Swiss Roll Combustors

What is Swiss Roll Combustor Technology?

The Swiss Roll combustor is a burner core surrounded by a spiral heat exchanger (Figure 1 Left). The concept was first proposed by Professor Weinberg in 1974[1].  The cold reactants travel into the center reaction zone through a spiral inlet channel.  After the exothermic reaction, the hot products travel out through the adjacent spiral outlet channel. The heat from the products is exchanged to the cold reactants via the spiral heat exchanger. The reuse of products’ thermal energy recirculated back into reactants results in an excess enthalpy process that raises the temperature above the classically defined adiabatic temperature, as shown in Figure 1 Right. Such conditions enable self-sustained combustion in either ultra-lean or ultra-rich conditions that are beyond the flammability limits without heat recirculation. 

Figure 1. Left: Schematic of Swiss-roll combustor. Right: Illustration showing how super-adiabatic combustion temperatures can be achieved.

Figure 1. Left: Schematic of Swiss-roll combustor. Right: Illustration showing how super-adiabatic combustion temperatures can be achieved.

Many studies have demonstrated the Swiss Roll’s superior performance in heat recirculation and its effect on promoting internal reactions [2]. Specifically, it has been shown effective at extending the flammability of a variety of fuels and enabling the achievement of equilibrium state concentrations i.e. fully complete combustion. 

WHICH FEATURES MAKE THE SWISS ROLL COMBUSTOR EXTREMELY THERMALLY EFFICIENT?

  • A large internal heat transfer to external heat loss area ratio enables highly efficient heat exchange in a very confined volume 
  • The curvature of the channel creates a centrifugal instability (Dean vortices) that further enhances heat transfer 
  • The reaction zone, which has the largest potential for heat loss, is surrounded by the spiral heat exchanger so most of the heat loss from the reaction can be recovered 
  • Long inlet channels with elevated temperatures provide a long residence time that may significantly reduce the formation of non-equilibrium state products 
Figure 2. Three dimension CFD modeling . Left: Boundary conditions. Right: Mesh file.

Figure 2. Three dimension CFD modeling. Left: Boundary conditions. Right: Mesh file.

ACT has been developing Swiss Roll-related technologies together with Professor Paul Ronney at the University of Southern California (USC). Extensive experimental work and detailed CFD-based modeling (Figure 2) has been performed to understand the effect of heat recuperation on the chemical reaction. A fuel reformer and incinerator are currently being developed. Unique material requirements drive innovation in fabrication methods including high-temperature brazing (Figure 3 Right) and 3D printing of metal reactors (Figure 3 Left). Novel additive manufacturing methods for advanced ultra-high temperature materials are currently under development. 

Figure 3. Different fabrication methods of Swiss-roll combustor developed at ACT. Left: 3D printing (stainless-steel). Right: Braze (Inconel).

Figure 3. Different fabrication methods of Swiss-roll combustor developed at ACT. Left: 3D printing (stainless-steel). Right: Braze (Inconel).

WHERE CAN SWISS ROLL TECHNOLOGY BE USED?

The unique, highly efficient operation and extension of flammability make Swiss Roll combustors an excellent solution for a multitude of thermal processes, some of which include: 

  • Self-powered incineration of harmful VOC and other fuel fumes that normally would not combust due to too small a concentration 
  • Ultra-efficient combustion process with near 100% fuel destruction 
  • Stabilization of intermittent combustion processes thanks to the substantial thermal mass of the solid 
  • Efficient radiant heat source stemming from super-adiabatic temperatures and enhances gas-solid thermal coupling
  • Autothermal fuel reforming uses energy within the fuel itself to drive the process 

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[1] Lloyd, S. and Weinberg, F., “A Burner for Mixtures of Very Low Heat Content,” Nature, 251, 47-49 (1974).
[2] Jones, A., Lloyd, S., and Weinberg, F., “Combustion in Heat Exchangers,” Proceedings of the Royal Society of London Section A, 360, 97-115 (1978).
[3] Chen, C. and Ronney, P., “Three-dimensional Effects in Counterflow Heat-recirculating Combustors,” Proceedings of the Combustion Institute, 33, 3285-3291 (2011). 

