Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode

Journal of the Energy Institute xxx (2015) 1e13

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Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode Hussain Sadig a, *, Shaharin A. Sulaiman b, Mior A. Said b a b

Department of Mechanical Engineering, Faculty of Engineering and Technology, Nile Valley University, P.O Box 26, Atbara, Sudan Department of Mechanical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 December 2014 Received in revised form 20 August 2015 Accepted 6 September 2015 Available online xxx

Producer gas from biomass gasification has a potential to cover a considerable part of power production in the future, the availability and variety of biomass put it as the fourth energy resource. The use of producer gas fuels in gas turbine engines can help mitigating problems related to fossil fuels depletion, emissions and biomass waste disposal. In this work, the effect of the staged combustion of a simulated low calorific value producer gas fuels on the performance and emissions of a single shaft micro gas turbine was investigated experimentally. In order to perform the experiments, the micro gas turbine system was characterized first with the liquefied petroleum gas (LPG) and then tested with two producer gas fuels, producer gas1 (10.53% H2, 24.94% CO, 2.03% CH4, 12.80% CO2, and 49.70% N2) and producer gas2 (21.62% H2, 32.48% CO, 3.72% CH4, 19.69% CO2, and 22.49% N2) in a dual fuel mode. Two injection methods were proposed and tested for producer gases including radially and axially injection. The tests were examined in terms of LPG fuel replacement, turbine entrance temperature, combustor efficiency, turbine efficiency, and emission characteristics at different LPG fuel replacement ratios. The study showed stable operation with a maximum LPG replacement of 42% and 56% for the radially injected producer gas1 and producer gas2, respectively. While for the axially injected producer gas fuels, the maximum achieved LPG replacement ratio was 38% and 52% for producer gas1 and producer gas2, respectively. A relatively higher efficiencies for the combustor and turbine with a remarkable reduction in NOx emissions were achieved when LPG fuel was replaced with producer gas fuels. On the other hand CO emissions were increased for both injection methods. © 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Gas turbine Producer gas Staged combustion Performance Emissions

1. Introduction Biomass is an essential energy source for mankind throughout the history. The availability and variety of biomass put it as the fourth energy resource after coal, oil and natural gas [1]. Since the amount of carbon which can be released from biomass is equivalent to the amount of carbon that can be absorbed during biomass lifetime, biomass is considered as carbon neutral [2]. Due to growing concerns on the environmental issues and fossil fuel depletion, a renewed attention has been given to extract energy from biomass. Substituting part of the traditional fuels with biomass have been considered as a promising way to meet the urgent environmental targets. Among all available renewable resources, biomass is the only resource that can be converted to gas, liquid and solid product by various conversion processes [3]. In Malaysia, the availability and variety of biomass make this type of fuel as one of the most promising solutions for power production in the future. In addition to the huge forest resources, Malaysia is the second largest producer and exporter of palm oil with 43% of total world supply [4]. Shown in Table 1 is the collected biomass from palm oil in Malaysia in 2005. Oil palm fronds (OPF) is the major component of palm oil residues, it basically consists of petiole and leaflets [4]. Researches show that the morphological structure of the frond fibers is comparable to those of hardwood. In the recent few years, an extensive research on oil palm fronds gasification was conducted in the * Corresponding author. Tel.: þ249 904988285. E-mail address: [email protected] (H. Sadig). http://dx.doi.org/10.1016/j.joei.2015.09.003 1743-9671/© 2015 Energy Institute. Published by Elsevier Ltd. All rights reserved.

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H. Sadig et al. / Journal of the Energy Institute xxx (2015) 1e13 Table 1 Collected biomass from palm oil in malayasis in 2005 [4]. Biomass component

Quantity available (million tons)

