Best energy consumption

When it comes to achieving the best energy consumption, what are the key factors a cement producer needs to address? In this article, extracted from the newly published Cement Plant Environmental Handbook (Second Edition), Lawrie Evans presents a masterclass in understanding and optimising cement plant energy consumption. By Lawrie Evans, EmCem Ltd, UK.

As control of sources, generation, distribution and consumption of energy is central to many current world issues, controlling the industry’s energy footprint is a matter of intense interest to governments. This is recognised in such initiatives as ISO 50001, the World Business Council for Sustainable Development’s Cement Sustainability Initiative, Energy Star in the USA, PAT in India and CO₂ taxes/trading in Europe and in other countries.

For the cement industry, there are three main drivers to energy consumption:
• electrical power
• fuel
• customer demand for high-strength products that require a significant proportion of high-energy clinker as a component.

For the producer, these factors have a significant influence on cost competitiveness, usually accounting for over 50 per cent of total production costs, so that accurately and continuously monitoring energy usage must be a way of life for any producer’s technical team. The introduction of CO₂ taxes in Europe and elsewhere adds a further twist to the story. For major groups, especially, decisions made in balancing maintenance, investments, operations and purchasing requirements all have to take into account the impact on their energy footprint.

Global scene

Globally a cement major such as Italcementi consumes annually some 6000GWh of power and 35,500,000Gcal of heat for a total of 5Mtpe. This is the same total energy as consumed by approximately 1.6m Italians or 0.6m Americans per year. For fuel-related energy costs, the worldwide industry has largely moved to efficient preheater/precalciner processes and has found many options to switch to cheaper fuels, with the global drive to alternative fuels still proceeding. For electrical energy, options to reduce unitary costs are much more limited in scope. Most countries still have power generation/distribution systems that are effective monopolies and the cement producer’s cost control capability is usually limited to selecting the appropriate contract and taking opportunities offered in lower-cost off-peak power tariffs, where they exist.

Figure 1 illustrates the wide variation in the cost of power across 14 countries. The average country cost of electrical power at an industrial level varies enormously. When the added complexity of on and off peak power costs, interruption clauses, supply charges versus energy charges, etc, are added, the evaluation of the benefits of energy saving investment can become very complex. Typical cement plant power costs can range from EUR39 to EUR170/MWh.

Mill designs

The most important first step in controlling energy consumption is to be aware of the relative importance of the process areas where most energy is consumed. Figure 2 shows a typical breakdown of electrical energy consumption at a cement plant. The most obvious area for attention is that of grinding, both raw and cement. In either case, grinding is, by design, a very inefficient process.

The ball mill has been the industry’s workhorse for over a century and despite its estimated meagre four per cent efficiency, little has changed over the years other than increases in the wear resistance of mill internals and the scale of the equipment. The addition of closed circuiting and progressively higher efficiency separators has improved cement product quality and produced higher outputs for a given mill size, but the case for adding or upgrading separators on energy saving alone has proved to be poor, unless the products are >4000Blaine. Starting from the 1970s, a new generation of mills appeared. Vertical mills (see Figure 3) were common for solid fuel grinding, generally with spring-loaded rollers. The principle of the new generation of vertical mill was to direct higher pressure from the grinding element to the material bed using hydraulic systems. From this approach the roller press, CKP (pre-grind vertical rollers) and Horomill ™ all developed.

Raw milling

The gas-swept vertical mill quickly became the raw mill of choice. Grinding energy was approximately 50 per cent of the ball mill and the drying capabilities allowed direct processing of materials of up to 20 per cent moisture content. The main energy issue was the high power consumption of mill fans, with pressure drops of 100mbar not uncommon with high nozzle ring velocities (>70m/s) and internal mill circulating loads of >1000 per cent. Manufacturers have countered this generally satisfactorily with pressure drops reduced by lower nozzle ring velocities and the addition of external spillage elevator recirculation systems plus higher-efficiency separators.

Better seal designs for mill roller assemblies and pull rods have reduced the inevitable inleaking air issue and its impact on power consumption. However, it remains a design where issues of wear and reliability are more challenging than for ball mills, and these issues have not diminished with increased scale. For raw grinding with relatively dry raw materials, the combination of the roller press and V separator is a viable alternative with far lower mill fan power.

Cement grinding

For cement grinding, the technology development away from ball mills has taken a different route. The development of roller presses in the 1980s took advantage of the benefits of higher-pressure grinding and many presses were retrofitted to ball mills as pregrinders. The main benefit was seen at lower Blaines as the first generation of presses suffered from stability problems when attempts were made to grind more finely by recirculating separator rejects. These problems are now largely resolved and the combination of a V and third-generation dynamic classifier separators together with a roller press can produce finished cement with high energy efficiency.

The Horomill and CKP systems have also enjoyed some market success and have provided good energy efficiency levels compared to ball mills. The vertical mill option has been slower to enter the cement grinding market. Grinding bed stability problems offered a challenge which the major manufacturers battled with, until finally a significant number of mills began to be installed in the late 1990s, and this has multiplied in the past decade. However, in pure energy efficiency terms, the benefit of grinding power reduction is countered by the very high power required by mill fans. In addition, the absence of the heat generated in a ball mill and the high volume of air required by the vertical mill have required the provision of waste heat from cooler exhausts and/or auxiliary furnaces to dry raw materials and achieve a limited dehydration of gypsum.

