1.1. Properties of Bio-LPG

On a commercial basis, two LPG grades predominate, commercial propane and the HD-5 (GPA, 2013; [7]). They exist in both liquid and gaseous states, making them a liquid-gas automotive fuel. Fortunately, greenhouse gas (GHG) emissions reduction and de-carbonization of the electric grid can be achieved by the adoption of off-grid biofuels, including bio-LPG and the ever-increasing batteries for example, in electric vehicles to reduce the enormously increasing global energy demand for fuel, [8]. The properties of LPG and other related automotive fuels are highlighted in Table 1.

 

It is noteworthy that LPG varies from country to country [12], during summer and winter [12], depending on the intent and suitability (please refer to Table 4 for some countries). There is a projected report that approximately 17%–33% of new vehicle sales and 8%–14% of the world's light vehicle fleet will be made up of light-duty EVs, by 2030 [28]. However, the challenges from fossil fuel alternatives include the illustrious advancement in electrification and are unique to heavy machinery, transatlantic aircraft, and long-distance terrestrial, marine, and aerial freight transit. These applications depend on energy-dense fuels because they need high power and energy-to-weight ratios. Still on the special need for elevated power, and the ratio of energy-to-weight for energy-dense fuels, petroleum distillate-rich branching and cyclic alkanes like aviation fuel (Jet-A), Rocketry fuel (RP1), and high energy density synthetic polymers such as hydroxyl-terminated polybutadiene and HTPB-rocketry serve military and commercial aerospace purposes [29], the water-way cargoes rely majorly uses bunker fuel which consist of denser fractions distillate of petroleum with notable NOx, SOx, and particulate matter emission during combustion process [30], but the resulting pollution from them impact the environment negatively. For example, Amer et al. [22] reported that shipping alone accounts for approximately 3% of global emissions with a release of annual emissions of 1 trillion Kg of CO2 and GHGs, whereas aviation is responsible for 2% on account of 9.15 billion Kg of CO2 in 2019 [31]. Although there will be a continual decline in the climate impact owing to the use of electric cars for light-duty transportation, the carbonizing impact of the heavy-duty and aerospace sectors may intensify as a function of global emissions in the long term. More so, petrol-based biofuel blends may cut down on harmful emissions, however, current renewable biofuels contribute approximately 0.5% to the global fuel supply [32]. So, the question is on how to produce so much LPG (commercial) utilizing a sustainable process while considering the fuel end product-host metabolism tolerance [22], fuel separation and purification, and bio-economic techniques. For these reasons, novel and sustainable alternatives to serve high energy demand sectors are urgently needed. There are no known commercial natural biosynthetic routes to propane, including the widely adopted aldehyde deformylating oxygenase (ADO) variants that are naturally occurring or developed synthetically with approximately 3–5 h−1 low turnover number (Kallio et al., 2010; [33]). Meanwhile, the sole commercially viable way of producing LPG continues to be the Neste process, labor-intensive catalytic chemical conversion of biodiesel waste (glycerol) dependent on H2-derived natural gas [34]. Hence, this research aims to highlight the synthesis techniques of bio-LPG and identify pathways for any prospective scale-up of bio-LPG, benefits, and general limitations of bio-LPG, as an automotive fuel. In addition, the novelty of this paper is evident in Bio-LPG potential for local air quality improvement by drastic elimination or reduction of emissions or transition to low-GHG energy for automotive vehicles, appropriate mitigation of global climate change, and energy security by reducing national dependence on petroleum are the central premise to foster climate protection. Whereas the main issue with alternatives to gasoline is the higher production of CO2 and NOx due to higher combustion temperature inside the cylinder, LPG has been reported to produce 18.74 and 25.92% lower CO2, and NOx emissions respectively as compared to gasoline [35], making studies on bio-LPG worth eternal devotion to as far alternative and renewable fuels are concerned.

