Exhaust emissions from liquid fuel micro gas turbine fed with diesel oil, biodiesel and vegetable oil

Applied Energy 101 (2013) 349–356

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Exhaust emissions from liquid fuel micro gas turbine fed with diesel oil, biodiesel and vegetable oil David Chiaramonti ⇑, Andrea Maria Rizzo, Adriano Spadi, Matteo Prussi, Giovanni Riccio, Francesco Martelli CREAR c/o Dipartimento di Energetica, Università degli Studi di Firenze and Consorzio RE-CORD via Santa Marta 3, 50139 Firenze, Italy

a r t i c l e

i n f o

Article history: Received 25 July 2011 Received in revised form 18 December 2011 Accepted 27 January 2012 Available online 18 February 2012 Keywords: Biodiesel Vegetable oil Biofuels Exhaust emission Micro gas turbine

a b s t r a c t Micro gas turbine units are reliable and versatile units for on-site combined heat and power production (CHP). Compared to internal combustion engines, CHP units based on micro gas turbines offer several advantages, among which the compactness, the high power-to-weight ratio, the lower pollutant emissions and maintenance costs. Depending on the specific type of gas turbine, also fuel flexibility could be better than diesel engines, as the fuel is continuously burnt in a hot environment and there is not possible mixing among fuel and lubricating oil. Within the framework of the EU-Russian Federation FP7 cooperative and specifically the Bioliquids-CHP project, a Garrett GTP 30–67 liquid fuel (diesel) micro gas turbine was characterised with diesel and then tested with different first generation biofuels, such as vegetable oil and biodiesel. An in-house test bench was designed, engineered, instrumented and built. In this research work, exhaust emissions from experimental campaign on the micro gas turbine run with diesel oil and biofuels are presented. Emissions were measured at various load. The experiments demonstrated that the MGT can be successfully operated with these biofuels, with emissions comparable to the standard diesel oil. The experiences gained on the operation of the micro gas turbine on first generation biofuels will serve as a basis for modifying the MGT to be operated with bio-oil from fast pyrolysis. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Significant attention has been recently given to micro gas turbines for decentralised renewable energy generation, thanks to their potential applications and excellent environmental and energetic performances [1]. In fact, the development and deployment of small scale distributed cogeneration systems fuelled by renewable fuels stimulated this renewed interest in micro gas turbines, as they represent an attractive option thanks to their performance in terms of efficiency, low emissions, and reliability (as well as a limited request maintenance) [2]. The use of liquid biofuels (namely ‘‘bioliquids’’) in engines for stationary power generation is rapidly diffusing in the EU, and a further large deployment can be expected in the near future [3,4]. Fuel properties and engine performances have been studied, tested and monitored in a large number of research works, and reviews are available in literature [5–8]. Commercial systems have been operated either on biodiesel (esterified vegetable oil) or on straight vegetable oil (e.g., as rape, sunflower, camelina, palm oil). Feeding liquid biofuels to micro gas turbines, however, is still at an experimental and not yet commercial stage: several research projects aim ⇑ Corresponding author. Tel.: +39 055 4796436; fax: +39 055 4796342. E-mail address: david.chiaramonti@unifi.it (D. Chiaramonti). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.066

at investigating such applications and technologies. The main reason is that micro gas turbines (MGTs) offer several advantages compared to compression ignition engines. Among others, typical problems in feeding vegetable oils (VO) to diesel engines can be summarised as follows:  possible contamination of the lubricating oil with the bioliquid (and therefore need for very frequent full substitution of the lubricating oil)  formation of deposits in the injector and some hot parts  the need to implement high frequency injection requires to achieve a very fast ignition in the combustion chamber (i.e. need for a sufficiently high fuel cetane number), etc. Several issues must be addressed in order to adapt MGTs to biofuels: in fact, while natural gas, kerosene, diesel oil or even clean biogas can be directly used in micro gas turbines without major modifications, biofuels from thermo-chemical conversion of biomass or very raw liquid biofuels such as vegetable oil require more significant re-design and greater changes of standard technologies before being used. In fact, these low quality renewable fuels present some critical factors mainly related to their unfavourable physical and chemical characteristics, such as low LHV, high viscosity, poor atomization, and corrosive properties: moreover, their chemical–

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Nomenclature vol.% %w/w cSt ppm rpm b

percentage by volume percentage by weight centiStokes part per million round per minute (rotational speed) compressor pressure ratio

List of abbreviations AC alternate current

BD CO FAEE FAME MGT NOx UHC VO

biodiesel carbon monoxide fatty acid ethyl ester fatty acid methyl ester micro gas turbine nitrogen oxides unburned hydrocarbons vegetable oil

physical characteristics tend to decay during storage, and fouling on the mechanical moving parts can occur due to their tar content or deposits. For example, viscosity largely affects the properties of the spray during injection, generally increasing mean droplet size diameter and decreasing spray cone angle [9], thus leading to increased spray penetration and poor combustion quality. Being most of these issues common to diesel engines, the experience acquired in the adaptation of engines to bioliquids during the lst decade can be transferred to the new sector of MGT modification. The use of bioliquids in MGTs offers several advantages, as:

The Biofuel-MGT test bench consists of a lightweight aluminium frame mounted on four rubber wheels; it is equipped with the engine and generator on the upper layer, fuels storages, batteries and controls on the lower one. The generator is connected to a resistive load which dissipates the generated power. In this set-up, the engine is equipped with three separate fuel tanks, to accommodate diesel, BD and VO. The following scheme summarises the main components of the test bench.

 possibility fuel preheating temperature higher than diesel engines  no possibility for VO/lubricating oil mixing due to the presence of air-lubrication of the rotating parts,  continuous injection and combustion  lower pollutant emission).

