Taking the heat

Introduction

Waste heat recovery is a way to lower electricity costs, thus strengthening the producer’s position in the international market. In recent years, an increasing number of cement plants have increased their interest in waste heat recovery (WHR). Turboden solutions, based on Organic Rankine Cycle (ORC) technology, help cement producers get the advantages of a WHR plant, while remaining focused on cement production.

Increasing plant efficiency

The electricity generated using the recovered waste heat does not add additional consumption of fuel, nor additional emissions. It is usually consumed inside the plant itself, thus avoiding any grid losses lowering the electricity purchased from the grid, and enhancing the energy efficiency of the plant.

A growing technology

ORC applications are becoming more widely used in WHR within energy-intensive industries, such as metals, cement, and glass. This is due to the ease of operation and efficiency in all operating conditions. In cement plants, where energy efficiency, greenhouse gas emissions, and competition are a constant concern, ORC systems offer the most reliable solution for generating power, by recovering waste heat streams under different working conditions, including dusty flue gas and relatively low temperatures . A growing number of ORC systems in the oil and gas, cement, steel, or glass industries testify to its in-built advantages, due to the working fluid peculiar properties. Minimum operation and maintenance costs also give ORC units a competitive edge in comparison to traditional steam turbines. In addition, the option to run them without water consumption offers a competitive solution when compared to traditional steam-based heat recovery systems.

Plant working principle

The thermal power contained in the cement waste heat streams, such as kiln combustion gas and clinker cooler air, can be converted into electricity using ORC turbogenerators.

The heat contained in the clinker cooler and preheater exhaust gas is typically transferred indirectly, via a thermal oil, saturated steam, or pressurised water circuit, to the ORC plant.

Normally, two separate heat exchangers are installed; their technical features are specifically suited to the different characteristics of the exhaust gas (sticky gas from the preheater and abrasive dust from the clinker cooler) and the high dust content.

The ORC plant produces electricity and low-temperature heat through a closed thermodynamic cycle, which follows the ORC principle.

In the ORC process, which is designed as a closed cycle, the organic working medium is preheated in a regenerator and in a preheater, before being vaporised through heat exchange with the hot source. The generated vapour is expanded in a turbine that drives an electric generator, transforming mechanical power into electricity. Leaving the turbine, the organic working medium passes through the regenerator that is used to preheat the organic liquid before vaporising. This increases the electric efficiency using internal heat recovery. The organic vapour then condenses and delivers heat to the cooling water circuit, or directly to an ambient air trough air condenser. After condensing, the working medium is brought back to the pressure level required for turbine operation by the working fluid pump, and then preheated by internal heat exchange in the regenerator.

The correct organic fluid can be selected depending on power size and temperature level.

The ORC module has a high level of automation and is designed to automatically adjust itself to the operating conditions. Variations on exhaust gas temperatures and flows will only affect the power output, and not the functionality of the system.

Under both normal operating condition and the shutdown procedure, ORC does not need to be supervised by personnel. No additional personnel are needed. The system is controlled using remote monitoring and requires minimal yearly maintenance activity.

The ability to design a WHR solution without water consumption is an attractive feature for cement plants, especially those located in dry areas.

Experience in cement WHR

Turboden ORC plants have been running in cement plants since 2010, when the ORC plant in Cimar, Ait Baha, Morocco, started up. This first unit in the cement industry was followed by five more units, mainly in Europe where cement plants are smaller and automatic systems are recognised as having advantages when compared to steam turbines.

Ciments du Maroc, Italcementi Group, Ait Baha, Morocco

A 2 MWe unit, recovering heat from preheater gas, was installed and started up in 2010. In 2015, the plant was hybridised with a parabolic solar collector, which increased the thermal power input to the ORC, thus producing additional electricity from a renewable source.

Alesd plant, Holcim Romania

A 4 MWe plant, recovering heat from both the preheater and clinker cooler gas stream, was started up in 2012 and has achieved more than 25 000 working hours.

