Efforts to fully decarbonize the cement industry will require carbon capture. There’s no way around that. But there are ways to make it easier. Oxyfuel technology increases the concentration of CO2 in the exhaust gas and so makes it simpler and less costly to capture. It’s thus currently the most techno-commercially feasible pathway to carbon capture at scale.

Carbon capture: a necessary pathway to decarbonized cement

Clinker production releases large amounts of CO2 as a result of the calcination of limestone.

Unlike energy-related emissions – which can be reduced e.g., by improving energy efficiency and using low-carbon alternative fuels – these process emissions are an unavoidable result of the process chemistry. The only way to avoid them is to capture them. This makes carbon capture, utilization, and storage (CCUS) the technological lever with greatest potential for carbon emissions reduction and a critical pathway to a fully decarbonized cement industry.

But carbon capture is a costly exercise. Part of this is because the CO2 concentration of standard cement plant exhaust gases is fairly low (mostly below 20%), which reduces the efficacy of capture technologies. Raising the concentration of CO2 in the exhaust gas is therefore an important step toward establishing commercially viable CCUS solutions at scale. And this is where KHD’s oxyfuel combustion concept comes into play.

What is Oxyfuel?

The pathway to CCUS at scale

Oxyfuel is essentially a way of operating the clinker production line to maximize the concentration of CO2 in the exhaust gas – and so to reduce the CAPEX and OPEX of carbon capture. This is achieved by replacing ambient air in the process with a mix of recycled preheater off gas and pure oxygen. This creates an oxygen-rich low-nitrogen combustion environment, which produces a CO2-rich exhaust gas, with concentrations increased up to 70% to 85%, depending on the specific plant conditions.

What is Oxyfuel?

Key equipment and processes: an overview

KHD has developed a complete oxyfuel concept, including oxyfuel-adapted kiln burner and clinker cooler solutions, as well as key gas recirculation processes.

  • Clinker Cooler

    The cooler can either be separated from or integrated into oxyfuel operation. Choice of configuration will be dependent on specific site conditions and requirements.

  • Kiln Burner

    Our oxyfuel burners deliver the same flame shape and heat liberation in the CO2-rich combustion conditions of oxyfuel operations as our regular kiln burners do so well in standard combustion conditions.

  • Main Recirculation

    Recirculating the exhaust gas from the preheater into the kiln generates a gas stream with high CO2 concentration that can be fed directly into the carbon capture unit.

  • Cooler Recirculation (Optional)

    Recirculating the cooler vent gas integrates the clinker cooler into oxyfuel operation, increasing CO2 yield and waste heat recovery, and eliminating the need for internal separation of recirculating gas and cooling air.

ISO Plant Illustrations

Diving deeper into the key equipment and processes

Oxyfuel operation is based on the established clinkerization process, although some adaptation of existing equipment in needed (notably of the kiln burners, clinker cooler, and seals) and some new equipment and processes are added (e.g., heat exchangers, air separation units, and gas recirculation cycles). However, plant equipment and processes remain fundamentally familiar, making oxyfuel operation appropriate for implementation at existing cement plants.

Diving deeper into the key equipment and processes

Kiln burners for oxyfuel

Oxyfuel affects combustion conditions in the kiln. Most obviously, the composition of the kiln gas is different as nitrogen is replaced by CO2. This impacts the heat capacity and burner firing rate mass flow due to the higher gas density of CO2 compared to nitrogen. Flames burn hotter, shorter, and brighter with a pure oxygen primary air stream, while fuel ignition characteristics are also altered.

These factors require some adjustment to burner design. To safeguard clinker quality, oxyfuel burners must create identical flame shape and heat liberation to those achieved in regular combustion conditions. Burner tubes and nozzle plates must be able to withstand the changes to the flame profile, and because the firing rate is higher with oxyfuel, a higher degree of control is needed compared to traditional burners.

Based on our process knowledge and longstanding expertise in burner design, we have developed an oxyfuel burner designed to overcome these challenges and deliver a similar heat transfer profile to traditional kiln burners.

Diving deeper into the key equipment and processes

The clinker cooler

Cooler operation in oxyfuel conditions is complex due to the traditional use of ambient air for cooling. This creates high potential for false air ingress into the recirculating gas stream. As a further complication, the cooler also recuperates thermal energy from the clinker for recycling into the kiln.

Latest system designs suggest possible solutions to this issue:

  • The use of a barrier between the front end of the cooler, where an oxyfuel atmosphere is maintained and the off gas is recirculated within the main recirculation loop, and the back end, where ambient air is used to complete cooling in the traditional way.
  • The use of an adapted cooling system with internal oxyfuel recirculation in the second cooling zone. Although requiring higher CAPEX, this option avoids a secondary emission source and so maximizes CO2 capture potential.

