Coal fuelled recips revisited

26 March 2020

Under a research project being carried out by Nexant and Bechtel, with funding from the US Department of Energy, an innovative concept for using coal slurry as fuel for piston engines is being investigated and a pre-FEED study for a modular power plant is underway. The proposal is to enhance the already high efficiency of the piston engine in simple cycle by employing turbocompounding (not to be confused with turbocharging) and the addition of a “bottoming” cycle.

Advanced natural-gas-fired reciprocating internal combustion engines (colloquially known as diesel engines), with close to 50% electric efficiency (lower heating value, LHV, basis), represent the most efficient technology for burning fossil fuels in a simple cycle configuration. Diesel engines rated up to 20 MWe are widely available for electric power generation, especially with cogeneration, across a wide load range, ie, from peaking and/

or distributed power to utility scale applications with multiple engines. The modular nature of such power plants makes them highly amenable to cyclic duty. In certain geographic locations with limited access to LNG and with smaller grids (eg, island nations), they can be used for base load duty firing diesel or number 2 fuel oil as well.

Interestingly, the first fuel in Rudolf Diesel’s mind for his internal combustion engine was coal dust (1890s). Not surprisingly, though, he quickly switched to petroleum-based liquid fuels, which were easier and safer for his engine. Since then, coal-fueled diesel engine research and development continued in a sporadic fashion in industrialised countries such as Germany, Japan, the United States and Australia. Coal slurry fuel was successfully injected, ignited, and burned

in test engines as well as commercial ones with good stack emissions in studies going back to the late 1960s (especially with locomotive applications in mind). Furthermore, coal-fired diesel engine technology, commonly referred to as Direct Injection Carbon Engine (DICE), can retain the efficiency advantage of its natural gas-fired brethren to a large degree. This has been verified by rigorous calculations, test-bench experimental studies and field experience obtained in engines fired with Orimulsion.

Turbocompounding and reheat

The efficiency of the engine-only (simple cycle) configuration can be further enhanced via turbocompounding and the addition of a “bottoming” cycle. Turbocompounding is different from conventional turbocharging in that the exhaust gas expander (turbine) in the former, in addition to driving the charge air compressor, contributes to the engine shaft output. In turbocharging, the compressor and expander constitute a self-balanced shaft with zero net power output. This distinction is illustrated in Figure 1. The numbers in the figure correspond to a modular power plant design with five engines (16 MWe each) and a hot gas expander with 1400°F inlet. Charge air is compressed to about 75 psia whereas exhaust gas pressure at the turbocharger expander inlet is about 60 psia.

A further enhancement of the turbocompound DICE combined cycle is via “reheat” combustion between the engine exhaust and expander inlet (see Figure 1). This further increases the expander output as well as exhaust gas energy for higher bottoming cycle contribution. The underlying thermodynamics is graphically illustrated in the temperature-entropy (T–s) diagram, Figure 2. In the figure, ideal internal combustion engine and gas turbine processes are represented by air-standard Atkinson and Brayton cycles, respectively. For the same “mechanical compression” pressure ratio (between state points 2 and 1) and cycle maximum temperature (state points 3 and 3A), constant volume heat addition (2 to 3A) of the Atkinson cycle results in higher efficiency via higher overall cycle pressure ratio (ie, higher mean-effective heat addition temperature and lower mean-effective heat rejection temperature). Combining the two cycles in a reheat configuration (the “hybrid” cycle) leads to a better approximation of the ultimate ideal, ie, the Carnot cycle {1-2C-3-4C-1}. The added benefit of the hybrid (turbocompound-reheat) cycle is higher bottoming cycle potential for even better approximation of the Carnot limit (ie, triangular area {1-4-4C-1} vis-a`-vis {1-4A-4D-1}).

Pre-FEED study

Nexant, Inc. and Bechtel Infrastructure & Power Corp. are currently being funded by the US Department of Energy to perform a pre-FEED (front-end engineering design) study for a modular power plant based on the hybrid cycle concept described above. The project is a part of the Coal FIRST (Flexible, Innovative, Resilient, Small, and Transformative) initiative, which aims to develop coal plants of the future that will provide secure, stable, reliable power with near zero emissions. A simplified system diagram of the proposed DICE-Gas Turbine Compound Reheat Combined Cycle (DICE-GT CRCC) is shown in Figure 3.

The coal feedstock is low-sulphur, subbituminous Powder River Basin (PRB) coal (less than 1% by weight sulphur), which is “micronised” and physically “beneficiated” to an ash content of 2.2% by weight on a dry basis. The coal–water slurry burned in DICE is 55% “micronised refined coal” (MRC) and 45% water by weight with an LHV of 14 513 kJ/kg.

