Combined Effect of a Catalytic Reduction Device with Waste Frying Oil-Based Biodiesel on NOx Emissions of Diesel Engines ()
1. Introduction
The emissions from internal combustion engines (IC) include but are not limited to; NOx, CO, hydrocarbons (HCs), and particulate matter. A singular IC engine such as an automotive engine will expel an insignificant amount of NOx into the atmosphere. However, collectively in a group, internal combustion engines emit most of the total anthropogenic NOx. For more than three decades, regulatory agencies have addressed NOx emissions; however, there is an appetite for more stringent NOx control measures beyond current requirements. The appetite for more stringent NOx control is based on the role that NOx emission plays in the development of ground-level ozone and photochemical smog (EPA, 2002 [1] ); (Clean Air Technology Center, 1999 [2] ). Biodiesel is a fuel type that continues to be evaluated as replacing diesel fuel in the automotive industry. Although it is sourced from many materials such as cooking oil and animal fat, it is cleaner, renewable. It can serve as a suitable replacement or an additive to diesel fuel. More so, it offers a high heat content, high density, and better lubricating properties (Barabas and Todoru, 2012 [3] ; Shahid and Jamal, 2011 [4] ; Murugesan et al. 2009 [5] ; Fasogbon and Asere, 2014 [6] ). In addition, biodiesel is very different from conventional diesel due to its physicochemical properties. However, with biodiesel, there is reduced emission of carbon monoxide (CO), particulate matter (PM), and hydrocarbon (HC), it also causes higher NOx emissions primarily because of the presence of oxygen in the oil (Murugesan et al., 2009 [5] ; Fasogbon, 2015 [7] ).
A literature review has shown that NOx emissions vary when diesel engines are fired with biodiesel produced from different organic sources. For example, Thangavelu and Thamilkolundhu, 2011 [8] evaluated the combustion and emission characteristics of compression ignition (CI) engine fueled with Jatropha-diesel oil blends and observed the emissions include but are not limited to; NOx emission and combustion characteristics of the blends to be comparatively higher than that of the baseline diesel. In addition, Vallinayagam et al., 2013 [9] also investigated a Kirloskar stationary CI engine fueled using pine oil blends while loading the engine with an eddy current dynamometer at varying loads. The study observed a significant reduction in NOx emission by 15.2% compared to pure diesel and concluded that pine oil biofuel positively impacts the atmosphere.
Although research is still ongoing on improving the quality and yield of biodiesel from waste frying oil (WFO), the idea of converting WFO to biodiesel originates from the perspective of a waste management approach (Banerjee et al., 2014 [10] ). The significant characteristics of WFOs concern high levels of free fatty acids, density, and viscosity. However, in producing biodiesel from WFOs, several factors such as; the type of oil source, duration of use, and the nature of the fried food products largely influence the quality of the biodiesel for use as fuel (Al-Kofahi, 2017 [11] ; Shaban, 2018 [12] ). Interestingly, with waste frying oil (WFO) biodiesel, studies have shown variations in NOx emission, for example, Al-Kofahi, 2017 [11] ; Guo et al., 2012 [13] ; and Sanli, 2018 [14] , reported an increase in NOx emission when using pure WFO biodiesel as against using pure diesel while Koçak et al., 2007 [15] , and Utlu et al., 2008 [16] , reported a decrease in NOx emission and some others have reported no significant effect on the NOx emission (Dennis, 2001 [17] ). The variations in the NOx emission as observed from using WFO biodiesels in the various studies were probably because of the increased sensitivity of NOx emission due to engine combustion conditions and the differences in the chemical properties of the WFO as well as the influence of the injection timing and the subsequent premixed and diffusion burn characteristics during combustion (Benjumea et al., 2011 [14] ; Guo et al., 2012 [18] ). However, with the SCR technology, NOx emission from diesel engines powered with WFO biodiesel has significantly reduced.