The reference fuel for MGT testing was commercial automotivegrade diesel. The measured biofuel density was equal to 830 kg/m3. Diesel composition was derived from literature [12]. Biodiesel (BD) is a mixture of fatty acid ethyl or methyl esters (FAEE or FAME): it is derived by modifying oil molecular structure through a trans-esterification process; trans-esterification involves a reaction in a triglyceride and alcohol in presence of a catalyst to produce glycerol and ester [13]. Biodiesel quality and property requirements are regulated by two international standards, i.e. ASTM D 6751 and EN 14214; whereas the European norm EN 14214 only accounts for FAME, ASTM D 6751 states that biodiesel is composed by ‘‘mono-alkyl esters of longchain fatty acids derived from plant oils or animal fats’’ and its definition does not depend on the oil or fat used to produce the biodiesel or the specific production process employed [12]. Plant- and animal-derived oils make up the majority of actual and potential sources for biodiesel production [13,14], and to date the vast majority of biodiesel is produced from rapeseed, palm, soy and sunflower oil. The use of esterified cooked/waste/ fried oils as feedstock for biodiesel production is also increasing in EU as they are double-counted towards compliance of EC targets. Biodiesel shows physical and chemical characteristics rather close to petroleum-derived diesel oil and is therefore rated as a strong potential alternative to diesel [15], however concerns about it sustainability exist, depending on the feedstock used for biodiesel production. The sample used for our tests was produced by Novaol (Italy) in April 2010 from a feedstock of mixed vegetable oils. From a chemical point of view, vegetable oil (VO) is a mixture of free fatty acid, di- and tri-glycerides, glycerol, phosphorus compounds and waxes. The ratio between linoleic and oleic acid (two fatty acids) is of paramount importance in diesel engine use, as the linoleic acid is a measure of the degree of saturation, i.e. the amount of double bound in the chain. The presence of an excessive number of double bonds can lead to the formation of deposits in engine hot parts (especially valves), while a very low number of double bonds makes the oil almost solid at ambient temperature (e.g. palm oil below 10 °C). Acid fats composition of vegetable oil depends on the specific type of seed. Refined vegetable oil from rapeseed was supplied by Novaol (Italy) in April 2010.

ASTM- or EN-compliant biodiesel is a liquid biofuel that is mainly used as a low-blend component for automotive diesel (only very rarely it is used in transport at higher percentage blend or even as neat biofuel) [10] and exhibits limited differences to the reference fossil fuel, i.e. diesel oil. Straight (or Pure) Vegetable oil is instead a low quality renewable fuel, and its use requires a deeper adaptation of the MGT given its physical properties (high viscosity, high density, poor lubricity, etc.) and chemical composition. The present work aims at investigating the behaviour of biomass-derived biofuels as fossil fuel substitute in micro gas turbines for power generation, through the construction of a dedicated test bench and the completion of experimental test campaign. The measurement of MGTs performances and main exhaust emissions is here used as a mean to evaluate the quality of the MGT conversion. The adaptation of the micro gas turbine to VO has already been addressed in previous papers [3,11] and will not be discussed in the present work. This paper concentrates on the characterisation of the performance of a micro gas turbine fed with vegetable oil, biodiesel, and their mixtures, and compares results with baseline performance on diesel fuel. The analysis is carried out by comparing fuel consumption and pollutant (CO and NOx) emissions, as indicators to evaluate the quality of the MGT adaptation and the combustion process. 2. Material and methods A test bench aimed at investigating the behaviour of a MGT when fed with bioliquids has been designed, built and equipped with instruments and actuators to implement MGT control and monitoring. Bioliquids such as Biodiesel (BD), Vegetable Oil (VO) and their mixtures are more difficult to ignite than standard diesel: moreover, they present some requirements in terms of material compatibility, and require higher flow rates to deliver equal power due to their lower energy content.

