TACKLING ELECTRICAL ENERGY EFFICIENCY IN INDUSTRIAL PLANTS

Electricity is an indispensable energy carrier in industrial plants, where it is used to power induction motors, provides energy for the heating of materials, in lighting applications, for activities such as welding and soldering, in induction furnaces and for many other uses. Individual industries tend to have quite different electrical energy demands, but it is safe to say that as electricity prices continue to escalate, most management teams have electricity savings firmly in their sights.

Why concern your organisation with energy efficiency?

Reducing electrical energy consumption is of interest to sustainability-focused organisations for the following reasons:

    

          In energy-intensive industries electrical energy efficiency is a powerful profit driver. This is becoming even more of an issue in countries such as South Africa, where electricity prices for industrial organisations have been escalating at in excess of 25%/annum for a few years in succession now;

        Every unit of electrical energy saved translates into a reduced impact on the environment. In South Africa, each kWh of energy produced is estimated to generate 1.015 kg of carbon dioxide equivalents¹ due to the coal-heavy nature of our energy mix. Then there are of course the large amounts of water required to generate power from coal, along with particulate emissions, mercury emissions and potential quality impacts on water resources. Further up the value chain, there are significant impacts arising from coal mining. From a life-cycle perspective, there are therefore few more meaningful things an organisation can do for the environment than to reduce electrical energy consumption.

     The above issues have social impacts, since increased profits can contribute towards job creation, both directly within a business and within its broader value chain, and environmental degradation generally tends to hurt socially vulnerable communities more. In emerging economies (and in first world countries too at times), power supplies can also be limited and energy efficiency therefore helps to make limited resources available to more consumers.

How to go about it? 

How should individual industrial power consumers begin an energy efficiency programme, and what are the important issues to consider when doing so? The nature of your industry will dictate where you will need to focus, but where should you start, and how are the savings actually going to be realised? I will hopefully answer some of these questions in this article.

Baseline your performance

The first thing you need to do is to analyse and understand your electricity bills and each cost element contained in them. When analysing an industrial site’s energy profile, I typically look at monthly bills stretching back over the most recent two year period. This gives me a sense of any seasonality in electricity consumption, the timing and scale of past price increases, and the tariff structure employed. Linking energy consumption to production levels is useful in terms of understanding crude energy intensity levels e.g. kWh/ton of production, and I typically construct trends of the data in order to provide insight into patterns. This exercise alone can immediately suggest savings opportunities. For example, there may be opportunities to shift load and achieve a lower average cost of electricity where time-of-use and maximum demand charges are applied.  

Assess the performance of individual processes and energy users

Conducting an energy audit of your operations at the process level should be your next activity in the quest to become more energy-efficient. There are typically a number of specialist companies you could approach to conduct such an audit. The audit should look at individual aspects of your operation and every point at which electricity is being used, but should also incorporate a “systems view” which takes potential impacts and synergies between individual areas into account. Hence, if you are reducing the energy requirements of a furnace, for example, it is not only about making the furnace more efficient, but also about reducing the levels of rework which that furnace needs to process. Solving a problem of that nature may require a review of downstream operations in relation to the furnace, and cannot be solved by looking at the energy efficiency of those operations and the furnace as separate entities.

An electrical energy efficiency audit will highlight areas of opportunity, and for each of these, the next step is to identify appropriate solutions. These solutions need not require large investment, and could include changes to shop floor work practices, the implementation of improved management systems, the performance of appropriate preventive maintenance tasks, changes to process set points and other approaches. The table below outlines a few examples of these types of solutions.

