Performance and Emission Characteristics of Hydrogenation Derived Renewable Diesel as Diesel Engine Fuel

Abstract

Growing anxieties about the continued depletion of fossil fuel reserves, improving the performance of diesel engines, and mandates to reduce greenhouse gas emissions have made the search for alternative fuels for diesel engines more imperative. Hydrogenation Derived Renewable Diesel (HDRD) is recognized as a sustainable, reliable, and cost-effective alternative to petroleum-based diesel (PBD) fuel for compression ignition (CI) engines. This may be because the physicochemical properties of HDRD are similar to that of PBD fuel. The current effort examines the performance and emission characteristics of HDRD in unmodified CI engines. Performance emissions characteristics such as power, torque, brake specific fuel consumption, thermal efficiency, nitrogen oxides, carbon monoxide, carbon dioxide, particulate matter, and exhaust gas temperature were interrogated and compared with that of PBD fuel in a CI engine. The outcome of the study shows that HDRD is better than biodiesel and a sustainable replacement for PDB fuel to achieve improved performance and reduced emissions of CI engines. Going forward, more investigations are needed to further simplify the preparation and democratize the utilization of HDRD as CI fuels for various applications.

1. Introduction

Fossil fuels, which originated from the anaerobic decomposition of carbon-rich dead plants and animals, have continued to dominate the energy source and drive the industrialized world. About 70–80% of the global energy consumption is gotten from fossil fuels [1]. Fossil fuels, comprising coal, oil, and gas, are non-renewable and the main contributor to global warming and climate change. Extraction, refining, and utilization of fossil fuels have caused unimaginable degradation of the environment. Also, going by the rate of consumption, the global oil reserves estimated to be 1.65 trillion barrels may be fully depleted within the next five decades [2]. Also, increased population, accelerated industrial revolution, and increased mechanized farming has continued to cause an increased utilization of fossil fuels and consequently increased emission. The global consumption of fossil fuels was recorded as 121, 531 Terawatt-hour (TWh), 129,855 TWh, and 136,131 TWh for 2010, 2015, and 2019 respectively. On the other hand, the total carbon dioxide (CO₂) emissions were documented to be 31.49 Billion Tonnes, 33.39 Billion Tonnes and 34.35 Billion Tonnes respectively (Figure 1). However, fuel consumption and CO₂ emission plummeted in 2020 due to the Covid 19-imposed lockdown. With the relaxation of various travel restrictions and increased commercial and industrial activities, fuel consumption and emissions are expected to increase substantially. This is expected to escalate environmental degradation and climate change.

Figure 1.

Global consumption (TWh) and CO₂ emission (billion Tonnes) from coal, oil, and gas 2010–2020. Adapted from [3, 4].

The use of biofuels is one of the panaceas for the unfavorable effects of fossil fuels in diesel engine applications. Biofuels are renewable fuels generated from fresh and living organisms. They usually occur in solid, liquid, or gaseous forms. Biofuels enjoy several benefits like renewability, ecological friendliness, feedstock accessibility, the elasticity of the production methods, and their amenability to existing fossil fuels pipeline infrastructure. Also, biofuels demonstrate matchless capability for the sustenance of the ecosystem [5, 6]. However, the high cost of production, increased NOx emission, and increased engine wear are major setbacks to the use of some biofuels. Also, the conflict between some of the feedstocks with the food chain, undeveloped production technologies, and unfavorable government policies have continued to militate against the wide production and utilization of biofuels in many jurisdictions. Notwithstanding these impediments and complications, biofuels remain a clean, safe, and sustainable replacement for fossil fuels and a strategic resource for CO₂ reduction and carbon mitigation to avert the ominous environmental catastrophe [5, 7, 8].

The transport sector utilizes more than 90% of the total fossil fuel products and about 28% of the total global energy and is a major contributor to the emission of dangerous gases [9]. Solid biofuels (wood chips, briquettes, sawdust), liquid biofuels (biodiesel, renewable diesel, bioethanol), and gaseous biofuels (biogas, biomethane, syngas) have been used as reliable and environmentally benign candidates for fossil-based fuels. The overall energy consumed in the transportation sector was 110 million terra joule (TJ) in 2015 while 129 billion liters of liquid biofuel were utilized in 2016 and the quantity is predicted to increase to 180 billion liters by 2050 [10]. The number of global on-road vehicles which was about 1.2 billion is projected to increase to 2 billion and 2.5 billion vehicles in 2035 and 2050 respectively [11]. Compression ignition (CI) engines because of their versatility, strength, and multi-faceted usage, have continued to be used as passenger vehicles, construction machinery, agricultural equipment as well as rail and heavy-duty trucks. Fueling these engines with petroleum-based diesel (PBD) fuel will exacerbate the detrimental effects on the health and environment.

