Thermal combustion and oxygen chemisorption of wood exposed to low temperature long term heating 7

Chapter 7: Conclusions Chapter Seven: Conclusions 7.1 Overview of work done In this study, works have been conducted to formulate a drying model that is suitable to examine low-temperature, long-term heating of wood Specific considerations have been given to evaporation occurring at the different phases of heating, liquid water movement in the porous medium and the combined diffusivity of both liquid and vapour transport on the temperature distribution in the low temperature heating of hygroscopic wood The role of water in the spontaneous combustion of wood was examined in the thermophysical framework of heat conduction; its effects were evaluated in terms of thermal conductivity, diffusivity, specific heat, latent heat of evaporation and heat of wetting An analytical model incorporating the moisture-mediated thermophysical terms based on heat conduction provides a framework to examine the experimental results on heating and ignition properties of green and preburn wood, each case representing the different degree of moisture and thermophysical values For preburn wood, it was preheated in oven to produce an average 52 % of preburn (i.e 48% unconverted virgin mass) The partially charred wood has halved its moisture content to 6.65% as compared to 13.83% of oven-dry green wood, and contained reduced specific heat capacity and thermal conductivity 204 Chapter 7: Conclusions Oxygen chemisorption was carried out to investigate the propensity of self-ignition of the low-temperature, long-term preheated wood chars Separate parameters of testing and evaluation model of reactive sites were carried out to elucidate the chemical reactivity of this amorphous cellulosic chars Oxygen chemisorption provides a chemical viewpoint to in addition to thermophysical approach to examine self-heating and spontaneous combustion of low-temperature, long-term preheated wood, targeted to minimize unwanted fires arising from the wood structures and construction in historical and modern buildings The next section presents the conclusions and contribution of knowledge to the thermal combustion and propensity of preheated wood chars exposed to low-temperature, long-term heating Recommendations are proposed therein with the respective research findings 7.2 Conclusions and Recommendations The research findings contribute to the body of knowledge on thermal combustion and oxygen chemisorption of wood in the following areas: evaporation, liquid water transport, role of moisture in thermal combustion and oxygen chemisorption 205 Chapter 7: Conclusions 7.2.1 Evaporation for low-temperature drying The study found that for low-temperature, long-term heating of wood, a different approach towards evaporation is required in the construction of a heating model The study indentified that for low-temperature heating model, the following key research findings that contributed to the understanding of low-temperature drying phenomenological modelling: - • For the initial drying phase of low-temperature drying with low heating rates, evaporation occurred at the boundary Hence, evaporation term was removed from the energy balance for the low drying model The simulated temperature-distance curve produced a uniform temperature distribution throughout the volume, which would be more realistic for the initial heating phase rather than an exponential temperature-distance time curve The study showed that re-formulation of evaporation at the boundary produced temperature distribution that concurred well with theoretical proposition for initial drying, where the large moisture content and water diffusivity contributed to a rather uniform temperature distribution within the domain • For prolonged drying, it was necessary to re-introduce the evaporation term into the energy balance as evaporation front has retreated into the domain Equilibrium approach was chosen over the non-equilibrium approach to formulate the rate of evaporation for the case of low-temperature, extended drying phase The simulations 206 Chapter 7: Conclusions results clearly demonstrated that equilibrium water-vapour assumption was more suitable for low-temperature, prolonged drying of wood than a fast heating scenario This was because the extended low temperature, constant heating of wood cubes at 200°C, coupled with moderate equilibrium moisture content of 14%, permitted the vapour pressure to reach equilibrium in both real time and over surface zones, where fast vapour convection would have perturbed the balance of water-vapour equilibrium, defying the equilibrium assumption which did not consider the rate limit The simulations illustrated the heat sink effect of an internal evaporation term • Future work: the findings from this study suggested that long-term heating could continue to adopt the equilibrium approach for water evaporation given the low temperature range and extended time The use of non-equilibrium approach would however improve the accuracy in terms of modeling the transition of non-equilibrium to equilibrium states However, measurement of vapour pressure in a small domain in a dynamic environment would be necessary to formulate the rate of evaporation to match the actual attainment of equilibrium time if the non-equilibrium approach is used More accurate dynamics measurement methods for vapour pressure in porous media would also be required 7.2.2 Liquid water transport To evaluate the effects of moisture transport on thermal combustion, this study found that free water movement should be modeled at the low temperature wood drying 207 Chapter 7: Conclusions A porous model was proposed to consider free water movement The research findings established key findings pertaining to