It is noteworthy that LPG varies from country to country [12], during summer and winter [12], depending on the intent and suitability (please refer to Table 4 for some countries). There is a projected report that approximately 17%–33% of new vehicle sales and 8%–14% of the world's light vehicle fleet will be made up of light-duty EVs, by 2030 [28]. However, the challenges from fossil fuel alternatives include the illustrious advancement in electrification and are unique to heavy machinery, transatlantic aircraft, and long-distance terrestrial, marine, and aerial freight transit. These applications depend on energy-dense fuels because they need high power and energy-to-weight ratios. Still on the special need for elevated power, and the ratio of energy-to-weight for energy-dense fuels, petroleum distillate-rich branching and cyclic alkanes like aviation fuel (Jet-A), Rocketry fuel (RP1), and high energy density synthetic polymers such as hydroxyl-terminated polybutadiene and HTPB-rocketry serve military and commercial aerospace purposes [29], the water-way cargoes rely majorly uses bunker fuel which consist of denser fractions distillate of petroleum with notable NOx, SOx, and particulate matter emission during combustion process [30], but the resulting pollution from them impact the environment negatively. For example, Amer et al. [22] reported that shipping alone accounts for approximately 3% of global emissions with a release of annual emissions of 1 trillion Kg of CO2 and GHGs, whereas aviation is responsible for 2% on account of 9.15 billion Kg of CO2 in 2019 [31]. Although there will be a continual decline in the climate impact owing to the use of electric cars for light-duty transportation, the carbonizing impact of the heavy-duty and aerospace sectors may intensify as a function of global emissions in the long term. More so, petrol-based biofuel blends may cut down on harmful emissions, however, current renewable biofuels contribute approximately 0.5% to the global fuel supply [32]. So, the question is on how to produce so much LPG (commercial) utilizing a sustainable process while considering the fuel end product-host metabolism tolerance [22], fuel separation and purification, and bio-economic techniques. For these reasons, novel and sustainable alternatives to serve high energy demand sectors are urgently needed. There are no known commercial natural biosynthetic routes to propane, including the widely adopted aldehyde deformylating oxygenase (ADO) variants that are naturally occurring or developed synthetically with approximately 3–5 h−1 low turnover number (Kallio et al., 2010; [33]). Meanwhile, the sole commercially viable way of producing LPG continues to be the Neste process, labor-intensive catalytic chemical conversion of biodiesel waste (glycerol) dependent on H2-derived natural gas [34]. Hence, this research aims to highlight the synthesis techniques of bio-LPG and identify pathways for any prospective scale-up of bio-LPG, benefits, and general limitations of bio-LPG, as an automotive fuel. In addition, the novelty of this paper is evident in Bio-LPG potential for local air quality improvement by drastic elimination or reduction of emissions or transition to low-GHG energy for automotive vehicles, appropriate mitigation of global climate change, and energy security by reducing national dependence on petroleum are the central premise to foster climate protection. Whereas the main issue with alternatives to gasoline is the higher production of CO2 and NOx due to higher combustion temperature inside the cylinder, LPG has been reported to produce 18.74 and 25.92% lower CO2, and NOx emissions respectively as compared to gasoline [35], making studies on bio-LPG worth eternal devotion to as far alternative and renewable fuels are concerned.

 

