Room-Temperature Plasma Hydrogenation of Fatty Acid Methyl Esters (FAMEs)

Abstract

The increasing demand for sustainable energy has spurred the exploration of advanced technologies for biodiesel production. This paper investigates the use of Dielectric Barrier Discharge (DBD)-generated low-temperature plasmas to enhance the conversion of fatty acid methyl esters (FAMEs) into hydrogenated fatty acid methyl esters (H-FAMEs) and other high-value hydrocarbons. A key mechanistic advance is achieved via in situ distillation: at the reactor temperature, unsaturated C18 and C20 FAMEs remain liquid due to their low melting points, while the corresponding saturated C18:0 and C20:0 FAMEs (with melting points of approximately 37–39 °C and 46–47 °C, respectively) solidify and deposit on a glass substrate. This phase separation continuously exposes fresh unsaturated FAME to the plasma, driving further hydrogenation and thereby delivering high overall conversion efficiency. The non-thermal, energy-efficient nature of DBD plasmas offers a promising alternative to conventional high-pressure, high-temperature methods; here, we evaluate the process efficiency, product selectivity, and scalability of this room-temperature, atmospheric-pressure approach and discuss its potential for sustainable fuel-reforming applications.

1. Introduction

Biodiesel, primarily composed of Fatty Acid Methyl Esters (FAMEs), is a renewable and biodegradable alternative to fossil fuels [1,2,3]. However, its widespread application is limited due to poor oxidative stability, cold flow properties, and compatibility with conventional diesel engines [4,5]. Notably, the hydrogenation of biodiesel feedstock to produce Hydrogenated Fatty Acid Methyl Esters (H-FAMEs) has shown promise in improving oxidation stability and engine performance characteristics [1,2].

Non-thermal plasmas have shown considerable promise in hydrogenation and fuel conversion applications. Specifically, Dielectric Barrier Discharge (DBD) technology, a non-thermal plasma generation method, presents a novel approach to overcome these economic and environmental limitations by enabling selective chemical transformations at lower temperatures and pressures [4,6,7,8,9,10,11,12,13,14]. In this paper, we describe a soft-condition, DBD-based system capable of selectively converting FAME to H-FAME at room temperature and atmospheric pressure.

Despite its environmental and technical merits, biodiesel (FAME) remains susceptible to oxidative degradation during storage and handling, leading to the formation of undesirable compounds such as peroxides, aldehydes, ketones, and carboxylic acids [2]. These by-products adversely impact fuel shelf life, engine compatibility, and combustion efficiency. Therefore, research focused on enhancing biodiesel stability through partial hydrogenation, with approaches such as plasma-assisted processing, is increasingly critical [1,15,16].

Plasma-assisted processing via non-thermal plasma like DBDs has emerged as a breakthrough technology capable of hydrogenating FAMEs while eliminating the need for expensive catalysts [6,17,18]. These electrically driven plasmas produce reactive species (e.g., electrons, ions, radicals) at relatively low temperatures, enabling chemical reactions under mild operating conditions. Thus, the ability of a plasma DBD to selectively hydrogenate biodiesel under ambient conditions, combined with its high reaction rates, electrification, and potentially reduced energy demand, makes it a sustainable, efficient, and electrified solution for biodiesel enhancement.

Plasma Dielectric Barrier Discharge for Process Applications

Dielectric Barrier Discharge (DBD) plasmas have emerged as versatile tools in a range of chemical processing applications—spanning energy and fuel upgrading, fertilizer production, and CO₂ conversion—offering unique advantages in reaction selectivity, milder operating conditions, environmental impact, and electrification. A plasma DBD operates by applying a high-voltage alternating current (AC) through a metal electrode stack across a dielectric barrier such as alumina. When the driven electrode is biased positively in the AC cycle, plasma electrons migrate towards and accumulate on the dielectric surface. As the secondary electrode potential turns negative, the electrons stream off with considerable energy as a result of the space charge and external field, ionizing the surrounding gas and sustaining the discharge generating a non-thermal plasma layer. Reactive species produced in this plasma layer interact with FAME molecules, facilitating partial hydrogenation and improving oxidative stability. In addition to selective reaction control, the benefits of plasma DBDs include lower reaction temperatures (room temperature), full electrification, and low-pressure operations. These advantages position DBD technology as a viable pathway toward next-generation industrial processes as electrification becomes more ubiquitous.

