What’s the best choice?

We all know that cement manufacturing is an extremely energy-intensive process and for exactly this reason final users are always keen to increase efficiency of their production plants. Recover of surplus of thermal energy produced during the clinker manufacturing is an attractive way to decrease the overall energy demand and the technology to convert this heat into power is well known and proven. Steam Rankine Cycles (SRC) cover 99% of the applications of WHR in cement plants worldwide, but are we all sure that SRC is always the right choice to maximize the return of investment? Organic Rankine Cycle (ORC) is recognised as a competitive alternative to steam technology, especially in sizes between 5 –15 MWe and in the last years it is getting more and more considered by stakeholders in cement plants. This is mainly due to few peculiarities of organic fluid technology, such as the capacity of efficiently recovering low temperature heat, performing turbines, no water consumption, zero waste to environment. There are other distinctive features of ORC that can drastically change the financial figures of an investment in WHR, especially in those countries where the cost of labour is significantly high: low operation and maintenance requirements, highly digitalized systems with no need of physical supervision, zero costs for water treatments. Let’s see in detail.

Typical OPEX with SRC

A standard SRC normally requires a staff of 12 to 15 people for daily operation and routine maintenance. Boilers are typically equipped with superheating sections and quality of feedwater (conductivity, Ph, TDS) is fundamental to avoid problems of corrosion, rupture for overheating of SH bundles, lack of efficiency for deposits on steam turbine blades. The water-steam cycle must be carefully operated with dosing of chemicals, daily sampling and lab analysis, proper care of water treatment (demineralizer section in particular). The waste-water treatment of blow-down before disposal to sewer is another point of attention for operators. Of course, maintenance on flue gas side of boiler is another important item, but this is not different from ORC technology. Even if the system is controlled by modern DCS, the presence and experience of operators is fundamental for smooth operation. Additionally, by nature SRC needs annual expenditures for supply of chemicals and water for refilling (if supplied from  deep well close to site, there are still electricity costs for supplying).

For the purpose of comparison between technologies, the following table offers a recap of OPEX calculated for an 8MW SRC of a 5000 TPD dry kiln line in Turkey. Of course, figures are site-specific and unit costs changes country by country and may vary significantly

Financial Figures in WHR investments

The financial figures of a WHR investment depend on multiple factors, mostly site-specific. Among them, the most important are:

  1. Price of electricity paid by the End User and saved with WHR installation. As a rule of thumb, the higher this value, the shorter the pay-back. This makes WHR investments more interesting in countries where the cost of electricity is normally higher than 65-75 $/MWh.
  2. The amount, quality (i.e. temperature) and availability during the year of the heat in waste gases. First these parameters naturally fix which between SRC and ORC is the most performing technology. SRC is usually preferred for large units (e.g. > 15MWel) and high temperatures, whereas ORC gives its best with low temperatures and sizes between 5 MWel and 15 MWel. In addition, ORC offers interesting solution for regrouping multiple kilns under one power unit thanks to an unbeatable efficiency at reduced loads. Second these values impact on CAPEX (how large are the equipment) to generate the expected performances. Third, small kiln plants (i.e.<1 MTon year) rarely offer acceptable pay-back conditions due to an high ratio of CAPEX Vs Power generated.
  3. CAPEX of imported equipment of WHR, which depends on size, technical site-related problems, technology used and of course country of origin. In this regard SRC by Chinese producers are normally cheaper than European/Israelian ORC. In this regard, currency fluctuation may play a heavy role on WHR financials.
  4. Erection costs, depending on the complexity of site installation and local labour costs.
  5. OPEX, that means all the costs coming after first start-up of WHR and connected to system operation and maintenance. Here the technology makes a great difference, as we can see in detail in the following analysis.

Typical OPEX with ORC

ORC typically presents a double close loop with thermal oil at recovery boilers side and organic fluid in the power unit, with intermediate heat exchangers. Both fluids are by nature self-protecting in terms of corrosion of steel, that is not subject to any oxidation, and the two circuits do not need any refilling during normal operation, unless in case of spillage due to accidental damages. During the first filling of the plant deaeration and drying procedures are applied to thermal oil and organic fluid. No other chemical or physical treatments are needed during the entire lifetime of both fluids, normally longer than 20 years. Being close loops, if air cooled condensers are applied, any additional water or waste treatment is needed. Finally, boilers are once-through, single phase fluid, without superheating sections, so very simple in operation and maintenance. Thanks to this simplicity and a high level of digitalization typically applied by ORC producers, a standard ORC does normally not require any additional employee for daily operation and maintenance. The existing staff is normally sufficient, once properly trained to the technology. In addition, OEM may offer remote surveillance services with annual fee.

For the same plant in Turkey used above, the following table offers a recap of OPEX for ORC.

Correct way to evaluate WHR Investment

A proper evaluation of investment in WHR must consider both the Pay-Back Period and the Overall Saving in a medium-long term period of operation. In this regard, both CAPEX and OPEX must be considered carefully and not only the first. Indeed, a simple calculation of Pay-back based on CAPEX generally tends to favour SRC, but in medium-long term period ORC can often result as a winning choice, thanks to lower OPEX.

In the example above, considering a typical inflation rate of 2% on costs, thanks to OPEX the ORC can generate in 10 years of operation a difference in future expenditure value of 3,65M$ respect to an equivalent SRC, that becomes 8,1 M$ in 20 years, that is the expected lifetime of such a plant. This advantage drastically rises in EU and US countries, where cost of labour is much higher than other countries.  In conclusion, even if the CAPEX of SRC can apparently be unbeatable, a careful analysis in a long-term period tends to reward the solution with lower OPEX.

Conclusion

The investment of a WHR must be anticipated by a precise evaluation of business plan in medium and long-term period, where the financial figures can be strongly dependant on OPEX. A highly digitalized and self-operating system like ORC may offer the right solution, especially in countries where OPEX are exalted by the cost of labour. Additionally, in the Post-COVID19, this solution can meet the increasing demand of customers of highly automated systems that can easily enter in remote controlled plants.

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