To increase the share of renewable fuels in the transportation sector energy mix, renewable energy sources and other less polluting fuels such as electricity, natural gas, bioethanol, propane, biodiesel, jet fuel, and biomethane have been tested. These renewable and less polluting energy sources have been found to meet the huge demand and requirements for bioenergy and secure the energy supply. For example, the deployment of electric vehicles has been plagued with the high cost, infrastructural deficit, and long duration of charging of the battery in many jurisdictions. The liquid biofuels have the advantage of being produced for wastes and other renewable sources with a low carbon footprint, thereby making them a more economically viable option [12]. Globally, more concerted efforts geared at increasing the production and utilization of renewable fuels are needed to achieve Sustainable Development Goals and ensure environmental sustainability. Also, more public awareness and education, targeted policy, and research and development (R & D) aimed at increasing the production and utilization of liquid biofuels should be intensified.

1.1 Motivation, aim, and scope

Concerns over the environmental, social, economic, and supply of world energy have been addressed by governments in various jurisdictions. Possible solutions include the introduction of biofuel into the energy mix by encouraging and incentivizing the production and utilization of biofuels. The desire to popularize the application of these biofuels, particularly for CI engines applications, has gained considerable attention in recent years. A lot of studies have been carried out and reported on the production and utilization of biodiesel and bioethanol as CI engine fuels. In previous research, Saravanan et al. [13], Khan et al. [14], Krishna et al. [15], and Shirneshan et al. [16], among several others investigated the performance and emission characteristic of biodiesel, ethanol, and biodiesel-ethanol blends on CI engines. The outcomes of their studies showed the benefits and shortcomings of the deployment of these renewable fuels in CI engines with particular attention to Hydrogenated Derived Renewable Diesel (HDRD). In their various studies, they confirmed the superiority of HDRD over biodiesel and PBD fuels for CI engine transport applications. Recently Chia et al. [12] and Kumar et al. [17] demonstrated their preference for HDRD over biodiesel, ethanol, and other liquid biofuels. They cited the superior heating value, excellent transport and storage stability, and non-corrosive nature of HDRD as some of the reasons.

Bearing in mind the ongoing efforts at finding more sustainable renewable fuels to power CI engines, and the various challenges encountered with the usage of biodiesel and bioethanol, the relevant question to ask is how has HDRD performed as an alternative fuel for CI engines? . How effective is HDRD as CI engine fuel from the standpoint of performance and emission characteristics? The motivation for this study is the desire to improve the quantum and quality of information and awareness on HDRD as a transportation fuel to assist consumers, fuel refiners, and engine manufacturers in making informed decisions in fuel selection. The current effort aims to investigate the performance and emission characteristics of CI engines fueled with HDRD.

Overall, the outcomes of this work will equip governments, policy formulating agencies, industry experts, researchers, and the general public with the requisite information on the application of HDRD in CI engines. It is also hoped that research funding bodies will be encouraged to provide more funds for future R & D to stimulate investigation into novel strategies for production and utilization of the HDRD. To achieve this, the article will be divided into subheadings to discuss HDRD as a renewable fuel, performance of HDRD in CI engine, emission characteristics of HDRD as CI engine fuel, implications of HDRD as CI engine fuel, and conclusion. The current effort is, however, limited to a desktop review of published literature on the performance and emission behavior of HDRD in diesel engines.

2. HRDD as a renewable fuel

HDRD, otherwise called renewable diesel, green diesel, and hydrotreated vegetable oil, is a second-generation liquid biofuel. HDRD is chemically identical to PBD fuel but not the same as biodiesel. Biodiesel, also referred to as Fatty Acid Methyl Ester (FAME), is a mono-alkyl ester mostly generated by the catalytic transesterification process, HDRD is a blend of straight-chain and branched paraffin hydrocarbons within the C₁₅–C₁₈ range. The similarities in properties of petroleum diesel and HDRD allow it to meet the automotive fuel specifications, seamless application of HDRD in CI engines, and use of the same transport infrastructure [18, 19]. The global production of HDRD grew from 1.5 billion liters in 2011 to 9.5 billion liters in 2017 and is projected to become 13 billion liters in 2024 [20, 21]. Also, due to attractive properties and advantageous utilization of HDRD, the production capacity and the share of biofuel production have been increasing since 2019, globally (Figure 2). This trend is expected to continue.

