Low Heating Value Fuels Research Articles

Impact of P-1 radiation model on simulated free jet flame characteristics of gaseous fuels: CFD with PDF approach

Simulation and analysis of a turbulent free jet flame erupting into still air are done using CFD. Using 2-D axisymmetric numerical modelling in ANSYS-FLUENT 14.5. Three distinct kinds of gaseous fuels are used: CH4, CO, and biogas (50% CH4 and 50% CO2). The effects of thermal radiation modelling utilizing the P-1 radiation model on the behavior of a free jet flame are investigated, and the impacts of air temperature and fuel velocity on the flame length are also provided. The findings demonstrated that the radiation modelling did not affect the temperature distribution and flame length for CO and biogas (i.e., lower heating value fuels). Nevertheless, the air temperature and fuel kind considerably impact the flame behavior. While the fuel inlet velocity (i.e., burner power) does not affect the flame length. Additionally, free jet flame velocity and length numerical correlations considering radiation modelling are predicted and presented with allowable errors. A comparison with earlier experimental correlation proved successful, with a maximum error of ?9.4%.

Effect of hydrogen addition on NOx formation mechanism and pathways in MILD combustion of H2-rich low calorific value fuels

Hydrogen is a major component of low heating value (LHV) fuels like syngas, coke oven gas, and blast furnace gas. These fuels can potentially replace methane in the industrial process under MILD combustion conditions. NOx formation mechanism in MILD combustion regime is different from conventional air combustion due to uniform temperature distribution and low oxygen concentration. Hence, it is important to investigate the effect of hydrogen addition inmethane (CH4) on different NOx formation mechanism and pathways under MILD combustion conditions. A well-stirred reactor (WSR) model with atmospheric pressure and long residence time was employed to simulate the ideal MILD combustion conditions with Chemkin Pro software and GRI Mech 2.11. H2 and CH4 blends ranging from 0% H2 (100% CH4) to 60% H2 (B60 = 40%CH4/60%H2) were selected as fuels to emulate MILD combustion of LHV fuels. The oxygen mass fraction was kept from 3% to 9%, and the fuel blends were altered. The results showed that hydrogen addition enhanced NNH route while decreasing prompt route and N2O intermediate route whereas thermal NOx route was insignificant for lower O2 mass fraction of 3% and 6%. It was observed that H2 addition reduced NO-reburning which led to more NOx emission. Moreover, the sensitivity analysis was conducted to analyze the pathway of NOx formation and to quantitively separate the relative contribution of the individual route on the total NOx formation, which was beneficial to control the NOx emission of LHV fuels during MILD combustion.

Investigation of fuel dilution in ethanol spray MILD combustion

MILD combustion is a promising technique for low heating value fuels. In this paper, spray MILD combustion of diluted liquid fuel is numerically studied. The modelling framework is based on the RANS approach, the EDC combustion model with a skeletal chemical mechanism, and the Lagrangian tracking of droplets. The validated numerical model is used to simulate the combustion of ethanol diluted with water is studied for different oxidizer temperatures and O2 concentrations under MILD condition. Results indicate that a low-level fuel dilution up to 5 percent has tendency toward expansion of MILD region, but higher degrees fuel dilution within the range of 5–20 percent results in a reduction of MILD combustion regions due to domination of physical effect contribution of addition of water to ethanol fuel compared to its chemical effect. Fuel water addition also leads to an increase in OH concentration due to H2O dissociation through O + H2O → 2OH, which assist the oxidation of fuel driven species, CO in particular. Changing the oxidizer stream temperature can lead to considerable changes in the flame structure: the typical flame shape is converted to a bifurcation style by reducing hot co-flow stream temperature from 1400 K to 1200 K. Moreover, for liquid fuel dilution, preheating and inlet air dilution do not lead to monotonic strengthening the MILD zone in spite of gaseous fuels.

Hydrogen concentration effects on swirl-stabilized oxy-colorless distributed combustion

