Biodiesel Blends

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Evaluation of the nitrous oxide emitted from diesel/ biodiesel blends during combustion in diesel engine at laboratory scale by photoacoustic spectroscopy technique Aline Martins Rocha, Marcelo Silva Sthel, Maria Priscila Pessanha de Castro, Geórgia Amaral Mothé, Wellington da Costa Silva, Victor Haber Perez, Marcelo Gomes Silva, Andreas Miklós, and Helion Vargas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef500294a • Publication Date (Web): 19 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014

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Energy & Fuels

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Evaluation of the nitrous oxide emitted from diesel/ biodiesel

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blends during combustion in diesel engine at laboratory scale by

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photoacoustic spectroscopy technique

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Aline M. Rocha†, Marcelo S. Sthel†, Maria P. P. Castro*†, Georgia A. Mothe†, Wellington C. Silva†,

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Victor H. Perez‡, Marcelo G. da Silva†, A. Miklos§, Helion Vargas†

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†

‡

Physical Sciences Department, State University of the North Fluminense, Rio de Janeiro, Brazil. Engineering Processes Sector, Food Technology Department, State University of the North

Fluminense, Rio de Janeiro, Brazil. §

Fraunhofer Institute for Building Physics, Stuttgart, Germany.

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*Corresponding Author:

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Profa. Dra. Maria Priscila Pessanha de Castro

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State University of the North Fluminense (UENF)

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Av. Alberto Lamego 2000. Pq California.

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Campos dos Goytacazes.

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Rio de Janeiro. Zip code 28013-602.

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Brazil.

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E-mails: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT

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The aims of this work was to evaluate the emission of nitrous oxide (N2O), using a diesel engine at

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laboratory scale without catalytic converter with blends diesel: biodiesel (%) of 95:5 (B5), 90:10

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(B10), 85:15 (B15), 80:20 (B20), 75:25 (B25) and 50:50 (B50). Measurements of N2O in the

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exhaust gas were made using an experimental setup based on photoacoustic spectroscopy using

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quantum cascade laser that was validated to be selective and sensitive in the range from 1.7 to 2.72

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ppmv. According to the experimental results the lowest N2O concentration was found to be in the

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B5 diesel/biodiesel blend. Also, formaldehyde and methane compounds were detected qualitatively

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indicating the possible presence of these chemicals in the combustion of diesel/biodiesel blends.

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INTRODUCTION

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Air pollution is actually considered an important environmental problem, especially as regards to

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emissions of greenhouse gases (global warming).1-3 These gases are causing changes in the global

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climate,4-12 with serious consequences for the environment, human society and biodiversity.13-16

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Nitrous oxide (N2O) is an important greenhouse gas17-19 and its concentration in the atmosphere has

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been growing since the industrial revolution, mainly due to the expansion of agriculture, including

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fertilization using nitrogen compounds.19-21 Another important source of N2O emissions is the

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transportation sector,22, 23 which uses fossil fuels. The IPCC report4 indicates as a probable solution

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to reduce greenhouse gas emissions, the use of non-fossil renewable fuels. The same report suggests

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the large-scale use of biomass as a factor to mitigate greenhouse gases (GHG) emissions.

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Brazil, is a tropical country, with a large amount of farmland, water resources, rainfall regularity,

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high biodiversity and developed agricultural technologies, and therefore has a great potential for

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bioenergy production,24, 25 which can be illustrated by the successful Brazilian Ethanol Program.26,

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Another important Brazilian program in the use of renewable fuels is the biodiesel program.

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Biodiesel is a renewable fuel with a potential to reduce greenhouse gas emissions. Thus, the

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Brazilian government instituted in 2005 the Brazilian Biodiesel Program (Law Nº 11.097 Natural 2


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Gas, and Biofuels Agency (ANP) – 2005, www.anp.gov.br). This law initially determined the

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addition of 2% biodiesel (B2) in diesel from January 2008. Biodiesel production in Brazil has

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grown considerably, leading to an addition of 5% biodiesel (B5) in diesel in January 1th, 2010.