The Selective Catalytic Reduction (SCR) method is an efficient approach to reducing NOx emissions from biodiesel-fueled CI engines (Sala et al., 2018 [19] ; Yang et al., 2015 [20] ). Its principle is based on injecting a reducing agent (urea) into the exhaust gas flow stream of an internal combustion engine. The urea immediately converts to ammonia, and the ammonia reacts with the nitrogen oxide in the presence of a catalyst to produce nitrogen and water as the exhaust (EPA, 2002 [1] ; Sinzenich, 2015 [21] ). With diesel fuel, the SCR system is considered a more effective means of reducing NOx emission (Sala et al., 2018 [19] ; Clean Air Technology Center, 1999 [2] ). However, the SCR technology application is faced with some challenges. One of such challenges is the need to achieve a threshold temperature of about 200˚C before injecting the urea solution into the hot exhaust gas, which in most cases is above the exhaust gas temperature (Kröcher, 2018 [22] ). To address this challenge, Sala et al., 2017 [23] in an experiment preheated and evaporated the urea solution before injecting it into the engine exhaust gas. In addition, while using biodiesel with SCR technology, because of the high concentration of impurities (potassium in the biodiesel), the catalyst [mostly V2O5-WO3/TiO2 (VWT) vanadium/titanium-based] has been observed to be deactivated due to the neutralization of the catalyst’s acid sites by the high basicity content of the potassium, thus decreasing the adsorption of NH3 (Kröcher, 2018 [22] ; Schill and Fehrmann, 2018 [24] ). Several studies have considered using SCR technology alongside biodiesel blends to further investigate and reduce the NOx emission from diesel engines. For example, Sundarraj et al., 2014 [25] achieved a maximum of 73.94% reduction in NOx emission using a urea-SCR system (with a urea concentration of 32.5%, at a constant flow rate of 0.75 lit/hr.) fitted to a CI engine operating at different loading conditions and fuelled with diesel-jatropha blends (25% of Jatropha and 75% diesel blends).
More so, Praveen and Natarajan, 2014 [26] fueled a CI engine using a diesel-ethanol blend and observed a 70% reduction in NOx emission while using a catalytic converter (TiO2)-coated catalyst with 5% urea concentration, at a constant flow rate of 0.75 litre per hr. as against 66% reduction in NOx emission obtained when the engine was fueled with pure diesel, and 76.9% reduction was also recorded in another study that combines both an SCR device and an exhaust gas recirculation (EGR) approach (Praveen and Natarajan, 2014 [26] ; Praveena et al., 2022 [27] ). Consequently, it can be concluded that biodiesel blends nonetheless, when coupled with an SCR technology, and without any engine calibration or modifications would produce a significant reduction in NOx emission in the range of 58% to 75% (Shi et al., 2008 [28] ; Praveen and Natarajan, 2014 [26] ; Sala et al., 2018 [19] ; Sundarraj et al., 2014 [25] ; Vallinayagam et al., 2013 [9] ; Yusuf et al., 2022 [29] ). In sum, a relevant review pertaining to this research is provided in [30] .
This study primarily observes the NOx emission from a diesel engine fueled using biodiesel blends obtained from waste frying oil (WFO) and investigates the influence of a small-scale selective catalytic converter on the NOx emission. The objective of the study is limited to examining the level of nitrogen oxide reduction with the catalytic converter when firing with a WFO biodiesel blend. It, however, does not cover the analysis of the biodiesel effect on the catalyst, and neither does it evaluate the engine performance while operating with the WFO biodiesel.
2. Materials and Methods
2.1. Biodiesel Production
This study produced biodiesel from waste vegetable cooking oil via a transesterification process as followed in previous studies by Dennis, 2001 [17] ; Guo et al., 2012 [14] ; Koçak et al., 2007 [16] ; and Tziourtzioumis et al., 2017 [31] . The waste cooking oil sample was collected from roadside bean cake and fried yam sellers on the streets of Agbowo, Ibadan, Nigeria. Using a simple transesterification batch process, 10.5 millilitres of the waste cooking oil sample was measured, filtered to remove residues and unwanted particles, poured into a 250 milliliter conical flask, and heated to a preselected temperature of 50˚C. A solution of potassium methoxide was equally produced using 0.25 g of potassium hydroxide pellet and 63 millilitres of methanol (catalyst concentration of 0.5% and an oil/Methanol mole ratio of 1:6). The potassium hydroxide pellet was stirred vigorously until it dissolved completely in the methanol mixture. The potassium methoxide solution was then mixed with the warm waste cooking oil, stirred vigorously with a mechanical stirrer while heating until 60˚C for about 50 minutes. The solution was kept to cool and settle for a day and later transferred to a gravity separating funnel until two distinct layers were visible. The glycerol and residual catalyst were removed, while the biodiesel was extracted, washed with warm deionized water, and heated to 30˚C to remove water. The WFO based produced biodiesel was characterised and the results presented in Table 1 and the expected standard for biodiesel properties. Diesel engine test rig for biodiesel-diesel blends with specifications is detailed in Table 2. The biodiesel blends were; pure diesel [B0]-0% biodiesel and 100% diesel [B100]; [B5]-5% biodiesel and 95% diesel; [B20]-20% biodiesel and 80% diesel; [B30]-30% biodiesel and 70% diesel; [B70]-70% biodiesel and 30% diesel [B100]-00% biodiesel and 0% diesel.
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Table 1. Characterization of Waste Frying Oil (WFO) biodiesel and comparison with standard biodiesel properties.
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Table 2. Test engine specifications.
