Experimental studies of self-sustaining thermal aquifer remediation (STAR) for non-aqueous phase liquid (NAPL) sources

Self-sustaining Thermal Aquifer Remediation (STAR) is a novel technology that employs smouldering combustion for the remediation of subsurface contamination by non-aqueous phase liquids (NAPLs). Smouldering is a form of combustion that is slower and less energetic than flaming combustion. Familiar examples of smouldering involve solid fuels that are destroyed by the reaction (e.g., a smouldering cigarette or peat smouldering after a wildfire). In STAR, the NAPL serves as the fuel within an inert, porous soil medium. Results from experiments across a range of scales are very promising. Detailed characterisation has focused on coal tar, a common denser-than-water NAPL (DNAPL) contaminant. Complete remediation is demonstrated across this range of scales. Visual observations are supported bychemical extraction results. Further experiments suggest that STAR can be self-sustaining, meaning that once ignited the process can supply its own energy to propagate. Costly energy input is reduced significantly. Comparison of large scale to small scale laboratory experiments, a volume increase by a factor of 100, suggests that STAR process efficiency increases with scale. This increase in efficiency results from reduced heat losses at larger scales while maximum the temperature achieved by STAR is unaffected. The research also demonstrates the controllability of STAR, where the termination of airflow to the reaction terminates the STAR process. The scale-up process provides important guidance to the development of full scale STAR for ex situ remediation of NAPL-contaminated soil

Scaling-up experiments of smouldering combustion as a remediation technology for contaminated soil

Self-sustaining Treatment for Active Remediation (STAR) is a novel, patent-pending process that uses smouldering combustion as a remediation technology for land contaminated with hazardous organic liquids. Compounds such as chlorinated solvents, coal tar and petroleum products, called Non-Aqueous Phase Liquids (NAPLs) for their low miscibility with water, have a long history of use in the industrialised world and are among the most ubiquitous of contaminants worldwide. These contaminants are toxic and many are suspected or known carcinogens. Existing remediation technologies are expensive and ineffective at reducing NAPL source zones sufficiently to restore affected water resources to appropriate quality levels. STAR introduces a self-sustaining smouldering reaction within the NAPL pool in the subsurface and allows that reaction to provide all of the post-ignition energy required by the reaction to completely remediate the NAPL source zone in the soil. Results from laboratory and field experiments have been very promising. Laboratory experiments have demonstrated STAR across a wide range of NAPL fuels and focused on coal tar to identify key parameters for successful remediation. Modelling has suggested that STAR efficiency will improve with scale as effects such as heat losses from boundaries become less significant. Observations from field experiments support the modelling theory - significantly lower relative air flow in a smouldering field experiment (330L) led to faster smouldering front propagation than observed in laboratory experiments (1L and 3L). Preliminary emissions monitoring by Fourier Transform Infrared (FTIR) spectroscopy has suggested that STAR emissions might be low enough to meet regulatory requirements, but further study is necessary. As emissions are expected to vary with each contaminant, activated carbon filters are being developed and tested in case emissions filtration is necessary. Experiments at all scales have demonstrated that STAR is controllable and self-terminating. Pilot-scale (2500L) field trials are underway to demonstrate STAR on excavated contaminated soil. The materials that will be studied in these trials are manufactured coal tar in coarse sand (which is the same material as used in the laboratory and field experiments) as well as two soils obtained from coal tar contaminated sites. This poster focuses on the scale-up to these field trials, including small scale characterisation, large scale performance, emissions monitoring and post-treatment soil analysis

Small-scale forward smouldering experiments for remediation of coal tar in inert media

This paper presents a series of experiments conducted to assess the potential of smouldering combustion as a novel technology for remediation of contaminated land by water-immiscible organic compounds. The results from a detailed study of the conditions under which a smouldering reaction propagates in sand embedded with coal tar are presented. The objective of the study is to provide further understanding of the governing mechanisms of smouldering combustion of liquids in porous media. A small-scale apparatus consisting of a 100 mm in diameter quartz cylinder arranged in an upward configuration was used for the experiments. Thermocouple measurements and visible digital imaging served to track and characterize the ignition and propagation of the smouldering reaction. These two diagnostics are combined here to provide valuable information on the development of the reaction front. Post-treatment analyses of the sand were used to assess the amount of coal tar remaining in the soil. Experiments explored a range of inlet airflows and fuel concentrations. The smouldering ignition of coal tar was achieved for all the conditions presented here and self-sustained propagation was established after the igniter was turned off. It was found that the combustion is oxygen limited and peak temperatures in the range 800-1080 °C were observed. The peak temperature increased with the airflow at the lower range of flows but decreased with airflow at the higher range of flows. Higher airflows were found to produce faster propagation. Higher fuel concentrations were found to produce higher peak temperatures and slower propagation. The measured mass removal of coal tar was above 99% for sand obtained from the core and 98% for sand in the periphery of the apparatus

Reducing The Computational Requirements for Simulating Tunnel Fires by Combining Multiscale Modelling and Multiple Processor Calculation