A typical comparison of three competing technologies is given in Table 1, demonstrating that an efficient ball mill/third-generation separator, CKP/ball mill/third-generation separator and vertical mill on a typical 4000Blaine limestone cement show little overall difference in energy consumption. Considering the higher capital cost, and more demanding maintenance and operating regime, there is no clear energy case to favour some of the modern variants.

Other mill debates

Even for solid fuel grinding, there has been a minor trend back to ball mills. This is most evident for petcoke grinding, where the demand for very low residues, and the very hard and sometimes abrasive nature of high-sulphur cokes has resulted in ball mill selection.

Many of the grinding design issues, which are still under debate, are usually very clear in other areas of process selection:
• high-efficiency process fans and low-pressure drop preheaters
• adequately-sized bag filters for the main exhaust to avoid high pressure drops and poor bag life
• avoidance of pneumatic transport systems
• low-energy raw meal homogenisation silos.

The main continued discussions are those of two- or three-fan systems for the raw mill/kiln or single filter for kiln and cooler, precipitator or bag filter for the cooler exhaust and two or three tyre kiln. For a bag filter on a separate cooler the main equipment energy efficiency issue is the air-to-air heat exchanger, but this is often substituted with a water spray in the cooler or more recently, by using a ceramic filter capable of operating at above 400°C.

Finally, in design terms, the most difficult decision is to avoid overdesign by applying too many safety factors. Post-commissioning audits often uncover a high contribution to poor energy efficiency from under-run equipment operating where it cannot perform efficiently.

In normal operations maintenance also plays a major part in ensuring energy efficiency. The impact of poor plant reliability upon overall electrical energy consumption is often under-estimated. In the kiln area, 100 short/medium stops (30 minutes to eight hours) per year can cost up to 5kWh/t clinker. The avoidance of inleaking air, correct alignment of motors, stopping compressed air leaks, etc are all part of the value of good maintenance.

In the key area of grinding there are important factors to control. For ball mills, ball charge level, lining and diaphragm condition must be monitored and maintained in near-optimum condition. Mill stops, defined as mill motor off, and measured by mean time between failures (mtbf), are frequently poorly recorded and the resolution of underlying issues is frequently not addressed.

Instability, where ball mill feed is stopped and the mill ground out, is also infrequently recorded or acted upon. When it comes to mill control, operators rarely concentrate on pushing mill production when the kiln is regarded as the key. Expert systems on mills should be universal and well tuned.

Grinding aids can give benefits of 5-15 per cent in production but need to be continuously evaluated for cost effectiveness. Unfortunately, their cost has risen more rapidly than the cost of energy in recent years and the economic balance has to be re-evaluated. The benefit of aids on cement flowability has to be considered, along with the added scope for reduction of cement clinker content with some modern additives. Correct timing on the maintenance of a first chamber cement mill lining and the successful implementation of an expert system on a cement mill both offer benefits in terms of power consumption (see case studies panel). Accurate process measurements are also key to energy saving opportunities. Air compressors are another area for attention. Often, these are multiple units operating on a cycle of on- and off-load. Replacement of one (of three) with a variable-speed type (see Table 2) can provide rapid payback. Even lighting and buildings offer excellent opportunities for power savings. Table 3 shows the 40-80 per cent energy savings that can be achieved by simply replacing old lighting systems. Buildings such as the new Italcementi Group Research and Innovation Centre (i.lab) in Bergamo, Italy, demonstrate that good building design creates significant savings.

Power generation

A major change has occurred in the last 20 years in the area of in-house generation of electrical energy by cement manufacturers, most significantly using waste heat recovery (WHR) from the pyroprocessing line. Figure 4 shows the areas suited to heat recovery for power generation, and WHR technology is already applied to preheater and cooler exhausts.

The modern technology originated in Japan in the 1980s, where high power prices and large-scale operations combined to produce useful economic returns, with most applications using steam boilers at the preheater exhaust. Little further development happened outside Japan until the turn of the century, when a combination of lower capital cost, Chinese equipment, and the idea to improve recovery by splitting cooler exhausts into higher and lower temperature streams combined to offer the paybacks necessary for the technology to expand, first inside China and then beyond.

The results of WHR have been impressive, eg, with the 19MW net achieved from a combined installation on two five-stage precalciner kilns (5500tpd and 7500tpd) in Thailand being typical. Options for the technology are evolving with other thermodynamic cycles being applied:
• steam Rankine cycle with various enhancements – the most widely applied technology
• organic Rankine cycle – a variety of organic fluids applied and favoured at
• lower gas temperatures
• Kalina (ammonia/water) cycle
• supercritical CO₂ cycle.

There are also further developments which can increase the power recovered, including recycling the lower temperature cooler exhaust, meal bypassing preheater stages to boost exit temperatures and the use of alternative fuels and excess air, also to boost preheater exit temperature and energy recovery. Other options for power generation can use the land owned by the cement plant for raw material reserves. These include wind farms photovoltaics, concentrated solar panels or growing and burning biomass either to boost power in a WHR system or for use in an internal, stand-alone power generation plant.

It is clear that the issues surrounding optimum electrical energy efficiency for a cement plant continue to be an active and exciting area for future development.

The unabridged version of this article was first published in International Cement Review, February 2015.