2.1. Diesel, biodiesel and vegetable oil for testing

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D. Chiaramonti et al. / Applied Energy 101 (2013) 349–356 Table 1 Selected results from analysis of tested BD and VO from Novaol and reference values from EN 14214 (FAME), DIN 51605 (rapeseed oil) and EN 590 (diesel). Property

Density 15 °C Cetane number Kinematic viscosity 25 °C 40 °C 60 °C 80 °C Flash point LHV HHV Iodine number Proximate analysis Carbon Hydrogen Nitrogen Oxygenb a b c

Unit

kg/m3 – mm2/s

°C MJ/kg MJ/kg gI2/100 g %w/w

Reference norms

Tested fuels

Diesel oil (EN590)

FAME (EN14214)

VO (DIN 51605)

Tested diesel

Tested biodiesel (from mixed oils)

Tested vegetable oil (rapeseed)

820–845 Min. 51

860–900 Min. 51

900–930 Min. 39

830 –

881 Comply

916.2 54

– 2.00–4.50 – – Min. 55 42a

– 3.50–5.00 – – Min. 120 37a

– Max. 36.0 – – Min. 220 36

–

Max. 120

95–125

3.62 2.62 – – – – 45.61 ± 0.08 –

5.93 3.66 2.51 1.81 – 40.2 39.87 ± 0.03 110

80.7 39.1 80.7 – 238.5 37.2 40.0 ± 0.05 84

– – – –

87c 13c

– –

– – – –

77.3 ± 1.4 12.1 ± 0.3 <0.01 10.6

77.5 ± 1.3 11.9 ± 0.2 <0.01 10.6

Typical value, not included in the norm. By difference. Not indicated in the norm, extracted from [12].

Mixtures of biodiesel and vegetable oils from 25%w/w up to 75%w/w VO in biodiesel were produced and tested in MGT. Selected results from the chemical analysis, along with the reference values from EN 14214 for FAME, DIN 51605 (pure rapeseed oil as transport fuel) and EN 590 (diesel) are reported in Table 1. Analysis of the vegetable oil according to DIN 51605 was carried out in a specialised analytical laboratory (ASG Analytic-Service Gesellschaft mbH, Germany). It can be seen that tested vegetable oil complies with standard, except for values of Iodine value and viscosity, that are slightly out of range, but to a very limited extent which seems not relevant for operation in the micro gas turbine. Analysis of the biodiesel was carried out at Novaol laboratory according to EN 14214 requirements. Properties of commercially available Diesel that could not be measured were assumed as those given in EN 590. Kinematic viscosity of diesel, biodiesel and vegetable oil was measured with a Lauda viscometer made by an Ubbelohde capillary tube controlled by iVisc software and a Proline PV 15 thermostatic bath filled with deionized water. Sample was introduced in the capillary tube and allowed to reach the selected temperature in the thermostatic bath before starting to analyze. Each measurement was carried out at the selected temperature for five times: two times in order to condition the capillary tube, then the last three times for analyses: an interval of 90 s occurred between every measurement. The kinematic viscosity of the biodiesel was measured at 25, 40, 60 and 80 °C, that of diesel was measured at 25 and 40 °C and the one of vegetable oil at 25, 40 and 60 °C. Higher heating value of diesel, biodiesel and vegetable oil was obtained by means of a Leco AC500 isoperibol calorimeter. A sample of about 0.7 g of liquid was weighed with a precision of 0.1 mg in a crucible: then the crucible and a nickel ignition wire were placed into the bomb. The bomb was closed and pressurised to 29 bar with high purity oxygen (99,999%), then settled into the bucket (previously filled with a fixed volume of distillated water). After a period needed to reach thermal equilibrium, the ignition was automatically started and temperature was measured by means of an electronic thermometer with accuracy of 0.0001 °C. The higher heating value automatically calculated by the instrument requires a correction according to the residual length of the nickel wire. Each measurement of higher heating value was repeated in triplicate and errors are given as half of the difference between the maximum and minimum results.

2.2. The micro gas turbine The micro gas turbine selected for the test is an overhauled military auxiliary power unit (APU) model Garrett GTP 30–67. It consists of the gear drive assembly, the centrifugal compressor and turbine rotating assembly, the combustor, the enclosing plenum and housing, the oil lubrication system, the fuel system and the electrical system. The gear drive assembly consists of a reduction gear train which drives the AC generator. The combustion chamber (CC) is a single silo, reverse flow type (see Figs. 1–3). The presence of a silo type combustion chamber is one of the main reasons for the selection of this specific type of MGT, as it allows easier modification to adapt the system to the unconventional fuels under evaluation. However, for the scope of the present research work, no modification has been applied to the MGT CC to run with BD, VO and mixtures. During operation of the engine at nominal speed, the flyweight-type controller in the engine Fuel Control Unit (FCU) regulates the fuel flow. The nominal fuel flow rate with diesel varies approx. between 18 and 33 l/h from full speed no load (FSNL) to full load, with resulting variations in the temperature of exhaust gas between 350 and 550 °C. A minimum of additional controls, external to the engine, are required in order to operate the engine: an electro-mechanical control panel was built to start and stop the

Fig. 1. Test bench block diagram.

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Fig. 2. Liner and schematic of the air passages and flow of the Garrett GTP 30–67 combustor.

