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 industrys 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 Developments 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 producers 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.
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 producers 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.
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 industrys 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.
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.
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.
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 400C.
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.
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.
Digital solutions do not help on their own. They need to be adopted as part of a bigger digital strategy that best aligns with your business processes and overall goals. This requires a holistic approach involving many disciplines and functional areas.As digital technologies are constantly and rapidly evolving, we invest in developing and integrating new, more effective solutions. We embrace this continuous development together with you, working with you to understand the opportunities for process improvements.
The cement production industry is realising the potential of connectivity-based technologies, often referred to as Internet of Things (IoT) technologies. We see it as the convergence of information and operations technology, providing valuable data about plant equipment. Understanding the advanced sensing and data analytics allows cement plant managers to make better decisions about the operation.
The ability to work with data is opening significant opportunities for process improvements in cement production. One example is condition-based maintenance. As cement plant equipment and processes are monitored digitally, it enables you to react quicker, make smarter decisions on when to repair or maintain equipment based on unique data specific to your operation. This ultimately increases productivity by reducing down-time.
FLSmidth develops customised solutions based on IoT technologies aimed at improving the performance of production equipment. The use of sensors in moving parts and network-based connectivity allows you to gather operating data from the equipment. This can be applied to improve up-time of cement kilns and grinding mills.
It also allows remote monitoring to oversee operations of quarry vehicles, for example, where key metrics such as fuel consumption and operating hours can be tracked. Data from vehicles can be analysed to predict potential failures.
By optimising processes and reducing resources in cement production, you can reduce your operations environmental footprint. Digital tools are not only useful in helping to streamline raw materials and consumables management, waste management and energy management; they are essential to helping your operations comply with local legislation. They help give you a detailed picture of your operations, giving you the opportunity to reduce consumption of energy and other materials.
Our digital solutions range from automation software systems and robotics technology to monitoring systems and advanced data analytics. An example is our advanced process control solutions, which many cement producers use to stabilise and optimise key cement processes. This delivers several potential advantages, such as:
The ability to predict quality is particularly important. High-quality cement requires homogeneous raw meal and consistent operations throughout the plant. Digital technologies enable you to monitor the input materials and adjust the operation of the grinding systems and pyroprocessing system to gain the best output.
As a pioneer in digitalisation in the cement industry, we have developed digital solutions for equipment control, process optimisation and plant optimisation since digital technologies were first introduced into industrial production processes. We help cement producers achieve greater levels of productivity, decrease cement plants environmental impact, and create ever-safer working conditions.
Applying data analytics to improve plant productivity is at the heart of our operations and maintenance services. We offer digital platforms that give easy access to key operational data, enabling you to take appropriate preventive or corrective actions, either automated or provided through remote services.Simulation technologies also allow actions to be tried out in a virtual environment. You can evaluate the impact on performance before implementation, saving costs and reducing risk.
FLSmidth provides sustainable productivity to the global mining and cement industries. We deliver market-leading engineering, equipment and service solutions that enable our customers to improve performance, drive down costs and reduce environmental impact. Our operations span the globe and we are close to 10,200 employees, present in more than 60 countries. In 2020, FLSmidth generated revenue of DKK 16.4 billion. MissionZero is our sustainability ambition towards zero emissions in mining and cement by 2030.
India: NCL Industries is planning to expand its 2.7Mt/yr Mattapalli plant in Suryapet district, Telangana, to 3.6Mt/yr capacity at a cost of US$13.5m. The work includes the installation of vertical roller mills to replace the plants ball mills. Times of India newspaper has reported that the company says that it will complete the expansion by 2022.
Its plan also involves the establishment of a new 660,000t/yr grinding plant at nearby Anakapalle, at a cost of US$26.9m. The producer will invest a further US$810,000 in setting up three new ready-mix concrete plants in Hyderabad and Vizag, bringing its total number of concrete plants in the state to eight.
Mrida, Yucatn, (June 24, 2021).- With a private investment of more than815 million pesosand the generation of455 jobs, this Wednesday 23th, the newgrinding plantof the cement companyHolcim Mxicowas inaugurated in Umn, Yucatn municipality.
The new factory has two raw material reception areas, 2 silos of 500 tons each for limestone and gypsum, and a building with feed hoppers where the ideal mixture is prepared to be fed to the cement mill.
Likewise, a cement mill for 650 thousand tons of cement per year, 2 cement silos of one thousand tons each, a packaging machine with a capacity of 90 tons per hour and a palletizer, and equipment that accommodates the bags on the pallets.
Together with the director of Holcim Mxico, Vila Dosal carried out the unveiling of the plate and the ribbon cutting of the new factory, which is the first of this company to be built in all of Latin America.
Our automatic production line for the grinding cylpebs is the unique. With stable quality, high production efficiency, high hardness, wear-resistant, the volumetric hardness of the grinding cylpebs is between 60-63HRC,the breakage is less than 0.5%. The organization of the grinding cylpebs is compact, the hardness is constant from the inner to the surface. Now has extensively used in the cement industry, the wear rate is about 30g-60g per Ton cement.
Grinding Cylpebs are made from low-alloy chilled cast iron. The molten metal leaves the furnace at approximately 1500 C and is transferred to a continuous casting machine where the selected size Cylpebs are created; by changing the moulds the full range of cylindrical media can be manufactured via one simple process. The Cylpebs are demoulded while still red hot and placed in a cooling section for several hours to relieve internal stress. Solidification takes place in seconds and is formed from the external surface inward to the centre of the media. It has been claimed that this manufacturing process contributes to the cost effectiveness of the media, by being more efficient and requiring less energy than the conventional forging method.
Because of their cylindrical geometry, Cylpebs have greater surface area and higher bulk density compared with balls of similar mass and size. Cylpebs of equal diameter and length have 14.5% greater surface area than balls of the same mass, and 9% higher bulk density than steel balls, or 12% higher than cast balls. As a result, for a given charge volume, about 25% more grinding media surface area is available for size reduction when charged with Cylpebs, but the mill would also draw more power.Get in Touch with Mechanic