Empty fruit brunches Fiber Shell Fronds and trunks Palm kernel Total

17.00 9.60 5.92 21.10 2.11 55.73

Biomass Energy Laboratory of Universiti Teknologi PETRONAS [5e7]. The results showed that the produced gas of OPF gasification is comparable to that of hardwood [5]. The main challenge of utilization of producer gas from gasification in energy production is due to its low energy density [8,9]. For the same amount of power, the flow rate of the low calorific value producer gas from biomass gasification may reach seven times that of natural gas on volume basis [10]. Thus, combustion of low calorific value producer gas requires combustors that provide high mixing quality with long residence time [11,12]. In order to improve the producer gas calorific value, different methods were investigated and applied such as using steam, oxygen and hot air as a gasification agent. In addition, the combination of biomass and other material which capable to produce higher combustible gases is another option. Co-gasification of coal and biomass can be considered as a mixed way for energy production from fossil fuels and renewable resources. Coal gasification is a process of producing a fuel-rich product. It has some advantage related to availability and low cost of coal, but still it has the same fossil fuels disadvantages. Extracting energy from biomass by gasification has an advantage as compared to coal gasification because it's environmentally friendly, but at the same time the produced gas has a low calorific value and it affected by wastes seasonal shortage [13]. The combination of coal and biomass in gasification is more attractive than their individual gasification. In recent years, some researchers have reported that the combination of both coal and biomass is more advantageous than their individual effects [13]. Biomassecoal gasification can allow using biomass and coal commercially with an environmentally friendly technique. In addition, it makes feasible utilization of biomass in energy production in larger scale, higher efficiency, and low specific operating costs [14]. Moreover, the use of Biomass and coal for gasification could help to provide a stable gasification condition and hence stable production of producer gas with a higher calorific value. Producer gas from gasification can be utilized in running internal-combustion engines. However, the main challenge of using it in SI and CI engines that these engines are very sensitive to tar and dust presence, which can affect the maintenance cost [12]. Moreover, using producer gas in such engines needs a lot of modifications, especially in the fuel system [15]. Another option for producer gas utilization for power generation is in fuel cells and Stirling engines, however, these technologies are not well established and still under development [16]. Gas turbines are a continuous combustion engines, which develop a continuous flame during operation. It is designed to run mainly on natural gas and oil. The attractive feature of gas turbine as compared to the other engines that it is capable to run with various types of fuels [17,18]. Micro-turbines are a small electricity generator with rated capacities of 25e300 kW [10]. Micro-gas turbines running on producer gas are an applicable option for distributed power generation, especially in rural areas where electricity is lacking. Compared to other internal combustion engines, micro gas turbine offers several advantages such as: high power to weight ratio, less complexity, low emissions and fuel flexibility [10,19,20]. Of all factors influencing gas turbine emissions, the most important factor is the temperature of the combustion zone. One way to reduce the gas turbine emissions is by controlling flame temperature. This can be achieved by using a combustors with variable geometry or by implementation of staged combustion. In the variable geometry systems, the combustion temperature can be controlled within narrower limits by switching air from one zone to another with the change in the engine power [21]. By contrast, the air flow distribution within staged combustors remain constant, the fuel flow is switched from one zone to another in order to maintain a constant combustion temperature [21]. A significant feature of producer gas fuels that they often have a high dilution level, lower flame temperatures and lower flame speeds than natural gas or other medium calorific value fuels which is good from standpoint of thermal NOx emissions [11]. The implementation of staged combustion techniques for producer gas fuels in gas turbine engines has advantages over other fuels from NOx emissions point of view since the temperature levels in producer gas fuels are lower as compared to gas turbine conventional fuels. Moreover, using of more than one injector to introduce the fuels into gas turbine combustor would improve the turbulence which is good for the combustion. Studies on running micro gas turbines on producer gas fuels were conducted by number of researchers [9,12,22e24]. The studies showed a stable operation with acceptable thermal efficiencies for gas turbine systems when producer gas fuels were used instead of the conventional fuels. The Department of Aerospace Engineering in the Indian institute of Science [22] conducted an experimental study to run a small scale Rovers gas turbine with producer gas. The study showed no difficulty in the start up the gas turbine system with producer gas fuel. Other studies were conducted by Rabou et al. [23,24] to investigate the performance of a micro gas turbine operating on biomass producer gas and mixtures of biomass producer gas with natural gas. They reported that the full nominal power of the turbine could be achieved when the calorific value of the producer gas and natural gas mixture reaches 15 MJ/Nm3 where the contribution of producer gas was two thirds by volume basis and only one third by energy basis. In order to improve the producer gas combustion characteristics by increasing the residence time, Al-attab et al. [9] proposed and fabricated a cyclonic combustor with a tangential flow at the inlet and outlet of the combustor. They achieved a 48% LPG replacement when cold producer gas was used while for hot producer gas a 72% LPG replacement was achieved. The producer gas combustibility, dynamic stability and emissions of the gas turbine combustor when natural gas fuel is replaced with a simulated producer gas were studied by a number of researchers [25,26]. The used test facilities were consisted of only combustor, ignition system and producer gas fuel supply system. They reported that there were no problems regarding dynamic or static instability. In addition, the investigation showed that the gas turbine combustion chamber temperature was in the allowable limits. Since these studies were Please cite this article in press as: H. Sadig, et al., Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.003