1.2. Bio-LPG overview

Bio-LPG (also known as futuria liquid gas) is an eco-friendly renewable energy solution that is chemically identical and compatible with all LPG products, with an 80% lower carbon footprint compared to conventional LPG, produced by bio-refining, power to gas (P2G), anaerobic digestion (AD), gasification techniques, etc. [36]. Bio-LPG can be employed to address rural and off-grid, cost-effective decarbonization, high-temperature industrial processes, household applications, etc. Based on applications, Bio-LPG can be employed as an environmentally benign substitute for ozone-depleting chlorofluorocarbons (CFCs and HCFCs) using refrigerant and aerosol propellant [37], cooking fuel [38], carbon footprint determinant in a non-homogeneous process [39]. In addition, it can proffer carbon footprint reduction through gas-powered heating combined with renewable thermal systems and hybrid systems as well as Autogas (LPG as transport fuel). Based on the hydrotreated vegetable oil (HVO) pathway, the projected capacity is forecasted to expand up to 2200 million kg/yr. by 2030 [40]. Synthesized quantities need to be scaled up to meet energy demands because this purified off-gas stream produces 50–80 kilos (5–8%) of bio-LPG for every 1000 kg of renewable diesel or kerosene. LPG (Autogas) is reported to have about a thousand applications and specifically powers about 27 million vehicles globally, mainly HD-5 (propane grade specification) vehicles containing at least 90% propane, 5% propylene, 5% butane, and 5% additional (butane and butylene) gases at most. When combusted, LPG generates 5.68 Kg of CO2 per 3.79 L as opposed to the 8.88 Kg and 10.18 Kg of carbon dioxide (CO2) emitted by gasoline and diesel fuels (GPA, 2013).

 

Based on environmental impact, industrial, agricultural, and commercial sectors that fuel their processes for space heating and vehicles with oil or coal, can switch to LPG because it presents a lower-carbon alternative to oil and coal, with an emission intensity of approximately 20% and 30–40% lower (GGR, 2019), respectively. As well as lowering carbon emissions, LPG can improve local air pollution since it is a clean burning fuel that produces almost zero particulate matter (PM) when combusted [6]. On public policies with the objective is to ensuring a smooth transition to low-GHG energy for motor vehicles, the idea of displacement of most of the fossil petroleum with low-GHG energy by 50–100% by 2050 would require that the majority of new vehicles sold in 2050 are alternative fuel vehicles, which is a huge task for a large-scale energy transition [41].

 

2. Bio-LPG production

Earlier techniques for Bio-propane synthesis were Bio-forming, production of gasoline and diesel from biomass [42], direct selective synthesis of LPG from synthesis gas produced from natural gas, coal, and biomass [43], a conversion process of DME from glycerol, a by-product from bio-diesel production and H2 to bio-methanol which can be further converted to bio-DME using commercially available dehydration technologies to LPG [44], or utilizing hydrogen from renewable sources, e.g., via biomass gasification, or from biogas, having material incompatibility with existing LPG [45]. Other bio-synthesis processes of LPG include microbial fermentation bioprocessing (e.g., amino acids) and chemical conversion of bio-derived raw materials including waste biomass (Sorigue et al., 2017; [36]), providing a green alternative to fossil-generated LPG. Fig. 1 gives a map highlighting the multiple potential pathways to bio-LPG production from separation techniques of fuel end-products during bio-LPG manufacturing.

 

Fig. 1

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Fig. 1. Map of potential routes to bio-LPG (bio-LPG study report, 2020).

 

The well-established synthetic techniques of bio-LPG development are the hydrotreatment and dehydration methods. However, only the hydro-treatment bio-LPG is commercially viable, while the dehydration has the potential to be expanded beyond a handful quantity. In addition, utilizing glycerine in fuel applications, is noteworthy for automotive fuel applications, being a good feedstock for chemical synthesis, as it is classified as a by-product. However, among the several feedstocks that may be hydrotreated to synthesize bio-LPG, including bio-oils, propylene, butylenes, and dimethyl ether (NREL, 2018), the bio-oil is the only one that is now used commercially. Table 1 highlights this. Processes including fermentation-to-LPG, alcohol-to-jet, biogas/biomethane conversion (oligomerization), glycerin-to-propane, power-to-x, and biosynthesis, however, have not yet scaled up significantly. The bio-LPG percentage varies depending on the method; a handful of them, including oligomerization, glycerin-to-propane conversion, and biosynthesis, produce 100% of propane (opisnet.com). For example, halomonas, a robust extremophile microbial chassis, was utilized in bio-LPG production under non-sterile conditions and using waste biomass as the carbon source, as presented by Amer et al [22] in a related study. Generally, bio-LPG production from biomass, biogas, or waste has a sustainable potential with a lower carbon footprint than fossil fuel LPG [46]. On the feasibility of bio-LPG in alignment with regulations and policies governing automotive fuels, there are hardly existing reports on private sector investment subsidy and regulation for refueling infrastructure, which may be a major contributor to the low volumes in availability from a commercial perspective.

