Experimental investigation on the effects of bioethanol addition in diesel-biodiesel blends on emissions and performances of a micro-cogeneration system

https://doi.org/10.1016/j.enconman.2019.01.097Get rights and content

Highlights

  • Pollutant emissions are reduced due to the use of bioethanol.

  • Smoke density level is strongly reduced thanks to bioethanol addition.

  • Electrical efficiency is increased due to improved combustion mechanisms.

  • Thermal performances are enhanced with all the diesel-biodiesel-bioethanol mixtures.

Abstract

Internal combustion engines are well spread in traditional power-generation systems: given their reliability, low-cost and performance it is desirable to avoid their full substitution in the short term, but, rather, to rely on the reduction of their environmental impact. This can be achieved by implementing solutions that involve minor modifications on the devices, such as the use of different fuels. In this work, the fossil fuels replacement approach has been applied to a micro-cogeneration system based on a small water-cooled compression ignition engine. The effect of bioethanol introduction as a diesel substitute in six diesel-biodiesel blends has been studied in terms of performances of the whole energy-conversion system and of the exhaust gas emissions. Positive results have been obtained showing that with a small amount (3%) of bioethanol, enhancements can be fulfilled both on performances (with approximately an average 13%-boost of the electrical and thermal efficiency) and emissions (reaching nearly a 80% of smoke opacity reduction).

Introduction

Global warming and depletion of fossil resources are boosting research and development projects, on a worldwide basis, to lead the contemporary energy transition to a more efficient energy production and a more environmentally-friendly future [1]. In this framework, combined heat-and-power-generation systems (CHP) represent a crucial path towards a more sustainable energy-conversion paradigm. Indeed, the concurrent use of two different (thermal and electrical) energy conversion modes ensures a higher global efficiency when compared to traditional thermal energy system [2].

Internal combustion engines (ICEs) are the most relevant thermal engines in the field of micro cogeneration. In the size range of 1–5 kW, thanks to their high power-to-weight ratios and low capital cost, ICEs are the most diffused (when compared to Stirling engines or organic Rankine cycle energy system) and suitable energy conversion system to be applied for cogeneration purposes [3]. On the other hand, sustainability of the primary energy sources and polluting emissions (together with noise and vibrations, especially for domestic applications use) are the main challenges of this technology [3]. A possible solution to limit the environmental footprint of ICEs is the use of alternative biofuels.

Biodiesel has been proposed and used in several studies [4], [5], [6], as alternative fuel in internal combustion engines. Its attractive properties in terms of biodegradability and non-toxicity represent a major factor of consideration together with its biological derivation, that plays a huge role in terms of overall life-cycle emission of carbon dioxide (CO2) [7]. Biodiesel can be produced from vegetable oils or animal fat feedstocks through alcohol transesterification [8]. The majority of the reviewed studies show the positive effect of the use of biodiesel on hydrocarbon (HC), carbon monoxide (CO) and particulate matter (PM) emissions. Nitrogen oxides (NOx) emissions are influenced by opposite types of phenomena. Biodiesel is characterized by the absence of aromatics compounds and by a high cetane number. Cetane number is, indeed, one of the most relevant parameters in terms of combustion quality in compression-ignition engines [9]. Its value is directly proportional to the length of the fuel molecules’ carbon chain [9]; a high cetane number is related to a short ignition delay time that consequentially leads to lower premixed combustion temperature. So, a reduction in NOx production could be observed. On the other hand, oxygen ensures higher reactants to the Zeldovich mechanism. NOx formation depends, indeed, directly on two factors: the concentration of reactants and their temperature [10]. Experimental studies [11], [12] agree that this hydrocarbon oxidation brings in a strong increase in NOx production. Such increase prevails over the effect of the limiting factors correlated to the NOx formation (such as the absence of aromatics and the high cetane number). Hence, it can be stated that, due to its high oxygen-content, biodiesel is responsible for an acceleration of NOx production [5]. However, oxygen content in biodiesel entails a positive effect in terms of reduction of particulate matter (PM) emissions, mainly caused by a reduced soot formation and an enhanced soot oxidation [12], [13], [14]. Nevertheless, under cold-start conditions this favorable effect is not always noticed and could be even reversed [12]. Given these emissions figures, a proposed control strategy for biodiesel combustion consists in delaying injection as a mean to limit the increase in NOx emissions, with a minor penalty in particulate emissions [12]. Some drawbacks have limited biodiesel diffusion in today’s market as a diesel-alternative fuel. Low volatility, high density, high viscosity and high pour point are commonly considered the main biodiesel disadvantages, affecting the fuel droplets formation and distribution, the atomization quality of fuel the fuel spray and the uniformity of the mixture [8], [9]. One of the methods for the enhancement of biodiesel properties as an alternative fuel for compression ignition engine is to blend it with other fuels, properly chosen in order to improve the overall characteristics of the fuel blends. Blending biodiesel with regular diesel is a typical mixing strategy [8] but other blending approaches can be used, to realize binary and ternary blends.

