Burning Rates and Instabilities in the Combustion of Droplet and Vapour Mixtures

It is well established that the laminar burning rate plays an important role in turbulent combustion and previous work at Leeds has suggested that the laminar burning velocity of an aerosol mixture is little different from that of a gaseous mixture at similar conditions. However, it has been shown that flames within well defined droplet suspensions (aerosols) more readily become unstable than for gaseous ones. Flame instabilities, characterised by wrinkling and cellular surface structure, increase the burning rate due to the associated increase in surface area. For gaseous mixtures, the effect has been shown theoretically and experimentally to be a function of Markstein number and critical Peclet number, which marks the flame radius at which cellularity is first observed. In aerosol combustion, the presence of liquid droplets has been shown to influence instabilities by causing earlier onset of cellularity than for gaseous flames. Therefore it is imperative to conduct a fundamental study to understand the complex interactions between droplets and combustion. In the present work, spherically expanding flames were employed to quantify the burning rates in gaseous and aerosol flames and to determine their differences. !so-octane-air aerosols were generated by expansion of the gaseous pre-mixture, based on the Wilson cloud chamber principle of expansion cooling, to produce a homogeneously distributed suspension of fuel droplets. Flames were centrally ignited for quiescent aerosols at near atmospheric pressures, drop sizes of up to 30 ~m and overall equivalence ratios between 0.8 to 2.0. The flame progress was monitored using high-speed schlieren photography, from which burning rates were determined. In turbulent studies, measurements were made for stoichiometric aerosols at root mean square turbulence velocities of between 1.0 and 4.0 m/s. For companson, gaseous combustion at conditions similar to those of aerosols were studied. From the laminar study, it was shown that the burning rate of lean mixtures is independent of droplet diameter. However, at higher equivalence ratios, the burning rate became a strong function of droplet diameter and equivalence ratio. This was associated with the onset of instabilities, which were, in tum, related to measured values of Markstein number and critical Peclet number for aerosol flames in a similar manner to those for gaseous flames. Heat loss from the flame due to droplet evaporation is probably the main reason for instabilities in aerosol flames. Interestingly, droplets which were assumed by previous workers to be fully evaporated at the flame front, were shown, at certain conditions in the present work,to survive behind the flame front. Thus other possible mechanisms for instabilities in aerosol flames could be important. For very rich mixtures, gaseous flames were shown to be partially smooth, slow, and were strongly affected by natural convection. Conversely, with the presence of droplets at similar mixture conditions, flames were found to be fully cellular and faster, with little sign of the effect of natural convection. This was suggested to be due to early instabilities caused by the presence of droplets, which, in tum, increased the burning rates. Oscillating flames, in which the flame speed and flame structure alternated between low and high values and smooth and cellular respectively, during flame development, were observed for some experimental conditions. These oscillations were most probably caused by aerodynamic interaction between droplets and gas motion ahead of the flame. This was examined using simultaneous laser sheet imaging and PIV analysis, with a simple model proposed by Atzler et al. (2001) which simulated aerodynamic interaction between droplets and gas phase motion ahead of the flame front. A dimensionless comparison between turbulent flames of aerosol and gaseous mixtures showed similar burning rates. The measurements were compared with existing turbulent burning velocity expressions and correlations. In general, these expressions are in quite good agreement with the present results, particularly at low stretch rate

On gasifier cookstove operation fuelled by different lignocellulosic biomass materials / Shaharin Anwar Sulaiman and Nurul Aisyah Mohd Zin

Gasifier based cookstoves are relatively clean and environmental friendly than that of direct combustion type. Though a few commercial designs are available, their capability in handling different biomass has not been known. The objective of this work was to characterize the basic operating properties of a gasifierbased biomass cookstove using different types of biomass fuels. The main characteristics evaluated were the efficiency of the stove. The biomass considered were oil palm fronds, dry leaves and pressed sugarcane. The efficiency of the stove was tested using standard boiling tests for 2.5 kg of water. The performance of each fuel was studied by analyzing the parameters involved during water boiling tests. It was demonstrated that the gasifier cookstove would be capable in handling different lignocellulosic biomass materials, although oil palm frond exhibited the highest thermal efficiency

Biomass Gasification

This chapter discusses the state of the art in biomass gasification studies. It initially gives a brief account on the energy potential and characterization of different biomass fuels. A review of the mechanisms of biomass gasification process and influence of major operating conditions on syngas composition and heating value (HV) was presented. Consideration of syngas quality requirements for different downstream applications and the means of achieving the same through optimum operation were highlighted. The theoretical studies of gasification process mainly focused on prediction of syngas composition and investigating influence of various operating conditions on process output. The equilibrium modeling assumes conditions of the ideal, well-stirred reactor with sufficiently long residence time to allow the reactions to reach equilibrium. Kinetic models present detailed information on the residence time and kinetic limitations; however, equilibrium models were widely used as a valuable tool in predicting the thermodynamic limits of chemical processes