Swiss-roll Volatile Organic Compound (VOC) Incinerator

Non-catalytic Reforming

  • Partial oxidation or fuel-rich reforming: The simplest way to convert fuels to hydrogen-rich syngas is by using air as an oxidizer. The products of this process can then be directly utilized by Solid Oxide Fuel Cells (SOFC), used in chemical processes, or mixed with other fuels.  Hydrogen (H2) and carbon monoxide (CO) are the major equilibrium state products in an oxygen-deficient (fuel-rich) environment.  However, the energy released from the exothermic partial oxidation reaction itself is typically insufficient to adequately raise the temperature necessary to drive the reforming reactions within the available residence time.  Therefore, catalysts are usually needed to accelerate the reaction rate and enable sustained reactions.  The use of catalyst allows the reformate to reach its chemical equilibrium state, where the optimal H2 and CO concentrations are achieved.  However, common fuels, such as JP-8 used in military logistics, contain sulfur, a notorious catalyst poison.  In addition, coking and degradation of catalysts are also technical challenges for catalytic-based fuel reformers.
  • The non-catalytic partial oxidation (Thermal Partial Oxidation or TPOX) reforming: This method can avoid the catalyst issues stated above and efficiently generate the desired reformate composition. One strategy to keep temperatures sufficiently high in TPOX systems is to use pure oxygen as an oxidizer to avoid the additional thermal mass of inert nitrogen in the air. While such a strategy has been incorporated in other industries for large-scale reformate production, air/oxygen separation is hardly practical at the portable scale.  It is possible to use air as an oxidizer by utilizing external energy sources, such as plasma, which promote the reforming reactions.  However, the electricity needs for such a system are subject to efficiency loss during the conversion process, and the need for an additional power source increases the size, weight, and system complexity.
  • Swiss roll heat recirculating combustor: The technology enabling air-breathing TPOX reformers without an expensive catalyst or external power source is the Swiss Roll heat recirculating combustor developed at Advanced Cooling Technologies, Inc.  The combustor effectively recuperates the heat released from the exothermic partial oxidation to the reactants and significantly increases the reaction temperature.  This enables the reformates to reach their chemical equilibrium state without the need for catalysts or external energy sources. Expensive catalysts and complicated subsystems are avoided in Swiss Roll combustors.  The long spiral inlet and outlet channels provide additional benefits for the reforming reaction, including enhanced mixing of reactants and increased residence time of the reaction.  
Figure 1. The flare of reformate from liquid fuel (n-heptane). The blue color indicates no visible soot formation in the reformate.

Figure 1. The flare of reformate from liquid fuel (n-heptane). The blue color indicates no visible soot formation in the reformate.

Figure 2.  Tests with a rich propane-air mixture.  Left: Without reforming reaction in the center, the downstream flare is yellow due to the soot formation of non-premixed combustion of propane.  Right: With reforming reaction in the center, the downstream flare is blue due to the absence of soot formation in an H2 and CO flame.

Figure 2.  Tests with a rich propane-air mixture.  Left: Without reforming reaction in the center, the downstream flare is yellow due to the soot formation of non-premixed combustion of propane.  Right: With reforming reaction in the center, the downstream flare is blue due to the absence of soot formation in an H2 and CO flame.

Figure 3. The Swiss-roll fuel reformer prototype under ultra-rich JP-8/air operation.

Figure 3. The Swiss-roll fuel reformer prototype under ultra-rich JP-8/air operation.

Fuel Compatability of Swiss Roll Combustor Technology

The Swiss Roll combustor is compatible with a wide range of fuels and is insensitive to the majority of contaminants. Furthermore, the compactness and robustness of the design allow for excellent system integration options, where reformed products can be used to improve other characteristics. The high conversion efficiency enables complex fuels to serve as hydrogen transport media. This allows to use of energy-dense fuels such as JP-8 (34.5MJ/L) or carbon-free Ammonia (11.5 MJ/L) to power hydrogen-burning systems without the need for high pressure or cryogenic H2 tanks (<10 MJ/L).  

Figure 4. Comparison of H2 mole fraction between CFD (FLUENT) and 1D reactor (CHEMKIN) modeling. Left: CFD mole fraction contour. Right: CHEMKIN PFR mole fraction profile of the outlet channel.

Figure 4. Comparison of H2 mole fraction between CFD (FLUENT) and 1D reactor (CHEMKIN) modeling. Left: CFD mole fraction contour. Right: CHEMKIN PFR mole fraction profile of the outlet channel.