the use of porous model and the effects of moisture transport on thermal combustion as follows: - • The porous slab was modeled by physical addition of momentum sink to the transport equations in a conservation model It offered a useful method to use heat and mass transfer governing equations for examining flow in porous structure The use of discrete pore model approach to study porous slab would not be able to incorporate governing equations that are built on the assumption that all phases are continuous Darcy’s law in the porous medium was modified to account for the effect of boundaries on the flow field and the inertial effects associated with the flow • The impact between free water movements on the development of temperature field, examined using the porous model, showed a strong correlation Free water movement was slow and stable at the longitudinal cross-sections measured at the centre, 20mm measured from surface and surface boundary The coupled velocity of free water and temperature field development in the initial phase did not produce any significant temperature rise in the heating of wood cube Free water movement at initial heating phase, when subjected to constant heating regime did not affect the spontaneous combustion of wood • For extended drying, the strong correlation between fluid flow and the temperature distribution illustrated the importance of fluid movement on temperature field 208 Chapter 7: Conclusions development Stronger fluid movement at low temperature, prolonged heating enhanced temperature development, partly explained the spontaneous combustion observed in the wood cube after being exposed for hours in the isothermal oven The results endorsed Babrauskas (2001) proposition that fluid flow has a deleterious effect on thermal stability, where spontaneous heating occurs more readily in wet than dry wood • Limitations: The drying model with internal evaporation successfully modeled fluid flows in terms of a combined moisture and vapour approach The combined fluid flow was introduced to overcome the problem of unknown transition rate between non-equilibrium to equilibrium between water to vapour, as equilibrium approach assumed instantaneous equilibrium without rate limit However, the use of combined diffusivity would not be able to differentiate between two types of moisture, and hitherto not able to provide information on the changes in each physical phase • Future work: For detailed modeling of flows in the heating of wood, both vapour and liquid water need to be considered However, correct modeling of vapour pressure or vapour equilibrium state is critical to the separate modeling of vapour and liquid water More work could be done to improve the theoretical knowledge of phase change between water and vapour in the heating process Further work could be carried out to investigate the interaction between convection and chemical reaction which impacts on the development of the concentration and temperature fields In 209 Chapter 7: Conclusions future models that consider chemical reactions such as pyrolysis, the influence of convection on the development of chemical reaction can be examined 7.2.3 The role of moisture in thermal combustion Thermal combustion, introduced by the Russian school (Zeldovich and FrankKamenetskii 1938, Frank-Kamenetskii 1969) which has been systematically adopted in the study of spontaneous combustion, is based solely on the consideration of pure heat conduction, without considering the thermodynamics of moisture effects This study incorporated moisture-mediated thermophysical properties within a revised thermal model to examine the role of moisture from thermophysical viewpoint The study established some important findings for thermal combustion model as follow: • Pure heat conduction model produced larger values of surface temperature at ignition Tig than the thermocouple-measured values for green wood At high heat fluxes ≥ 20kW/m2, the pure heat conduction model yielded exceedingly high temperatures ranging from 430°C to 465°C The revised analytical model incorporating moisturemediated thermophysical terms, however was able to produce surface temperatures that were in better fit with the experimentally measured surface temperatures The significance of moisture in thermophysical terms can also be illustrated by the results obtained from the preburn wood samples The good agreement in both the computed values with measured temperatures in preburn wood can be explained in terms of reduced moisture, which lowered the values of thermophysical properties 210 Chapter 7: Conclusions • This study has proposed the use of complimentary error function erfcβ for β  and complimentary function erf β for β  in the calculation of cooling modulus The proposed change yielded good results for piloted ignition of both green and preburn wood However, the revised thermal model could not accurately predict the surface temperatures for spontaneous ignition mode, particularly for green wood This was because without the presence of pilot to provide the energy to ignite the combustible-gas mixture, flaming occurs entirely upon the attainment of sufficient surface temperature; hence, moisture, other than its effects on thermophysical terms, must have acted thermodynamically to affect the ignition of green wood • This study experimentally yielded a lower critical heat flux at 11kWm-2 for green wood, and 12kWm-2 for preburn wood, when measured up to hours for green wood and ¾ hour for preburn wood These experimentally derived critical heat fluxes are lower than the stipulated design value of 12.5 kWm-2 obtained for correlation of ignition times less than 20 minutes duration For green wood, theoretically  ′′  ′′ extrapolated qcr derived using 1/ tig vs qe is within 10% of the measured