In nitrogen fertilizer production, DBD plasma technology is being explored as an innovative route for nitrogen fixation and ammonia production that circumvents the harsh conditions of the Haber–Bosch process. Plasma-driven N₂ fixation can occur at atmospheric pressure and near-room temperature, leveraging energetic electrons to dissociate N₂ and H₂ (or even H₂O) and form NH₃ or ${\mathrm{NO}}_3^{-}$ under far milder conditions. This approach allows full electrification of nitrogen fertilizer production—the DBD reactor is powered by electricity, enabling the use of renewable energy and decentralized, modular plants—thereby potentially reducing the need for natural-gas-derived H₂ and high-temperature furnaces. The combination of plasma with catalysts (plasma catalysis) has been shown to enhance ammonia yields and guide the reaction along more selective pathways, improving efficiency and suppressing unwanted by-products. Current plasma-based nitrogen fertilizer synthesis remains less energy-efficient than the century-old Haber–Bosch process. This highlights the ongoing challenge of improving the energy efficiency of plasma processes.

DBD plasmas offer a means to activate the very stable CO₂ molecule at ambient conditions, opening pathways to convert CO₂ (often with co-reactants like H₂ or CH₄) into value-added fuels and chemicals without the extreme heat input normally needed. Non-thermal DBD discharges generate a wealth of reactive species (electrons, ions, radicals) and vibrationally excited states that can drives endothermic reactions at low bulk-gas temperatures. This enables processes such as CO₂ splitting or hydrogenation to proceed via alternative reaction mechanisms with lower apparent activation energies and greater selectivity. For example, in plasma-catalytic CO₂ hydrogenation, the synergy between DBD plasma and catalysts has achieved notable conversions and selectivities at reduced temperatures—one study reported ∼$12\%$ CO₂ conversion with 73% CH₄ selectivity at only 25 °C using a Ru/Al₂O₃ catalyst in a DBD reactor and observed that the addition of plasma allowed methanol synthesis to occur at reaction temperatures ∼120 °C lower than in the purely thermal catalytic process. Such results illustrate how DBD plasmas can promote targeted reaction pathways (e.g., favoring CH₄ or CH₃OH formation) while operating under conditions unattainable for traditional thermal processes, thereby potentially integrating CO₂ utilization with intermittent renewable-electricity sources [19].

Biodiesel produced via transesterification remains overwhelmingly feedstock-driven: a decade-long Iowa model shows soybean-oil biodiesel production costs ranging from USD 2.28 to 7.33 gal⁻¹ between 2013 and 2022, with soybean oil itself accounting for ∼92% of operating expenses [20]. Hydrogenation-derived renewable diesel (HDRD) adds high-pressure hydro-processing and hydrogen supply. A techno-economic analysis of an 812 million L yr⁻¹ plant in Alberta reported minimum HDRD costs of USD 0.85–1.09 L⁻¹ (USD 3.20–4.10 gal⁻¹), depending on whether camelina-meal co-product revenue is realized [21]. Independent bottom-up industry modeling indicates that a new U.S. green-field hydro-processing facility must earn about USD 4.6 gal⁻¹ in sales revenue to satisfy a 10% IRR, effectively quantifying the incremental hydrogenation cost layer atop the lipid feedstock [22]. Collectively, these figures imply that hydrogenation increases lipid-to-diesel conversion costs by roughly USD 1 gal⁻¹ relative to conventional biodiesel, underscoring the importance of feedstock innovation and low-carbon hydrogen for future cost reduction.