ENERGY EFFICIENCY PROBLEMS THAT DO NOT REQUIRE INVESTMENT FOR THEIR SOLUTION

ENERGY EFFICIENCY OPPORTUNITY	SOLUTION
Electrically heated process baths are operated at too high a temperature.	Reduce temperature set point by 5 deg. C
Refrigeration plant evaporative condenser is not removing enough heat from the refrigerant. Tubes were found to be excessively scaled up and some nozzles were blocked.	Institute a chemical de-scaling programme for the condenser tubes and a blowdown procedure for the cooling water.
Too many factory lights were found to be left on unnecessarily at night.	Add more switches to allow areas to be individually controlled, and institute a checklist for plant operators for implementation when the plant is shut down at night.
Compressed air reticulation system was found to have excessive air leaks.	Institute a regular leak detection and maintenance programme.
Compressed air pressure too high.	Reduce compressor pressure set point.
V-belts on drive systems slipping excessively.	Measure belt tension and correct regularly, taking care to check alignment at the same time.
Air conditioners set at too low a temperature during the warm months.	Increase the air conditioner temperature set points.
Production systems running on idle for long periods due to poor planning.	Investigate bottlenecks in material flow and improve planning processes.

Technological solutions are widely available to improve energy efficiency, and it is worthwhile to keep up to date with latest developments through regular engagements with OEM’s, and attending industry engagements (seminars/conferences). There is also a wealth of information available on the internet, but take care to verify manufacturers’ claims.

Understand the risks associated with individual solutions

Whether solutions are technological or not, remember to assess the risks involved with the implementation of any solution. Reducing the temperature of a process bath in an electroplating operation could have implications for drag out losses, for example, and dropping air pressure on a site could affect the operation of individual machines. While not all such risks can be quantified beforehand, make a point of evaluating the impacts of changes made so that if there are unintended consequences, these can be rapidly detected and dealt with. Where there are significant capital costs involved in your solution, consider life cycle costs, not just capital outlay. While operating costs tend to dominate life cycle costs for electrical equipment such as lighting, consider also environmental impacts, and the challenges that could be associated with the safe disposal of used components.

A useful risk management strategy is to pursue limited implementation, which allows an assessment of post-implementation performance prior to making larger financial commitments. This is particularly important when working with novel technologies, but of course cannot be used to assess risks which only present themselves after a long period of time, since implementation would be unduly delayed. Another option would be to insist on client references, and to contact these clients to get their views on the merits of individual energy efficiency products.

Evaluate implemented options and use feedback constructively

Finally, each implemented sub-project should be reviewed against the criteria which justified its implementation in the first place. This aspect of implementation, which you could call “performance testing”, is in my experience the most neglected part of energy efficiency project implementation. Are those new lighting arrangements really using the amount of energy expected, and is the light provided sufficient and sustained? If not, what is the problem? As with all continuous improvement, improvements in energy efficiency are also subject to the Plan-Do-Check-Act cycle.

I will review individual energy efficiency opportunities and how to practically assess these in future posts.

References

1. Letete, Guma and Marquard, “Information on climate change in South Africa: greenhouse gas emissions and mitigation options”, University of Cape Town Energy Research Centre.

TAKING A SYSTEMS VIEW OF RESOURCE EFFICIENCY – A SIMPLE EXAMPLE FROM THE BREWING INDUSTRY



Making the linkages between sub-systems is

important for overall resource efficiency

A fundamental principle of Cleaner Production is to optimise systems rather than individual system elements. When partitioning a system into sub-systems for analysis, it is important to draw a boundary around the broader system you are analysing to ensure that in dealing with resource efficiency problems in one part of the system, you are not introducing unintended consequences elsewhere. I will use the example of centrifugation in the brewing industry to show how resource efficiency impinges on other operational issues such as product quality and throughput, and how taking too narrow a view can result in unexpected problems. If a systems view is not taken, these problems are difficult to solve, and could actually result in a negative overall efficiency position for the system despite apparent efficiencies in an individual area.

