Experimental study of the gaseous and particulate matter emissions from a gas turbine combustor burning butyl butyrate and ethanol blends
Introduction
The gaseous pollutants and particulate matters (PM) from burning fossil fuels have attracted ever increasing research attentions on the development of new fuel formulations [1], [2], [3], advanced engine design and calibration methods [4], [5], [6], and effective after treatment technologies [7], [8]. Increasingly strict emission regulations have been proposed and adopted including Euro V and VI requiring a non-volatile particle number (PN) emission limit of 6 × 1011 particles/km to complement the mass-based limit for PM emissions from light-duty diesel vehicles [9], [10]. Similar to vehicular emission regulations, the Committee on Aviation Environmental Protection (CAEP), a technical committee of the International Civil Aviation Organization (ICAO) Council has recently proposed amendments of number-based particle limits regarding the non-volatile Particulate Matter Standard [11]. The studies on aviation emission characteristics and their mitigation technologies are much limited compared with vehicular emissions. It is conceivable that the emissions from aviation gas turbine engines will become a hot topic in the light of the upcoming aviation emission regulations.
The main gaseous pollutants from aero-engines are carbon monoxide (CO), nitrogen oxide (NOx) and unburnt hydrocarbon [12], [13]. Fu et al. and Kyprianidis et al. [14], [15] employed lean-burn combustors in aero-engines and obtained significant NOx reduction. Zhang et al. [16] introduced a novel double-vortex combustor for gas turbine engines burning kerosene and lower emissions of CO, NOx and UHC especially at high inlet temperature have been achieved. Numerical study was conducted by Hamed et al. [17] in order to figure out the method to reduce NOx emission of aero-engine combustor. Results indicated that the increase of the axial distance of the stabilizer and the number of holes could significantly hinder NOx generation in the combustor [17]. Xing et al. [18] also summarised researches on reducing NOx with increasing thermal efficiency via flameless combustion technologies. However, these methods of reducing gaseous emissions depend on retrofitting current aero-engines, which increase the costs of commercial application.
Another major pollution from aero-engines is the PM emissions including soot and volatile particles, which now contribute about 4.9% of total anthropogenic PM emissions, which have drawn increasing attentions in recent years [19], [20]. Ultrafine particles (smaller than 100 nm) are harmful to human health because they can penetrate deeply in the lung and alveoli [21], [22]. The size distributions and chemical compositions of PM emissions from gas turbines or aero-engines are the main research topics of scientists at present. The formation of ultrafine particles is highly correlative with fuel properties and engine operational conditions [13], [20], [23]. Lobo et al. [20] studied PM emissions of a JT8D-219 engine burning kerosene Jet A at various conditions. Results demonstrated that the mean diameter increased with increasing engine thrust and the PN emissions experienced a U shaped line when the engine power raised from about 4% to full level. Huang et al. [23] tested the aviation kerosene JP-8 and several renewable fuels in a jet engine. Higher PN concentrations were found at 85–100% power level than that of 4–7% power due to a higher fuel air ratio and the presence of aromatics content [23]. Timko et al. [24], [25] also demonstrated that PM emitted from aero-engines during take-off and landing played a dominant role in the ultrafine particle emissions (4–100 nm). And the majority of total PN concentrations was the nucleation mode particles (5–50 nm) [24], [25]. However, in terms of engine operational parameters, previous researches have focused more on heavily sooting conditions (such as take-off, climb) and conditions that primarily affect the airport air quality (such as landing, taxiing). Limited researches have been conducted on the PN emissions under cruising and ground idling conditions, which represent the two longest durations of engine operation time.
The ion analysis can identify the water-soluble inorganic component such as metal ion, sulphate, nitrate and ammonium, which are the chemical source of toxicity in PM [26], [27]. Popovicheva et al. [28] tested kerosene with the sulphur content of 0.11% in an aero-engine and reported that sulphates (SO42−) and organic ions dominated the water-soluble fractions of soot emissions. Kinsey et al. and Mironova et al. [29], [30] demonstrated that SO42‑ was the largest single component ion in particle emissions from aero-engines. Cl−, NO3−, NH4+, K+, and Na+ as well as other metal elements were also found and their sources were considered the compositions of kerosene, engine lubrication oils and abrasion from engine wearing components [31], [32].