Across all these applications, DBD plasmas contribute to lower-temperature processing by enabling the use of waste or renewable feedstocks (CO₂ and N₂ from air- and biomass-derived oils), allowing for the possibility of electrification and modernization of these processes. However, there are still significant challenges and future directions to address. Plasma processes in general suffer from energy losses (e.g., power dissipated as heat or light) and can face issues like electrode erosion or, in hydrocarbon systems, carbon deposition (coking) that can deactivate catalysts. The underlying plasma–catalyst interactions are complex and not yet fully understood, which hinders the rational design of efficient plasma reactors and catalysts. Moreover, the energy efficiency and throughput of DBD systems need to be improved for industrial scalability—for instance, while non-thermal plasmas excel at selective activation, converting a sufficient fraction of reactants without excessive power input remains an active research area. Ongoing efforts are therefore focused on optimizing reactor designs (e.g., electrode configurations, pulsed-power schemes), enhancing plasma–catalyst synergy (through tailored catalyst materials that capitalize on the unique plasma environment), and coupling plasma processes with renewable power to realize economically viable manufacturing. This combination of fundamental research and engineering development is expected to further unlock the potential of DBD plasmas in process technologies.

2. Materials and Methods

The experimental schematic is shown in Figure 1. A DBD plasma was generated at atmospheric pressure and moderate temperatures (below 50 °C) through a DBD electrode stack consisting of a metal sheet electrode grid and ground plane sandwiched between an alumina dielectric barrier. This electrode is connected to an alternating current (AC) power supply with a peak voltage of ±5 kV and driving frequency of 20 kHz. An aluminum reaction chamber with an adjustable sample stand allows for gas containment and sample exposure adjustments, with additional details of the plasma reactor setup and plasma parameters and efficiencies described in Zhong et al. [23].

The production of Fatty Acid Methyl Esters (FAMEs) from vegetable oil was conducted through a transesterification process that used methanol and sodium hydroxide as a catalyst. Initially, vegetable oil was heated to approximately 60 °C to reduce its viscosity and facilitate proper mixing. Next, sodium hydroxide (NaOH), amounting to $1\%$ by weight of the oil, was dissolved in methanol at a molar ratio of 6:1 (methanol to oil). The prepared methanol–NaOH solution was then gradually added to the preheated vegetable oil under continuous stirring. The mixture was maintained at 60 °C and stirred vigorously for a duration of 30 min to ensure the transesterification reaction proceeded to completion. Post-reaction, the mixture was transferred to a separation funnel and allowed to settle, resulting in the formation of two distinct layers. The upper layer, which contained the FAME (biodiesel), was carefully separated from the lower layer, which comprised glycerol and any un-reacted methanol.

We used argon and hydrogen as process gases for our reactor, with flow controlled by calibrated mass flow controllers (MKS Instruments) operating in the range of 10 to 100 sccm, to load the reactor. For the plasma processing of the FAME, a glass slide was used to hold the liquid sample approximately 2 mm beneath the DBD plasma surface. For each experiment, the glass slide was prepared with a thin layer of FAME to ensure even exposure to the plasma over the reaction zone.

The experimental process involves the conversion of FAME to H-FAME using a hydrogen–argon or argon plasma and the breakdown of H-FAME. The FAME sample is placed on a glass slide approximately 2 mm beneath the plasma surface. The reactor is supplied with 7 percent hydrogen gas (H₂) and argon (denoted as $H_{2}plasma$ throughout the manuscript), controlled by a calibrated mass flow controller (MKS Instruments).

The high-voltage alternating current is applied across the dielectric barrier, generating non-thermal plasma discharges that produce reactive hydrogen species. These species facilitate the hydrogenation of FAME to H-FAME under atmospheric pressure and temperatures below 50 °C. For the pure argon atmospheric condition, the FAME sample is placed on a glass slide 2 mm beneath the plasma surface, but this time, the reactor is supplied with argon gas (Ar), controlled similarly by a mass flow controller. These reaction products are analyzed using an HP 7890/5975 GC–MS, Agilent Technologies, Santa Clara, CA, USA (a single-quadrupole MS with an electron impact ionization source) to determine their composition and yield. Data from GC-MS is shown in Appendix A-

Table A1. GC–MS data on argon plasma FAME experiment.

