Characteristics Of Smoldering Of Piled Rice Husk And Char Rod

Smoldering is slow,low-temperature,and flameless burning of solid fuels,which has broad application prospects in the field of biomass-waste thermal conversion due to the advantages of low temperature of solid phase and high energy-efficiency.Rice husk is a typical biomass waste,and the silica content is very high in its ash.As far as we know,there are no experiments using smoldering technology to produce silica in literatures.In addition,the process of smoldering of piled biomass fuels like rice husk is unsteady,which is bad for treatment of flue gas and the large-scale application of smoldering technology.Therefore,it is necessary for us to study the mechanism of steady smoldering.Since the steady propagation of smoldering is present in the combustion of rod-shaped fuels,char rods were chosen for analysis in this study.Mechanism on steady propagation of smoldering based on characteristics of char cone was studied.This study provided basis for investigation on steady smoldering.(1)Smoldering experiments of dried and moist(moisture content of 51%)rice husk in a newly designed smoldering apparatus was performed to analyze the smoldering characteristics of rice husk.The temperature history inside the fuel bed during smoldering and the content of each component of flue gas at different times were recorded.Results showed that the temperature and content of flue gas vary significantly with time,suggstiong that the smoldering process is unsteady.The maximum temperature is about560°C,lower than those in most of combustors.The duration of high temperature in moist fuel bed is shorter than that in dried fuel bed,and the maximum temperature of the former(about 50°C)is slightly lower that the latter.Some volatiles from the lower part of the fuel bed adhere to its upper ash during piled smoldering.(2)The silica content and specific surface area of it were analyzed,and comparison on the silica content and specific surface area between smoldering silica and prepared by burning of dried and moist rice husk in muffle furnace was performed.Results showed that the silica content and specific surface area of ash from smoldering of moist rice husk were86.4%and 84.9 m~2/g,respectively.Compared to our experiments,they are close to smoldering of moist rice husk(89.0%,67.7 m~2/g);different from muffle furnace burning(600°C,2 h)of rice husk.The specific surface area is higher than those from most industrial methods(from 11.4 to 39.3m2/g).After smoldering of washed and dried rice husk,the quality of silica(93.5%,145.9 m~2/g)has great improvement.Smoldering technology has great potential in mass product of high quality silica from rice husk.(3)Char rods with different materials(cypress wood+elm bark;elm bark)and diameters(2～7 mm)were prepared,and natural smoldering experiments of char rods were performed.Propagation velocity,height and temperature field of char cone were measured and comparison between char cone and overventilated Burke-Schumann flame was performed.Results showed that smoldering propagates steadily in all prepared char rods and the propagation velocity decreases with increasing diameter and ash content of char rods.Height of char cone is proportional to the mass flowrate of carbon,similar to the relationship between flame length and volume flowrate of gas fuel in overventilated Burke-Schumann flame.(4)Mechanism on steady smoldering based on characteristics was analyzed.A model was established based on energy conservation and oxygen transport through ash and flue gas to describe the steady smoldering in char rods.Shape and temperature field of char cone were calculated using a self-written code and sensitivity analysis of parameters to height and temperature of char cone were performed.Results showed the maximum error between calculation and experiment was 26%,suggesting that the proposed model could describe steady propagation of smoldering in char rods appropriately.The main parameters affecting the height and temperature of char cone are activated energy and emissivity of fuels,respectively,and the height of char cone is more sensitivity than temperature field to parameters.