Figure 2.

Global HDRD production capacity and share in biodiesel production 2019–2022. Adapted from [22].

To meet up with the growing demand and utilization of HDRD, many commercial production plants have been installed and commissioned using advanced technologies (Table 1). Figure 3 shows the producer, capacity/year, and country of location of HDRD plants, worldwide. The HDRD is usually produced through catalytic hydroprocessing, decarboxylation, and/or decarbonylation of triacylglycerol. During hydroprocessing, hydrogen is applied for the removal of oxygen from the triglyceride molecules through decarboxylation and hydrodeoxygenation, depending on the catalyst selection and process conditions [24]. This can be accomplished either through a co-processing arrangement of a distillate hydroprocessing unit or by building a standalone unit as shown in Figure 4. Figure 5 shows the reaction pathways for HDRD production.

Company	Location	Capacity (tonnes/year)	Technology/process
Neste	The Netherlands	1,000,000	NExBTL
Neste	Singapore	1,000,000	NExBTL
Diamond Green; Diesel	USA	900,000	Ecofining™
UOP/Eni	Italy	780,000	Ecofining™
Total	France	500,000	Vegan® by Axens
Petro oil & Gas	UAE	500,000	UOP Renewable jet fuel process
Neste	Finland	380,000	NExBTL
REG Inc	USA	250,000	Dynamic Fuels LLC
AltAir Fuels	USA	130,000	Ecofining™
UPM Biofuels	Finland	100,000	UPM BioVerno
Petro oil & Gas	UAE	500,000	UOP Renewable jet fuel process

Table 1.

HDRD production plants [20, 21, 23].

REG = Renewable Energy Group, UAE = United Arab Emirate, USA = United States of America.

Figure 3.

Locations, company, and capacity/year of major HDRD plants [21].

Figure 4.

Schematic diagram of HDRD production by hydroprocessing. Adapted from [25].

Figure 5.

Reaction pathways for HDRD production [26].

Generally, HDRD can be synthesized from feedstocks such as sugar, starch, or cellulosic materials through various techniques like catalytic conversion, Biomass to Liquid, and pyrolysis. Also, vegetable oil, waste cooking oil, waste animal fats, recovered fats, and other triglycerides-bearing oils are converted into HDRD by pyrolysis and hydroprocessing. The outcome of the use of some renewable feedstocks such as waste cooking oil, animal fats, algae oil, jatropha oil, and Karanja oil have shown high product yield under moderate production conditions (Table 2). The conversion of triglycerides to HDRD through hydroprocessing entails chemical reactions such as hydrogenation, decarboxylation, decarbonylation, and hydrodeoxygenation reactions [12]. HDRD is produced in line with the methods and specifications of the American Society for Testing and Materials (ASTM) D975 and the European Committee for Standardization EN 590 [27]. Table 3 shows the specifications and International Standards Organizations (ISO) test method for HDRD.

Feedstock	Production process	Yield (%)	Remark
Waste cooking oil	Deoxygenation	>95	Cheap and readily available feedstock
Waste cooking oil	Hydrodeoxygenation	43.8	Low-cost and non-edible feedstock
Waste cooking oil	Hydrotreatment	100	High product yield; Waste to fuel
Karanja oil	hydroprocessing	80	Nonedible oil
Karanja oil	hydrogenation	100	High product yield
Karanja oil	Hydrotreating	82.6	Non-edible oil
Algae oil	Hydroprocessing	80	Non-edible feedstock
Palm oil	Hydrodeoxygenation	100	High product yield
Palm oil	Deoxygenation	>95	Readily available feedstock; Easy conversion method
Palm oil	Hydrodeoxygenation	100	High product yield
Palm oil	Hydrodeoxygenation	>89	Readily available feedstock
Animal Fats	Deoxygenation	90	Waste to fuel, cheap feedstock
Animal Fats	Deoxygenation	100	High product yield
Animal Fats	Deoxygenation	94.2	High product yield, non-edible feedstock
Jatropha oil	Hydroprocessing	98.5	High product yield

Table 2.