Biofuels are recognised as alternative energy resources for aero application, which could effectively mitigate the pollutant emissions from gas turbines or aero-engines. Chiaramonti et al. [33] tested diesel fuel, vegetable oil and biodiesel in a modified micro gas turbine and found that the combustion of vegetable oil generated comparable emissions with diesel fuel [33]. Habib et al. [34] tested four types of biofuels and their blends with Jet A in a gas turbine engine and demonstrated that biofuels decreased thrust-specific fuel consumption, CO and NOx emissions. Mendez et al. [13] selected butanol as a typical biofuel and observed less NOx and CO emissions. Seljak et al. [35] investigated the emissions of liquefied lignocellulosic biofuels in a gas turbine and found out the NOx and PMs are both reduced but the CO and UHC are increased. Nevertheless, a number of biofuels have shortcomings such as high viscosity, high surface tension and poor thermal stability, which may exert a negative impact on atomisation and combustion [36]. Jenkins et al. [37] and Chuck et al. [38] examined certain properties of several single-composition biofuels and compared with fossil fuel counterparts. Results suggested that butyl butyrate as a qualified biofuel surrogate, has similar viscosity, flash point, distillation profile and low temperature behaviour to kerosene (Jet A) [37], [38]. Thus the butyl butyrate has the potential to be used in a blend and fully compatible with aviation kerosene. However, experimental work on the combustion performance in gas turbine burning butyl butyrate-based biofuels has been rarely found in the literature.
In summary, gaseous and particulate matter (PM) emissions from gas turbine engines, which are highly correlative with the fuel compositions and operating conditions of engines, are drawing concerns due to the adverse effects on health and environment. However, most research work has not mentioned the information on PM number (PN) concentration at cruising state and idling state of aero-derivative gas turbine engines. In addition, biofuels have the advantage in reducing most pollutant emissions, yet most of them have poor viscosity, distillation profile and low temperature behaviour, which have negative impacts on atomization and combustion. Given the above considerations, a series of experiments on a gas turbine combustor were conducted to analyse the characteristics of CO, NOx, unburnt hydrocarbon (UHC) and PM emissions of biofuels consisting of ethanol and butyl butyrate, which has closed thermal properties to aviation kerosene. Two conditions of the combustor were operated to represent the cruising state and idling state of a gas turbine engine respectively.
Section snippets
Test rig and measurement instruments
The gas turbine combustor consists of a high-pressure air source, a low-pressure air source, a combustion chamber, a fuel delivery system, and a cooling system. The schematic of the test combustor rig is show in Fig. 1. The pressure of air source is from 0 to 7 MPa and the air temperature can be heated up to 600 K. The fuel supply system with the injection pressure at 2 MPa consists of a main feed line and a secondary feed line respectively for the primary combustion and pre-combustion. The K-type
Gaseous pollutant emissions
The emissions of CO, NOx and UHC were measured at condition 1 (cruising state) and 2 (idling state). As the outlet temperature is an important factor indicating the combustion temperature, which influences both gaseous emissions and PM emissions, the average outlet temperature of the combustion chamber was measured and drawn in Fig. 3. The average outlet temperature of RP-3 were 1234 °C and 801 °C under condition1 and 2, respectively, whilst biofuels exhibited noticeable lower (about 19.5%)
Conclusions
This paper reports the gaseous and particulate matter (PM) emissions and icon analysis of burning a promising bio-jet fuel (butyl butyrate) for aviation engine using a gas turbine combustor. Two engine operational conditions (cruising and idling state) were conducted to study the potential of using butyl butyrate-based biofuels as alternative clean bio-jet fuels for aero-engines application. The results drawn from this work can be summarised as,
- 1.
The concentration of CO emissions from biofuel
Acknowledgement
This research is supported by National Natural Science Foundation of China (91641119 and 51306011). The financial supports from the SAgE doctoral Training Award NH/140671210 and from Chinese Scholarship Council under No. 201508060054 are also acknowledged.
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