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conducted at atmospheric pressure, it was lack to give any information on the performance of the gas turbine system at the pressurized condition which is the real condition of gas turbines. Mandl et al. [27] studied the characterization of fuel bound nitrogen in the staged combustion of producer gas. In this work, the air staging is based on the introduction of the combustion air by two separate streams, secondary and tertiary air and the combustion chamber was divided into two zones. In the first combustion zone the producer gas combustion was considered as oxygen deficient zone while in the second combustion zone the remaining combustion air was supplied in order to provide a complete burn out of the flue gas. The results showed almost a complete burn out of the flue gas with no significantly change in NO concentrations along the investigated positions in the reducing zone of the combustion chamber. The present work aims to study the effect of the staged combustion of a simulated low calorific value producer gas fuel on the performance and emissions of a single shaft micro gas turbine system. In order to perform the investigation, a low cost turbocharger based single shaft gas turbine was designed and fabricated at the Biomass Energy Laboratory of Universiti Teknologi PETRONAS. First, LPG was used as benchmark fuel to characterize the system, then two producer gas were tested. For producer gas injection, radially and axially injection configurations were proposed and tested. The adaptation of the micro gas turbine system to the producer gas fuels was addressed in terms of LPG replacement, turbine efficiency, turbine inlet temperature, efficiency, combustor pressure and exhaust emissions. 2. Experimental setup The single shaft micro-gas turbine is consisted of combustor, compressor and turbine; a schematic diagram of the experimental rig is shown in Fig. 1. In order to reduce the cost and due to the complication in the gas turbine parts fabrication, the basic idea for the development of the experimental rig was based on the utilization of vehicle turbocharger to be used as a compressor and turbine. 2.1. The combustor The combustor is a 50 kW thermal input tubular type with a 259 mm diameter and D/L ratio of 2. The air enters the combustor radially via a three inch diameter inlet located at the bottom side of the combustor where it mixes with the fuel and then combust. The exhaust gas exit axially at the top of the combustor and enter to the turbine through a conical nozzle with 100 mm high. The LPG fuel distributor, flame holder and igniter are attached to the combustor bottom cover. 2.2. Turbocharger-based gas turbine Based on its suitability to operate within the air flow rates and pressure ranges required in this work, Garrett GT2854R turbocharger shown in Fig. 2 have been selected and mounted to be used as a single shaft turbine. Since the selected turbocharger has a journal bearing and rotate with a high rotational speed, a vehicular oil pump driven by one-horse power electric motor was used to provide the turbocharger

Fig. 1. Schematic diagram of the experimental rig.

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Fig. 2. Garrett GT2854R turbocharger.

with the needed lubrication. In order to insure that the turbocharger bearing is well protected of overheating, the selected turbocharger is also provided with a water cooling line which circulates the cooling water around the turbocharger bearing. 2.3. Fuel system The fuel supply system for the dry simulated producer gas and LPG was installed for this study. The fuel system allows for testing of LPG and dry producer gas separated and in dual fuel mode. Two producer gas fuels with different compositions and gasification method were selected to replace LPG fuel. Producer gas1 is produced by pre-heated air gasification of oil palm fronds OPFs in a fixed bed gasifier [5]. While producer gas2 is produced by co-gasification of coal and pine saw dust in a fluidized bed gasifier, oxygen rich air and steam were used as a gasification agent [28]. Table 2 shows the compositions and the low and high calorific value of LPG and the tested producer gas fuels. For LPG injection, the fuel which supplied from the cylinder passes through a pressure regulator, control valve and flow meter then injected into the combustion chamber via eight holes with 1 mm diameter distributed in 100 mm diameter stainless steel circular pipe. The LPG distributor shown in Fig. 3 was mounted on the bottom side of the combustion chamber; in addition to LPG injection it served as a flame holder. In order to initiate the combustion, an ignition circuit which consists of electric coil and spark plug was used. The producer gas, which supplied from the cylinder is passed through a pressure regulator and flow meter then injected into the combustion chamber. In order to study the effect producer gas injection direction, two injection configuration was proposed in this work includes radial and axial producer gas injection. In the radial injection configuration which shown in Fig. 4a, the producer gas is mixed with air in the compressed air line 100 mm before the entrance to the combustion chamber. The air fuel charge in this configuration can be considered as a partially pre-mixed. For the Axial producer gas injection shown in Fig. 4b, the producer gas is injected axially in the same direction of LPG fuel. The axial flow injection has a less residence time as compared to the radial flow injection and the air fuel charge in this configuration can be considered as non-premixed type. 3. Test method In this work, the tests were conducted in order to study the effect of producer gas staged combustion on the performance and emissions of the micro gas turbine system. The combustion was started on LPG then a gradual replacement of LPG with producer gas was applied. The flow rate of the used fuels was measured using calibrated CONCOA flow meters. A calibrated Pitot tube and pressure gauge were attached in Table 2 Producer gas and LPG fuel compositions. Constituents H2% CO% CH4% CO2% N2% C3H8% C4H10% Lower heating value (kJ/kg) Higher heating value (kJ/kg)

Producer gas1 [5] 10.53 24.94 2.03 12.80 49.70 e e 5015 5273

Producer gas2 [28] 21.62 32.48 3.72 19.69 22.49 e e 7769 8282

LPG e e e e e 30 70 45,897 49,716

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Fig. 3. LPG distributor and the combustor bottom cover.