 

2.1. Scalability of microbial pathways

Most proofs of concepts are limited to lab-scale production in vivo (low technology readiness level) and do not demonstrate detailed approaches to commercial-scaled bio-LPG production. According to Amer et al. [22], scaled production of bio-LPG is more economically feasible using Halomonas. They reported that scaled production of polyhydroxyalkanoates using Halomonas is at a 65% cost saving compared to E. coli. Halomas bio-alkane commercially viable process is reportedly achievable by Halomonas cultivation around a coastal environment with on-site seawater, an anaerobic digester (AD) for VFA production, and optionally a cyanobacteria photobioreactor for CO2 fixation and VFA supply (see Fig. 2 for typical prototype).

 

Fig. 2

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Fig. 2. Futuristic prototype bio-LPG production hub.

 

Where processes; (1) seawater intake and pre-treatment; (2) biomass accumulation fermentation system; (3) anaerobic digestion (AD) plant for volatile fatty acid supply; (4) photo-bioreactor for propane production; (5) propane purification; (6) propane compression and liquefaction; (7) local propane distribution by road and rail; (8) local propane usage by heavy industry such as power generation or steel mills; and (9) waste biomass treatment and fish feed production [22].

 

In addition, steps from typical halomonas to bio-LPG production include strain identification by gene encoding amplification and sequencing (phylogenetics), for example, Amer et al. [22] reported that their process of sampling and microbial cultures was prepared to generate inoculum slurries from both terrestrial soil and marine sediment, with seawater. Then, samples were incubated, under micro-aerobic/anaerobic conditions in tightly sealed headspace vials without shaking at 4 °C for 11 weeks, before analysis for propane in the headspace above the live cultures was conducted. On isolation, individual bacterial species were isolated by repeated colony picking and sub-culture on solid media at 4 °C for 10 weeks under anaerobic conditions. This highlights steps for which bio-LPG from the oceanic environment (Fig. 3) using microbial culture should be explored deeper with the intent that the bio-LPG production route could be deployed to address energy insecurity, clean air, and carbon management issues.

 

Fig. 3

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Fig. 3. Steps on the typical microbial generation of bio-LPG through Halomonas strain [22].

 

3. Benefits and limitations of LPG

Apart from the general benefits of LPG (bio-LPG) which include extended engine life due to reduction of wear on its components, for example, rings and bearings, etc. (see Table 6) by the action of lubrication, high octane rating which controls complete combustion and hence, engine knock [47]. The low stoichiometric fuel-air ratio and density reduce the specific fuel consumption and exhaust emissions. When burnt, its low carbon molecular content including particulate molecules produces low carbon dioxide (CO2) emissions compared to conventional fuels. In addition, LPG (bio-LPG) based engines produce high thermal efficiency and improved fuel economy, high compression ratio, and subsequently high thermal efficiency without denotation compared to gasoline (Chaichan, 2019). Based on region and feedstock, LPG is mostly composed of propane and butane in varying ratios (see Table 2). According to Table 5, LPG (Bio-LPG) is seen as a replacement for gasoline and diesel to achieve better exhaust emission [48,49], replacement for marine fuel since it reduces emission and fuel costs [50], despite the high level of studies and appreciation of electric propulsion as a good fuel alternative in sole electric or hybrid propulsions, Jeong et al. [51], confirmed that the impact of battery application is far from zero-emission shipping from a life cycle perspective, concluding that electric propulsion systems are not necessarily adjudged eco-friendly. Meanwhile, LPG is adjudged to have storage and transportation benefit, better emission characteristics, low sulphur content, easy adaptation for engine or fuel change, and above all, unlike its renewable form, bio-LPG is reported to be sustainable [22].