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The Brazilian government has proposals to increase biodiesel percentage to 10% biodiesel in diesel

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(B10) after 2014 and 20% (B20) of biodiesel in diesel after 2020. Therefore, environmental

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assessments must be carried out continuously, especially regarding to the emission of gaseous

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pollutants from the exhaust of diesel vehicles. In this paper, we present a study of nitrous oxide

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emissions in a bench-scale engine, where mixtures of biodiesel in diesel were used in the following

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proportions: 5 % (B10), 10 % (B15), 15 % (B20) and 25% (B25) 50 % (B50). The soybean

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biodiesel was used in this study, because nowadays it represents more than 72 %, of the total

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biodiesel produced in Brazil.28

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In order to distinguish nitrous oxide (N2O) from other pollutants, a molecule-selective technique,

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such as infrared absorption spectroscopy, can be employed. Versatile near-infrared (NIR) lasers

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work at near room temperature and allow simple, robust, low-cost, and portable detection of trace

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gases. Specifically, NIR diode lasers, which excite overtones and combination vibrations, have been

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applied in connection with absorption spectroscopy29, 30 or photoacoustic detection schemes.31-33

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With the recent development of quantum-cascade lasers (QCLs), compact solid-state radiation

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sources are available, covering the important infrared (IR) region with specific molecular

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absorption lines. In addition, spectral regions where water vapor has a very low absorption

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coefficient can be selected. These regions are known as atmospheric windows.

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Pulsed quantum-cascade distributed-feedback (QC-DFB) lasers provide quasi room temperature

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operation, combined with a high spectral selectivity and sensitivity, real-time measurement

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capabilities, robustness, and compactness. For this reason, QCLs are ideal for the development of

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compact trace gas analyzers that are also suitable for in situ measurements. In recent years, several

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studies about important trace gases have been performed using these devices.34-37 3


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In this work, was reported the photoacoustic measurements of nitrous oxide using a QC-DFB laser.

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This research motivation comes from the need for simple, sensitive, and spectrally selective devices

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for measuring traces of nitrous oxide.

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EXPERIMENTAL SECTION

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In the photoacoustic technique, the samples, placed inside a resonant cell, are exposed to the

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incidence of modulated light, in an appropriate wavelength, to induce absorption. The resonant

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absorption of radiation is followed by a non-radiative relaxation process; that is, the absorbed

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energy is converted into translational kinetic energy of the gas molecules, generating a modulated

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heating in the gas sample. This heating induces pressure waves and a sound signal is produced,

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which can be detected by highly sensitive microphones, located inside the resonator tube of the

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photoacoustic cell. These microphones convert this sound signal into an electrical signal, which is

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filtered and detected by a lock in amplifier. The photoacoustic signal S(λ) produced by a single

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absorbing gaseous species diluted in a non-absorbing gas can be expressed by Eq. 1 according to

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Sigrist31:

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=

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where S(λ) is the photoacoustic signal generated in the microphones by single absorbing species

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surrounded by a non-absorbing gas, C is the cell constant, which depends on the cell geometry, the

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microphone responsivity and on the nature of the acoustic mode, P(λ) is the laser power at the

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wavelength λ, Ntot is the total density of molecules (~2.5×1019 molecules/cm3), c is the gas

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concentration and σ (λ) is the cross- sections of molecules considering a pressure of 1013 hPa at 20

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°C.

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The experimental setup (Fig.1) employed a QCL with an emission band ranging from 7.71 µm to

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7.88µm as the excitation source and a resonant differential photoacoustic cell as the detector. The

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laser emission lines were tuned by changing the diode temperature, which was set by a temperature

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control unit. The laser was fed applying pulsed current (14.0 mA) with a repetition rate of 400 kHz

(1)

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(2.5 µs), and a pulse duration of 50 ns (duty cycle of 2 %). The pulsed QCL light beam was gated

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by an external transistor-to-transistor logic (TTL) signal at 3.89 kHz to excite the first longitudinal

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acoustic mode of the resonant photoacoustic cell and was focused into the cell by a germanium lens

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(focus ~30.7 mm and diameter ~10.35 mm).