Performance of some renewable feedstocks for HDRD production [12].

Property	Unit	EN 590	ASTM D975	Test method
Density @15°C	kg/m^3	820–845	—	EN ISO 3675, EN ISO 12185
Kinematic viscosity @ 40°C	mm^2/s	2.0–4.5	1.9–4.1	EN ISO 3104
Flashpoint (Closed cup)	°C	55	52	EN ISO 2719
Cloud point	°C	—	W: −5 °C; S: 3 °C	—
Cold filter plugging point	°C	—	W: −15 °C; S: −5°C	—
Cetane number	—	51	40	EN ISO5165
Cetane index	—	46	40	EN ISO 4264
Water and sediment	% vol	0.02w/w	0.06	EN ISO 12937
Total contamination	ppm	24	—	EN ISO 12662
Carbon residue	wt %	0.3	0.36	EN ISO 10370
Total ash	wt %	0.01	0.01	EN ISO 6245
Total sulfur	mg/kg	10	15	EN ISO 20846, EN ISO 20847,; EN ISO 2088
Lubricity @ 60°C	WSD, microns	460	520	EN ISO 12156-1
Copper strip corrosion	3 h @ 50 °C	No. 1	No. 3	EN ISO 2160

Table 3.

Specifications and testing methods of HDRD [28, 29].

W = winter, S = summer.

The cetane number of HDRD, a measure of the ignition quality of diesel fuel in CI engines, is usually between 820 Kg/m³ and 845 Kg/m³ and higher than PBD fuel and biodiesel. The high value of cetane number allows a CI engine fueled with HDRD to operate with higher thermal efficiency and at a lower fuel consumption [12]. The lower value of density, compared with biodiesel or PBD fuel indicates reduced volumetric heating value and increased fuel consumption. The high lubricity of HDRD ensures minimum engine wear, noiseless running, and smooth engine operation [12, 20].

CI engines are a form of an internal combustion engine. As heat engines, CI engines convert the chemical energy in the fuel into mechanical work [30]. The diesel fuel is passed into the engine through a fuel injector into the cylinder and mixed with preheated air where the mixture auto ignites due to the movement of the piston. The piston reciprocates between the Bottom Dead Center (BDC) and the Top Dead Center (TDC). The application of HDRD in CI engines makes the engine behave in a certain way and the efficacy of the fuel is measured in line with some set performance criteria and emission characteristics.

3. Performance of HDRD in CI engines

Desirous to find solutions to the obvious inadequacies in the utilization of PBD fuel in CI engines, HDRD has been used by various researchers. However, the major performance criteria used in measuring the performance characteristics include power, torque, fuel consumption, thermal efficiency, and mean effective pressure. For example, the brake specific fuel consumption (BSFC) is an important performance metric that measures the conversion of the fuel to useful work while mechanical efficiency calculates the effectiveness of the engine as the ratio of the brake power to the indicated power. On the other hand, the brake thermal efficiency (BTE) measures the ability of the engine to efficiently convert the chemical energy in the fuel to useful work.

Using these performance criteria, the engine metrics of HDRD is compared with that of PBD fuel when used in a CI engine are compiled in Table 4. When HDRD was used to power a 6.5 liters, indirect-injection, water-cooled military diesel engine, it was reported that an increase in load led to increased fuel consumption and improved brake mean effective pressure (BMEP). Also, at a given fuel consumption threshold, an increment in engine speed caused a reduction in the brake torque. It was also reported that the best BSFC was achieved at high loads and low engine speed. This is because at low speed, engine friction is reduced and fuel consumption is minimized. When compared with PBD fuel, the application of HDRD resulted in better engine performance in all the engine metrics measured [31]. Also, Ogunkoya et al. [32], Mangus et al. [33], and Kim et al. [34] reported that their respective tested CI engines fueled with HDRD presented lower BSFC when compared with PBD fuel. They attributed the lower BSFC to the lower viscosity and the impact of high heating value which allows for better fuel atomization.