Fig. 4. Producer gas injection configuration (a) Radially injection (b) Axially injection.

the compressed air line and used to measure the flow rate and pressure of the air leaving the compressor. Six K type thermocouples were attached to monitor the temperature along the combustor, flue gas leaving the turbine and compressed air leaving the compressor. For exhaust gas emission analysis MRU VARIO plus industrial infrared gas analyzer was used. The used gas analyzer has an accuracy of 0.02%, 5% and 5% for CO, NO and NO2 respectively. In order to test the effect of producer gas injection direction, the system was started on LPG which is the reference fuel, then a gradual replacement of LPG with a radially injected producer gas fuels was applied. After that, a similar test at same conditions were repeated for the axially producer gas injection. The producer gas replacement ratio was calculated based on the following relation:

Producer Gas Replacement Ratio ¼

minimum heat required by LPG for single mode ðkWÞ  minimum heat required by LPG for dual modeðkWÞ  100 minimum heat required by LPG for single modeðkWÞ

(1)

4. Results and discussions 4.1. LPG and producer gas mixtures calorific value Figs. 5 and 6 show an investigation on the mass and volumetric lower calorific value of LPG and the producer gas fuels mixtures at different producer gas volume percentage. As it can be seen from the figures, the mass and volumetric calorific value of LPG/producer gas mixtures are lower than those of LPG fuel. The calorific value of LPG/producer gas mixtures decreases with the increase of the producer gas fraction. Due to the high percentage of CH4, H2 and CO in producer gas2 as compared with producer gas1, the calorific value of LPG/producer gas2 mixture is less affected by the increase of the producer gas ratio in the mixture. Practically, for better utilization of producer gas fuels in gas turbine engines in power generation; there is a need to reduce the drop in the calorific value of the fuel by increasing of producer gas flow rate. In order to replace the natural gas, which is a common fuel for gas turbines with a low calorific value producer gas, the producer gas volumetric flow may reach around seven times that of natural gas. Increasing of producer gas flow rate is always accomplished by designing of a producer gas nozzles/injectors and combustors which are capable to handle this huge amount of producer gas flow. By this way less amount of the fuel with the high calorific value can be used to make up the mixture and hence improve its calorific value. 4.2. LPG replacement with producer gas fuel Fig. 7 shows a comparison of the maximum achievable producer gas replacement percentage, energy share and total heat input required for the radially and axially injected producer gas fuels. Based on LPG fuel, the minimum amount of heat required for the stable operation of Please cite this article in press as: H. Sadig, et al., Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.003

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Fig. 5. Variation of mass lower calorific value against producer gas fraction.

Fig. 6. Variations of volumetric lower calorific value against producer gas fraction.

Fig. 7. Maximum achievable producer gas replacement percentage, energy share percentage and total heat input for the tested fuels.

the single shaft micro gas turbine system was 21.56 kW with a total flue gas mass flow rate of 0.0209 kg/s. For the radially producer gas injection, the maximum achievable LPG replacement was 42% and 56% with a total flue gas mass flow rate of 0.0215 kg/s and 0.0216 kg/s for producer gas1 and producer gas2, respectively. The maximum producer gas energy share was found to be 27% and 47.5% for producer gas1 and producer gas2, respectively. This is because producer gas2 has a relatively higher calorific value as compared with producer gas1 which allows for a higher LPG replacement and hence a higher energy share. Please cite this article in press as: H. Sadig, et al., Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.003