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The photoacoustic data analysis was performed by the lock-in technique using a lock-in amplifier

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(model Stanford SR 850) with a 300 ms time constant. The laser power was monitored by a power

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detector (OPHIR, 3A-SH-ROHS) and the gases flows were controlled by electronic mass flow

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controllers (model MKS, 247), one of 50 sccm as full scale limit and another of 200 sccm.

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Fig. 1 here

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Photoacoustic cell calibration and sensitivity measurements

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The calibration and sensitivity measurements of the photoacoustic cell were performed obtaining

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the cell coupling constant C in the Eq. (1). This was carried out using a 5 ppmv certified mixture of

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nitrous oxide in N2 furnished by White Martins (High Purity > 99.998 %) and diluting it in nitrogen.

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Using two electronic mass-flow controllers, the first one to control N2 flux (200 sccm) and the other

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to control N2O flux (50 sccm), connected in parallel to the gas inlet of the photoacoustic cell, the

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initial N2O concentrations were diluted with pure nitrogen until the lowest investigated

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concentration of 83 ppbv of the (N2O), as shown in Fig. 2. The measured background, obtained by

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flushing the photoacoustic cell with pure nitrogen, was 1.09 µV, with mean value standard

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deviation of 0.23 µV.

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Fig. 2 here

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Intending to perform the photoacoustic spectrum of N2O, a standard mixture of 5 ppmv N2O in

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nitrogen was investigated in the QCL temperature range from 2 °C to 30 °C, which corresponded to

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a wave number range from 1304.056 to 1298.49 cm-1. This spectrum is displayed in Figure 3 in the

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black line. In order to assess the influence of water absorption in this range, water vapor was

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introduced into the cell, free from N2O, and the same temperature variation was applied on the 5


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laser. The results are displayed in Figure 3 in which the absorption peaks of nitrous oxide and water

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were identified in the used wave-number range.

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Fig. 3 here

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Our system was able to detect several absorption lines for N2O, which are noticeable in Figure 3 as

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six intense and well-defined peaks (black line in Fig. 3). Also absorption peaks of water (in red)

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were found (Fig. 3). The following N2O concentration measurements were performed keeping the

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laser temperature constant at the temperature of 19.8 °C (indicated by arrow in Fig. 3) which did not

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overlap any water absorption peak. At this emission line, the laser power is 1.5 mW for an

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excitation current of 14.0 mA and high stability was observed during the entire experiment.

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Bench-scale motor characteristics and operation procedure

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The gas samples were collected from the exhaust of a bench-scale diesel engine (TOYAMA

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Model TD70F, 6.7 HP) at laboratory scale without a catalytic converter and were stored in

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previously evacuated metallic canisters (SUMMA Andersen Instruments). The canisters are made

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of stainless steel and the samples were taken to the laboratory and coupled to our photoacoustic cell

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inlet. The gas sample was then pulled into this cell by a mechanical pump (Ambient Volatile

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Canister Sample AVOCS). Filters were used to remove the particulate matter larger than 2µm. The

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sample collecting procedure was performed in two sequences in the diesel engine: the first one with

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the bench-scale engine turned on and at low rotation speed (3000 rpm) and the second one, with the

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engine turned on and at high rotation speed (9000 rpm). The fuel-air ratio was 23:1 for both speed

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conditions and the motor was operated at same conditions for all samples. This procedure was

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adopted in the collection of samples from the diesel/biodiesel blends B5, B10, B15, B20, B25, B50.

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The measurements started in B5 because it is the diesel Brazilian standard and were carried out in

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quintupled.

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RESULTS AND DISCUSSION

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The N2O emissions from binary diesel/biodiesel blends for both modes of engine operation (low

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and high rotation speed), these results expressed average values with their corresponding standard

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deviation are shown in Table 1. While, the Figure 4 shows N2O concentrations emitted during this

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experiment. The formation of nitrous oxide comes from the combustion of components of air (O2,

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N2) or from the combustion of the components of the fuel itself.