RT (min)	Label	Area	Total Area (%)	Start Time (min)	End Time (min)	Conc. (mg/mL)	Percent Conc. (%)
5.282	Methyl Palmitate (C16:0)	4,113,941	12.85	5.249	5.332	0.0021	12.85
6.940	Methyl Oleate (C18:1)	1,806,675.833	5.64	6.911	6.975	0.0009	5.64
7.216	Methyl Stearate (C18:0)	25,505,049.5	79.68	7.169	7.298	0.0128	79.68
9.430	Methyl Eicosenate (C20:0)	584,537	1.83	9.416	9.452	0.0003	1.83

,

Table A2. GC–MS data on hydrogen–argon plasma FAME experiment.

RT (min)	Label	Area	Total Area (%)	Start Time (min)	End time (min)	Conc. (mg/mL)	Percent Conc. (%)
5.280	Methyl Palmitate (C16:0)	23,439,017	9.15	5.242	5.339	0.0117	9.15
7.219	Methyl Stearate (C18:0)	228,553,686	89.26	7.159	7.356	0.1142	89.26
9.431	Methyl Eicosenate (C20:1)	4,064,765	1.59	9.401	9.488	0.0020	1.59

,

Table A3. Average composition of untreated FAME.

FAME	Average Composition (%)
C16:0	5.52
C18:2	19.43
C18:1	70.42
C18:0	2.75
C20:1 (cis–)	1.18
C20:0	0.71
Total	100.00

.

3. Results

The experimental results demonstrate the successful conversion of FAME to H-FAME using a plasma DBD under ambient room-temperature 7 percent hydrogen–argon and argon gas conditions. The process exhibits high selectivity towards H-FAME formation. The results (shown in

Table 1. Change in FAME composition after plasma treatment (6 h H₂ and 6 h Ar).

FAME	Base (%)	6 h H2/Ar (%)	6 h Ar (%)	Δ 6 h H2/Ar (% Change)	Δ 6 h Ar (% Change)
C16:0	5.52	8.00	9.15	+2.49	+3.63
C18:2	19.43	0.00	0.00	−19.43	−19.43
C18:1	70.42	0.00	0.00	−70.42	−70.42
C18:0	2.75	90.70	89.26	+87.95	+86.51
C20:1 (cis–)	1.18	0.00	0.00	−1.18	−1.18
C20:0	0.71	1.30	1.59	+0.59	+0.88

) also show the potential for plasma DBDs to further upgrade H-FAME into higher and lower hydrocarbons, which are valuable components for advanced biofuels and chemical feedstock. Throughout this manuscript, all fatty acid compositions refer to their methyl ester (FAME) forms. For brevity, C16:0 refers to methyl palmitate, C18:0 refers to methyl stearate, C18:1 refers to methyl oleate, C18:2 refers to methyl linoleate, C20:0 refers to methyl arachidate, and C20:1 refers to methyl gadoleate, unless otherwise specified.

One of the primary benefits of using plasma DBD technology is its ability to operate at low temperatures, typically below 50 °C. This low-temperature operation is particularly advantageous as it prevents thermal degradation of the feedstock and products, ensuring the integrity and quality of the resulting biodiesel. Moreover, the mild reaction conditions reduce the formation of undesirable by-products, contributing to the high selectivity and efficiency of the process.