Choice of cooler configuration will be dependent on individual plant requirements. Our experts are on-hand to discuss the options and support you to make the best decision.

Conventional sealing in a realistic state of maintenance after several years of operation.

Conventional sealing but all locations equipped with best possible mechanical design, best practice installation, and optimised maintenance resulting in gap width reduction of 50%.

Conventional sealing with realistic gap width reduction from of 25% improved mechanical design, installation, and maintenance.

Sealing zone is isolated from ambient conditions via a hood or housing, which is flushed with recirculating waste gas, preventing ingress of ambient air.

Diving deeper into the key equipment and processes

The false air problem

False air is a challenge in traditional operating conditions, since it increases energy consumption, reduces productivity, and can contribute to process instability. It is however a particularly critical concern for oxyfuel operation because it dilutes CO2 concentration and thus massively increases the cost of downstream carbon capture. Measures are therefore needed to minimize false air indraft. Our solutions include traditional sealing enhancement measures and also innovative new sealing technologies.

Our experience in the design, implementation, and optimization of complete cement plant has given us an in-depth understanding of process gas flows and the challenge posed by false air. We are thus well placed to recommend solutions that reduce air in-leakage to the lowest possible levels and achieve best possible oxyfuel system efficiency.

Diving deeper into the key equipment and processes

Main recirculation

Carbon capture is much more cost effective when CO2 concentration is high. Oxyfuel operation delivers this by increasing CO2 concentration of cement plant exhaust gas to more than 85%. This is achieved by re-routing CO2-rich preheater exhaust gas to the kiln, where it replaces nitrogen-rich ambient air. Oxygen for combustion is carefully controlled to provide optimum burning conditions in the kiln and calciner.

As gas cleaning and preparation is done within the recirculation loop, the result is a gas stream with high CO2 concentrations, ready for the capture unit. Efficiency of the concentration process depends on the appropriate technologies for the kiln burner, clinker cooler, and sealing solutions to prevent ambient air ingress from diluting the recirculated process gas.

Diving deeper into the key equipment and processes

Cooler recirculation (optional)

The cooling process requires more cooling air than can be recirculated to the kiln. Additional cooling air is thus fed into the back section of the clinker cooler. When ambient air is used here, there is a risk this additional cooling air will mix with recirculating air at the transition zone in the cooler, diluting CO2 concentration.

One solution is to separate the recirculation zone from the back end of the cooler using two separate housings or complicated internal separation devices. In contrast, cooler vent gas recirculation allows the complete cooler to be integrated into oxyfuel operation. It’s a low-cost solution that also provides additional waste heat recovery and CO2 yield.

Comparing alternative carbon capture technologies

The most efficient option for carbon capture

Oxyfuel is the most energy-efficient technology for concentrating CO2 in the clinkerization process. That’s not just our opinion: it’s been shown to be so by the CEMCAP project who evaluated and compared the specific primary energy consumption per CO2 avoided (SPECCA) of all main carbon capture technologies.

Existing cement plants can also be converted to oxyfuel operation: another key benefit. And although it’s not yet been applied at commercial scale in the cement industry, the technology involved – from the ASU to the carbon capture unit – it well understood and in commercial operation in other industries.

And when it comes to other carbon capture options? Our engineering and process expertise is not limited to oxyfuel. We can assist you implement your pathway to carbon capture, whether that’s via oxyfuel or another route, such as membrane-assisted CO2 liquefaction (MAL), the chilled ammonia process (CAP), the MEA process, or something else.

Why KHD?

Your partner for
carbon capture

That said, successful implementation of oxyfuel in practice will pose challenges. Each implementation will have to be tailored to the specific plant layout and process conditions. This all highlights the need for an expert partner when undertaking the oxyfuel journey.

Here at KHD, we are committed to delivering the technological innovations needed to achieve Cement beyond Carbon. From alternative fuels to plant efficiency upgrades and advanced digitalization solutions, we have the experts and experience to make it happen. We have also long recognized the importance of oxyfuel technology: our experts have been involved in its development since 2010. So, if you’re looking for a partner to support you when it comes to designing and implementing oxyfuel at your cement plant, look no further.

We also recognize that oxyfuel is not the only carbon capture technology available. From MAL to CAP, the MEA process, or something else, we have the knowledge and skills available to evaluate, design, and implement whatever your preferred technology route.

If you have any questions about implementing oxyfuel or carbon capture at your plant, reach out to a KHD expert today at oxyfuel@khd.com or just hit Get In Touch and complete the contact form. And let’s build the future of cement, together.


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