Swiss Roll Technology Innovation

The Swiss roll reformer concept has been experimentally and numerically demonstrated at ACT (Figure 2 and Figure 3).  Reforming liquid fuels such as n-heptane (Figure 1) and JP-8 (Figure 3) has also been demonstrated. The work on JP-8 fuel is concluded in Patent No. 9595726 for hydrocarbon fuel reforming under fuel-rich conditions. Notably, the reformer used in that project was entirely 3D printed in stainless steel, which enabled high internal complexity, leading to even higher efficiency compared to the non-3D printed reformer. Current work on Swiss roll combustors studies innovative new applications for this technology in unique environments and complex systems, such as undersea energy harvesting. 

United States Patent US 9,595,726 B2

United States Patent US 9,595,726 B2

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Waste Gas Incineration

Many industrial processes create a byproduct of low concentration air contaminants collectively called volatile organic compounds (VOC). Despite the low concentration, VOC emissions remain directly harmful to humans and damage the overall environment due to the large scale of the industries contributing to the VOC production.

Industries Contributing to VOC Emissions

    • Oil and gas processors
    • Chemical plants
    • Landfills
    • Large-scale cattle farming

The Environmental Protection Agency (EPA) provides specific guidelines for addressing VOC emissions. The most common approach is incineration, i.e., burning unwanted hydrocarbons and other VOC in the contaminated air. However, this process requires supplemental fuel as the VOCs are insufficiently concentrated to sustain the flame. Furthermore, a poorly executed incineration can create more harm than good (NOx production from hot spots, improved spreading of unburned VOC that slipped through, etc.).  

How Can Swiss Roll Technology Address VOC Emissions?

Figure 1. Operating principle of heat-recirculating Swiss-roll incinerator.

Figure 1. Operating principle of heat-recirculating Swiss-roll incinerator. VOC-contaminated air (blue) enters the device and is preheated as it travels to the core, where combustion occurs. Hot air and combustion products (green) continue outwards, sharing their thermal energy with the new portion of VOC-contaminated air.

The unique thermal design of Swiss roll allows it to address the identified issues of classical incinerators. The self-enclosing spiral section enables the recycling of the heat from products back into the reactants, as shown in Figure 1. The hotter the reactants, the easier it is to burn and sustain the flame. In technical terms, the excess enthalpy recuperated from the products extends the lean flammability limits in the core, enabling a steady-state ultra-lean operation. The experiments performed at ACT supported this principle, as shown in Figure 2. The ACT lean Swiss roll incinerator was able to operate at almost four times lower fuel load than it is usually possible with using traditional burners. Furthermore, the overall temperature is lower, preventing NOx and other secondary emissions. ACT was able to test a range variety of Swiss roll incinerator sizes (right of Figure 2) and confirmed no change in combustion performance while addressing different ranges of flow rates. In practical incineration cases, the ultra-lean operation dramatically reduces the supplementary fuel requirements and, in certain VOC-heavy cases, avoids the fuel addition entirely.

Figure 2: Results of Swiss-roll incinerator efforts at ACT:

Figure 2: Results of Swiss-roll incinerator efforts at ACT: (left) large 1’ and small 2” units presented side-by-side (right) lean operational limits for propane and biogas fuels proven significantly lower than conventional values over a range of flow rates. That means our incinerator can operate without the supplementary fuel for a significantly wider range of conditions.

The compact, thermally efficient design, with no moving parts, makes the Swiss roll an attractive option for waste gas incineration applications.

Waste Gas Incineration Innovation

ACT has been awarded an ARPA-E Reducing Emissions of Methane Every Day of the Year (REMEDY) project, where a Swiss roll incinerator will be applied to oil field gas flares. The goal is to improve the VOC destruction efficiency beyond the industry-standard 98% level. The remaining 2% represent effectively doubles the flare stack’s effective carbon footprint due to flare gas’s very high Global Warming Potential (GWP). TheA recent climate report by ABC2021 UN Environment Programme Global Methane Assessment identified reducing methane emissions as the most effective method of fighting climate change. The proposed system, shown conceptually in Figure 3, aims to reach 99.9% flare gas combustion, significantly reducing harmful emissions.  

Figure 3: The flare incineration system for ARPA-E REMEDY project:

Figure 3: The flare incineration system for ARPA-E REMEDY project: (left) conceptual view (right) operational principle with flare gas entering directly to the center. The unique thermal design enables 99.9%+ methane destruction efficiency.

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