There are five DICEs in the plant, each rated at 15.7 MWe and 42.5% (net LHV) efficiency. Natural gas is burned in the reheat combustor (18% of total plant fuel energy input). Net plant output is 106 MWe at a net LHV efficiency of 45.6%. Figure 4 shows a conceptual layout of the plant.

It is highly unlikely that burning coal without carbon capture is a feasible proposition – certainly not in Europe and North America. One additional advantage of the DICE combined cycle with reheat combustion is increased CO2 content of the stack gas (9.5% by volume compared with about 4% for conventional gas turbine combined cycles), which makes it amenable to post-combustion CO2 capture with an amine-based chemical absorption-desorption process, the most mature and readily available capture technology. Rigorous system modelling has shown that generation of enough steam in the heat recovery steam generator (HRSG) to satisfy the demand of the desorption tower (the “stripper”) reboiler (a kettle-type evaporator) requires supplementary (duct) firing. However, the oxygen content of the DICE exhaust gas (about 10% by volume) is insufficient to sustain reheat combustion and duct firing without supplementary air. This adds cost and complexity to the system. Consequently, in order to minimise the requisite engineering development cost and simplify the system, the introductory version of the DICE combined cycle with post-combustion carbon capture does not include a reheat combustor. With carbon capture add-on, DICE-GT CRCC output drops to about 83 MWe with net LHV efficiency of 31.2% (30.4% in higher heating value or HHV).

Building on proven technology

To a large extent, DICE-GT CRCC is based on mature, off-the-shelf technology with some modifications. (One exception is the reheat combustor, manufactured to spec by a qualified supplier, which, however, is not included in the introductory offering.) The stock diesel engine is a medium speed (500 rpm), large bore (460 mm) heavy fuel oil (HFO) fired V18 engine available from manufacturers such as Wa¨rtsila or MAN. The turbocharger module of the engine is removed to make it ready for turbocompounding (see Figure 1). (Charge air to all engines is supplied by the main air compressor – see Figure 3.)

For coal–water slurry fuel, retrofit considerations include atomiser nozzle wear, piston ring jamming, abrasive wear, ignition delay and exhaust valve seat wear. Also important are fuel system issues, eg, blockage, fuel stability and corrosion.

Known solutions to atomiser nozzle wear include diamond compact or silicon carbide nozzles and the use of lower speed engines (with increased time for combustion giving a higher tolerance to poor atomisation). Good atomisation (MRC slurry is like house paint in thickness), low speed and large bore cylinders with large clearances are key to the prevention of piston ring jamming. Abrasive wear can be reduced by plasma-spray carbide coatings. Ignition delay (due to the slow burning characteristics of the MRC slurry) and knocking problems can be rectified by pilot injection of diesel fuel to ensure reliable ignition, particularly at low load.

The bottoming cycle is very simple with a one-pressure (no reheat) HRSG and single-casing, condensing steam turbine with high and low-pressure casings (HP and LP, respectively).

During operation with capture, the LP turbine is idle separated from the powertrain via SSS clutch. Exhaust steam from the HP turbine at 60 psia is sent to the stripper reboiler. Particulate removal is accomplished by a “third stage separator” commonly used in fluid catalytic cracking (FCC) applications for the same purpose to satisfy the requirements of the hot gas expander (also common in FCC applications).

The HRSG contains an SCR/CO catalyst module to scrub NOx and CO from the stack gas. Coal beneficiation can remove the non-organic sulphur from MRC so that SOx in the stack gas can be removed in a direct contact cooler upstream of the capture block.

DICE-GT CRCC can burn coal efficiently (comparable to ultra-supercritical boiler-steam turbine technology available at gigawatt scale) and cleanly at 100 MWe scale without a need for exorbitant investment in R&D.

Past efforts came to naught not due to insurmountable technical difficulties but rather due to wide availability of cheap oil and, later, natural gas for transportation and electric power generation applications. Another factor is the cost of manufacturing MRC and storing the MRC slurry in large quantities to facilitate experimental runs of long durations (eg, 6000 hours of demonstration run planned as part of the US DOE’s clean coal technology programme).

Presently, in the face of rising public opposition to fossil fuels in general and coal in particular, large engine manufacturers in the developed world are (understandably) leery of embarking on coal-fired diesel development. Still, the reality is that coal will continue to be a major energy source in other parts of the world for the foreseeable future. This translates into a lucrative market for DICE-based power plants (especially with large centralised MRC-slurry fuel production facilities akin to oil refineries).

Author: S. C. (John) Gu¨len, Bechtel Fellow

Figure 1. Turbocharging (left) versus turbocompounding (right)
Figure 2. Temperature–entropy diagrams of ideal, air-standard RICE and gas turbine cycle
Figure 3. DICE-GT compound reheat combined cycle (showing state points, see Fig 2)
Figure 4. Conceptual layout of modular power plant

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