Oxy-colorless distributed combustion is a novel combustion technique to achieve more uniform and controlled thermal field, enhance flame stability, and reduce emissions including combustion noise. Colorless distributed combustion (CDC) conditions was achieved through controlled entrainment of hot reactive gaseous species. This allowed for reduction of oxygen concentration to provide improved mixture preparation for distributed combustion with the reaction zone distributed across the entire volume of the combustor. In this study, oxy-colorless distributed combustion was examined for fuel flexibility using different mixtures of methane and hydrogen diluted with 10% each of N2 and CO2 to represent low heating value fuels. The oxidizer used was an O2-CO2 mixture that eliminated the formation of NOx from the combustor. The evolved OH* chemiluminescence signatures were recorded for various hydrogen concentrations in the fuel at equivalence ratios in the range of 0.6–0.9 under distributed combustion condition. At Φ = 0.9, transition to CDC initiated at oxygen concentrations of 17%, 19% and 21% for fuels having a hydrogen concentration of 60%, 50% and 40% (on volume basis), respectively. Differences in transition point were attributed to higher flame speed associated with more hydrogen in the fuel. Gaseous fuel having high hydrogen concentration provided more distributed OH* flame structures at lower oxygen concentration while the flame stability enhanced at higher oxygen concentration. Oxy-colorless conditions provided only 10–30 ppm CO even at high oxygen concentration and high (0.9) equivalence ratio. The results showed fuel flexibility from different heating value gaseous fuels having high hydrogen content. The results provided very low CO emission and enhanced flame stability under oxy-colorless distributed combustion for the low heating value fuels using O2-CO2 mixture as the oxidant.

Industrial gas turbine engine response and combustion performance to fuel changeovers in compositions and heating values

Extension of gaseous fuel flexibility for an industrial gas turbine engine is reported in this paper. A Siemens SGT-400 engine with 13.4 MW rating with hybrid combustion system configuration was string tested to operate on and switch between three different gaseous fuels with temperature corrected Wobbe Index varying between 45 MJ/m3, 38 MJ/m3 and 30 MJ/m3. The alteration of fuel heating value was achieved by injection or withdrawal of N2 into or from the fuel system. The results show that NOx decreases for fuels with lower heating value at the same engine output power and most importantly the engine can maintain stable operation on, and switching between these three different fuels with heating values changeover rate faster than 10% per minute without shutdown or change in load condition. The effect of gaseous fuels with different compositions and heating values on turbine entry temperature, NOx emission and combustion was further investigated in a high pressure combustion testing rig and it is found that the peak temperature and temperature gradient of turbine entry temperature are reduced by application of lower heating value of fuels. It is also noted that for the fuels having the same heating value different diluent, either CO2 or N2, has insignificant effect on hot gas temperature profile, however, different diluent does have impact on NOx emission and NOx emissions is lower by up to 5 ppm if CO2 is selected as diluent compared to N2 as diluent. The physical and chemical mechanisms for NOx reduction for gas turbine engines operating on lower heating value fuels have been identified: improving fuel and air mixing by stronger fuel to air momentum ratio and enhanced turbulence level, slowing chemical reaction by decreasing laminar flame speed and reduction of thermal NOx due to decrease in adiabatic flame temperature and flame shifted to downstream.

Effect of the Regenerator Efficiency on the Performance of a Micro Gas Turbine Fed with Alternative Fuels

In this paper a validated in-house MATLAB© model was used to assess the behaviour of the Turbec T100 MGT when operated with alternative low lower heating value (LHV) fuels; moreover, the effect of the cycle humidification is assessed. In both the aforementioned cases, the flow rates through the turbomachines, their operating points, and the effectiveness of the recuperator might change and determine performance losses. In particular, the recuperator in a MGT is a crucial component that allows to achieve good thermodynamic performance, also in presence of low compression ratios, and its performance can strongly influence the final output of the machine. Therefore, the aim of the work is to evaluate the effect of the variation of the operating conditions on the performance of the recuperator and, therefore, of the whole MGT. The use of alternative fuels with low LHV and of steam injection shifts the operative points of the turbomachines without strongly affecting their isentropic efficiency; in general, compression ratio is reduced and the flow rate of the compressor is reduced. Therefore, attention must be paid for the compressor stall limit. The recuperator shows a slight variation of the temperature of the fluids, but a higher efficiency is recorded as the flow rate are typically reduced and a better heat recovery performance can be obtained.

Modeling and Assessment of a Biomass Gasification Integrated System for Multigeneration Purpose

The use of biomass due to the reduction in greenhouse gas emissions and environmental impacts has attracted many researchers’ attention in the recent years. Access to an energy conversion system which is able to have the optimum performance for applying valuable low heating value fuels has been considered by many practitioners and scholars. This paper focuses on the accurate modeling of biomass gasification process and the optimal design of a multigeneration system (heating, cooling, electrical power, and hydrogen as energy carrier) to take the advantage of this clean energy. In the process of gasification modeling, a thermodynamic equilibrium model based on Gibbs energy minimization is used. Also, in the present study, a detailed parametric analysis of multigeneration system for undersigning the behavior of objective functions with changing design parameters and obtaining the optimal design parameters of the system is done as well. The results show that with exergy efficiency as an objective function this parameter can increase from 19.6% in base case to 21.89% in the optimized case. Also, for the total cost rate of system as an objective function it can decrease from 154.4 $/h to 145.1 $/h.