Centrifugation is a common unit operation in industry and is used to separate materials of different densities through the application of centrifugal force. It is applied globally in the brewing industry to separate yeast from green beer, with the beer than passed on to additional clarification processes such as filtration. Disc stack centrifuges use conical discs stacked on top of each other, with the product to be clarified passing through the spaces between the discs. As the centrifuge bowl rotates at high speed, yeast is pushed against the inner surfaces of the discs and is forced downwards into solids pockets in the bowl. The clarified beer passes up the centrifuge and exits, while separated yeast accumulates in the centrifuge. This yeast is periodically removed by momentarily opening the bowl and then shutting it to prevent an excessive loss of beer.  This process (called “de-sludging”, “discharging” or “ejection”) is well illustrated by this short video

There are a number of ways to initiate a centrifuge discharge. One way is through the use of a timer, in which case the centrifuge bowl will open and shut at a pre-determined frequency. This is not really desirable, since it leads to variations in the concentration of the yeast slurry discharged and in the clarity of the centrifuged beer. The second is through the use of in-line instruments which measure the clarity of the beer passing through the centrifuge, and then prompt a discharge when this clarity level becomes unacceptable i.e. when there is too much yeast in the beer exiting the centrifuge. Turbidity meters are an example of a typical instrument that can be used in this application, and work by measuring the amount of light absorbed by the beer, which is considered to be inversely proportional to its clarity. The turbidity level in the beer exiting a centrifuge tends to increase in the periods between discharges as the bowl fills with yeast and more and more yeast is carried over into the clarified beer stream.

From a resource efficiency point of view, the objective here is to discharge the yeast from the centrifuge in as concentrated a form as possible while meeting the clarity requirements of the beer being processed. The more concentrated the yeast is, the less beer there is associated with the yeast, and beer is the valuable resource we wish to preserve in this process. It is also likely that any beer associated with waste yeast will ultimately be disposed of to the brewery effluent system, which would either impose an additional load on the brewery’s wastewater treatment plant or that of the local municipality. 

Centrifuges can typically achieve a maximum yeast concentration of 80% by mass in the centrifuge discharge, and it is common practice to dilute this yeast with water to permit pumping.  It is a simple matter to measure yeast concentration in the diluted yeast slurry (through centrifugation in a laboratory) and then to use the alcohol concentration of the supernatant, together with the alcohol concentration of the beer being processed to back-calculate the yeast concentration at the centrifuge discharge. This should generally be done at defined volume intervals over the course of a batch, along with measurements of the yeast count in the incoming beer (expressed as 10^6 cells/ml).   

The factors (outside of design) that impact on the efficiency of a centrifuge are many, and include:

• Beer flow rate – higher flows reduce the residence time in the centrifuge, which reduces separation efficiency. Also, a centrifuge has a maximum solids (i.e. yeast) removal capacity by virtue of the capacity of the bowl and the maximum discharge frequency permitted. If the rate of solids inflow exceeds this maximum removal capacity, there will be excessive yeast carryover into the clarified beer. The solids inflow rate depends on the beer flow rate and the yeast concentration in the beer being processed. This latter factor can vary significantly over the course of a batch – see my comments about this later.

·    Inlet pressure – high inlet pressures can lead to excessive losses of product, since effectively the volume discharged increases above that desired. Yeast and clarified beer are forced out of the bowl during discharge instead of yeast only.

·    Turbidity set point – clearly, higher set points promote a thicker yeast discharge from the centrifuge. Remember however that the clarity requirements still need to be met.

·    Discharge volume – modern centrifuges are equipped with timers which control the volume discharged (obviously assuming a fixed beer inlet pressure).

·    Operating water pressure – there needs to be sufficient pressure to keep the bowl closed when required, and to close it as quickly as required.

There are also equipment-related factors that can cause the bowl to leak, though such problems can be detected by alarms (e.g. an alarm indicating low bowl speed due to the braking action of the leaking product or an alarm highlighting a reduced back-pressure) and of course, the entire control philosophy employed relies on instruments that are working correctly. A useful way to track centrifuge losses is to measure the frequency of operation of the yeast removal pump, with frequent running indicating large losses. It should be obvious from these points that the various controls available in operating a centrifuge have to be considered in concert.