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Table 1 here

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Since biodiesel is a mixture of mono-alkyl esters and consequently oxygen is present in their

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composition in despite of hydrocarbon components of petrodiesel38,39 and also, as nitrogen is not

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present in both, i.e., it comes from the atmospheric air, the N2O emission from the B50 (50%

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biodiesel) sample was the highest among all the samples. For this sample, we can also notice a

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small change in emissions levels between low and high motor rotation speed. The result obtained

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for nitrous oxide emissions from the sample B5 (5% biodiesel) was the lowest among the samples.

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This result is also consistent, since there is less oxygen availability due to lower fraction of

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biodiesel. For this sample, there was a slightly higher emission in the high rotation mode. The

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samples above B5 showed a similar behavior with emission levels over 2.0 ppmv, except to B20

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which concentration emission were 1.9 ppmv at low motor rotation speed. For the sample B10,

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nitrous oxide emission was higher than those observed to samples B15, B20 and B25 when high

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motor rotation speed was used, which is an anomalous behavior, not consistent with increased

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presence of oxygen in samples with more biodiesel. We can also note that, after the sample B5, the

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sample with the lowest nitrous oxide emission was the sample B20, which is interesting because

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this mixture is indicated as the upper limit of biodiesel in diesel without major changes in vehicle

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engines. Therefore this mixture generates less nitrous oxide emissions, being environmentally

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better.

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Fig. 4 here

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In Figure 5, the absorption spectra of the theoretical and experimental chemical species present in

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this study are described. The spectra (a) and (b) correspond to theoretical spectrum of methane40

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and nitrous oxide, respectively. Otherwise, the experimental spectrum for formaldehyde (diluted to

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50 ppmv in nitrogen), nitrous oxide (diluted to 5 ppmv in nitrogen) and diesel/biodiesel B5 blend

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are shown in the spectra (c), (d) and (e), respectively. The superposition of peaks 1, 3, 4 and 7

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present in the spectrum diesel/ biodiesel B5 blend sample (e) with the peaks of the spectra (b) and

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(d), which corresponds to the theoretical and experimental spectra of the gas nitrous oxide.

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Therefore, this gas is present in emissions from the combustion of diesel engine using a

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diesel/biodiesel (B5) blend. All other binary blends (B10, B15, B20, B25, B50) exhibited the same

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behavior, showing the presence of this gas in all of them. Peak 2 in present in the spectrum (c),

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probably indicates the presence of formaldehyde, since the second peak of formaldehyde can not be

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identified (see spectrum (e)) because it was not possible to extend the spectral scanning range of the

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laser used in the experiment.

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Fig. 5 here

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The peaks 5 and 6 probably indicate the presence of methane, as there is a overlap with the peaks of

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the spectrum (a). Both formaldehyde and methane were not subject of this paper, so they are

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presented qualitatively, indicating the possible presence of these chemical species in the combustion

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of diesel and biodiesel as already described in the literature.41, 42 The objective of this study was to

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detect nitrous oxide emitted from binary samples of biodiesel and diesel. This study indicates that it

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is necessary to carefully evaluate the environmental impacts of these emissions, because this study

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was performed in the laboratory (pilot), with a bench engine. However, in Brazil, millions of trucks

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and buses are traveling with B5 biodiesel fuel, using more powerful engines, generating volumes of

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gases greater than the engine studied here. Therefore, nitrous oxide emissions can be much greater

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than we estimated, generating serious environmental problems.

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CONCLUSION 8


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Photoacoustic detection of nitrous oxide was carried out using a quantum-cascade laser in order to

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measure the emission from combustion of diesel/biodiesel blends in an engine at laboratory scale.

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This technique is very sensitive and selective for the detection of this gas. The detection limit of the

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gas in our photoacoustic sensor (experimental arrangement) was approximately 83 ppbv of N2O.