Rohožník plant, CRH Slovakia

A 5 MWe ORC unit has been working since the beginning of 2014. It is connected to both preheater and clinker cooler.

Carpat Cement plant, HeidelbergCement Group, Fieni Romania

A 4 MWe ORC unit was started up during 3Q15. This WHR plant was designed to work without water consumption.

Switzerland

A 2.5 ORC was successfully put into operation at the end of 2016, recovering heat from a clinker cooler gas stream.

Piacenza plant, Cementi Rossi, Italy

A 2 MWe plant is under construction. It is the first direct exchange unit. Start-up is planned for the end of 2017.

Reaping the benefits

Within the cement industry, the working ORC-based WHR plants have demonstrated that there is no need for additional personnel, which allows employees to focus solely on cement production.

Maintenance costs are also minimised in comparison to steam technology, due to the characteristics of the organic fluids, which are not corrosive and lead to dry expansion (no erosion of turbine blades).

The low rpm of the turbine helps to maintain low maintenance costs. No blow down or substitution of heat carrier and organic fluid is needed during the life of the plant. Furthermore, no major overhauls are foreseen in the life of ORC plants (more than 20 years).

In the future, cement processes will be increasingly efficient, thus leaving lower temperature exhaust gas available for heat recovery plants, which can be easily exploited using ORC technology. ORC for cement is typically from 2000 kWe to 10 – 15 MWe.

Innovation

New solutions that aim to minimise investment costs, while increasing electrical efficiency are under study for cement application. In particular, direct heat exchange between exhaust gas and organic fluid will be implemented. Direct heat exchange is already a viable and proven solution for clean heat sources, such as exhaust gas coming from reciprocating engines, gas turbines, and hot rolling mills. Two direct exchange ORCs are working in connection to reciprocating engines in Italy, and one ORC is recovering the heat from a hot rolling mill in Singapore. Other direct exchange ORCs are under construction and in the development phase. The ORC has also demonstrated flexibility when high thermal load variation (e.g. in hot rolling mill application the ORC can operate at 5 – 10% of the nominal load, and it remains in operation also when the furnace is in stand-by).

Starting from December 2014, Turboden has been involved in the European project, TASIO. TASIO’s main object is to develop a new solution for recovering waste heat in energy-intensive processes within the industrial space and transforming it into useful energy. The project addresses the implementation of a full demonstration of a direct exchange heat recovery system for electrical energy generation at the Cementi Rossi plant in Italy. Commissioning is scheduled for 4Q17.

Direct heat exchangers can make use of both known solutions for cement hot gas stream, and features developed for the direct heat exchanger in other applications. The design of the direct heat exchanger takes into consideration the peculiarities of cement hot gas streams, including high dust content (up to 80 g/Nm3), type of dust (sticky or abrasive), and possible peak operation. Advantages include a more compact layout, lower O&M, higher electric output, and lower investment cost. A direct heat recovery exchange solution, if compared to a traditional thermal oil-based solution, will lead to a 10 – 15% increase in net output. At present, the barriers limiting the implementation of direct exchange solutions are related to layout constraints (i.e. the heat exchanger must be close to the ORC skid).

Conclusion

Thanks to its ability to recover heat at low temperatures, even for relatively small plants, together with good electrical efficiency and high flexibility, in addition to the need for minimum management and maintenance, may be the ideal technological solution for the effective and profitable implementation of heat recovery systems within the cement process.

In this context, WHR with ORC technology has been implemented successfully in Europe over the last ten years. As demonstrated by more than 300 working Turboden installations, ORC can be adapted for installation in industrial environment, such as cement plants, and represents an alternative to the traditional and more complicated WHR system with a steam turbine.

This article was first published in World Cement. To receive your free copy, click here.

Read the article online at: https://www.worldcement.com/special-reports/03072017/taking-the-heat/