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As compared to the radially injection, the axially injected producer gas fuels showed less capability to replace LPG fuel. The maximum achievable LPG replacement ratio was 38% for producer1 with a producer gas share of 21% on energy basis and total flue gas flue rate of 0.0202 kg/s. While for producer gas2, the maximum achievable LPG replacement ratio was 52% with a producer gas share of 47.9% on thermal heat input basis and total flue gas flue rate of 0.0218 kg/s. This is due to the effect of the higher residence time and the partially premixed combustion regime associated with the radial configuration. Since the axially producer gas injection has a lower residence time as compared with the radially injection it is expected to replace less part of LPG fuel. In the axially producer gas injection, a part of the producer gas might not find the enough time to complete the combustion. Moreover, in the radial producer gas configuration LPG and producer gas fuel were injected at two places along the combustion flow path. Products from the first combustion zone are mixed with fuel and air in a subsequent combustion zone, which provides a better mixing and combustion characteristics as compared with the axially producer gas injection. The effect of producer gas calorific value was noticeable in the obtained values for the maximum achievable LPG replacement ratio. Because producer gas2 has a higher calorific value as compared with producer gas1, it was capable to replace a higher amount of LPG fuel for both injection configurations. The minimum heat input required for the stable operation of the single shaft micro gas turbine was 21.56 kW, 16.23 kW and 18.29 kW for LPG, LPG/producer gas1 and LPG/producer gas2 radially injected, respectively. The minimum heat input required for the stable operation of the single shaft micro gas turbine was less in the dual fuel operation mode as compared to that of LPG fuel mode. The reason for this is due to the increase of the mass flow rate in the dual fuel operation mode as compared with LPG fuel operation mode. Since the turbine output power is a function of flue gases mass flow rate, the effect of incombustible gases was noticeable in improving of the turbine output power and hence in the reduction of minimum thermal heat input. The inert gases such as nitrogen and carbon dioxide, which considered as a major part of the producer gas would gain a part of the heat released at the combustion chamber and hence it would contribute in the output power during the expansion process at the turbine blades. Since the producer gas fuels have a low calorific value as compared with LPG, it was very important to increase the volumetric flow of producer gas fuels during LPG replacement tests. The maximum producer gas to LPG volumetric ratio for the single shaft micro gas turbine were 6.95% and 13.7% for producer gas1 and producer gas2, respectively which were achieved at the maximum LPG replacement ratio. The higher volumetric ratio of producer gas2 compared with producer gas1 was due to higher stability in the operation of the micro gas turbine when the producer gas2 was used which resulted in a higher LPG replacement as compared with producer gas1. Shown in Fig. 8 is the actual net volumetric calorific value of the LPG/Producer gas fuels at the different LPG replacement ratio for the radially injected producer gas fuels. As it can be seen from the figure that the micro gas turbine operated stably with a net calorific value at least 17,262 kJ/m3 and 11,533 kJ/m3 for producer gas1 and producer gas2, respectively. 4.2.1. Combustor temperature The temperature of the micro gas turbine working fluids at different points are measured for LPG and dual fuel operation mode, including compressed air temperature, combustor temperature, turbine inlet temperature and flue gas temperature. Combustor temperature is the most important factor which affects the performance and emission of the gas turbine engine. The factors of prime importance to combustor temperature are excess air, adiabatic flame temperature, burning rate and initial temperature and pressure. Shown in Fig. 9 is a comparison of the temperature distribution along the combustion chamber at the maximum achievable LPG replacement ratio of the single shaft micro gas turbine dual fuel operation mode. The temperature was collected and recorded from the first three thermocouples which distributed axially along the combustion chamber wall. For the dual fuel operation mode, the combustion chamber temperature profile for producer gas2 was higher as compared with producer gas1. This is due to the effect of the concentrations of the combustable and incombustible constitutes in each producer gas fuel. Since producer gas2 contains a higher percentage of hydrogen and carbon monoxide it is expected to yield higher temperatures as compared with producer gas1. Another factor which may affect the combustor temperature is the dilution effect of nitrogen and carbon dioxide. Due to the higher presence of carbon dioxide and nitrogen in producer gas1 as compared with producer gas2, the dual fuel operation mode of producer gas1 fuel might associate with a more temperature reduction as compared with producer gas2. For both tested producer gas fuels, the axial injection showed higher temperature as compared with the radial injection. This is because of the diffusion combustion associated with the axial injection where fuels and air were injected separately.

Fig. 8. The actual volumetric calorific value of the LPG/Producer gas at the different LPG replacement ratio for the radially injected producer gas fuels.

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Fig. 9. Temperature distribution along the combustor at the maximum achievable LPG replacement ratio.

4.2.2. Turbine inlet temperature (TIT) Turbine inlet temperature is one of the main parameters which affects both efficiency and performance of gas turbine engines. Due to the metallurgical limitations a special consideration is always given to the turbine inlet temperature in gas turbine system design. Practically, the available materials which can be used in the fabrication of turbine blades might limit the maximum temperature at which the gas turbine system can operate. In this work since vehicles turbocharger was used as turbine, more attention was given to turbine inlet temperature to insure the over temperature does not occur. Shown in Fig. 10 is the effect of LPG replacement with the radially and axially injected producer gas fuels on the turbine inlet temperature. Generally, the highest turbine inlet temperature was found to be for LPG fuel operation mode, then the temperature starts to decrease with the increase of the producer gas share in the fuel. It is normal that the combusted gas temperature is affected by some factors such as burning velocity, excess air ratio and adiabatic flame temperature. For producer gas fuels, the diluents effect of noncombustible constitutes is an important factor since a significant part of these fuels is CO2 and N2. Thus, it is a common trend that the turbine inlet temperature decreases with the increase of producer gas share because of the dilution effect of N2 and CO2. For the same replacement ratio, the temperature reduction of producer gas1 due to the increase producer gas share was higher as compared with producer gas2, this is due to the higher H2 and CO concentration in producer gas2 as compared with producer gas1. In addition, the presence of N2 in producer gas1 is higher as compared with producer gas2 which might affect in a further turbine inlet temperature reduction. As shown in the figure, the turbine inlet temperature for the axially injected producer gas fuels were found to be higher as compared to the radially injected producer gas fuels. This is because of the effect of the diffusion combustion associated with axially configuration, where the producer gas and air were injected separately into the combustor primary zone in unmixed state. As stated by many authors that diffusion combustion has a higher temperature as compared with the pre-mixed combustion. For the radially injected producer gas, the producer gas injection configuration may produce a subsequent combustion zone, which provides an advantage for lean operation condition and hence a lower temperatures. 4.2.3. Combustor pressure The combustion chamber pressure is very important parameter for the performance, efficiency and emissions of gas turbine engines. Generally, the gas turbine thermal efficiency is relying on the compression ratio only. In addition, the combustion chamber pressure has a great influence on gas turbine emissions since the increase of combustion chamber pressure would accelerate the chemical reaction rates. In this work, the combustor pressure for the dual fuel operation mode was measured and compared with that one of LPG fuel. Shown in Fig. 11 is the effect of LPG replacement with producer gas fuels on the combustor pressure for the axially and radially injected producer gas. In gas turbine engines, the main factors which affect the combustion chamber pressure are the combustor temperature and the compressor power. Generally, the combustion chamber pressure characteristics were governed by pressure law which stated that the exerted pressure is proportional to its temperature. In addition, the combustor pressure relies totally on the compressor power which also affected by the combustor temperature. For dual fuel operation mode, the combustor pressure remains in the same LPG pressure levels for around 30%e40% of the total LPG replacement ratio for all tested producer gas fuels then it starts to decrease. This is due to the effect of the combustor temperature reduction, which resulted of the increase of the producer gas share in the total heat input. In this setup the maximum allowable combustor pressure reduction due to producer gas replacement was around 10%, further reduction may effect in system shut down or it may lead to unstable operation and a noticeably audible booming noise which is probably because of the blow-off and re-ignition as mentioned by Rabou [24]. For all tested fuels, including LPG fuel, the combustor pressure was very low, ranging from 2.6 to 2.8 psi (17.92e19.31 kPa), this is because the turbine is not loaded. Since the turbine is driving only the compressor, a considerable part of compressor power would convert to a kinetic energy which appeared in turbine speed. Increase of compressor speed would effect in the increase of the admitted air and hence increase of air fuel ratio, which would lead to a combustor temperature reduction. As shown in Fig. 10 the axially injected producer gas showed better combustor pressure stability as compared to the radially injected producer gas fuels. This is due to the relatively higher temperature of the axially injected producer gas fuels.