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The lowest N2O concentration was found to be in the exhaust from the B5 blend. Brazil has a large

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program of renewable energy use in the transport sector. This program proposes the use of biodiesel

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in blends with diesel, with a progressive increase in the use of biodiesel.

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Also, in the absorption spectrum of the samples, absorption peaks have been identified

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(qualitatively), probably corresponding to methane and formaldehyde. These gases are likely to be

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detected in the exhaust of diesel engines. However, the quantification of these gases was not the

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subject of this work. Future studies may focus on assessing and quantifying these gases.

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This study is important for environmental assessments of the Brazilian biodiesel program, as this

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work indicated the presence of N2O (powerful greenhouse gas) emissions in all diesel/ biodiesel

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blends.

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ACKNOWLEDGEMENTS

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The authors are grateful to the Brazilian agencies “Carlos Chagas Filho” Research Foundation of

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the Rio de Janeiro State (FAPERJ), The National Council for Scientific and Technological

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Development (CNPq) and Coordination for the Improvement of Higher Level Personnel (CAPES)

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for their financial support.

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Atmospheric Workstation): 1996 edition. J Quant Spectrosc Ra 1998, 60, (5), 665-710.

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Table 1. Nitrous oxide concentration at different binary mixtures for the two modes of operation of

327

the diesel engine (low and high rotation). The measurements were carried out in quintupled and the

328

results expressed average values with their corresponding standard deviation. Engine operation mode Diesel/ biodiesel

Low rotation

High rotation

(ppmv)

(ppmv)

B5

1.70 ± 0.05

1.90 ± 0.11

B10

2.05 ± 0.23

2.36 ± 0.26

B15

2.18 ± 0.14

2.07 ± 0.11

B20

1.90 ± 0.06

1.99 ± 0.05

B25

2.04 ± 0.16

2.18 ± 0.04

B50

2.72 ± 0.06

2.64 ± 0.05

blend samples

329

15


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Figure captions

331

Fig. 1. QCL-based photoacoustic experimental setup for nitrous oxide detection.

332

Fig. 2. Calibration curve of a certified mixture of 5 ppmv of nitrous oxide in pure nitrogen.

333

Fig. 3. Comparison of the experimental spectrum of N2O and water.

334

Fig. 4. Nitrous oxide concentration at in different binary diesel/ biodiesel blends.

335

Fig. 5. Absorption spectra of different pure gases/ vapors and combustion gas from diesel/biodiesel

336

B5 blend. Symbols: (a) theoretical spectrum of methane40, (b) theoretical spectrum of nitrous

337

oxide40, (c) experimental spectrum of formaldehyde, (d) experimental spectrum of nitrous oxide,

338

and (e) experimental spectrum of diesel/biodiesel B5 blend sample.

16


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Energy & Fuels

Fig. 1. QCL-based photoacoustic experimental setup for nitrous oxide detection. 62x49mm (144 x 144 DPI)


Energy & Fuels

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Fig. 2. Calibration curve of a certified mixture of 5 ppmv of nitrous oxide in pure nitrogen (Regression coefficient Adj. R2=0.9974). Symbol: - Adjusted curve. 143x100mm (300 x 300 DPI)


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Energy & Fuels

Fig. 3. Comparison of the experimental spectrum of N2O and water. 76x60mm (96 x 96 DPI)


Energy & Fuels

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Fig. 4. Nitrous oxide concentration at in different binary diesel/ biodiesel blends. Symbols: ■ Low rotation; ■ High rotation. 68x54mm (96 x 96 DPI)


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Energy & Fuels

Fig. 5. Absorption spectra of different pure gases/ vapors and combustion gas from diesel/biodiesel B5 blend. Symbols: (a) theoretical spectrum of methane40, (b) theoretical spectrum of nitrous oxide40, (c) experimental spectrum of formaldehyde, (d) experimental spectrum of nitrous oxide, and (e) experimental spectrum of diesel/biodiesel B5 blend sample. 73x115mm (96 x 96 DPI)


Biodiesel Blends

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