خودآتشگیری مخلوط های N-بوتانول و اتانول با سوخت دیزلی و بیودیزلی در محفظه احتراق حجم ثابت

ABSTRACT Autoignition of blends of n-butanol and ethanol with diesel or biodiesel fuels in a constant-volume combustion chamber

The continued depletion of oil reserves, the fluctuating but net increase in fuel prices, the new policies promoting the use of advanced biofuels and the increasingly harder restrictions on emissions for vehicle engines, have led researchers to seek new forms of energy that reduce the dependence of fossil fuels. In this sense, new fuels from waste or lignocellulosic materials or alternative production techniques appear to be able to reduce life-cycle greenhouse emissions and thus to contribute to restrain global warming.

Diesel fuels can be blended with bioalcohols as a means to introduce a renewable fraction and to provide certain oxygen content. This renewable fraction could be additional to that already included in many diesel fuels with some biodiesel content, as it is usual in many countries. Some studies, performed in different setups and under different conditions, have reported reductions in emissions of particulate matter and carbon monoxide when diesel engines operate with ethanol and butanol blends [1], [2], [3], [4], although not unanimous trends have been reported in nitrogen oxides and unburned hydrocarbons especially at low loads [3], [5], [6], [7], [8]. The hydroxyl group of the alcohol molecule contributes to reduce soot formation and consequently particulate emissions, even more than other functional groups with similar oxygen content [9], [10], [11], [12], [13]. Among the alcohols to be used in diesel blends, ethanol and butanol have proved to have a significant potential to reduce life-cycle greenhouse gas emissions, as far as they can be produced from biological processes. Specifically, butanol can be produced from various methods, among which acetobutylicum fermentation (ABE) has superior interest, as far as the bio-acetone obtained as co-product is valued as such [14]. Ndaba et al. [15] suggest that some chemical conversion routes are preferable to the conventional ABE process, because the reaction proceeds more quickly compared to the fermentation route and fewer steps are required to produce n-butanol. Zheng et al. [16] made a review on the latest advances in butanol fermentation particularly from the perspective of genetic engineering and fermentation technology.

A previous work was published about blending stability, lubricity, viscosity and cold filter plugging point as the key properties of ethanol and butanol blends (among others) with diesel fuel [17]. Additionally, among the properties affecting the combustion process, cetane number is a limiting one. Alcohols exhibit low cetane numbers, and therefore, only minor concentrations of these alcohols in the blends are recommended for use in unmodified diesel engines. The higher cetane number of butanol with respect to ethanol, together with its better miscibility, higher heating value and lower hydrophilic character, suggest that n-butanol is a better renewable component than ethanol in diesel blends [18], [19], and consequently, its maximum concentration in diesel blends could be increased with respect to that recommended for ethanol.

Cetane number of a fuel is defined as the concentration of n-hexadecane in heptamethylnonane providing the same autoignition delay as that of the fuel. However, blends of these reference fuels do not provide autoignition times proportional to their concentrations. Similarly, blends of alcohols with diesel or biodiesel blends are not expected to provide autoignition times proportional to their composition. Instead, the most reactive components (those with shorter autoignition times) contribute to shorten the autoignition time more that the least reactive ones to enlarge it [20]. Therefore, experimental autoignition studies with these blends are necessary to evaluate their autoignition behavior.

Other studies have been previously published reporting autoignition results from alcohol blends in diesel or biodiesel fuels, or surrogates, in constant-volume combustion chambers, but far from modern diesel-like injection conditions [21], [22], [23].

The impact of ethanol and butanol concentrations on the autoignition time of diesel and biodiesel blends is analysed in this study. The effects of the initial temperature and pressure, as well as that of the equivalence ratio, are also studied for 10% alcohol blends.