2.2. The Selective Catalytic Converter and Reagent
The study developed a catalytic converter similar to those previously developed by Tan et al., 2020 [32] ; and Bhaskarrao and Shinde, 2015 [33] . The catalytic converter had a honeycomb structure with a cylindrical shell with length and diameter, approximately 92 mm and 69 mm, respectively. It had a converter volume of 0.3393 litres (339.23 cc), designed for an exhaust gas volume flow rate of 0.006349 m3/sec. It had a platinum catalyst and a wash-coat coated with an alloy of Al2O3. The reagent system, however, consists mainly of the following components: a storage tank, a dc pump, piping, a time relay-delay module to regulate the injection timing of the warm aqueous urea solution, an atomizer nozzle, and a 12-volt battery to power the dc pump and the relay-delay module. The study utilized a warm anhydrous aqueous urea of 30% concentration (volume/volume %) at 40˚C stored in a plastic container tightly covered to prevent contamination and for ease of handling and simplicity of design. Although the scope of this study does not cover the analysis on the effect of reagent concentration on the converter, however, the reagent set-up follows a similar approach and set-up put together in the study by Sundarraj et al., 2014 [25] ; Vallinayagam et al., 2013 [9] ; and Kumar et al., 2021 [34] .
2.3. Experimentation
Praveena et al., 2022 [27] ; Kumar et al., 2021 [34] ; and Vallinayagam et al., 2013 [9] developed a similar experimental set-up to those used in this research, as shown in Figure 1. The experiment was conducted using a four-stroke, air-cooled C.I engine with the specification given in Table 2, fueled with biodiesel blends
[B0], [B5], [B20], [B30], [B70], and [B100], and operating at a constant engine speed of 1900 rpm. The engine was connected to a Megatech DG2 dynamometer for loading varying from 25%, 50%, 75%, and 100% full load, a PCA 3 Bacharach Gas analyzer also connected to a computer for data collection, and the catalytic converter connected at the exhaust gas outlet tail end of the diesel engine, as shown in Figure 1. The warm aqueous urea stored in a container was metered and injected (using a time relay-delay module and a dc pump connected to a 12-volt battery) into the exhaust gas flow stream through a fine atomizer nozzle fixed upstream of the exhaust gas flow. The experiment maintained the procedure over time while varying the engine load; the NOx emission data were collected, and the graphs presented the results.
3. Results and Discussions
The exhaust gas temperature and NOx emission against different load ranges are plotted for the base fuel diesel [B0], pure biodiesel [B100], and the various biodiesel blends [B5], [B20], [B30], and [B70].
3.1. Exhaust Gas Temperature at Different Load Ranges
The necessary information on performance characteristics of relevant systems could be found in the work of Utlu et al., 2008 [15] . Although performance characteristics are not the focus of this work, the extraction of exhaust gas temperature provides an understanding of NOx emission. Figure 2 depicts the exhaust gas temperature relative to the load percentage revealing an increasing trend.
3.2. NOx Profile without the Use of the Catalytic Converter
Figure 3 shows a plot of NOx emission against biodiesel-fossil diesel blends without catalytic converter as an add-on technology at different loadings. It was observed that an increasing percentage of biodiesel in biodiesel-fossil diesel blends leads to an increase in NOx emission. And this observation is in tandem with Shahid et al., 2011 [4] and Fasogbon, 2015 [7] . The study observed that the oxygen richness of biodiesel could have been responsible for the increasing content of NOx; as the higher the exhaust gas temperature, the higher the NOx emission. Thus, the oxygen content/richness of Waste Frying Oil-based biodiesel must have supported combustion; thereby leading to high NOx emission emanating from high exhaust gas temperature.
3.3. NOx Profile with the Use of the Catalytic Converter
Even though the increasing percentage of biodiesel in biodiesel-fossil diesel blends leads to an increase in NOx emission, as in the case of Figure 3, the injection of a catalytic converter as an add-on technology/device significantly reduces NOx emission, as shown in Figure 4. At higher engine loads which is equivalent to higher exhaust gas temperature and NOx emission, there were higher reductions of NOx; this is because, at the higher temperature, Ammonia
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Figure 2. Plot of exhaust gas temperatures at various loads.
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Figure 3. Plot of NOx emissions for biodiesel blends without the use of the catalytic converter device.
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Figure 4. Plot of NOx emissions for biodiesel blends with the use of the catalytic converter device.
(Urea reagents) do show better reaction with NOx. This observation is in line with the work of Sala et al., 2017 [23] .
4. Conclusion
This study ascertained the NOx emission profile of a diesel engine powered with a Waste frying oil-based biodiesel at different blends and further evaluated a catalytic converter’s NOx emission reduction efficiency. With the conclusion that a combined effect of waste frying oil-based biodiesel and catalytic converter as add-on technology will yield a significant NOx emission reduction.