Alcohol fuels addiction represents a valid strategy to improve biodiesel use in compression-ignition engines. Due to the low density and viscosity of alcohol, biodiesel blends with alcohol overcome some of the drawbacks of the pure biodiesel fuel [9]. On the other hand, alcohols generally have a cetane number lower than pure diesel fuel. Typically, diesel engines require a cetane number between 45 and 60; ethanol, methanol and butanol have, instead, cetane numbers in the range of 8 – 25, which determines poor auto-ignition properties and serious combustion-stability problems [9]. Hence, the use of pure alcohols as diesel additive in compression ignition engines is very rare. Therefore, the addition of alcohol to biodiesel blends, to realize binary or ternary fuel blends depending on the presence of regular diesel, could meet both pure fuels needs when used in compression ignition engines. Ethanol is commonly used in combustion as an alternative fuel for spark ignition engines [15], or as an additive to diesel fuel, thanks to its miscibility with diesel fuel [9], [14], [16]; it can be obtained starting from coal and petroleum-based fuels, as well as from renewable sources as corn, sugar beets, sugar cane, sweet sorghum, barley, cassava or agricultural residuals such as raw materials, waste woods and straw [11]. According to Mofijur et al. [17] the use of both biodiesel-diesel and biodiesel-diesel-ethanol blends shows, in most of the recent research activities, a reduction in CO emissions, due to the high oxygen-content of biodiesel and ethanol, an increase in NOx emissions, which can be however reduced with exhaust gas recirculation (EGR), and an increment of hydrocarbons (HC) and PM emissions.

Engine performances are strongly affected by the combustion strategy management, the type of engine and the amount of ethanol present in the blend [18], [19]. The addition of ethanol decreases the low heating value (LHV) and, hence, the energy density of the fuels blend; on the other hand, kinematic viscosity decreases, enhancing the quality of combustion thanks to the improvement of the spray droplets pattern dynamic. Therefore, different indications can be found in literature, addressing performances enhances [20], limitations [18] or stability [9] to ethanol addition in diesel-biodiesel blends. The proposed work fits in this framework and aims at better understanding the controversial effects of ethanol addition in diesel-biodiesel blends on performances and emissions in micro-cogeneration systems (below 10 kW). Given the actual state of the art, thermal recovery characteristics are neglected most of the times, and full CHP characterizations are hard to be found in the small-scale.

Therefore, this work, based on the fossil fuel substitution approach, presents the following novelties: i) it is an investigation on a wide range of biodiesel shares in binary and ternary mixes; ii) it gives an insight on the cogeneration performance and emissions of a small-scale diesel engine fuelled with alternative fuels; iii) it shows how the use of a low percentage of bioethanol (3%) can take advantage of the positive effects of alcoholic fuels, avoiding performance derating due to their lower LHV and reliability issues, due to their physical and chemical properties.

Section snippets

Experimental test bench

An integrated cogeneration-system test bench has been set-up in the Biofuels and Bioenergy Lab of the Free University of Bolzano. A water-cooled naturally-aspirated single-cylinder diesel engine (Farymann 15 W430, see Table 1) is coupled with a synchronous, brushless, AC-powered, watercooled electrical generator. Commercially known as Paguro 4000, this marine diesel genset has been modified with the implementation of an external cooling water-circuit to recover the thermal power losses in a

Performances

Electrical efficiency ηe is obtained through the measured output electrical power and has been used also to quantify the Specific Fuel Consumption (SFC). Fig. 3 shows the trend of the electrical efficiency for the tested binary and ternary blends.

Among all the binary blends, B75 has shown the highest values of electrical efficiency, with a peak value of 22.15% at full load (load condition: E). The second and third best mixtures at the same load resulted to be B15 and B50. At load A, B100

Conclusions

An experimental characterization of a micro-CHP system has been carried on, aiming at monitoring emissions and cogeneration performances with the use of different fuel blends in a single-cylinder water-cooled compression-ignition engine. A set of six diesel-biodiesel mixtures has been tested and the effect of bioethanol introduction as a diesel substitute has been studied. The system has been tested under five different load conditions, ranging from 0.74 to 3.7 kW. Enhancements on both

References (31)

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