So what has all this got to do with the need for a systems approach you ask? Centrifuges are typically employed as part of a much bigger process system. For example, consider the use of a centrifuge downstream of fermentation in the lager brewing process. As shown below, the centrifuge would remove yeast remaining in fermented beer, which is then transferred into the storage/maturation process. After storage, the beer is typically filtered and then filled into containers.

Maximisation of the yeast concentration in the discharge of the centrifuge would require the use of a high turbidity set-point, implying significant carryover of yeast into the clarified beer stream.  While some yeast is required for the storage/maturation process to remove residual oxygen and for continued flavour development, too much yeast can cause a range of quality problems, including:

• Beer haze (cloudy beer and which results in excessive filter circulation, reduced throughput and increased energy and powder consumption at filtration);

• Foam problems (since yeast contains proteolytic enzymes which can be released into the beer as the yeast autloyses – beer foam is principally comprised of high molecular weight protein materials);

• Yeasty off-flavours in the final product, which affect drinkability.

Yeast transferred to storage along with the beer being processed will continue to sediment out, and a portion (that which settles out) is typically removed prior to filtration. However, some yeast will remain in suspension and excessive yeast carryover from the centrifuge will translate into a higher yeast count at filtration, and hence a more rapid increase in filter inlet pressure than would be the case with lower yeast counts (assuming the brewery uses constant-flowrate powder filtration). This results in increased product loss at filtration, and overall, losses could therefore be far more than the product “saved” through operating the centrifuge at a high outlet turbidity. In addition to product loss, filtration problems would increase the usage of filter powder (a non-renewable resource), increase the cleaning cycles required per unit volume of beer filtered (incurring increased cleaning chemical and energy usage) and would also increase the electrical energy required (as pressure drop across the beer filter bed increases due to increased yeast load, so does the power required to maintain the required beer flowrate). The point is that the "optimisation" of a centrifuge in isolation can be to the significant detriment of the system within which that centrifuge operates.

Taking this thinking further, this "systems approach" should also extend upstream of the process being optimised. Yeast management practices upstream of the centrifuge can have a significant impact on the resource efficiency of the centrifugation process if we consider that the amount of yeast carried over to storage/maturation can be considered a limiting factor, or constraint. The extent of this carryover depends on the turbidity setpoint employed, as discussed earlier, but also depends heavily on the amount of yeast that is in the beer being processed. Yeast flocculates and then sediments to the bottom of the fermenter once the nutrients required for yeast growth are removed from the green beer. The majority of this yeast is typically removed some time before the beer is transferred through the centrifuge.  Immediately prior to transfer, it is good practice to remove additional sedimented yeast. If this is not done, the first portion of beer being centrifuged will contain an excessive amount of yeast, and even with discharges at maximum frequency, a centrifuge will not be able to achieve the targeted turbidity level since the bowl will fill very quickly. The result could therefore be excessive yeast carryover. As a consequence, the only way that the overall yeast count at maturation could then be achieved would be to run the centrifuge at a low outlet turbidity for the remainder of the transfer, which translates into discharge at low yeast concentration, excessive ejections and therefore excessive loss of product. One control philosophy could be to use an algorithm to reduce the beer flow rate based on a combination of outlet turbidity and discharge frequency (discharge volume could also be varied depending on the capabilities of the centrifuge concerned), but it is still necessary to use good yeast management practices, since otherwise overall brewery throughput could be compromised.  This is particularly true where the transfer process is a constraint.

In conclusion, process losses at breweries are not just about minimising losses in each area, but about finding the right balance between individual areas such that the losses in the overall system are optimised. This logic extends beyond the few limited areas discussed here, and can in fact even be extended to the balancing of  losses in the manufacture of brewing raw materials such as malted barley with how those losses impact on subsequent process losses in brewing. The value of a systems approach in resource efficiency cannot be underestimated, and I will illustrate this further in future posts using examples from other industries. The implications for manufacturing organisations as regards how sustainability performance is measured and managed are fascinating and often counter-intuitive.

Copyright © 2013, Craig van Wyk, all rights reserved