The atomizer is a pressure swirl type, fitted at the top of the liner and consists of a single orifice that generates a diffusive flame. With the support of computational fluid dynamic (CFD) simulations, the aerodynamic pattern of the combustor was evaluated. The analysis showed that approximately 35% by mass of the air flow enters the primary zone, and that 15% passes through the swirl vanes, causing the recirculation flow. Through the wall of the flame tube, adjacent to the combustion zone, a number of holes allows for further 9% of the total air entering the primary combustion zone. Air flows through these holes and through the swirlers and generates the recirculation zone, which has the effect of stabilizing the flame. The remaining amount of air flows through the annulus (the gap that lies between the flame tube and the outer casing of the combustor) into the dilution zone. The fuel is injected into the CC through a single injector positioned in the centre of the recirculation. The high level of turbulence in the primary zone promotes the mixing of the fuel droplets and the air–fuel mixture . During start up, and until the MGT reaches 40% of the rated speed, the spark placed on the surface transverse to the axis of the CC assist the ignition the fuel.

Fig. 3. Measurements sites on the test bench.

Table 2 Standard MGT fuels and their LHV. Fuel

LHV (MJ/kg)

Note

JP-4 JP-5 Kerosene Diesel

43.2 42.8–43.2 43.2 42

Standard Alternative Alternative Emergency

Table 3 Specifications of Garrett GTP 30–67 and fuel consumption at standard conditions according to the engine manufacturer [16]. Property

Unit

Value

Turbine wheel sustained speed Output drive shaft speed Fuel consumption full load (see level) Fuel consumption no load (see level) Fuel inlet temperature at FCU Fuel inlet pressure at FCU

rpm rpm kg/h kg/h °C bar

52,870 8000 32.8 19.1 57 0.34–1.38

engine, to handle ignition cut-out situations, and to monitor operation. This APU-derived MGT could be operated on several fossil fuels with a viscosity up to 15 cSt, which is about 4–5 times the corresponding value for diesel. A summary of possible feeding fuels is reported in Table 2 along with their lower heating values (LHV). The machine runs at the nominal rotational speed of 52,870 rpm and compression ratio (b) of 2.3. The three-phase, brushless generator is rated for 25 kVA at 400 Hz, and is connected to the engine through a reduction gear train. A summary of the main MGT specifications is reported in Table 3. The CC is equipped with a spark plug for fuel ignition, a drain valve for the fuel purging, an atomizer for the fuel injection; the CC is assembled tangentially to the rotation plane of the turbine.

2.3. The biofuel test bench 2.3.1. Fuel feeding lines The viscosity of VO is about one order of magnitude higher than diesel, whereas BD is only approximately twice than diesel. To cope with this aspect, the biofuel under testing is heated up to the required process temperature by two Watlow Cast-X cartridge heaters. The heating procedure is split into two phases, in relation to the maximum working temperature of the FCU that is limited to 57 °C. The heaters consist of a cylindrical aluminium body, which embeds a 316 stainless steel tubing arranged in a spiral in which the fluid flows. In the heater core there is a cylindrical cartridge heating of 500 W for the first step and 1500 W for the second. To withstand the increased aggressiveness of the biofuels under test, the new fuel lines were built in 1/800 OD stainless steel tubing (AISI 316). During operation, fluid is withdrawn from one of the three storage tanks through a 40 lm cartridge filter from an alternative boost pump, battery powered. It passes through the metering section, consisting of a positive displacement oval-gear flow metre and a by-pass, then goes through a first auxiliary heater before entering the FCU. The FCU increases the fluid pressure to the rated value for injection, and regulates the fluid flow rate to maintain a constant rotational speed. Before entering the injector, fluid passes the admission valve, which is electrically actuated, and the second auxiliary heater, which raises the temperature of the fluid to improve atomization. Switching from one fuel to another is performed manually, operating the respective admission valves. 2.3.2. Power section A brushless generator (8000 rpm CCW, 120/208 V, 3 phase, 400 Hz) is connected to the compressor wheel through a reduction

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gear train; the frequency of the delivered electrical power is proportional to the engine’s rpm. For testing purpose, the MGT was equipped with a resistive load made of non-inductive resistors, which are intrinsically insensitive to frequency variations and with on-board control unit and replace its important function with an ad hoc device, i.e. a separate exciter for the voltage regulation of the generator. This device recognises the frequency and amplitude of one of the three phases of the generator, and regulates accordingly the excitation current of the brushless generator. It is protected from under -and over-voltage, and features a low-speed protection, i.e. the excitation is inhibited until a minimum rpm is achieved. A package of two 12 V/40 Ah lead-batteries connected in series provides the DC power to the boost pumps of each fuel line. 2.4. Measurements and data acquisition In order to determine the performance of the engine when operated on conventional and unconventional fuels, a number of measurements were carried out on the machine: pressure, fluid flow, gas species concentration, temperature, and rotational speed. Data acquisition on the test bench is performed through three National Instrument USB modules, supported by the NI c-DAQ 9178 chassis. The installed modules are NI 9401 for digital input/ output, NI 9207 for voltage and current measurements, NI 9213 for thermocouple acquisition. Data acquisition hardware is managed by LabviewÒ software. Gas composition is a fundamental source of information for both combustion analysis and indirect measurement of air flow rate. In general terms, and depending on the type of fuel, the presence of unburned hydrocarbon (UHC) in the flue gas is an indication of low quality atomization, carbon monoxide (CO) suggests incomplete mixing or cold zones in the combustor, while NOx can indicate the presence of hot spots and temperature peaks. Measurement of oxygen concentration in exhaust can be used to estimate the air flow rate once the proximate analysis and flow rate of the fuel are known, in a similar way to that adopted by EPA method 19 [17]. From the fuel composition, one can calculate a F-factor (Fd) as follows:

F d ½Sm3 %=kg ¼

%C  kc þ %H  kh þ %N  kn þ %S  ks  %0  ko 100

where the amount of Carbon, Hydrogen, Sulphur, Nitrogen and Oxygen are indicated by % and kc, ko, ks, kn and kh are constants. The gas flow rate (Q) at the stack is then obtained by:

" # Sm3 20:9 _ _ fuel  F d  ¼ qfuel  m Q 20:9  ½%O2 M h _ fuel and qfuel are fuel flow rate and density respectively, and where m [%O2]M is the measured O2 concentration in the exhaust. In the present study, the air/gas flow rate is used to calculate mean flow velocities in the sections of interest for CFD analysis on the combustor annulus and turbine outlet. The exhaust temperature is acquired with a K-type thermocouple to make estimation on turbine stage efficiency. A group of anti-inductive ceramic resistors composes the threephases resistive load section. This part of the system converts to heat the electric energy produced by the generator; in this section, each phase is monitored. Current, frequency and voltage measurements are carried out with a single multimeter designed to work at 400 Hz. Two pressure measuring points are located respectively on the compressor discharge and on the fuel line just before the injector. Pressure transducers have a maximum total error range

353

at full scale equal to ±2%, and an accuracy of ±0.5% of the reading. Temperature measurements are carried out through T and K type thermocouples, which present an outer wire diameter of 0.25 mm and 3.0 mm respectively. Three T-type thermocouples are used to measure environmental, compressor delivery and fuel temperatures (after each auxiliary heater), whereas K type is adopted for measuring the turbine outlet temperature. All thermocouples are stainless steel sheathed. Fuel flow rate is measured by a positive displacement oval flow metre. Since the slip between the rotors and the measurement chamber wall is minimal, the flow metre is essentially unaffected by changes in viscosity and lubricity of the liquid. Meter’s body, rotors, rotor’s shaft and bearings are made of 316L (stainless steel) and O-ring is in VitonÒ. The metre is calibrated, and a precision of less than 1% of the reading is ensured. The analysis of gas concentration is implemented through an on-line gas analyser, which measures CO, O2, and NO concentrations. Resolution and accuracy of electrochemical sensors are respectively 0.1 vol.% and ±0.1 vol.% (O2 sensor), 1 ppm and ±4% (CO sensor) and 1 ppm and ±5 ppm (NO sensor). It is well known that in MGTs the vast majority of NOx is constituted by NO [18]; in this work, therefore, NOx concentration is calculated from the measured NO concentration assuming that NO2 account for only the 3% of the total. The probe to pick the flue gas samples is located at the turbine discharge cone, two centimetres after the access for the temperature measurement. In addition to environmental implications, exhaust gas composition mainly in terms of O2 contents plays an important role in the indirect measurement of airflow. 3. Experimental results 3.1. Test plan The MGT mounted on the test bench was initially characterised in terms of main operating parameters: hourly consumption of fuel, fuel injection pressure, exhaust temperature, and O2, CO and NOx concentrations at the stack were measured. In particular, for the present study the latter parameters have been chosen as indicators for a qualitative evaluation of the combustion process. Once the standard diesel–fuel fed MGT was characterised, the experimental activity focused on the use of BD,VO and their mixtures. The influence of fuel preheating temperature on the concentration of CO and NOx in the exhaust was measured for BD at various temperature, namely 80, 100 and 120 °C, whereas in case of diesel only one test was carried at a preheating temperature of 120 °C. However due to safety reason (related to fuel flash point) is not recommended to maintain so high temperature of diesel fuel prior to injection, and therefore only one load condition was tested for a short time. The test conditions are summarised in Table 4. Measured concentration of pollutants are not comparable if reported as absolute value because they depend strongly on excess of air used in the combustion process. Usually from gas turbine (working with large air-excess) the measured data are corrected to 15% Oxygen level to prevent the concentration of pollutant being achieved by dilution of the exhaust with air. The formula that was adopted to refer CO and NOx concentration is the following:

½XR ¼ ½XM

20:9  ½%O2 R 20:9  ½%O2 M

Where [X] is the gas specie concentration, R the reference, M the measured, [%O2]R the reference O2 concentration in vol.% and [%O2]M is the measured O2 concentration in vol.%.