Figure 2 shows the differential changes seen in the FAME composition after a 6 h $7\%H_2/Ar$ and a 6 h $Ar$ plasma treatment. The results indicate a substantial shift from unsaturated to saturated fatty acids in the plasma over six hours, regardless of whether the environment contains hydrogen (H₂) or is pure argon (Ar). Notably, the unsaturated C18:2 (methyl linoleate) and C18:1 (methyl oleate), which together comprise nearly 90% of the baseline FAME profile, both drop to 0.00% after six hours, signifying a complete disappearance. Correspondingly, there is a dramatic increase in C18:0 (methyl stearate) from 2.75% to roughly 90%, suggesting extensive saturation of the C18 chain. Lesser but still appreciable increases in C16:0 (methyl palmitate) and C20:0 (methyl arachidate) also reinforce the overall shift toward saturated fatty acids.

Interestingly, the changes under hydrogen (H₂) and argon (Ar) are similar, implying that the observed saturation and loss of unsaturated FAMEs may not be solely driven by hydrogen gas but could also involve other reaction conditions (e.g., plasma catalytic, thermal, or oxidative processes) that lead to the conversion or disappearance of unsaturated species over time.

Figure 3 shows a comparison of Fatty Acid Methyl Ester (FAME) profiles in untreated samples (FAME) versus samples subjected to 6 h hydrogen or argon plasma treatments. The base FAME composition was dominated by C18:1, accounting for 70.42%, followed by C18:2 at 19.43%, C16:0 at 5.52%, C18:0 at 2.75%, C20:1 at 1.18%, and C20:0 at 0.71%. Hydrogen plasma treatment drastically altered the composition, eliminating C18:2 and C18:1 while increasing C18:0 to 88.49%, a 3117.8% increase. Argon plasma also reduced C18:1, but retained 5.64%, while C18:0 increased to 79.68% (+2797.5%). Both treatments resulted in complete elimination of C20:1, from 1.18% to 0.00% (−100%).

C16:0 (methyl palmitate) increased from 5.52% in the base sample to 9.85% (+78.4%) in hydrogen-treated FAME and 12.85% (+132.8%) in argon-treated FAME. The greater increase in C16:0 with argon plasma treatment suggests a potential selective effect of the treatment process on shorter-chain fatty acids.

C18:2 (methyl linoleate) was completely removed in both hydrogen and argon plasma treatments, declining from 19.43% to 0.00% (−100%). C18:1 (methyl oleate) was eliminated in the hydrogen-treated FAME, dropping from 70.42% to 0.00% (−100%), whereas argon-treated FAME retained 5.64%, representing a −92.0% reduction. C18:0 (methyl stearate) increased significantly: in hydrogen-treated FAME, it rose from 2.75% to 88.49% (+3117.8%), while in argon-treated FAME, it increased to 79.68% (+2797.5%).

C20:1 (methyl gadoleate) was removed entirely, declining from 1.18% in base FAME to 0.00% (−100%) in both plasma treatments. C20:0 (methyl arachidate) increased slightly, from 0.71% to 1.66% (+133.8%) in hydrogen-treated FAME and 1.83% (+157.7%) in argon-treated FAME.

The following proposed mechanism for enhanced hydrogenation from the plasma system is presented below and represented in Figure 4.

1. 

Plasma-phase activation of the ${\mathbf{H}}_{\mathbf{2}}/\mathbf{Ar}$ feed

$$\begin{array}{ccc}\hfill \mathrm{Ar}+e^{-}& ⟶{\mathrm{Ar}}^{*}+e^{-}\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{metastable}\right)\end{array}$$

(1)

$$\begin{array}{ccc}\hfill {\mathrm{Ar}}^{*}+{\mathrm{H}}_2& ⟶\mathrm{Ar}+{\mathrm{H}}_2^{+}+e^{-}\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\left(\mathrm{Penning}\right)\end{array}$$

(2)

$$\begin{array}{ccc}\hfill {\mathrm{H}}_2^{+}+e^{-}& ⟶2\mathrm{H}·\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{diss}.\mathrm{recomb}.)\end{array}$$

(3)

$$\begin{array}{ccc}\hfill {\mathrm{Ar}}^{*}+{\mathrm{H}}_2& ⟶\mathrm{Ar}+2\mathrm{H}·\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{chemi}-\mathrm{dissoc}.)\end{array}$$

(4)

$$\begin{array}{ccc}\hfill {\mathrm{H}}_2+e^{-}& ⟶{\mathrm{H}}_2^{*}+e^{-}\hfill & \hfill \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}(\mathrm{vib}.\mathrm{exc}.)\end{array}$$

(5)

2. 

