Improving energy efficiency in cement manufacturing
The industry has reduced its overall specific electrical energy consumption (SEC - Electricity), in cement production from 80 kilowatt hour/tonne of cement (kWh/t cement) in 2010 to 76 kWh/t cement in 2017. The steady increase in the production of blended cement (72 per cent) in 2016 and 2017 and improvements in the clinker factor have mostly driven this reduction. A decrease in clinker quantity in cement implies lower energy consumption for grinding. Despite increased use of petcoke and alternative fuels, frequent stop-starts due to market conditions and low capacity utilisation, the specific electrical energy use of clinkerisation (kWh/t clinker) decreased by nearly 1 kWh/t clinker compared to the baseline (2010).
The thermal SEC increased to 744 kcal/kg clinker in 2017 compared to 725 kcal/kg clinker in 2010. The increase in thermal SEC can be attributed to increased petcoke use, increases in alternative fuel use and frequent start/stops due to low market demand.
Market changes & improvements observed From 2010-2017, we have observed the following sectoral dynamics with respect to electrical and thermal energy-efficiency improvements.
Preheater and kilns: companies made a number of improvements in preheaters and kilns, such as the installation of platforms and alternative fuel feeding systems, nitrogen oxides (NOx) control systems, waste heat recovery from kiln shell radiation, energy-efficient blowers for kiln and calciner coal feeding systems and optimizing phase density, the pre-mixing of fuel and raw meal to improve heat transfer and reduce NOx, and modifications in cyclones based on computational fluid dynamics (CFD). The installation of alternative fuel feeding systems and NOx control systems results in increases in SEC.
Latest generation high-efficiency clinker coolers: improvements include the installation of high-efficiency clinker coolers by certain plants, recirculation systems to maximise cooler recuperation efficiency and excess power generation through WHRS installed on the cooler side, heat shields to improve cooler recuperation efficiency and the installation of latest generation efficient cooler grates.
Grinding systems: improvements include secondary classification in the grid cone, the use of CFD to improve classification efficiency, multi-drive systems, vibration-based sensors for filling control, the adoption of high-pressure grinding rollers (HPGR) instead of vertical roller mills (VRM) for slag and cement grinding, the optimization of air flow by providing angular flow channels below rollers, and the installation of latest generation classifiers.
Process fans: the major improvements observed are the installation of high-efficiency fans, preheater systems with fan SEC as low as 4 kWh/t clinker, and the use of slide gates instead of dampers for major fans with variable frequency drives (VFD).
Auxiliary equipment: auxiliary equipment such as vertical conveyors, tri-lobe blowers, three-phase transformers for increased collection efficiency in electrostatic precipitators, lower head pumps to cool water circuits with booster pumps for specific application, aluminum piping to reduce the pressure drop, water-cooled condenser coils for packaged air conditioning and the installation of screw chillers instead of compression chillers.
The Indian government announced PAT, an innovative, market-based trading scheme, in 2008 under its National Mission on Enhanced Energy Efficiency (NMEEE) in the National Action Plan on Climate Change (NAPCC). PAT aims to improve energy efficiency in industries by trading in energy-efficiency certificates in energy-intensive sectors.
Under the PAT scheme, designated consumers (DCs) are assigned targets for reducing their specific energy consumption. The target reduction for each DC is based on its energy-efficiency quotient during the baseline year (2010), such that energy-efficient DCs have a lower percentage reduction target compared to those that are less energy efficient.
The first PAT cycle period ran from 2012 to 2015 and included 85 cement plants. The target allocated to these plants was to reduce energy consumption by 0.815 million tonnes of oil equivalent (MTOE). The plants in PAT cycle 1 surpassed the energy saving targets and achieved savings of 1.48 MTOE, which is around 81 per cent higher than the savings target.
Under the PAT scheme, energy from alternative fuels is accounted for as zero. During PAT cycle 1, the most significant contributions were from the use of alternative fuels and waste heat recovery. PAT has further enhanced energy efficiency in the cement sector and, as a result, the sector is currently among the best globally. Some of the key measures that cement manufacturers have taken in India to achieve their PAT targets include: the installation of co-processing platforms and pre-processing systems to feed-in alternative fuels, resulting in reductions in carbon emissions; and the upgrading of conventional clinker cooler systems with latest generation ones.
PAT cycle 2 will cover 111 cement plants. The average reduction target for the sector in cycle 2 remains similar to that in cycle 1 (4-5 per cent). It will be challenging for the sector to achieve these targets since most of the easy changes have already been made. The appropriate pricing of Energy Saving Certificates (ESCerts) is crucial to ensuring the continued effectiveness of the PAT scheme.
Challenges to implementation
Over the last seven years (2010-2017, Indian cement companies reported several technical, financial and regulatory barriers to the adoption of technological advancements to improve electrical and thermal energy efficiency. Factors such as layout constraints, high moisture content in limestone and the burnability index of raw mix pose technical barriers for preheaters and kilns.
One of the major limitations in the installation of high-efficiency clinker coolers is the uncertainty around estimating the guaranteed benefits for retrofit installations. Additional shutdown time requirements for retrofits is another. In financial terms, incremental costs for new installations and overall costs for retrofit installations are also a challenge.
In terms of technology, there are still a few challenges, such as the capacity limitations of a roller press, whereas possibilities of higher efficiency are by voltage regulator module. Retrofitting costs to upgrade grinding technology are very high and have long payback periods, i.e., 6 -10 years (if only energy savings are considered). Challenges associated with retrofitting uniflow burners with advanced multi-channel burners include high costs and long payback periods. We have observed that many plants have installed the latest energy-efficient burners, which have shown good reductions in NOx generation and primary air consumption as low as 3.5 per cent.
In terms of energy-efficiency improvements in process fans, a few technical challenges are associated with layout in facilities where the ideal duct system cannot be accommodated. Moreover, the cost of retrofitting in certain cases could be high. Higher investment and operating costs for the latest auxiliary equipment could be a deterrent.
Examples of energy-efficiency improvements with the latest technological interventions:
The Indian cement industry is a pioneer in adopting the latest technologies to improve productivity and energy efficiency.
A plant in Madhya Pradesh has upgraded its conventional clinker cooler with the latest generation energy-efficient clinker cooler. This has resulted in reductions of 15-20 kcal/kg of clinker in specific heat consumption. The power consumption of the cooler section was also reduced by 0.59 kWh/t of clinker.
A company in Chhattisgarh has commissioned a new clinkersation line of 10,000 tonnes per day (tpd) with a six-stage preheater. The new clinkerisation unit is a state-of-the-art plant that is highly automated and has a latest generation energy-efficient clinker cooler. The specific heat consumption of the kiln is 697 kcal/kg clinker and the specific energy consumption of the kiln section is 13.0 kWh/t clinker.
A plant in Rajasthan has installed a medium voltage variable frequency drive (VFD) instead of a grid rotor resistance (GRR) for a kiln hybrid bag house fan, which has resulted in savings of around 160 kW. The investment incurred was Rs 6 million, with a simple payback period of 10 months.
SOURCE: Excerpts from the Low Carbon Technology Roadmap for the Indian Cement Sector: Status Review 2018, published by World Business Council for Sustainable Development (WBCSD).