Fuel tested	Engine parameters	Test conditions	Result of the test	Remark	Ref.
HRDR and PBD	6.5 L, WC, indirect injection	Varying engine speed and load	↑ BMEP; ↑ BSFC at high load and low engine speed	HDRD was found better than PBD fuel	[31]
HDRD and PBD	1C, 1S, DI, AC	Constant engine speed at varying load	Higher mechanical efficiency and BTE with increased load; BSFC decreased with increased engine load	HDRD performed better than PBD fuel	[32]
HDRD and PBD	1S, NA, 0.435 L, 6.2 kW, common rail	Varying engine load and speeds	Reduced BSFC at all load and speeds	HDRD is better than PBD fuel	[33]
HDRD, biodiesel, and PBD	1.5 L passenger car, intercooler	Varying engine load and speeds	Reduced BSFC at all load and speeds	Lower fuel consumption than biodiesel and PBD fuel	[34]
HDRD and biodiesel	1C, 4S, DI, 4.3 kW	Varying loading	↑BSFC at higher loading; ↑ BTE as loading increases	HDRD performed better than biodiesel	[35]
HDRD, biodiesel, PBD, and their blends	1C, 4S, DI, water-cooled	Varying engine loads	Higher BSFC, BTE, and EGT than biodiesel and PBD	HDRD is preferred over biodiesel and PBD	[36]
HDRD and PBD fuel blends	1C, 4S, common rail AVL 501 heavy duty engine	Varying engine loading conditions	↑BSFC increased by 2.8% compared with PBD; ↑HDRD displayed better BTE	HDRD performed better than PBD in heavy duty CI engines across engine loads	[37]
HDRD and PBD blends	1C, 4S, common rail Ricardo Hydra light duty engine	Varying engine loading conditions	↑Better BSFC compared with PBD; ↑HDRD displayed better BTE	HDRD was adjudged a better fuel than PBD in light duty CI engines across engine loads	[37]
HDRD and biodiesel blends	4C, DI, WC, 1.9 TDI diesel engine	Varying engine loads	↑ Improved BTE and BSFC across the loading condition	HDRD performed better than biodiesel and the blends	[38]

Table 4.

Performance of CI engine fueled with HDRD.

↑ = increased, ↓ = decreased, L = liters, C = Cylinder, S = Stroke, DI = Direct injection, NA = Naturally aspirated, AC = air-cooled, WC = water-cooled.

In research, Janarthanam et al. [35] compared the engine performance of HDRD with that of biodiesel in a vertical single-cylinder, four strokes, and 4.3 kW Kirloskar engine across engine loads. They reported that HDRD showed higher BSFC and BTE, particularly at higher engine loads. They attributed these results to the kinematic viscosity and calorific value of HDRD. Similarly, Singh et al. [36], compared the performance of HDRD with biodiesel and PBD blends in a single cylinder, four strokes 3.5 kW direct injection water-cooled test rig at various engine loads. They reported a higher BSFC, BTE, and EGT with HDRD than with biodiesel and PBD blends, as shown in Figure 6. According to them, higher calorific value and cetane index of HDRD compared with biodiesel and PBD accounted for these results. Though HDRD has not been widely used in CI engines, a few reported cases show that HDRD is a better alternative to PBD fuel when compared with biodiesel. The properties of HDRD are a major factor propelling the application of HDRD as a viable and effective substitute for PBD fuel.

Figure 6.

BTE, BSEC, and EGT of HDRD at various engine loads [36].

Similarly, Preuß et al. [37] tested HDRD and its blends on both light and heavy duty single cylinder CI engines and compared the results with PBD fuel under various operating conditions. The heavy duty research engine was equipped with an AVL 501 single cylinder engine while the light duty research engine had a Ricardo Hydra engine equipped with a Volvo NED4 cylinder head. The authors reported that the use of HDRD in both light and heavy duties engines led to improved BTE and BSFC for all the engine loading conditions. They attributed these results to the high oxygen content and lower heating value of HDRD compared to PBD and their blends. Using HDRD and biodiesel blends in a 4 cylinder 1.9 TDI CI engine text bed, Shepel et al. [38] reported that HDRD generated better BTE and BSFC than biodiesel and its blends. The results are due to the heating value and the specific heat of combustion of HDRD. This result confirms the assertion that HDRD is a better fuel than biodiesel for transportation applications of CI engines.