Development of a 100 kW plasma torch for plasma assisted combustion of low heating value fuels

Most thermal power plants need an auxiliary power source to (i) heat-up the boiler during start up phases before reaching autonomy power and (ii) sustain combustion at low load. This supplementary power is commonly provided with high LHV fossil fuel burners which increases operational expenses and disables the use of anti-pollutant filters. A Promising alternative is under development and consists in high temperature plasma assisted AC electro-burners. In this paper, the development of a new 100 kW three phase plasma torch with graphite electrodes is detailed. This plasma torch is working at atmospheric pressure with air as plasma gas and has three-phase power supply and working at 680 Hz. The nominal air flow rate is 60 Nm3.h−1 and the outlet gas temperature is above 2 500 K. At the beginning, graphite electrodes erosion by oxidizing medium was studied and controlling parameters were identified through parametric set of experiments and tuned for optimal electrodes life time. Then, a new 3-phase plasma torch design was modelled and simulated on ANSYS platform. The characteristics of the plasma flow and its interaction with the environing elements of the torch are detailed hereafter.

Experimental investigation of submerged flame in packed bed porous media burner fueled by low heating value producer gas

Combustion inefficiencies and high pollutants emissions keep motivating researchers to enhance combustion technology. Producer gas fuel from biomass gasification with its low heating value and high CO content requires a special combustor design for efficient burning. Porous media burner (PMB) has been widely investigated and proven to be well suited for low heating value fuels lean combustion. This study aims at performance investigation of PMB fueled by producer gas from biomass gasification. A downdraft gasifier system along with a PMB burner and heat recovery unit has been developed. The PMB comprises two layers of 10mm and 20mm diameter upper and lower alumina spheres packed, respectively. With PG heating value of about 5MJ/m3, lean to ultra lean stable combustion was achieved with equivalence ratios (Φ) in the range of 0.33<Φ<0.71. Combustion layer temperature was in the range of 1300–1550K. The lowest recorded emissions from the PMB were 6ppm and 230ppm for CO and NOx respectively. The heat recovered from the burner was utilized in hot air production of 7kWth that can be used for drying process in small industries. Maximum heat recovery heat exchanger effectiveness was about 93% with overall system efficiency of 54%.

GAS TURBINE PERFORMANCE ANALYSIS OPERATING WITH LOWHEATING VALUE FUELS

Usually, power plants work with gas turbine designed to fire natural gas; however, there are possibilities to use other types of gaseous fuels with different calorific values that may be available close to the power plant site. These fuels can be gases obtained from steel (from blast furnaces and coking plants), from gasification processes of coal or biomass, among others. In this work, a gas turbine performance was evaluated at different operational conditions in order to verify the technical feasibility of burning low calorific value fuels. A gas turbine designed to operate with natural gas was used as a reference, the model was built and the performance evaluated at design and off-design conditions using a commercial computer program, GasTurb 11®. A good agreement was obtained between the model operating with natural gas and the available data from open literature, at design and off-design conditions. The model was simulated using low heating value fuels under the same conditions established for natural gas. A reduction in compressor’s surge margin was identified when using low heating value fuels as well as an increase in power output. Therefore, for safe operation a strategy for recovering the surge margin was adopted. In this study the control strategy adopted was bleed air at the compressor discharge. This control strategy presents a technical viability and ensures that the gas turbine operates with the same surge margin level as when using natural gas.

Non-equilibrium plasma assisted combustion of low heating value fuels

This paper describes the effects of non-equilibrium air plasma generated by a dielectric barrier discharge (DBD) on the combustion of low heating value fuels. The experimental results indicate that addition of a very small amount of energy to the air flow in the form of DBD significantly improves the flame stability. Moreover, main combustion characteristics such as flame propagation speed, combustion intensity and lean blow-off limits are also enhanced by the effect of plasma. Some active radicals such as excited O atom and excited N2 molecule are observed by spectrograph in the discharge area. Based on the results of numerical investigation we can conclude that these active radicals generated in discharge area can accelerate the production rate of active OH radical which plays a key role in the oxidation process of low heating value fuel, and thus the whole combustion process is accelerated.