Transport to the liquid film

Atomic hydrogen radicals ($\mathrm{H}·$) convect through the 1 mm gap and diffuse the last few hundred microns to the FAME surface.

3. 

Step-wise radical hydrogenation of C₁₈ FAME chains

Note: Methyl linoleate (C18:2 FAME) has double bonds at positions 9 and 12 (from the carboxyl end).

1.

First hydrogenation: C18:2 FAME ($\Delta 9,12$) → C18:1 FAME ($\Delta 9$)

Selective hydrogenation of the $\Delta 12$ double bond:

$${\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_4\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3+\mathrm{H}·⟶{\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_4\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{H}·{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3$$

(6)

$${\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_4\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{H}·{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3+\mathrm{H}·⟶{\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_7\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3$$

(7)

(methyl oleate, C18:1 FAME)

2.

Second hydrogenation: C18:1 FAME ($\Delta 9$) → C18:0 FAME

$${\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_7\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3+\mathrm{H}·⟶{\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_7\mathrm{C}\mathrm{H}·{\mathrm{C}\mathrm{H}}_2{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3$$

(8)

$${\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_7\mathrm{C}\mathrm{H}·{\mathrm{C}\mathrm{H}}_2{\left({\mathrm{C}\mathrm{H}}_2\right)}_7{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3+\mathrm{H}·⟶{\mathrm{C}\mathrm{H}}_3{\left({\mathrm{C}\mathrm{H}}_2\right)}_{16}{\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}\mathrm{H}}_3$$

(9)

(methyl stearate, C18:0 FAME)

4. 

Phase change-aided separation

Saturated products (C18:0, C20:0; $T_m\approx 37$–47 °C) crystallize, withdrawing themselves from further attack, while unsaturated C18:1 and C18:2 remain liquid ($T_m<$ −10 °C), exposing fresh double bonds to $\mathrm{H}·$ influx.

5. 

Overall stoichiometry for C₁₈ FAME conversion

$$\mathrm{Methyl}\mathrm{linoleate}(\mathrm{C}18:2\mathrm{FAME})+2\begin{array}{c}\hfill {\mathrm{H}}_2\end{array}⟶\mathrm{Methyl}\mathrm{stearate}(\mathrm{C}18:0\mathrm{FAME})$$

Argon stabilizes the discharge, provides Ar^* metastables for energy transfer, and removes Joule heat, collectively enabling the hydrogenation process even without external H₂.

4. Discussion

As previously discussed, saturated fatty acid methyl esters have higher melting points (e.g., 37–39 °C for C18:0 methyl stearate and 46–47 °C for C20:0 methyl arachidate) that are near or above the reactor temperature (25–40 °C). Once formed, these saturated products tend to solidify and deposit on cooler surfaces (e.g., the glass slide (Appendix A-Figure A1) or reactor walls). This physical separation removes the saturated species from the main reaction zone, driving equilibrium toward further saturation of the remaining unsaturated species. In essence, the saturated FAMEs are continually “distilled out” of the reaction mixture as solids.

Although direct hydrogenation is likely the dominant mechanism, plasma environments are known to generate energetic electrons, ions, and radicals that can induce bond cleavage. This can lead to mild “cracking” of longer-chain FAMEs. For example, partial scission of a C20:1 chain could yield smaller-chain intermediates that might more readily hydrogenate into C16:0 rather than simply converting to C20:0. The fact that C16:0 shows a greater increase under argon plasma—where hydrogen radicals are less abundant—suggests that these side reactions could be promoted by the highly reactive plasma species rather than just hydrogen alone. Further investigation into these mechanisms will be explored in future work.