4. Emission characteristics of HDRD in CI engines

Kim et al. [34] reported the outcomes of the exhaust test carried out on a passenger car with an intercooler fueled with unblended HDRD and compared the results with when PBD fuel was used. They reported a reduction in particulate matter (PM), nitrogen oxide (NOx), carbon monoxide (CO), and total hydrocarbon content (THC) emissions. They attributed these results to the properties of HDRD which allows more complete combustion. Mangus et al. [33] also reported the same pattern of results affirming that the CI engine fueled with HDRD emits less NOx, CO, PM, and THC than when the same engine is fueled with PBD fuel under the same engine speed and load. In another research, da Costa et al. [39] reported that a single cylinder power generation CI engine operated with HDRD synthesize from sugarcane emitted less CO, HC, NOx, and PM when compared with PBD fuel. The same pattern of results was reported by Ogunkoya et al. [32], Vojtisek-Lom et al. [40], and Na et al. [41] who, in their separate studies, affirmed that CI engines fueled on HDRD emitted less CO, CO₂, HC, NOx, and soot.

However, Karavalakis et al. [42] and Gysel et al. [43] reported a slight increment in the CO, CO₂, NOx, and PM emissions in their studies, as shown in Table 5. The higher PM was attributed to the higher cetane number of the tested HRDR fuel which promoted the growth of the diffusive combustion. The higher NOx and PM emissions eliminate the benefits of the aromatic-free characteristics associated with using HDRD fuels. The emission of two greenhouse gases, CO₂ and N₂O were found to be lower with the use of HDRD. This is one of the benefits of the application of HDRD in CI engines. Janarthanam et al. [35] studied the emission characteristics of compared the engine performance of a vertical single cylinder, four strokes, 4.3 kW Kirloskar fueled with HDRD and biodiesel. They reported lower emissions of CO, HC, NOx, and smoke due to higher methyl esters and oxygen contents of the tested HDRD. Similarly, Singh et al. [36], compared the performance of HDRD with biodiesel and PBD blends in a single cylinder, four strokes 3.5 kW direct injection water-cooled test rig at various engine loads. They reported that HDRD generates lower CO, UHC, and smoke but higher NOx emission compared with biodiesel and PBD and their blends (Figure 7). Reduction in CO, CO₂, and smoke emissions were due to higher oxygen content and cetane index of HDRD while the increment in NOx emission was attributed to the higher cetane index, ignition delay, higher cylinder temperature, and pressure as compared to biodiesel and PBD [36]. When HDRD and biodiesel blends were tested a six-cylinder, 6.37 L Mercedes-Benz CI engine equipped with a turbocharger and intercooler, HDRD generated less NOx but more PM emissions. The NOx and PM emissions generated from HDRD were found to be lesser than that from PBD and within the acceptable Euro III limit, as shown in Figure 8 [44].

