Over the past decades, the European lime industry has invested heavily in energy savings technology and continuously improved plants to reduce their energy consumption.
While the heat of reaction, the energy required for the chemical reaction to take place – for a typical quicklime quality – is 3.03 GJ/tonne. Theoretically, the potential for energy efficiency improvement is, therefore, limited to 29%; the rest of the fuel is needed to provide the energy for the chemical reaction. Limestone always contains impurities and a driving force is always required to get the reaction going.
We estimate that implementing existing technology and future innovation could lead to a reduction of 16% in fuel intensity by 2050. This would be achieved mainly by building new – highly efficient – kilns, and retrofitting existing kilns. By 2030, the projected total decrease of fuel intensity is estimated to be 8%.
The main methods of this increased fuel efficiency would be
Switching from horizontal to vertical kilns
Lime can be produced in different kinds of kilns. Over the past decades, new kiln types have been developed and existing kilns have been improved. Newer vertical kilns are considerably more energy efficient than horizontal kilns. In Europe, 80% of lime is produced in vertical kilns. Some existing horizontal kilns have been refurbished, but they cannot achieve the same energy efficiency as a new vertical kiln. In the coming years, old kilns are likely to be replaced by new, more efficient ones.
Installing heat exchangers in horizontal kilns
In horizontal kilns heat exchangers can be used to recover some of the heat from the flue gases produced in the kiln and use this heat to preheat the feed limestone. This could generate – on average – a fuel saving of around 25%. However, this potential reduction is only applicable to horizontal kilns.
Parallel flow regenerative kilns
There are several types of vertical kilns. The Parallel flow regenerative (PFRK) type is the most efficient one. However, they cannot always produce all types of products to fully satisfy market needs and sometimes cannot process the smallest particles; therefore, they are not always the best solution from a resource efficiency point of view. Further innovation could increase the applicability of PFRKs enabling them to handle smaller particles and, thus, making them more resource efficient.
Improved use of waste heat
Waste heat from the kiln can be used to dry limestone or in the milling process. In addition, the waste heat can be used in other industrial processes, in other sectors with a heat demand or they can be used to heat buildings and generate electricity. However, many lime plants are located in remote rural areas, which makes distributing the excess heat to other industries or as a residential heating source difficult at times.
Energy recovery in hydration
The production of hydrated lime and dolime is an exothermal reaction, i.e. it generates heat. The heat resulting from the production of hydrated lime amounts to about 1.2 GJ/tonne of CaO. This heat could be used in industrial processes or heating buildings if there is such a demand in the remote areas where the plants operate. This option will require further R&D to work out how to extract the heat without affecting the production process and quality of the product.
A range of other measures could also lead to increased fuel efficiency. These include efficient kiln insulation lining, optimised combustion processes, improved process and input control, optimal change-over and further improved maintenance procedures.
Significant steps have been taken over the years to reduce our electricity consumption. Increasingly efficient engines and other technological progress have made a real difference but more stringent environmental legislation and increased controls in modern plants have offset some of these gains. Nevertheless, we remain committed to finding ways to reduce our power consumption. The efficiency of motor systems is – conservatively – assumed to have a saving potential of 10%. Optimising cooling and grinding, for instance, could lead to further energy efficiency gains.
This is an extract from the report 'A Competitive and Efficient Lime Industry' published by the European Lime Association.
Read the article online at: https://www.worldcement.com/europe-cis/18122014/a-competitive-and-efficient-lime-industry-42/