Plasma-derived radicals can facilitate not only hydrogen addition but also radical–radical coupling or rearrangements. When unsaturated C20:1 species lose their double bond(s) through hydrogenation or undergo partial cleavage, the resultant fragments may recombine to form either C16 or other shorter-chain fatty acids/methyl esters. Over extended reaction times, these processes can accumulate appreciable amounts of new chain-length distributions (e.g., an increase in C16:0).

Methyl arachidate (C20:0 FAME) has a melting point of roughly 46–47 °C, placing it at the edge of solidification within the reactor. Once hydrogenated (C20:1 → C20:0), the newly formed C20:0 may precipitate almost immediately, effectively pulling it out of the reaction zone and curbing further transformations or cracking pathways. The net result is a modest net increase in C20:0 but a greater propensity for these newly formed molecules to drop out from the fluid phase.

Although shifts in C16:0 and C20:0 are discernible, the most striking change remains the near-complete conversion of C18:1 and C18:2 to C18:0. This suggests that the C18 species are the easiest to hydrogenate (owing to direct addition across the double bonds), and once formed, solid C18:0 readily deposits. Because these hydrogenation steps are highly favorable, the residual unsaturated C18s continue reacting until they are almost entirely depleted.

The shift in C16:0 and C20:0 is the product of interlinked factors under plasma conditions: (1) direct hydrogenation of unsaturated double bonds, (2) phase-driven removal of newly formed saturated species, and (3) possible mild chain scission or rearrangement induced by energetic plasma radicals. Together, these processes contribute to the overall conversion of unsaturated fatty acids to saturated forms and, in certain cases, the redistribution of chain lengths. Further research of these chain-scission and recombination pathways—especially under different plasma gases or optimized reactor temperatures—will be key to fine-tuning product selectivity and improving the yields of specific saturated (or partially saturated) FAMEs for advanced biofuel applications.

The plasma DBD process eliminates these major cost drivers by operating at atmospheric pressure and room temperature without catalysts, reducing capital equipment costs compared to conventional methods. Additionally, the elimination of catalyst preparation, removal, and regeneration steps further reduces both processing time and operational complexity, making plasma-based hydrogenation potentially economically competitive for small-to-medium scale biodiesel producers.

5. Conclusions

Plasma Dielectric Barrier Discharge (DBD) technology offers a transformative pathway for upgrading biodiesel by enabling the efficient conversion of FAME to H-FAME and higher-value hydrocarbons. The scalability of such technologies requires some possible improvements in electrode design, power systems, and reactor engineering, however the possibility for large scale DBD systems is realizable [24]. This energy-efficient, non-thermal approach circumvents many challenges associated with conventional systems, offering a more sustainable and electrified alternative. A key advantage of plasma DBD technology lies in its scalability; modular reactor designs allow for facile expansion to accommodate larger throughputs or diverse feedstock types while maintaining consistent process conditions. Moreover, combining plasma DBD systems with additional thermal stages or heterogeneous catalysts holds promise for further enhancing reaction rates, selectivity, and overall product quality. The ability to tune plasma chemistry through the use of different gases also provides an avenue for tailored production of specific biofuel components. Future research will focus on refining reactor configurations and operating parameters that allow for increased energy efficiencies and surface reaction scaling, developing novel catalyst formulations that allow for increased selectivity of specific reaction pathways, exploring hybrid systems that couple plasma DBD with other processing steps, and conducting pilot-scale as well as techno-economic assessments to demonstrate the feasibility of large-scale deployment. By addressing these areas, plasma DBD technology stands poised to significantly advance sustainable biofuel production, offering a versatile, adaptive platform capable of meeting the evolving demands of the global energy market. Additionally, it offers the potential for fully circular agricultural systems, where vegetable feedstocks are converted into fuels that, in turn, power the machinery used to cultivate and harvest those same crops.