ARTICLE pubs.acs.org/EF
Comparison of Ethanol and Butanol as Additives in Soybean Biodiesel Using a Constant Volume Combustion Chamber Haifeng Liu,†,‡ Chia-fon Lee,*,‡ Ming Huo,‡ and Mingfa Yao† † ‡
State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States ABSTRACT: To investigate the effects of different alcohol additives in biodiesel fuel on the spray, combustion characteristics, and soot formation and oxidation, a detailed comparative study between the butanolbiodiesel blend and the ethanolbiodiesel blend was carried out in an optical constant volume combustion chamber. Two different volumetric blend fuels were tested in this study with different ambient temperatures at the start of injection (from 800 to 1200 K). The volumetric ratios were the 20% butanol/80% soybean biodiesel referred to as B20S80 and 20% ethanol/80% soybean biodiesel referred to as E20S80. Results demonstrated that the microexplosion occurred for B20S80 and E20S80 fuels at 800 and 900 K ambient temperature because of the volatility difference between the additives (butanol or ethanol) and the base fuel (biodiesel). The E20S80 fuel presented higher peak pressure and shorter combustion duration compared to the B20S80 fuel. The autoignition was earlier for the B20S80 fuel at 1000 and 1200 K ambient temperature, while the autoignition of the B20S80 and E20S80 fuels was nearly the same at 800 K ambient temperature. The E20S80 fuel had a lower flame luminosity compared to the B20S80 fuel. The soot distribution was increased downstream of the spray jet with a higher ambient temperature for both tested fuels, and E20S80 had a lower value of normalized time-integrated soot mass (NTISM). Therefore, E20S80 has more advantages to reduce the soot emission compared to the B20S80 fuel. Also, increasing the ambient temperature from 800 to 1200 K led to a rapid increase in the value of NTISM for both tested fuels. Therefore, a lower ambient temperature with the piston at top dead center (TDC) should have more advantages to combustion and soot control in a real diesel engine.
1. INTRODUCTION Diesel fuel is a specific fractional distillate of petroleum fuel oil and is largely consumed by the transportation sector. Alternatives that are not derived from petroleum, such as biodiesel, are increasingly being developed and adopted. Biodiesel is defined as the monoalkyl esters of long-chain fatty acids derived from various feedstocks, such as vegetable oil, animal fat, algae, etc. Biodiesel has many similar properties to diesel fuel, and it can blend with conventional diesel fuel in any proportion. Studies with various biodiesel blend ratios or the neat biodiesel have demonstrated that biodiesel-fueled diesel engines could reduce emissions of carbon monoxide (CO), total hydrocarbons (THCs), and particulate matter but slightly increase brakespecific fuel consumption because of the reduction in heating value, while the power output for biodiesel was almost the same as that for diesel fuel.19 Most of the studies reported a slight increase in nitrogen oxides (NOx) emissions using biodiesel fuels. However, this NOx emissions problem could be eliminated by advanced injection strategies and exhaust gas recirculation (EGR).2,4,9 In addition, studies have also demonstrated that biodiesel had low or no greenhouse gas emissions (in net) by the well-to-wheels analysis.10,11 Thus, it can be seen that biodiesel is able to be used as a substitute for diesel fuel in a diesel engine with lower harmful gas emissions. Most of the properties of biodiesel fuels are comparable to those of diesel fuel, except for cloud point and pour point, which indicate that the cold flow behavior of a fuel is poor. In addition, the viscosity of biodiesel is higher than that of the diesel fuel, which will affect the spray characteristics and subsequent combustion r 2011 American Chemical Society
processes. Studies have demonstrated that the low-temperature flow properties of biodiesel could be improved by blending ethanol with various volumetric ratios (020%).1214 Further, a 20 vol % ethanol blending into biodiesel has been reported to achieve improved combustion with a reduction in CO, THC, NOx, and smoke emissions without affecting the break thermal efficiency.12,15 The addition of ethanol with a 1030% blend ratio decreased the droplet size and improved the atomization performance of biodiesel fuel because of the lowered kinematic viscosity.1618 On the other hand, butanol is a very competitive alcohol to be applied in diesel engines and is becoming popular recently. Similar to ethanol, butanol is a biomass-based fuel that can be produced by alcoholic fermentation of the biomass feedstocks. A study has shown that the solubility of dieselbiodieselbutanol was higher than the solubility of dieselbiodieselethanol at temperatures of 1030 °C,19 and another study has also demonstrated that ethanoldiesel depicted poor blending stability compared to butanoldiesel blends.20 The miscibility of butanol with biodiesel was excellent compared to ethanol at a wide range of operating conditions.21,22 Furthermore, fuel properties illustrated in Table 1 indicate that butanol has the potential to overcome the drawbacks brought by ethanol in diesohol fuels,1926 i.e., higher heating value, less vapor lock tendency because of lower volatility, less ignition problems because of Received: January 20, 2011 Revised: March 28, 2011 Published: March 28, 2011 1837
dx.doi.org/10.1021/ef200111g | Energy Fuels 2011, 25, 1837–1846
Energy & Fuels
ARTICLE
Table 1. Properties of Ethanol, Butanol, Soybean Biodiesel, and the Blend Fuels properties
ethanola
butanola
soybean biodiesel
B20S80
E20S80
(ASTM method)
(ASTM method)
(ASTM method)
molecular formula
C2H5OH
C4H9OH
CH3OOCR
cetane number
8
25
51 (D 613)
49.2 (D 976)
47.8 (D 976)
octane number
108
96
oxygen content (wt %)
34.8
21.6
10
12.32
14.96
density at 15 °C (g/mL)
0.795
0.813
0.887 (D 1298)
0.871 (D 1298)
0.869 (D 1298)
autoignition temperature (°C)
434
385
363b
flash point at closed cup (°C) lower heating value (MJ/kg)
8 26.8
35 33.1
173.9 (D 93) 37.53 (D 240)
36.64
35.38
boiling pointc (°C)
78.4
117.7
329.7 (D 1160)
318.6 (D 86)
303.5 (D 86)
stoichiometric ratio
9.02
11.21
12.5
12.24
11.80
latent heating at 25 °C (kJ/kg)
904
582
200b
276.4
340.8
4.0 (D 445)
3.68 (D 445)
3.08 (D 445)
flammability limits (vol %)
4.319
1.411.2
saturation pressure at 38 °C (kPa)
13.8
2.27
viscosity at 40 °C (mm2/s)
1.08
2.63
total glycerin (wt %) free glycerin (wt %)
0.142 (D 6584)