Fuel tested	Engine parameters	Test conditions	Result of the test	Remark	Ref.
HDRD, biodiesel, and PBD	1.5 L passenger car, intercooler	Varying engine load and speeds	↓reduced CO, NOx, PM, and THC emission	HDRD emits less dangerous gases	[34]
HDRD and PBD	1S, NA, 6.2 kW, common-rail	Varying engine load and speeds	Emission of less NOx, CO, PM, and THC	Emission of less dangerous gases	[33]
HDRD and PBD	1C, 4S, NA, AC,	Varying engine loads	Less CO, HC, NOx, and PM	Emission of fewer pollutants	[39]
HDRD and PBD	1C, 1S, DI, AC	Varying load	Lower emission of CO, CO2, HC, NOx, and soot	HDRD generates fewer pollutants than PBD fuel	[32]
HDRD and PBD	6C, turbocharged, WC, common rail	Varying engine loads and speeds	Lower HC, CO, CO2, NOx	HDRD produces less toxic emissions	[40]
HDRD, biodiesel, and PBD	Freightliner truck with 2000 C15 Caterpillar engine	Test cycles	Reduced CO, THC, PM, and NOx	HDRD generates fewer exhaust gases than biodiesel and PBD fuel	[41]
HDRD and PBD blends	6C, 2014 Cummins ISX15 400ST diesel engine	Engine load and blends	Lower, CO, CO2, THC; Higher NOx, PM	The use of HDRD provides some emission benefits	[42]
HDRD and PBD	12C, 4S, Caterpillar D398 engine	Engine load	Reduced NOx; A slight increment in CO, CO2, and PM	The use of HDRD provides some emission benefits	[43]
HDRD and biodiesel	1C, 4S, DI, 4.3 kW	Varying loading	Reduced CO, HC NOx, and smoke emissions	HDRD generates lesser exhaust gases than biodiesel	[35]
HDRD, biodiesel, PBD and their blends	1C, 4S, DI, water cooled	Varying engine loads	↓CO, UHC, and smoke opacity emissions; ↑ NOx emission	HDRD generates lower CO, UHC, smoke but higher NOx emission compared with biodiesel and PBD	[36]
HDRD, PBD, and biodiesel	6C, 2014 model year Cummins ISX15 400ST diesel engine	Varying engine loads	↓CO, CO2, and smoke emissions; ↑ NOx emission	HDRD generates lower CO, CO2, smoke but higher NOx	[40]
HDRD and biodiesel blends	6C, 6.37 L, Mercedes-Benz engine turbocharger and intercooler	Varying engine loads	↓ NOx emission; ↑ PM emission	Lower NOx but higher PM emission than PBD and biodiesel; Emissions within Euro III limits	[44]
HDRD and PBD fuel blends	1C, 4S, common rail AVL 501 heavy duty engine	Varying engine loading conditions	↑ Slight increment in NOx emission; ↓ About 50% reduction in PM and soot emissions	The use of HDRD contributed to improved air quality	[37]
HDRD and PBD blends	1C, 4S, common rail Ricardo Hydra light duty engine	Varying engine loading conditions	↑ Slight increment in NOx emission; ↓ About 50% reduction in PM and soot emissions	HDRD fuel ensured lower soot and improved air quality	[37]
HDRD and biodiesel blends	4C, DI, WC, 1.9 TDI diesel engine	Varying engine loads	↑ 8% CO2 emission; ↓ 15% CO emission; ↓ 18% smoke emission; ↓ 14% HC emission; ↓ 19% NOx emission	HDRD was more ecologically beneficial than biodiesel fuel	[38]
HDRD and PBD fuel	Euro 3, 51 kW Fiat Panda vehicle	Varying engine loads	↓27% HC; ↓ 30% NOx; ↓18% CO; ↓ 3% CO2; ↓5% PM	HDRD will contribute to the attainment of air quality and environmental sustainability	[45]

Table 5.

Emission characteristics of CI engine fueled with HDRD.

↑ = increased, ↓ = reduced, L = liters, C = Cylinder, S = Stroke, DI = Direct injection, NA = Naturally aspirated, AC = air-cooled, WC = water-cooled.

Figure 7.

Emission characteristics of HDRD at various engine loads [36].

Figure 8.

NOx and PM emissions of HDRD [44].

Similarly, light and heavy duty CI engines were fueled with HDRD and PBD fuels blends across various engine loads. The light duty engine was fixed a single cylinder, common rail, Ricardo Hydra, and Volvo NED4 cylinder head engine while the heavy duty engine consisted of a single cylinder, common rail, AVL 501, and Volvo D13 cylinder head. The outcome of the emission characteristics showed HDRD a slight increment in NOx emission and reduction in PM and soot emissions for both engine types fueled with HDRD across tested engine loads [37]. Shepel et al. [38] reported a reduction in CO, smoke, HC, and NOx emissions when HDRD was tested in a 4 cylinder, direct injection, water cooled, 66 kW, 1.9 TDI diesel engine test blend and the results compared with biodiesel fuel. There was, however an increment of 8% in CO₂ emission which was a result of the higher oxygen content of HDRD compared to other tested fuels. Similar results were obtained when Dobrzyńska et al. [45] tested both HDRD and PBD fuels on a Euro 3, 51 kW Fiat Panda vehicle. They recorded a 27% reduction in HC, 30% in NOx, 18%, in CO, 3% in CO₂, and 5% in PM emissions. They concluded that the adoption of HDRD as fuel for CI engines, particularly in the transport sector will reduce the emission of environmentally hazardous gasses, ensure cleaner air quality, and ultimately improve human health.

5. Implications and justifications

The increased utilization of HDRD as transport engine fuel has triggered renewed interest in R & D and funding of the production infrastructure across the globe. Also, the share of HDRD in global biofuel moved from about 5% in 2019 to about 10% in 2021 is pointed to its increased global production capacity. This trend, which is expected to continue, typifies the concerted efforts by countries to increase their share of renewable fuel in their energy mix. The simple production method, low cost feedstock, ecofriendly nature, improved performance, and moderate emission generated from HDRD, in comparison with PBD and biodiesel makes HDRD a fuel of the future.

