ARTICLE pubs.acs.org/EF
Combustion Characteristics and Soot Distributions of Neat Butanol and Neat Soybean Biodiesel Haifeng Liu,†,‡ Chia-fon F. 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, Illinois 61801, United States § Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China ‡
ABSTRACT: The combustion characteristics and soot distributions of neat butanol and neat soybean biodiesel were explored in an optical constant-volume combustion chamber using natural flame luminosity and forward illumination light extinction methods under various ambient temperatures (800 and 1000 K) and oxygen concentrations (21%, 16%, 10.5%). Results demonstrated that butanol had a higher normalized peak pressure compared with the biodiesel. Soybean biodiesel autoignited earlier than butanol at 21% and 16% oxygen, while a reverse trend was found at 10.5% oxygen. The oxygen concentration had little effect on the autoignition timing for butanol when it was between 16% and 10.5% at 8001000 K. The natural flame luminosity reduced with lowered oxygen concentration and the flame distribution was notably increased at 10.5% oxygen. At 800 K ambient temperature, there was no soot formation detected for butanol, while the net soot release for soybean biodiesel was reduced with the decrease of oxygen concentration. At 1000 K ambient temperature, the net soot release increased for both butanol and soybean biodiesel with the decrease of oxygen concentration. Compared with butanol, soybean biodiesel had a higher value of normalized time integrated soot mass (NTISM) under all conditions. The value of NTISM increased about 16 times from 800 to 1000 K for the soybean biodiesel at 10.5% oxygen, indicating that low oxygen concentration will deteriorate combustion and increase the difficulty of soot emissions control for the soybean biodiesel under a higher ambient temperature.
1. INTRODUCTION Conversion of biomass to biofuels has recently gained significant political and scientific interest owing to concerns about climate change, global energy security, and petroleum supply shortage in the foreseeable future. Diesel fuel is largely consumed by the transportation sector, therefore some biofuels have been produced to substitute for diesel fuel and reduce the dependence on diesel refined from petroleum. Among the biofuels, biodiesel is the primary alternative to diesel fuel for compression ignition engines because it has properties similar to diesel fuel and it can be blended with diesel in any proportion. Biodiesel-fueled diesel engines could reduce emissions of carbon monoxide (CO), total hydrocarbons (THC), and particulate matter but slightly increase brake specific fuel consumption due to the reduction in heating value, while the power output for biodiesel is almost the same as that for diesel fuel.19 Most studies reported slight increase in nitrogen oxides (NOx) emissions for using biodiesel fuels. However, this NOx emissions problem could be eliminated by advanced injection strategies and exhaust gas recirculation (EGR).2,4,9 Meanwhile, alcohols, mainly ethanol and to a much lesser extent methanol, have also been considered as alternative fuels for diesel engines.1017 Methanol can be produced from coal- or petrolbased fuels with low production cost, yet its solubility in diesel is within a very narrow range. On the other hand, ethanol is a biomass-based renewable fuel, which can be produced by alcoholic fermentation of sugar from vegetable materials, such as corn, sugar cane, sugar beets, barley, and other agricultural residues. Therefore, ethanol has the advantage over methanol as a renewable energy source. Butanol is a very competitive alcohol to be applied in diesel r 2011 American Chemical Society
engines and is gaining popularity recently. Like ethanol, butanol is a biomass-based renewable fuel that can be produced by alcoholic fermentation of biomass feedstocks. The physical and chemical properties of methanol, ethanol, butanol, diesel fuel, and soybean biodiesel are listed in Table 1.1821 These properties indicate that butanol has the potential to overcome the drawbacks of lowcarbon alcohols in several aspects, i.e., higher heating value, less vapor lock tendency due to lower volatility, less ignition problems due to lower latent heat, good intersolubility with diesel without any cosolvents, higher viscosity resulting in less wear problems in fuel supply system, safer usage in high temperatures due to higher flash point and lower volatility, and virtually no corrosion to existing pipelines. In addition, butanol has also a number of synergies with ethanol as follows: production from the same agricultural feedstocks as ethanol, minor changes in fermentation and distillation process as ethanol, and assisting in the conversion of vegetable oils into biodiesel like ethanol. In this sense, butanol is a better alternative than ethanol for a diesel engine. Investigations of butanol as diesel engine fuel have been conducted by several research groups. Yao et al.22,23 reported that the impacts of pilot and post injection on a heavy-duty diesel engine by using butanol blend fuels were similar to those found by using neat diesel. The in-cylinder pressures did not show much difference for various blends (015% vol), whereas higher butanol fraction in the blend resulted in a little higher premixed Received: December 22, 2010 Revised: May 4, 2011 Published: May 04, 2011 3192
dx.doi.org/10.1021/ef1017412 | Energy Fuels 2011, 25, 3192–3203
Energy & Fuels
ARTICLE
Table 1. Properties of Alcohols, Diesel Fuel, and Soybean Biodiesel property
methanola
ethanola
butanola
dieselb
soybean biodiesel
molecular formula
CH3OH
C2H5OH
C4H9OH
C12C25
CH3OOCR
cetane number
3
8
25
40 min
51 (D 613)
octane number
111
108
96
oxygen content (% weight)
50
34.8
21.6
density (g/mL) at 20 °C
0.796
0.790
0.81
0.820.86
0.887 (D 1298) at 15 °C
10
autoignition temperature (°C)
470
434
385
230
363c
flash point (°C) at closed cup
12
8
35
64
173.9 (D 93)
lower heating value (MJ/kg) boiling pointd (°C)
19.9 64.5
26.8 78.4
33.1 117.7
42.5 282338
37.53 (D 240) 342 (D 1160)
stoichiometric ratio
6.49
9.02
11.21
14.3
12.5
latent heating (25 °C kJ/kg)
1109
904
582
270
200c
flammability limits (% vol)
6.036.5
4.319
1.411.2
0.65.6
saturation pressure (kPa) at 38 °C
31.69
13.8
2.27
0.3
viscosity (mm2/s) at 40 °C
0.59
1.08
2.63
1.94.1
4.0 (D 445)
total glycerin (% weight)
0.142 (D 6584)
free glycerin (% weight) sulfated ash (% weight)