Fuel dilution and liquid fuel operational effects on ultra-high thermal intensity distributed combustor

Ultra-high thermal intensity colorless distributed combustion has been examined for methane and methane diluted with air, nitrogen or carbon dioxide gas and liquid fuel (ethanol) burning in air at a fixed thermal load. Fuel was diluted with inert gases and air to simulate the combustion of low heating value fuels with special focus on the emission of pollutants. Reverse cross-flow configuration has been investigated at very high thermal intensities in the range of 156–198MW/m3atm with specific focus on the exhaust emissions, and distribution of intermediate radical species emission and flowfield from within the combustor using novel but simplified geometry for easy transition to practical applications in ultra-high thermal intensity gas turbine engine applications. The ultra-high combustion intensity demonstrated here is significantly higher than that used in current stationary gas turbine engines. Numerical simulations are also performed under non-reacting conditions to understand the effects of fuel injection in cross flow configuration for both undiluted and diluted fuel cases. Ultra low NOx emissions are achieved for both the novel premixed (3ppm) and non-premixed (8ppm) mode using methane as fuel at equivalence ratio of 0.6. Carbon monoxide level of about 100ppm was obtained in both novel premixed and non-premixed modes of combustion at equivalence ratio of 0.6. Dilution of fuel significantly reduced NO emissions to 2ppm from 8ppm and slightly increased CO emissions in non-premixed mode at equivalence ratio of 0.6. Dilution of fuel suggested much lower OH* suggesting favorably mixed oxidizer prior to ignition that results in lower NO emissions. Liquid fuel (ethanol) was also examined and very low NO emission of about 6ppm was obtained in direct-injection mode and 2ppm in premixed–prevaporized mode at equivalence ratio of 0.6. CO emission of about 200ppm was observed in both the modes at equivalence ratio of 0.6.

3D Numerical Combustion Simulation of Designed Low Heating Value Fuel Combustor for Further Detail Modulation

This study applied three-dimension numerical combustion simulation method to design the specifications of a low heating value (LHV) fuel combustor. According to the discussion on design parameter system, quantity of main jets is the governing factor for flame stabilization of the primary combustor zone. Combustor outlet temperature factor improves with increase of load, and can be reduced to 0.33 under full load. However, load increase may slightly change recirculation flow structure and weaken flame stabilization. The findings showed that the combustion strength of near-field area greatly increased, and the recirculation flow structure center moved to the liner well when combustor used propane as fuel. This also caused deterioration of temperature factor.

Effects of diluents on knock rating of gaseous fuels

Concerns on energy security, emissions, and the recent hike in the price of fossil fuels have prompted the rapidly growing interest in the use of various alternative and renewable fuels, including low heating value fuels such as land-filled gases, biogases, coal bed methane gases, and others. Generally, the low heating value (btu) fuels contain substantial amounts of diluents such as carbon dioxide, nitrogen, water vapour, and other trace gases in the fuel composition. The present contribution describes the results of the investigation of knock in a single-cylinder variable compression ratio (CR) spark-ignition engine fuelled with gaseous fuels, such as natural gas, methane, and hydrogen, in the presence of different amounts of diluents, specifically carbon dioxide and nitrogen, in the fuel mixture in order to represent closely the general composition of land filled and biogases in practice. The knock characteristics of the fuels were quantitatively evaluated in terms of the methane number using various methods. Generally, the addition of either diluent carbon dioxide or nitrogen in the fuel mixtures augmented the knock resistance characteristics extending the engine operational limits. With every 10 per cent increase of carbon dioxide in the fuel mixture, the CR was increased by one point, whereas for a 25 per cent of nitrogen content in the fuel mixture, the CR was augmented by a half point in the operating conditions considered.

Low Heating Value Fuel Combustion: Flamelet Combustion and NO Formation Models

This study has performed the numerical simulations of turbulent low-heating-value fuel (LHVF) combustion and NO emissions using a flamelet model and a newly developed NO formation model. The emphases of this study are that the flamelet model employs a flamelet library specialized to LHVF combustion reaction, and a NO formation model involves not only the Zeldovich mechanism but also Fenimore mechanism. The calculated results of temperature and chemical species concentrations are compared with the measured ones. In consequence, it is clarified that the flamelet model can predict temperature and chemical species distributions of experimental results well, and the NO formation model developed in this paper shows good agreement with experimental results quantitatively.