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Fig. 10. Effect of LPG replacement with the radially and axially injected producer gas fuels on the turbine inlet temperature.

Fig. 11. Effect of LPG replacement with producer gas fuels on the combustor pressure for the axially and radially injected producer gas fuels.

4.2.4. Combustor efficiency In this work, the combustor efficiency was calculated based on Equation (2) which represents the ratio of the increase in the working fluid thermal energy content to the theoretical heat input.

hcc ¼


_ p 3 ðT 3  T 2 Þ mC  100 _ _ ½mðLHVÞ LPG þ ½mðLHVÞPG

(2)

_ Cp ; LHV, T2 and T3 are the mass flow rate, specific heat, fuel lower heating value, combustor temperature and compressed air where m, temperature, respectively. Shown in Fig. 12 is the effect of LPG replacement with the radially and axially injected producer gas fuels on the combustor efficiency. In the beginning, the combustor efficiency starts to decrease with the increase of the producer gas share in the fuel, this behavior continued till LPG replacement ratio reached around 5%. Beyond this point, the behavior is converted to a different trend in which the combustor efficiency increases with the increase of LPG replacement. This is due to the change in the amount of total mass flow rate associated with the increase of the producer gas share. For a small LPG replacement ratio less that 5% the contribution of producer gas mass flow rate was relatively low as compared to the total mass flow rate. On the other hand, the increase of the producer gas share would affect in a relative increase in the total mass flow rate, which would effect in the increase of the thermal energy content of flue gases and hence improve of the combustor efficiency. For the same replacement ratio, the axially injected producer gas fuels showed a relatively higher combustor efficiency as compared to the radially injected producer gas fuels. This is due to the effect of the combustor temperature. In comparison with LPG fuel, the combustor efficiency improved during the dual fuel mode operation mode with 2e15% for LPG/producer gas fuel1 while for LPG/producer gas2 fuel the improvement was in the range of 1e5%. Generally, due to the high excess air ratio which associated with gas the turbine engine operation, the combustor efficiency of gas turbines may reach 99%. In this work, the maximum obtained combustor efficiency for the dual fuel operation mode was 79%, which obtained for the radially injected producer gas2. This is may be due to the combustion chamber high thermal loss.

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Fig. 12. Effect of LPG replacement with producer gas fuels on the combustor efficiency for the radially and axially injected producer gas fuels.

4.2.5. Turbine efficiency In this work, the term turbine efficiency refers to the ratio of the thermal energy gained by the cold air in the compressor to the turbine thermal energy input. For the single shaft micro gas turbine, since the compressor and the turbine are mounted on the same shaft, the total power produced by the turbine will be used to drive the compressor. Equation (3) was used to calculate the turbine efficiency [9].

 _ p ðT 2  T 1 Þ a mC  _ p ðT 3  T 4 Þ Flue gas mC 

hT ¼ 

(3)

where T1, T2, T3 and T4 are the atmospheric temperature, compressed air temperature, combustor temperature and exhaust temperature, respectively. Shown in Fig. 13 is the effect of LPG replacement with radially and axially injected producer gas fuels on the turbine efficiency. Based on Equation (2), it is clear that turbine efficiency is affected by flue gases total mass flow rates and temperature differences across the turbine and the compressor. For the dual fuel mode operation, the increase of the producer gas fuels share has a contradictory effect on the turbine efficiency. Since the turbine efficiency is highly affected by the mass flow rate, the higher flow rates associated with producer gas dual fuel operation mode would affect in the improvement of the turbine efficiency. On the other hand, this would reduce the flue gas temperature, which consequently would reduce the efficiency. This is because of the dilution effect of the inert gases in the producer gas fuel. In this work, the turbine efficiency was improved with about 1e12% for LPG/producer gas1 while it was around 8e15% for LPG/producer gas2. For producer gas1 the turbine efficiency started to fall down beyond 40% replacement, this is due to the reduction in the turbine inlet temperature.