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Table 4 Summary of the test runs. Run

Fuel (–)

Preheating temperature (°C)

Load (kW)

1 2 3 4 5

Diesel Diesel BD BD 25% VO, 75% BD 50% VO, 50% BD 75% VO, 25% BD VO

20 120 20 80–100–120 120

0–25 20 0–25 0–20 0–20

6

120

5–20

NOx concentration [ppm] @15%O2

354

3.2. MGT performance with diesel oil

diesel flowrate [l/h]

35 30 25 20 15 10 10

15

20

25

net power output [kWel]

CO concentration @ 15% O2 ref. [ppm]

2500 2000 1500 1000 500 0 5

10

15 10 5 0 0

5

10

15

20

25

Fig. 6. NOx emissions at 15% O2 versus electrical load for the diesel fed MGT.

operates. Being the main objective of this work the comparison of the emissions levels for different fuels, this first characterisation was essential to define the baseline for comparisons with VO, BD and their mixtures. With respect to NOx emissions, the measured value was extremely low under all tested conditions, and only few tens of ppm were detected: this is probably due to a low temperature in CC. It has to be pointed out that the accuracy of the gas analyser is comparable to the measured NOx concentration, and therefore these results should not be seen in absolute terms. 3.3. MGT performance with biodiesel at different fuel pre-heating temperature

CO concentration [ppm] @15% O2

Fig. 4. Fuel flow rate versus electrical load of MGT fuelled by diesel.

0

20

Biodiesel feeding exhibits consistently higher carbon monoxide emissions than those measured with diesel fuel at the same fuel injection temperature (ambient temperature: here fuels were not heated), as showed in Fig. 7. This effect can be explained by two main reasons: the lower adiabatic flame temperature of biodiesel, as calculated by Glaude et al. [22], and worse fuel atomization, mainly due to the slightly higher viscosity (Table 1), which leads to higher average drop diameters. Nevertheless, as the operation of the turbomachine approaches the full load condition, the difference is considerably reduced, while at partial load these effects are more evident. It has to be remarked that at the same load, the flow rate of biodiesel is higher than that for diesel, as expected due to the difference in calorific value between the two fuels (37 MJ/kg against 42 MJ/kg), which is in agreement with literature [23]. In order to understand the effect of fuel injection temperature on emission performances of biodiesel, measurements were

40

5

25

net power output [kWel]

Fig. 4 shows the trend of fuel consumption in case of diesel fuel. During these tests the fuel was injected in the CC without preheating; the fuel flow rate increases with the electric load almost with a linear trend, from 19.2 l/h up to 33.7 l/h. As regards exhaust emissions, CO decreases with load whereas NOx emissions increase (Figs. 5 and 6) as expected [18,19]. In fact, higher electrical load corresponds to higher temperature in the combustion chamber, which improves the combustion in terms of reduced emission of UHC and CO however higher temperature also implies some increase in NOx. The CO emissions level is rather high compared to common commercial MGT standards [20]: for instance, CO concentration of 25 and 6 ppm (at 15% O2) were recently measured by the authors in a Capstone C30 fed with diesel respectively at 15 and 30 kW electrical output [21]; in our case, the difference can be due to the very small size of the engine, the simple technology of the injection system and the large air excess at which the MGT

0

30

15

20

25

net power output [kWel] Fig. 5. CO emissions @ 15% O2 versus electrical load for the diesel fed MGT.

3000 Diesel Biodiesel

2500 2000 1500 1000 500 0 0

5

10

15

20

25

net power output [kWel] Fig. 7. Diesel–biodiesel CO emissions (at 15% O2) versus electrical load.

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carried out at increasing injection temperature, from 20 °C to 120 °C, from no load to 20 kW. Fig. 8 reports CO emissions at various loads for several biodiesel and diesel preheating temperatures, namely 80, 100 and 120 °C for BD and 120 °C for diesel. Without preheating, feeding the MGT with biodiesel results in increased CO emission when compared to diesel, that is approx. +30% at all loads. Fuel preheating significantly reduces CO concentration; in particular, from the experimental analysis it can be seen that for both fuels a reduction of 63–64% can be obtained by preheating either biodiesel or diesel up to 120 °C when compared to test performed at 20 °C. For biodiesel, a significant reduction of CO in the exhaust can also be obtained by either increasing the load, or increasing the preheating temperature. For example, referring to biodiesel at 20 °C, preheating the fuel at 80 and 120 °C led to a 52% and 64% in CO concentration respectively. The suggested explanation for the effect of preheating on CO concentration is that as the fuel temperature increases, the viscosity decreases and the fuel atomization is improved. This effect is more noticeable at lower loads where the injection pressure is lower and the effect of temperature is more relevant. 3.4. MGT performance with VO and comparison with BD and diesel oil

2500

2000

2000 1500 1000 diesel_120°C BD_80°C BD_120°C

Diesel_20°C BD_20°C BD_100°C

500 0

10

5

0

15

20

net power output [kWel]

CO concentration @15%O2 [ppm]