Fischer-Tropsch process for the production of hydrocarbon products and the generation of electric power : Price, J. G. PCT Int. Appl. WO 02 60,841 (Cl. C07C1/00), 8 Aug 2002, ZA Appl. 2001/909, 1 Feb 2001. 31

Catalytic combustion of low heating value fuels is a promising method for electricity production combining the use of a renewable fuel with ultra-low emissions. In the present work, catalytic combustion of a low heating value gas has been studied over monolithic catalysts in an atmospheric 30 kW pilot catalytic combustor connected to a wood pellet gasifier. The results have been compared to similar tests with a model gas mixture and a series of test in a laboratory scale reactor for monolithic samples. Various catalyst configurations have been tested, such as precious metal-based catalyst impregnated on modified alumina washcoats and hexaaluminate washcoats. Cordierite monoliths with various cells per square inch were used as supports, but hexaaluminate extruded monoliths were also used. The catalysts were combined in different segment series.The results show that it is possible to ignite the low heating value gas at compressor outlet temperatures without the use of pre-burners over precious metal catalysts. Generally, trends and phenomena obtained in laboratory reactor operating close to isothermal could be confirmed in the pilot operating at close to adiabatic conditions. Further results, such as deactivation by sulphur compounds, fuel-NOx conversion and emissions of carbon monoxide and total hydrocarbons, are addressed in the paper.

Mathematical modelling of catalytic combustors fuelled by gasified biomasses

Catalytic combustion of low heating value (LHV) fuels from gasification of biomasses has attracted attention in recent years in order to reduce net CO 2 emissions in the atmosphere. A European project, ULECAT, has been undertaken to investigate this matter along with combustion of diesel fuel since 1996. This paper describes part of the work performed in the mathematical modelling task of this project in connection with the analysis of atmospheric pilot scale experiments. Simulation results obtained with a single channel 2D model of the monolith catalyst are presented and compared with experimental results obtained in an atmospheric pilot scale facility. From this analysis a quantitative description of the role of the different governing phenomena in determining the combustor performance is derived and discussed to provide insight into the catalyst behaviour under severe conditions. The most critical parameters in the simulations are also identified and discussed.

Catalytic combustion of low heating value gas mixtures: comparison between laboratory and pilot scale tests

Catalytic combustion of low heating value fuels is a promising method for electricity production combining the use of a renewable fuel with ultra-low emissions. In the present work, catalytic combustion of a low heating value gas has been studied over monolithic catalysts in an atmospheric 30 kW pilot catalytic combustor connected to a wood pellet gasifier. The results have been compared to similar tests with a model gas mixture and a series of test in a laboratory scale reactor for monolithic samples. Various catalyst configurations have been tested, such as precious metal-based catalyst impregnated on modified alumina washcoats and hexaaluminate washcoats. Cordierite monoliths with various cells per square inch were used as supports, but hexaaluminate extruded monoliths were also used. The catalysts were combined in different segment series. The results show that it is possible to ignite the low heating value gas at compressor outlet temperatures without the use of pre-burners over precious metal catalysts. Generally, trends and phenomena obtained in laboratory reactor operating close to isothermal could be confirmed in the pilot operating at close to adiabatic conditions. Further results, such as deactivation by sulphur compounds, fuel-NO x conversion and emissions of carbon monoxide and total hydrocarbons, are addressed in the paper.

98/01542 Utilization of wastes and low-heating value fuels in cement kilns: Schingnitz, M. et al. Ger. Offen. DE 19,608,093 (Cl. C10J3/00), 4 Sep 1997, Appl. 19,608,093, 2 Mar 1996, 10 pp. (in German)

98/01542 Utilization of wastes and low-heating value fuels in cement kilns: Schingnitz, M. et al. Ger. Offen. DE 19,608,093 (Cl. C10J3/00), 4 Sep 1997, Appl. 19,608,093, 2 Mar 1996, 10 pp. (in German)

Autoignition of Low-Heating Value Gases in a Direct-Injected Diesel Engine

Abstract The autoignition of gaseous mixtures representative of coal derived low-heating value fuels in a direct-injected diesel engine was theoretically investigated over a pressure and temperature ranges of 10 to 50 atm and 800 to 1000 K respectively- A computer model was validated with experimental data from an engine operated on direct injected synthetic coal gas. The computed results demonstrated the importance of the characteristic time associated with the chemical kinetics when compared to the fluid mixing. High gas temperatures resulting from high compression ratios, high boost, high inlet air temperatures or other ignition aids are needed to ignite these gases in a compression-ignition engine in a timely manner. The predicted sensitivity of the autoignition delay time to changes in the engine operating conditions was significant for temperature but almost negligible for pressures up to and slightly in excess of 30 atm