4.3. Flue gas emissions Carbon monoxide CO and oxides of nitrogen NOx are the major pollutant of gas turbine combustors. A significant feature of producer gas fuels that they often contain high percentage of diluents such as CO2 and N2 which affect in lowering of flame temperature and flame speed. In addition, since the producer gas fuels used in this work contain a considerable amount of N2 and CO2, it is expected to have a lower flame temperature as compared to LPG. Furthermore, since a significant part of producer gas fuel is CO, the residence time is a very important factor in producer gas combustion to achieve CO oxidization. From the standpoint of thermal NOx emissions, the high dilution levels associated with producer gas fuels combustion is advantageous. On the other hand, lowering of combustion temperature has a diverse effect on CO production. In this work, the concentration of CO and NOx emissions of the flue gas leaving the turbine for the LPG replacement tests are recorded and discussed. 4.3.1. CO emissions Generally, CO is the product of incomplete combustion. Given sufficient time, and at high enough temperature, it will be further oxidized to carbon dioxide CO2. Completing the oxidization process of CO is a matter of providing the enough residence time at high temperature, which is challenging in the case of producer gas fuels since it has a relatively low temperature and need extended residence time to complete the combustion. On the other hand, the presence of hydrogen in the producer gas fuels has a favorable effect on the oxidation of CO. This because the CO oxidation step involving hydroxyl radicals is much faster than the steps involving O2 and O, the CO oxidation in the presence of hydrogen can be described as follows [29]:

CO þ O2 /CO2 þ O

(4)

O þ H2 /OH þ H

(5)

Please cite this article in press as: H. Sadig, et al., Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.003

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11

Fig. 13. Effect of LPG replacement with producer gas fuels on the combustor pressure for the axially and radially injected producer gas fuels.

OH þ H 2 /H2 O þ H

(6)

CO þ H2 O/CO2 þ OH

(7)

CO þ OH/CO2 þ H

(8)

The CO oxidation reaction shown in Equation (4) is slow and does not contribute significantly to the formation of CO2, but rather serves as the initiator of the chain sequence [29]. As it can be seen that the presence of hydrogen in the producer gas fuels would affect in the formation of OH radicals, which are very important in the CO oxidation chain sequence, Thus, Equation (8) is the key reaction for CO oxidation in the overall scheme. Shown in Fig. 14 is the effect of LPG replacement with producer gas fuels and producer gas injection direction on the micro gas turbine CO emissions. The lowest amount of CO emissions was found to be at 0% LPG replacement. For the radially and axially producer gas fuel injection, CO emissions showed a tendency to increase with the increase of the LPG replacement ratio. The maximum emitted CO for the dual fuel operation mode was 47 ppm and 157 ppm for producer gas1 and producer gas2, respectively which were achieved at the maximum LPG replacement. This is a common result that the CO emissions increase with the decrease of the combustor temperature. For the dilution with non-flammable gases such as CO2 and N2, these constitutes affects in the decreasing of flame temperature which would affect in slowing of the reaction rate of H2 and CO and hence increase of CO emissions. At the same replacement ratio, the CO emissions for producer gas2 were found to be higher as compared with producer gas1 for the axial and radial injection. This is because of the higher CO concentration in producer gas2 as compared with producer gas1. The different trends of CO emissions for the radially and axially injected producer gas fuels are due to the effect of residence time and combustion chamber temperature. The non-premixed producer gas combustion has a higher temperature as compared with pre-mixed producer gas temperature, this clearly appeared in the combustion chamber and turbine temperatures which are shown in Figs. 9 and 10. The effect producer gas injection direction was noticeable in the micro gas turbine emissions. Since the radially injected producer gas has a higher residence time as compared with the axially injected producer gas which would allow more time for CO for a further oxidation to

Fig. 14. Effect of LPG replacement with producer gas fuels on CO emissions.

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H. Sadig et al. / Journal of the Energy Institute xxx (2015) 1e13

