A Study Of Fammability Of Layered Fuels And The Conditions To Form Flaming Combustion

The research into the initiation and spread of the crown fire has been recognized as the most important topic in forest fire prevention and control sector.As a key technique for blocking forest fire including crown fire,the well-known biological fire-prevention belts are worthy of investigation in terms of their fire blocking mechanism and the associated scientific basis.However,so far the existing work focuses on the ignitibility and combustibility of the leaves from individual plant species,which cannot give convincible explanation on the fire retardant capacity of the biological fire-prevention belts.The broadleaves in tree crown are naturally formed as layered fuels in case of a wildland fire.As the result,the study on the flammability of the layered leaf samples can not only enhance the understanding on the spread of crown fire,but also provide the scientific basis for the development of the technique of biological fire-prevention belts and its further application.The pyrolysis characteristic and variations of heating values for various leaves were both explored,which can quantify their potential fire risk and provide the basic data for subsequent discussions.The kinetic data for pyrolysis of the plant samples were retrieved by using global single component reaction model(GSCRM),and then were compared with those obtained by the multi-component reaction model.The apparent activation energy for pyrolysis of leaf samples varies from 43 to 80 kJ mol-1,whereas that of the stems(twigs)ranges between 84 and 110 kJ mol-1.Further analyses revealed that the apparent activation energy obtained from GSCRM is essentially dominated by the cellulose content of the samples,which does not correspond to the energy barrier of any reactions occurring during plant pyrolysis.Nevertheless,GSCRM is still useful in the theoretical modeling of plant pyrolysis because of its practicability and reasonable accuracy exhibited.27 woody plant species including coniferous and broadleaf trees as well as shrubs were adopted,and the measurements were conducted through the proximate analysis,ultimate analysis and the detection of the heating values.Test results of the leaf samples indicate that their higher heating values on air-dried basis(HHVd)vary in a broad range from 17.48 to 24.01 MJ kg-1,with the average value of 21.30 MJ kg-1.The values of the HHVd are very close for the samples from the same family,and the ash content on air-dried basis plays negligible role in altering the magnitudes of the HHVd for the samples studied.Further analysis indicates that the HVd closely relies on the content ratio of volatile matter to the fix carbon(VMd/FCd).The ratio VMd/FCd increases with an increase in the atomic O/C ratio of the leaf samples,whereas the content ratios of fix carbon to the combustible substances fall in the range of the lignin contents stored in the plant foliage.An increase in the atomic O/C ratio and atomic H/C ratio implies a decrease in the lignin content stored,which leads to a reduction in a higher heating value on air-dried basis.It was found that the higher heating values of these leaf samples on air-dried basis can be precisely determined by the empirical formulas proposed as following:HHVd=18.93VM_d+36.95FC_d and HHV_d=0.4478C_d+1.4072H_d-0.2837O_d,which provide the basis for fast evaluation of the heating values of the leaf samples on received basis.The combustion characteristics of layered-leaf samples were explored by using the standardized cone calorimetric.Under the same radiant heat flux the samples underwent combustion in two major modes.Some of the samples appeared short time flame and followed by smoldering,whereas the others maintained smoldering throughout the measurements.With an increase in the radiant heat flux or volatile matter content of a sample,the combustion mode of the samples may shift from smoldering to flaming combustion at initial stage of measurement.Further analysis confirmed that the flaming combustion formed at initial stage of measurement mainly relies on the pyrolysis process of the sample surface layer.The initial volatile matter content of a sample and the rate of temperature rise at the surface layer are major factors governing the mass flux of volatiles generated and subsequently determining whether the sample undergoes flaming combustion.In the light of the characteristics of the external heat source and the layered feature of a leaf sample,the leaf sample was split into two major parts,the surface layer with thermally-thin property and the inner layers with thermally-thick property.An integrated model was then developed to describe the energy and mass balance of the two parts in the ignition process.By applying this model to tracing the heat transport during the ignition of a fresh leaf sample,it is found that the surface layer of the sample play an important role in absorbing external radiant heat,leading to a relatively low portion of the heat flowing into the inner layers of the sample.As a result,the contribution of inner layers to the formation of a flame during the ignition of the sample is negligible.The role of the surface layer in attenuating the portion of the net heat flowing into the inner layers is slightly affected by the level of the external radiant heat flux,but more significantly affected by the water content of the sample itself.The conditions to form the flaming combustion were then explored by taking into account the pyrolysis reaction in the ignition model,where the critical mass flux of combustible volatiles was adopted as an ignition criterion.Further analysis confirmed that the combustible volatiles during ignition were mainly sourced from the surface layer,and the proportion of the volatiles released by the surface layer would increase with an increase in the external radiant heat flux or a decrease in the moisture content.As the moisture content of leaf samples increases,the mass flux of volatiles stemmed from the inner layer also increases,which leads to the decrease in the critical radiant heat flux for forming flaming combustion.For comparison,the mathematical model for the ignition process of woody materials was established,which can visually present the difference of the ignition phenomena between the high-density materials and layered combustibles.It was observed that,for a specific type of wood materials,the net energy absorbed by a solid during ignition Eig decreases with an increase in incident heat flux or a decrease in local wind speed,whereas an increase in the moisture content of the solid results in an enlarged Eig.The ratio of the net energy to the apparent energy absorbed during ignition Eig/Ea varies in a broad range,essentially independent to the wood types and their moisture content.Further analyses revealed this ratio can be described precisely by a function of the parameter yig that specifies the environmental conditions for a solid to ignite.This finding paves the way for improving the ignition criteria based on the energy requirement for a solid to ignite.