A modern cement plant burns fuel to raise limestone-based raw meal to sintering temperature in a rotary kiln, and it recovers much of the heat internally. Kiln exhaust gas preheats the incoming meal in a tower of cyclones, and air that has cooled the clinker returns to the kiln and calciner as combustion air. Even so, two streams leave the process with useful energy: the gas at the top of the preheater and the excess air vented from the clinker cooler. Energy audits of dry-process kilns consistently identify these streams, together with radiation from the kiln shell, as the main heat losses (Engin and Ari, 2005; Madlool et al., 2011).

Generating electricity from the two gas streams is an established practice, recognised in the European reference document on best available techniques for the sector (Schorcht et al., 2013). Steam cycles are the usual choice for large kilns with hot exhaust. The organic Rankine cycle, or ORC, replaces water with an organic working fluid of lower boiling point and is favoured where the source is cooler or the unit smaller (Tchanche et al., 2011; Quoilin et al., 2013). An early ORC installation on clinker cooler air at a German cement works was described by Legmann (2002).

A plant engineer or consultant asked to judge such a project faces a sequence of decisions, each of which is treated somewhere in the literature but seldom in one place. This article arranges them into a procedure. It is methodological: it proposes what to do and in what order, and it does not report the assessment of an actual kiln.

An ORC has the same four processes as a steam Rankine cycle: pumping, evaporation, expansion and condensation. Organic fluids differ from water in ways that suit modest temperatures. Many have a saturated vapour line with positive slope on a temperature-entropy diagram, so the vapour leaves the expander superheated without any need for superheating at the inlet, and droplet erosion of the turbine is avoided. Their low enthalpy drop permits simple, slow-running turbines with few stages. The technology and its history are surveyed by Colonna et al. (2015) and in the handbook edited by Macchi and Astolfi (2017).

Studies of cement plants have compared the options. Khurana et al. (2002) drew up an energy balance for a plant and estimated the cogeneration potential of its waste streams. Wang et al. (2009) compared several power cycles for cement waste heat on the basis of exergy and optimised their parameters. Karellas et al. (2013) analysed a water-steam cycle and an ORC for the same duty and concluded that the steam cycle is the more efficient when the exhaust gas is hot, with the ORC gaining the advantage as the source temperature falls. The procedure below takes these studies as its foundation.

The two sources differ in character. Preheater exit gas is the larger and steadier stream. Its temperature depends chiefly on the number of cyclone stages, since each additional stage transfers more heat to the meal and leaves cooler gas, and it carries fine raw meal dust that is soft but abundant. Cooler vent air is cleaner in composition but carries abrasive clinker dust, and both its flow and its temperature swing with the state of the clinker bed. Some plants extract air from an intermediate point along the cooler to obtain a hotter stream for recovery, at the cost of modifying the cooler. These differences govern the design of the heat exchangers and explain why the two streams are usually treated separately until the choice of layout in Stage 3.

The procedure has five stages, shown in Figure 1 with the output that each passes to the next. The stages are sequential, but a result at a later stage, such as an unacceptable cost, will often send the assessor back to an earlier one.

The weight given to each stage will vary. At a plant with wet raw materials, Stage 2 may show that little preheater heat is left and the project reduces to the cooler air alone, which is cooler and more variable; an ORC is then the natural candidate and the comparison with steam can be brief. At a large kiln with dry feed and a short preheater tower, the exhaust is hotter and the steam reference case in Table 1 must be worked through with the same care as the organic fluids. Plants that already have a long record of process data can complete Stage 1 at a desk. Others will need a measurement campaign, and the uncertainty of gas flow measurement in large dusty ducts should be carried through the later stages and not hidden.

The procedure can also be scaled to the decision at hand. For a first screening, Stages 1 and 2 with a generic conversion efficiency are sufficient to show whether a project deserves study. The full sequence is intended for the feasibility stage that precedes requests for tenders.

The procedure has been assembled from published methods and has not been tested by applying it to a plant. Its usefulness would be shown by a comparison of its predictions with the measured output of an operating unit, in particular the annual energy figure of Stage 4, which depends on part-load models that are less mature than design-point calculations (Lecompte et al., 2015). Steady-state modelling may misjudge a source as unsteady as cooler vent air, and dynamic simulation might be required. The procedure also treats the ORC in isolation. Changes to the kiln line itself, such as an additional preheater stage or a more efficient cooler, reduce the waste heat and may be the better investment, and reviews of energy saving in cement production set waste heat recovery alongside such measures (Madlool et al., 2011). An assessment should state that alternative openly.

An organic Rankine cycle on cement kiln waste heat is proven technology whose merit at a given plant depends on local facts: how much heat remains after drying, how steady it is, what electricity costs and what cooling is available. The five-stage procedure proposed in this article orders the questions so that those facts are established before a cycle is designed, and so that the cycle is judged on annual net electricity and cost and not on design-point efficiency. It is offered as a framework for feasibility work, open to correction when tested against operating installations.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.