CO2. On the other hand, the staged combustion, which applied in the radially injected producer gas has adverse effect on CO formation by lowering the combustion chamber temperature. Hence it would allow the flames to be less adiabatic, which consequently would affect in increasing of CO emissions. 4.3.2. NOx emissions NOx emissions are mainly produced by the oxidization of nitrogen, which is the major part of air used in combustion or in the fuel. Generally, fuels burning occur at temperatures high enough to oxidize atmospheric nitrogen, producing NO which can further oxidize to produce NO2. Shown in Fig. 15 is the effect of LPG replacement with the radially and axially injected producer gas fuels on NOx emissions, the highest NOx emissions were found to be for the LPG fuel operation mode. Generally, the replacement of LPG with producer gas fuel showed a less NOx concentration as compared with LPG. The reduction in NOx concentration for the radially injected producer gas1 fuels was in a range of 37.5%e62.5%, while it was 43.4%e51.5% for the radially injected producer gas2. This is because of the temperature reduction associated with the dual fuel operation mode. The different trends of NOx emission for producer gas1 and producer gas2 replacements may be due to the different concentrations of N2 and CO2 dilutes in each producer gas fuel. As mentioned by Lee et al. [30] that the amount of NOx reduction at the same dilution ratio is larger for CO2 as compared with N2. Furthermore, the amount of nitrogen in the producer gas fuel would affect in the emitted NOx. Generally, the presence of N2 in the fuel has two effects on flame: first, dilution causes a decrease in adiabatic flame temperature; and second, N2 decreases the amount of soot formed in the flame. For nonluminous (nonsooting) fuels such as CO and H2 which are the main component of the producer gas fuels, the decrease in adiabatic flame temperature is high. Thus, a higher N2 percentage would affect in more adiabatic flame temperature reduction. Although the relatively lower temperature of the axially injected producer gas1, it showed higher NOx emissions as compared with the axially injected producer gas2, this is may be due to the higher concentration of fuel nitrogen in producer gas1 fuel. The effect of staged combustion, which applied in the radially producer gas injection was noticeable in the emission of NOx. As compared with the axially injected producer gas fuels, the radially injected producer gas fuels showed less NOx emissions. This is because the radially producer gas injection configuration allows for fuel staging which facilitate for additional pre-mixed fuel to be injected downstream into a secondary combustion zone. The products from the first combustion zone are mixed with the radially injected mixture of the producer gas and air in the secondary combustion zone, which provide and advantageous for lean operation of the second zone. This would help to keep the combustion process lean at all operating conditions and hence lowering of the combustion temperature and NOx emissions. As shown in the figure, NOx emission behavior during LPG replacements tests can be described based on two main zones. Firstly, NOx emissions started to decrease with the increase of producer gas percentage till around 20% replacement, and then it switched to a different form in which NOx emission remains at almost same levels. This because the introduction of producer gas fuel into the combustion chamber has greater effect on temperature reduction which would affect in NOx reduction. Since a considerable part of the resulted NOx is thermal type and generated at high temperatures, further increase of producer gas percentage has less effect on the resulted NOx.

4.4. Coefficient of variation of the measurements The reproducibility of the micro gas turbine results was checked by conducting three number of experiment at given operating conditions. The standard deviation for the carried out measurements for the turbine inlet temperature, combustor pressure and exhaust gas analysis are shown graphically by the error bars. In addition, the coefficient of variation for all measurements was calculated. For all measured parameters, more than 90% of the collected data were found to have a coefficient of variation of less than 10% which shows a reasonable accuracy for the collected results.

Fig. 15. Effect of LPG replacement with producer gas fuels on the NOx emissions.

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H. Sadig et al. / Journal of the Energy Institute xxx (2015) 1e13

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5. Conclusions In this work, the effect of the staged combustion of producer gas fuels on the performance and emissions of a single shaft micro-gas turbine were studied experimentally. Two different producer gas fuels, namely producer gas1 (10.53% H2, 24.94% CO, 2.03% CH4, 12.80% CO2, and 49.70% N2) and producer gas2 (21.62% H2, 32.48% CO, 3.72% CH4, 19.69% CO2, and 22.49% N2) were tested to replace LPG which was the reference fuel. In order to study the effect of producer gas staged combustion, two injection configuration were proposed and tested for producer gas fuels including axially and radially injection. From the results:  Based on the thermal heat input, a maximum LPG replacement of 42% and 56% with stable operation were achieved for the radially injected producer gas1 and producer gas2 respectively. While for the axially injected producer gas fuels the maximum achievable LPG replacement was 38% and 52% for producer gas1 and producer gas2, respectively.  For both tested producer gas fuels injection configurations, the combustor and turbine efficiencies showed a remarkable improvement when LPG was replaced with producer gas fuels. In comparison with LPG fuel, the combustor efficiency was improved during the dual fuel mode operation mode with 2e18% for LPG/producer gas1 while for LPG/producer gas2 fuel the improvement was in the range of 1e5%. The turbine efficiency was improved with about 1e12% for LPG/producer gas1 while it was around 8e15% for LPG/producer gas2.  On emissions, a remarkable reductions in NOx emissions were recorded for LPG replacement tests. As compared with the axially injected producer gas fuels, the radially injected producer gas fuels showed less NOx emissions. On the other hand, the CO emissions were exponentially increased. Acknowledgments The author would like to acknowledge the support of Universiti Teknologi PETRONAS in this work, which is internally funded by URIF No. 15/2012. The thanks extend to MR. 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Please cite this article in press as: H. Sadig, et al., Effect of producer gas staged combustion on the performance and emissions of a single shaft micro-gas turbine running in a dual fuel mode, Journal of the Energy Institute (2015), http://dx.doi.org/10.1016/j.joei.2015.09.003