CO concentration [ppm] @15% O2

The following activity in the tests campaign aimed at investigating the behaviour of the MGT fuelled by VO and mixtures of BD and VO. Concerning vegetable oil, due to its physical properties rather distant from diesel and biodiesel (mainly in terms of viscosity and surface tension), tests with different VO/BD blends (from 25% to 100% of the mixture) were initially carried out. These tests were

performed with fuel preheating: temperature after pre-heating was limited to 120 °C. In case of preheating temperature below 120 °C, the combustion of VO or VO/BD mixtures was very critical, and some flameouts occurred. However, at idle conditions, even at 120 °C VO preheating temperature, it was not possible to keep the MGT in operation, as flame-out occurred (probably due to the very low temperatures in the CC that did not allow proper combustion). CO emissions of VO, BD and their mixtures (25%, 50% and 75%) versus load for fuel injection temperature of 120 °C are reported in Fig. 9. It was found that the effect of VO on combustion quality, in terms of CO concentration, can be seen from 25% VO in blend with a substantial increase, mainly at lower loads. At 20 kW, CO concentrations for 50%, 75% and 100% VO are almost identical, and correspond to almost the double than biodiesel at 120 °C, whereas the blend with 25% VO shows an intermediate behaviour. However, it must be observed that preheating biodiesel at 120 °C is a very unusual situation, as injection conditions for biodiesel are normally the same as for diesel oil, since their viscosity is comparable. This is a remarkable difference to VO adapted prime movers, where a consistent fuel preheating is always adopted. In this respect MGTs offer advantages towards diesel engines, as VO can be heated above the usual 70–80 °C in diesel engines. This is a typical limitation for alternative engines, where the spill return flow from the injector can cause degenerative modifications to the biofuel quality due to cyclic overheating. This situation does not occur in MGTs, where fuel form the injection in not returned to the fuel tank. Fig. 10 summarises CO emissions for VO and BD, both preheated at 120 °C, at 10, 15 and 20 kW load. Under all circumstances, VO generates approximately the double amount of CO compared to BD.

1800 1600

VO_120°C

1400

BD_120°C

1200 1000 800 600 400 200 0

10

15

20

net power output [kWel]

Fig. 8. CO emissions at different fuel preheating temperatures versus load for biodiesel and diesel.

2500 2000 1500 1000 VO 100 % VO 50 % BD 100%

500

VO 75 % VO 25 % Diesel

0 0

5

10

15

20

net power output [kWel] Fig. 9. CO emissions @ 120 °C (15% O2) versus load for Biodiesel-VO and their blends.

CO concentration @15%O2 [ppm]

CO concentration [ppm] @15% O2

Fig. 10. BD-VO CO emissions (15% O2) at various electrical load.

1200 1000 800 600 400 200 0 Diesel (20 °C)

BD (20 °C)

Diesel (120 °C) BD (120 °C)

VO (120 °C)

Fig. 11. CO concentration al 20 kW for diesel and BD (preheated at 120 °C and at 20 °C) and VO (preheated at 120 °C).

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D. Chiaramonti et al. / Applied Energy 101 (2013) 349–356 Table 5 Fuel consumption (20 kW load). Fuel flow rate (l/h) Diesel BD BD (Tinj = 120 °C) VO (Tinj = 120 °C)

30.5 33.05 32.5 32

Fig. 11 reports CO emissions at 20 kW for diesel and BD (preheated at 120 °C and at 20 °C) and VO (preheated at 120 °C). It can be seen that fuel injection temperature has a great effect on CO emissions for any fuel, while at the same preheating temperature and without special adaptations to the turbomachine VO generates significantly higher CO emissions (+118% compared to diesel at the same temperature), whereas the difference between BD and diesel is not as large (+28% compared to diesel). Nevertheless, this figure also shows that preheated VO can behave similarly to diesel and biodiesel at standard conditions (20 °C), which is a remarkable result. Measured fuel consumption is reported in Table 5, which shows the relevant effect of the type of fuel on the mass flow rate: comparison of diesel and BD (at environmental temperature injection) shows a difference of 9%, a slightly lower variation than expected considering the fuel differences in terms of LHV (see Table 1); in case of BD, it is worth to note that when the injection temperature is increased to 120 °C the fuel mass flow rate is decreased by 11%. Finally the difference between BD and VO (at 120 °C of injection temperature) is negligible, as expected from the very small difference in terms of LHV. 4. Conclusions A micro gas turbine, originally designed for liquid fossil fuels, has been characterised and then tested with various biofuels (bioliquids) and blends, such as vegetable oil, biodiesel and their mixtures. The scope was to examine variations in exhaust emissions so to derive a qualitative understanding of the minor modifications implemented on the MGT. A test bench has therefore been designed and developed for the specific case. The test bench was initially operated with the reference diesel oil to determine a baseline condition for comparison with bioliquids. Then, the MGT was fed with biodiesel (with and without fuel preheating), with pure vegetable oil, and mixtures of these two (25%, 50% and 75% of vegetable oil in mixture), and exhaust emissions were monitored. Compared to diesel, biodiesel, vegetable oil and biodiesel/vegetable oil mixtures showed higher levels of CO in the exhaust under the same fuel feeding conditions. However, while biodiesel can be used without pre-heating of fuel, the use of vegetable oil as fuel requires preheating at temperature of at least 120 °C. Lower fuel temperatures do not allow smooth operation of the MGT. Generally, CO emission from all fuels can be considerable reduced by fuel preheating, expecially at partial load. One of the most important conclusion of the present work is however that preheated VO (at 120 °C) generates CO emissions very similar to diesel at standard (20 °C) conditions: this is a very promising result, as it demonstrates that a properly modified VO MGT can be operated with the same environmental performances as a standard diesel MGT. Biodiesel, being instead very close to diesel oil in terms of physical and chemical properties, did not show particular difficulties in its use, even in the unmodified standard turbomachine. From the operational point of view, VO and VO/BD mixture preheating was found to be a necessary condition to achieve stable operation, and to avoid engine shut-down. Depending on the fluid viscosity, the temperature of the fluid to avoid flameout is equal to ambient temperature for both diesel and biodiesel, whereas it is equal to 120–130 °C for VO.