minimum  ′′  ′′ heat flux of 11kW/m2 The extrapolation using qe / tig vs qe is around 20% of the measured minimum heat flux The results suggest that the data for thick fuels should  ′′ be examined by plotting 1/ tig vs qe However, both extrapolation methods did not yield as good a proximate critical heat flux for piloted ignition preburn wood as compared to that of green wood, when judged using the 10% rule 211 Chapter 7: Conclusions • Future work: It could be possible to further elucidate an even lower critical heat flux following the introduction of the new class of potential hazard of preheated wood However, more work is need to elucidate the mechanisms, both experimental and theoretical, that cause the ignition data at lower heat fluxes to deviate from that obtained in high fluxes A secondary trend still dominated at the lower heat fluxes for both green and preburn wood using thermal correlation A better physicochemical understanding is required to address the secondary mechanism before any extended correlation could be realisable 7.2.4 Oxygen chemisorption To evaluate the propensity of self-heating, gas-solid reactions provided the chemical viewpoint in addition to the thermophysical modelling studies The key findings pertaining to oxygen chemisorption, the reactivity, and changes in chemical functional groups of preheated wood chars were listed as follow: • Oxygen chemisorption largely conformed to Elovich kinetics Because of the low heat treatment temperature, wood chars were invariably very reactive Only wood chars tested at low chemisorption temperatures equal and below 140°C could be interpreted for the full run for chemisorption Cellulosic char created at 300°C at partial pressure of oxygen pO2 = 160mmHg has compatible reactivity as that of more graphitic, well carbonised chars found in other studies such as brown coals char 212 Chapter 7: Conclusions (tested at pO2 = 380mmHg ) and pure carbon (tested at pO2 = 760mmHg ) temperature range from 110°C to 155°C The changes in oxygen partial pressures did not significantly affect the rate of chemisorption as temperature did Cellulosic wood chars reported activation energy of 16 kcal/mol for chemisorption The apparent activation energy in cellulosic wood chars showed that there are different types of active sites responsible for chemisorption in wood chars and that of the more mature and graphitic carbon chars Different theoretical models are needed to understand chemisorption and combustion behaviours of cellulosic wood chars from coal chars • Contrary to the popular belief that only inert-heated chars are capable of chemisorption, this study found that air-preheated wood chars at 150°C for 30days also showed some weight gain due to gases adsorption, and thereafter ensued by a precipitous weight loss, indicating ignition For air-preheated wood chars heated at 140°C for 50 days, chemisorption occurred after some period of isothermal heating, leading to rapid gasification at the end of the chemisorption run Nonetheless, there is no conclusive evidence at this point to predict as to when chemisorption might occur in air-preheated samples, and if ignition might take place at all, unlike inert-heated samples where chemisorption is immediate upon exposure to oxygen and in certain cases, leading to ignition of wood chars at low temperature • Fourier-transform Infra-red Spectroscopy showed that the low temperature chars prepared at 140oC and 150oC for both types of woods in general have very similar chemical composition with the fresh wood samples, except that these char samples 213 Chapter 7: Conclusions contained higher concentration of carbonyl groups at around 1700 cm-1 Hydroxyl groups (3600-3300 cm-1) were progressively eliminated as charring temperature increased These groups were totally eliminated in wood char samples when heated at 200°C for days The study showed that wood chars that exhibited increased propensity for self-ignition were those heated at 140°C and 150°C The heat treatment temperature at 200°C therefore sets the upper limit on pyrophoria of wood chars because the primary hydroxyl group responsible for reactivity of wood chars was preferentially oxidized • Lower ignition temperatures have been found for the long-term air-preheated wood char samples in this study The Semenov model of thermal ignition analysis showed that the different ignition temperatures, which arose from a combination of different preheating duration and heat treatment temperature were almost kinetically comparable Despite the fact that chemisorption led to a lower ignition temperature, the observed range of lowered ignition temperatures was not kinetically significant on wood pyrolysis, neither did the change in ignition temperature suggest a switch in pyrolysis pathway in wood decomposition • Future work: To further elucidate the carbon-oxygen reaction, temperatureprogrammed desorption studies could be carried out to study the transient rates of consumption of oxygen and production of CO2 and CO The role of water content in wood char oxidation can be further investigated to expound the interaction between water and chemisorption in wood char 214 ... the body of knowledge on thermal combustion and oxygen chemisorption of wood in the following areas: evaporation, liquid water transport, role of moisture in thermal combustion and oxygen chemisorption. .. addition to thermophysical approach to examine self -heating and spontaneous combustion of low- temperature, long- term preheated wood, targeted to minimize unwanted fires arising from the wood structures... structures and construction in historical and modern buildings The next section presents the conclusions and contribution of knowledge to the thermal combustion and propensity of preheated wood chars exposed