The outcome of most research showed that HDRD increased the BTE of the tested CI engine by more than 20%. For example, Kumar et al. [17] reported an increase in BTE from 21 to 23% at lower engine loads. However, at higher loads, a 34%, 36% and 32% increment were recorded at 40%, 80%, and 100% engine loads, respectively. In terms of emission, most studies reported a reduction of about 15%, 30%, 35%, and 75% reductions in NOx, HC, CO, and smoke emission at full load conditions. However, some authors reported that the NOx is unchanged while some reported an increased NOx emission from HDRD fueled CI engine. The aggregate of opinions suggests that HDRD performs better and generates fewer emissions than biodiesel, and PBD. This is very significant because it justifies increased investment in the production and utilization of HDRD.

When compared with HDRD with PBD and biodiesel, available information shows the preference for HDRD by most researchers and consumers. For example, using the major performance criteria, HDRD performed better as ICEs fuel than PDF and biodiesel. Also, the cost of production of HDRD is comparably lower than that of biodiesel. Just like biodiesel, HDRD is generated from lignocellulosic biomass, waste oils, and animal fats. HDRD is not only ecofriendly, and cost effective but also safeguards the environment by emitting fewer toxic gases. Table 6 compares the performance, emission, production, and application of PBD, biodiesel, and HDRD.

Parameter	PBD	Biodiesel	HDRD	Ref.
Engine performance	Poor performance in ICEs	High BSFC at high load and low engine speed Good thermal efficiency	BTE improved by 20–23% Better mechanical efficiency Lower BSFC	[32, 33]
Emission Characteristics	High emission of CO, CO2, smoke, and PM	Low emission of CO High emission of NOx	HC reduced by 30% CO was 35% lower NOx reduced by 15% Smoke reduced by 75%	[33, 41]
Renewability	Nonrenewable	Renewable though can affect food chain	Renewable	[36]
Sustainability	Not sustainable	Ecologically and environmentally sustainable High water consumption	Highly sustainable Improved sustainability by using waste oil and fats as feedstock Biooil and biodiesel can be used as feedstock. Lower water footprint	[46]
Cost of production	High cost of crude oil exploration High cost of refining	Moderately high	Reduced cost of production	[35]
Production infrastructure	Complex and expensive	Costly	Can be upgraded	[36, 46]
Application	Used in trucks and stationary engines Used for earth moving equipment and agricultural machinery Used for house heating	Alternative fuel for ICEs Viable liquid biofuel turbine.	Useful in ICEs Viable bio-jet fuel for aviation gas turbine. Supplemental fuel for turbines.	[47]

Table 6.

Comparison of PBD and biodiesel with HDRD.

6. Conclusions

The utilization of HDRD as CI fuel is to assist in energy security and provide sustainable and environmentally friendly alternatives to the use of PBD fuel. Though the use of biodiesel, bioethanol, and biogas have been well established with notable advantages, HDRD is to help fill the performance gap created by these renewable fuels. The use of HDRD ensures better engine performance creating more options for running a CI engine. One of the disadvantages of using biodiesel in an unretrofitted CI engine is the emission of NOx. The use of HDRD emits less NOx in most cases. In other to reduce the emission of NOx, the concentration of hexadecane and dodecane in the fuel should be increased. This can be achieved during production by altering the feedstock after production by the addition of additives. This however negates the idea of the carbon chain length effect on NOx emission [48].

In the current effort, the performance and emission characteristics of using HDRD in a CI engine have been presented. HDRD is a sustainable replacement for PBD fuels and a more effective renewable fuel than biodiesel. The application of HDRD in CI engines allows improved mechanical efficiency, BTE, and reduced fuel consumption across all engine loads and speeds. CI engines fueled with HDRD are reported to generate less CO, CO₂, NOx, and PM when compared with PBD fuel. Though the production process for HDRD is more complex and expensive than biodiesel due to the high temperature and pressure involved, the overall advantage of using HDRD surpasses that of biodiesel.

Going forward, more investigations are needed to simplify the production process of HDRD to domesticate the procedure. More awareness is needed to popularize the production and utilization of HDRD among the population. There should be tax holidays and other incentives for the producers of HDRD as a way to encourage its production and utilization. Governments, across jurisdictions, should provide more funds for R & D in the feedstock, production techniques, standardization, and utilization of HDRD for various applications.

Conflict of interest

The authors declare no conflict of interest.