In addition, despite the implementation of fuel preheating on the turbomachine, at idle state the MGT could not always run on pure vegetable oil. NOx emissions for all fuels did not differ in a significant way, and the extremely low measured values are only slightly higher than the instrument accuracy. Based on this experience, and in good agreement with literature, high load (and therefore CC temperatures) is the most appropriate condition to operate fuel switching. Based on this research work, bioliquids appear to be a promising opportunity for operating micro gas turbines in renewable decentralised cogeneration systems; however, further tests and long duration trials are needed to evaluate potential issues of forming and carbonisation, and to assess the feasibility and reliability of fossil fuel substitution with bioliquids in stationary micro-power generation. Acknowledgements Authors wish to thank the European Commission for the partial financial support received for research activities through the European project Bioliquids-CHP (FP7), as well as the project coordinator BTG. We gratefully acknowledge Novaol for providing biofuels samples. References [1] Pilavachi P. Mini- and micro-gas turbines for combined heat and power. Appl Therm Eng 2002;22:2003–14. [2] Obernberg I. Trends and opportunities of micro-CHP technologies based on biomass combustion. In: Proceeding of the 18th European biomass conference and exhibition, Lyon, ETA-Florence Renewable Energies, Italy; 2010. p. 1–9. [3] Chiaramonti D, Prussi M. Pure vegetable oil for energy and transport. Int J Oil, Gas Coal Technol 2009;2. [4] Demirbas A. Competitive liquid biofuels from biomass. Appl Eng 2011;88:17–28. [5] No S-Y. Inedible vegetable oils and their derivatives for alternative diesel fuels in CI engines: A review. Renew Sustain Energy Rev 2011;15:131–49. [6] Sidibé SS, Blin J, Vaitilingom G, Azoumah Y. Use of crude filtered vegetable oil as a fuel in diesel engines state of the art: Literature review. Renew Sustain Energy Rev 2010;14:2748–59. [7] Ramadhas AS, Jayaraj S, Muraleedharan C. Use of vegetable oils as I.C. engine fuels–A review. Renew Energy 2004;29:727–42. [8] Misra RD, Murthy MS. Straight vegetable oils usage in a compression ignition engine—A review. Renew Sustain Energy Rev 2010;14:3005–13. [9] Chiaramonti D, Oasmaa A, Solantausta Y. Power generation using fast pyrolysis liquids from biomass. Renew Sustain Energy Rev 2007;11:1056–86. [10] EN 14214:2008+A1:2009 – Automotive fuels – Fatty acid methyl esters (FAME) for diesel engines – Requirements and test methods; 2008. [11] Prussi M, Development of a multi-fuel micro gas turbine for pure vegetable oil. University of Florence; 2010. [12] Tuttle J, von Kuegelgen T. Biodiesel handling and use guidelines. 4th ed. National Renew Energy Lab; 2009. [13] Gupta KK, Rehman A, Sarviya RM. Bio-fuels for the gas turbine: A review. Renew Susta Energy Rev 2010;14:2946–55. [14] Drapcho C, Nghiem J, Walker T. Biofuels Engineering Process Technology 2008. [15] Ahmad AL, Yasin NHM, Derek CJC, Lim JK. Microalgae as a sustainable energy source for biodiesel production: A review. Renew Sustain Energy Rev 2011;15:584–93. [16] Technical Manual – Field maintenance instructions. Garrett/Honeywell 1974. [17] EPA Method 19 – SO2 Removal & PM, SO2, NOx Rates from Electric Utility, n.d. [18] Boyce MP. Gas turbine engineering handbook. second ed. Gulf Professional Publishing; 2002. [19] Lefebvre AH. Gas turbine combustion. second ed. Taylor & Francis, Ltd.; 1993. [20] Kolanowski BF. Guide to microturbines. Fairmont Press, Inc.; 2004. [21] Prussi M, Chiaramonti D, Riccio G. Numerical and practical experiences on a 30 kW MGT fed by pure vegetable oil. In: Proc. of the 17th European biomass conference and exhibition. Lyon, Hamburg, ETA-Florence Renewable Energies, Italy; 2009. p. 2057–2063. [22] Glaude P-A, Fournet R, Bounaceur R, Molière M. Adiabatic flame temperature from biofuels and fossil fuels and derived effect on NOx emissions. Fuel Process Technol 2010;91:229–35. [23] Nascimento M, Lora E, Correa P, Andreade R, Rendon M, Venturini O, et al. Biodiesel fuel in diesel micro-turbine engines: Modelling and experimental evaluation. Energy 2008;33:233–40.