Multiscale modelling of tunnel fires that uses a coupled 3D (fire area) and 1D (the rest of the tunnel) model is seen as the solution to the numerical problem of the large domains associated with long tunnels. The present study demonstrates the feasibility of the implementation of this method in FDS version 6.0, a widely used fire-specific, open source CFD software. Furthermore, it compares the reduction in simulation time given by multiscale modelling with the one given by the use of multiple processor calculation. This was done using a 1200 m long tunnel with a rectangular cross-section as a demonstration case. The multiscale implementation consisted of placing a 30 MW fire in the centre of a 400 m long 3D domain, along with two 400 m long 1D ducts on each side of it, that were again bounded by two nodes each. A fixed volume flow was defined in the upstream duct and the two models were coupled directly. The feasibility analysis showed a difference of only 2% in temperature results from the published reference work that was performed with Ansys Fluent (Colella et al., 2010). The reduction in simulation time was significantly larger when using multiscale modelling than when performing multiple processor calculation (97% faster when using a single mesh and multiscale modelling; only 46% faster when using the full tunnel and multiple meshes). In summary, it was found that multiscale modelling with FDS v.6.0 is feasible, and the combination of multiple meshes and multiscale modelling was established as the most efficient method for reduction of the calculation times while still maintaining accurate results. Still, some unphysical flow oscillations were predicted by FDS v.6.0 and such results must be treated carefully

Computational study on self-heating ignition and smouldering spread of coal layers in flat and wedge hot plate configurations

Porous fuels have the propensity to self-heat. Self-heating ignition has been a hazard and safety concern in fuel production, transportation, and storage for decades. During the process of self-heating ignition, a hot spot forms in the fuel layer and then spreads as a smouldering fire. The understanding of hot spot and smouldering spread is important for prevention, detection, and mitigation of fires. In this paper, we build a computational model that unifies the simulation of self-heating ignition and smouldering spread by adopting a two-step kinetic scheme obtained from literature. The model is validated against hot plate experiments of coal in both flat and wedge configurations. The comparison shows that the model predicts the minimum ignition temperature (Tig) and transient temperature profiles reasonably well. The simulation results demonstrate that the hot spot originates at the hot plate and then spreads towards the free surface due to oxygen consumption. In the wedge configuration, the simulations show that the height of maximum temperature point decreases with wedge angle, and that the influence of wedge angle can be explained by the heat transfer. This model brings together two combustion phenomena (self-heating ignition and smouldering) that were traditionally studied separately and analyses the transient behaviour of hot spot and smouldering spread in detail. It deepens our understanding of self-heating fire and can help mitigate the hazard.</p

Gains and Threats from Smouldering Combustion to Biochar Production and Storage

Poster presented at the 2008 Conference of the International Biochar Initiative, Newcastle, 9th Sept 2008Biochar is an environmentally beneficial way of locking carbon emissions into a solid phase and storage for very long periods of time. On the one hand, smouldering combustion can be used in a pyrolysis reactor to provide energy-efficient conversion of biomass into biochar. This reactor design could run with minimal or zero energy costs and high yield since the smouldering process would provide the energy supply released from the slow oxidation of a fraction of the biomass itself. On the other hand, smouldering fires of organic soils occurs at a global scale and are a mayor menace to biochar storage fields and can reduce them to ashes. This is about the only hazard that can led to an accidental release to the atmosphere of the stored carbon in the biochar. The technology to enhanced production and safer storage of biochar is currently being developed at Edinburgh from fundamental knowledge on smouldering combustion

From Pyrolysis Kinetics to Models of Condensed-Phase Burning

invited paperThe state-of-the-art of fire modelling is currently hindered due to a poor capability to model the burning of solid fuels. Current fire modelling tools provide good predictions of the thermal effects of a fire (e.g. the resulting thermal environment) but fail to predict properly the fire development (e.g. flame spread and fire growth). The consequence is that current fire modelling cannot predict the transient evolution of the heat release rate of a fire. The development of fire spread models that can accurately predict the ignition and burning of solid-fuels will be a major advancement. In this work, the effects of the kinetic parameters of a one-step and a two-step pyrolysis are studied by combining it with a simple heat transfer model. The issue of the required level of complexity in the models for parameter-estimation is a concern. Simplifications are required where the necessary precision does not warrant the inclusion of higher levels of complexity. This paper advocates for the use of blind predictions in combination with sensitivity studies and the identification of the simplest model that can predict the experimental data. Results are presented here in that direction

FireGrid: Forecasting Fire Dynamics to Lead the Emergency Response

The predictions of future events has fascinated humanity since the beginning of history. This attraction has permeated into science and engineering, where several disciplines has emerged providing the capability to forecast dynamics within some lead time. Weather forecast is the most important and the most developed one. The concept of FireGrid is a paradigm shift in the response to emergencies, providing the fire service with essential sensor information. This information will also include forecast of the fire dynamics. But many questions remain to be answered. This talk explores the potential methodology to forecast fire dynamics in enclosures, drawing lessons also from weather forecast. www.firegrid.or

IMPROVED TRAVELLING FIRES METHODOLOGY - iTFM

Current design codes and most of the understanding of behaviour of structures in fire are based on small enclosure fires. The World Trade Centre Tower fires in 2001 have highlighted the need of a more realistic design tools to represent fires in large compartments. Following the events Travelling Fires Methodology (TFM) has been developed by Stern-Gottfried and Rein to account for the travelling nature of fires. In this study the TFM is refined to account for more realistic fire dynamics. Equations are introduced to reduce the range of possible fire sizes. The analytical equations describing reducing far-field temperatures are presented. The concept of flame flapping is introduced to account for variation of temperatures in the near-field region due to natural fire oscillations. The need for more fundamental research and experimental evidence in large compartments for further development of and improvements on TFMis highlighted