Study on Tar Generated from Downdraft Gasification of Oil Palm Fronds

One of the most challenging issues concerning the gasification of oil palm fronds (OPF) is the presence of tar and particulates formed during the process considering its high volatile matter content. In this study, a tar sampling train custom built based on standard tar sampling protocols was used to quantify the gravimetric concentration of tar (g/Nm3) in syngas produced from downdraft gasification of OPF. The amount of char, ash, and solid tar produced from the gasification process was measured in order to account for the mass and carbon conversion efficiency. Elemental analysis of the char and solid tar samples was done using ultimate analysis machine, while the relative concentration of the different compounds in the liquid tar was determined making use of a liquid gas chromatography (GC) unit. Average tar concentration of 4.928 g/Nm3 and 1.923 g/Nm3 was obtained for raw gas and cleaned gas samples, respectively. Tar concentration in the raw gas sample was found to be higher compared to results for other biomass materials, which could be attributed to the higher volatile matter percentage of OPF. Average cleaning efficiency of 61% which is comparable to that of sand bed filter and venturi scrubber cleaning systems reported in the literature was obtained for the cleaning system proposed in the current study

Syngas-Enriched hydrogen production via catalytic gasification of water hyacinth using renewable palm kernel shell hydrochar

Syngas produced from biomass gasification has emerged as a highly promising substitute for conventional fossil fuel, catering to various industrial applications while ensuring minimal greenhouse gas emissions. Water hyacinth (WH) has been a major concern due to its invasive nature and uncontrollable growth which impedes aquatic growth and urban management. Fortunately, WH is a potential biomass feedstock due to the comparable cellulose and hemicellulose contents alongside high carbon content and high calorific value which reflects good biofuel properties. Therefore, this study aims to investigate the conversion of WH biomass via catalytic air gasification for syngas-enriched hydrogen production using palm kernel shell hydrochar (PKSH). A parametric study was conducted in a lab-scale fixed-bed downdraft gasifier based on the response surface methodology coupled with Box-Behnken design (RSM-BBD). The combined interaction effects of the influencing parameters investigated are temperature (600–800 °C), biomass particle size (2–6 mm), catalyst loading (0–10 wt%), and air flow rate (1–3 L/min). Temperature was revealed to be the primary factor with significant influence on the H2 and CO output. Maximum syngas (30.09 vol%) compositions of 11.14 vol% H2 and 18.95 vol% CO were obtained at 800 °C with a particle size of 6 mm and air flow rate of 2 L/min alongside 5 wt% PKSH catalyst loading

Evaluation on methods in estimating the photovoltaic performances affected by module operation temperature in tropical region

This research work is intended to evaluate the reliability of commonly utilized empirical correlations of module operation temperature in estimating the photovoltaic performances in tropical region. The Nominal Operation Cell Temperature (NOCT) model, Tropical Field Operation Cell Temperature (tFOCT) model and the experimental back module temperature were selected for evaluation purposes. The models were evaluated by comparing the performance characteristics of a 250W monocrystalline photovoltaic module installed at University Malaysia Pahang. The monocrystalline back module temperature and power output as well as the environmental data including both solar irradiation and ambient temperature were monitored to assist the analysis. Based on the 5 consecutive day experimental data, results indicated that the module operation temperature estimated by tFOCT model had the closest value to the experimental back module temperature. Whereas, the temperature estimated by NOCT model showed the highest deviation up to 25.8% from the experimental back module temperature. However, in terms of estimating the photovoltaic module power output, the NOCT model had the closest value to the experimentally measured power output. The results also indicated that utilizing the back module temperature often mislead the estimation of photovoltaic module power output. In addition, the deviation of estimated power output from NOCT model, tFOCT model and back module operation temperature as compared to the experimental power output were 15.4%, 18.87% and 21.2%, respectively. Thus, the NOCT model demonstrated better estimation of power output as compared to the experimental result than tFOCT model, and back module temperature. However, better estimation method for tropical regions is still needed because three methods evaluated in this study shows deviation of more than 15.4% from the measured power output

Effects of Swirl Bubble Injection on Mass Transfer and Hydrodynamics for Bubbly Flow Reactors: A Concept Paper

Bubble flow reactors (BFR) are commonly used for various industrial processes in the field of oil and gas production, pharmaceutical industries, biochemical and environmental engineering etc. The operation and performance of these reactors rely heavily on a range of hydrodynamic parameters; prominent among them are geometric configurations including gas injection geometry, operating conditions, mass transfer etc. A huge body of literature is available to describe the optimum design and performance of bubbly flow reactors with conventional bubble injection. Attempts were made to modify gas injection for improved efficiency of BFR's. However, here instead of modifying the geometry of the gas injection, an attempt has been made to generate swirl bubbles for gaining larger mass transfer between gas and liquid. Here an exceptionally well thought strategies have been used in our numerical simulations towards the design of swirl injection mechanism, whose paramount aspect is to inhibit the rotary liquid motion but facilitates the swirl movement for bubbles in nearly stationary liquid. Our comprehension here is that the swirl motion can strongly affect the performance of bubbly reactor by identifying the changes in hydrodynamic parameters as compared to the conventional bubbly flows. In order to achieve this bubbly flow, an experimental setup has been designed as well as computational fluid dynamic (CFD) code was used with to highlight a provision of swirl bubble injection by rotating the sparger plate

Study on Thermal Performance Assessment of Solar Hot Water Systems in Malaysia

Solar Hot Water Systems (SHWS) are gaining popularity in Malaysia due to increasing cost of electricity and also awareness of environmental issues related to the use of fossil fuels. The introduction of solar hot water systems in Malaysia is an indication that it has potential market. However, there is a need for a proper methodology for rating the energy performance of these systems. The main objective of this study is to assess the thermal performance of several SHWS subject to four different locations in Malaysia using combined direct measurement and computer modelling using the TRNSYS simulation program. The results showed distinct differences in performance of the systems as a result of locations and manufacturers. The findings could be used further in developing an acceptable rating system for SHWS in Malaysia