stack loss is 250,000 BTU/hour, stored heat loss is 75,000 BTU/hour, furnace wall loss is 50,000 BTU/hour, opening loss is 25,000 BTU/hour and conveyor loss is 20,000 BTU/hour – for a total loss of 520,000 BTU/hour. Eout is the difference between Ein and the total loss, which is 480,000 BTU/hour, making the energy efficiency of the system 48%. Over half of the original natural gas energy input is lost and not providing useful value-added work in the process.
The energy intensity of a manufacturing process is the amount of energy that is required to produce one logical unit of product (e.g., kWh/ton metal melted at a foundry, MMBTU/bbl of oil refined at a refinery, MMBTU/pound of polymer produced at a chemical plant). Energy intensity provides an order-of-magnitude estimate of the significance of energy in the production process, and it varies widely from industry to industry.
Each step of the transformation process should add value with minimal waste. Every step requires some type and amount of energy to carry out the transformation. Certain steps require a large amount of energy, while others require very little. Outlining each step and the required energy inputs is useful for planning and prioritizing energy projects.
To understand transformation, consider the process that produces a vase from a lump of clay. Figure 2 depicts a step in the process with energy inputs from manual human labor and possibly an electric motor to turn the wheel. This step adds value by transforming the clay into a useful shape. Table 1 presents the full details of this transformation. The steps that require a kiln are the obvious big energy users and would be logical candidates to evaluate for energy savings.
The same approach of evaluating each transformation step can be applied to complex manufacturing systems. A process block diagram is useful for outlining the transformation steps in a process. Consider the process to manufacture formaldehyde (Fig. 3), which will be referred to throughout the remainder of the article.
The 10 Steps
Step 1. Identify the Raw Materials
Some industrial processes have one main raw material, while others have dozens or even hundreds. Raw materials can come into the process at many places along the transformation journey. To determine the type and amount of energy required in the system, first consider these aspects of the raw materials:
- Type of material (e.g., metal, chemical, mineral, textile, vegetable, finished goods)
- Physical state (e.g., solid, liquid, gas, subassembly)
- Delivery method (e.g., tanker ship, tanker truck, common carrier, railcar)
- Delivery storage (e.g., dry bulk, tank farm, warehouse, sacks, pallets, cardboard boxes)
Defining the raw materials and their details is an initial step in creating a process block diagram. We will follow these materials on their journey to their final destination while evaluating the energy use at each point along the way.
Step 2. Identify the Final Products
The final product is the destination of the transformation journey. Manufacturing plants are in the business of making money, so raw materials are brought in, transformed into something useful and then sold for a profit. The plant adds value, hopefully very efficiently, to the raw materials and produces a final product of a designated design and quality.
Step 3. Tour the Plant
There are many possible ways to get from Point A (raw materials) to Point B (final product). Touring the manufacturing site with process operators and maintenance personnel as guides is essential to defining the transformation steps and developing the process block diagram. The walk-through should ideally be conducted chronologically, from raw materials to finished products.
When you tour the plant, bring a blank process block-diagram template that indicates the raw-material starting point and final product end point and has empty boxes in between to take notes. The tour may last anywhere from a couple of hours to a couple of days, depending on the size and complexity of the plant. Ask your tour guides questions and get their contact information for follow-up requests. Record or photograph nameplate data for large pieces of equipment that you know consume a lot of energy.
Step 4. Develop the Process Block Diagram
You have done your homework and completed a detailed tour of the manufacturing site. Now you are ready to flesh out the process block diagram. Use your notes, conversations, utility data and possibly some online research to document the transformation steps in the process. The product of your work should look something like Figure 3, which shows the basic steps necessary to transform methanol and air into formaldehyde. Next, evaluate each block to identify the energy components, including energy inputs, energy wastes, energy-recovery possibilities, energy-efficiency opportunities and new technology opportunities (steps 5-9).
Step 5. Identify Energy Inputs
Each step of the process block diagram must be reviewed to identify the primary energy inputs required to perform the transformation. Energy inputs may be direct energy (e.g., electricity, natural gas, propane and fuel oil) or derived energy (e.g., compressed air, steam and chilled water). Repeating this analysis for every step helps to produce an overall qualitative energy-usage model.
Completing an energy-input analysis for each block in the diagram creates an overall picture of the process-energy consumption. If available, information to help quantify the energy input is valuable, including motor horsepower, actual metered cubic feet of natural gas, electric process sub-metering, etc.
Step 6. Identify Energy Wastes
Energy is wasted to some degree in every step of the manufacturing process. Major wastes should be identified when you are analyzing the process block diagram. Identifying process waste streams is the first step to minimizing them, recovering valuable energy from them and reducing their environmental impact.
Step 7. Identify Energy-Recovery Possibilities
The energy waste streams should be examined for their energy-recovery potential. Observations of the waste streams in Figure 3 might include:
- The waste tail gas from the scrubbers is already being used to generate site steam, which is good.
- In the tail-gas boiler, hot stack flue gas could be used to preheat tail-gas boiler combustion air and tail-gas boiler feedwater.
- In the tail-gas boiler, hot blowdown could be used to preheat tail-gas boiler makeup water.
- There may be an economical way to extract valuable methanol or formaldehyde from the scrubber wastewater.
Step 8. Identify Energy-Efficiency Opportunities
Each block in the process block diagram should be evaluated for energy-efficiency opportunities. Depending on the energy input for the process operation, a variety of options may be available to reduce energy consumption. The motors, compressed air, chilled-water supply and boiler/steam supply in Figure 3 have the potential for energy-efficiency improvements.
Step 9. Identify New Technology Opportunities
Implementing new or existing process technologies can provide energy savings in addition to those identified in step 8. A goal is to reduce energy intensity, and a different technology may lower the energy required to transform one logical unit of product. Look for opportunities to improve process equipment with new technology.
In Figure 3, for example, the wastewater from the scrubbers could be handled differently. In the current process, the wastewater is drained away to an on-site wastewater-treatment plant. Membrane filters or reverse-osmosis systems could clean the wastewater and extract valuable methanol or formaldehyde from the stream. The energy and financials would need to be evaluated to see if the idea is feasible, but this type of new-technology investigation can frequently provide energy-efficiency and energy-intensity solutions.
Step 10. Implement Solutions
After you perform the process energy analysis and develop the process block diagram, the next and most important step is to implement some of the energy-saving solutions you have identified. Savings will not be realized until the results are actually applied.
Your analysis will produce a detailed set of opportunities for energy improvements. Compile the results in a table or spreadsheet so they can be evaluated, prioritized, budgeted and tracked for implementation. Then repeat the approach periodically for continual improvement.
Pursuing energy improvements often produces benefits in other areas as well. These non-energy benefits may include greater plant productivity, higher product quality, fewer process bottlenecks, improved worker safety, more available floor space and lower emissions and waste-stream volumes.
When exploring new technologies, a combination may produce the best energy-saving results. Depending on the process, there may be numerous technology opportunities available. Any idea must be subjected to rigorous energy and financial analyses to prove its feasibility prior to implementation.