Study On Heating Method Based On Burning Cave

In rural areas of China, agricultural production brings a lot of crop waste each year, such as straws, corn cob, dead branches or leaves and so on. Besides a small amount for cooking, most of the crop waste can only be treated by the way of burning, this method not only causes environmental pollution and traffic accidents, but also becomes a waste of resources. Burning cave for heating which is very popular in rural areas of Northern China in recent years makes full use of these agricultural wastes, significantly improves indoor thermal environment of rural house in the cold areas. However, most of the existing houses with burning cave for heating are built depended on farmers'experience, so there are some problems, such as low thermal energy utilization efficiency, unreasonable heat transmission way, flue gas leakage and so on. Therefore, fuether study of smoldering and design for burning cave has important practical reference value for improving rural energy status and easing the pressure on the environment.First, this study carried out field measurement on thermal and humidity environment and IAQ of the houses with burning cave for heating.The time of field measurement is 2009 and 2010 winter, and the measurement parameters include temperature and relative humidity of the room with burning cave for heating, surface temperature of Kang and floor heating with burning cave, building thermal characteristics, outdoor temperature and relative humidity, concentration of CO/CO2 and PM10 during the daily life and so on. According to the measurement results, this study further analyses indoor thermal environment of house with burning cave for heating, operation control strategy and existing problems, and discusses the key influence factor of burning cave design and heat transfer mode. Measurement results show that while the outdoor average temperature is below-10℃, the indoor temperature of house with burning cave for heating can reach at about 15℃. However, most of rural house with burning cave for heating are built depended on farmers'experience, lacking professional guidance, so there are some problems, such as low thermal energy utilization efficiency, unreasonable heat transmission way, flue gas leakage and so on.Secondly, combustion process and heat transfer mechanism were studied.This part discussed the uneven level of top and bottom surface temperature of top floor of burning cave, experimental results show that the effect of the location of ignition point to the uneven level of top and bottom surface temperature of top floor of rectangular burning cave is not large, and the uneven level of circular cross-section burning cave is the best. The time interval of the same horizontal level reaching the maximam temperature is large, higher moisture content leads to the lower burning rate, the moisture content of fuel increased by 60%, the burning rate of smoldering reduced by 19%. Two sides of the lateral wall of burning cave is not filled with the soil, leading to large heat dissipating capacity of the lateral, the efficiency of smoldering were 26.2% and 15.4%. Ash deposition can reduce the combustion temperature, in general, the temperature drop caused by ash deposition will not exceed 100℃.Meanwhile, by the study of operation control and the fuel form of a small smoldering furnace, we can find that the better sealing of the experimental furnace, the more difficult to ignite, so it is very important to strengthen the intensity of ignition heat source. The condition that the corn cob with compaction density of 208.83kg/m3 and bulk density of 171.65kg/m3 could occur stable smoldering is that maximum diameter of fuel rods is 34mm, the corn cob of 40mm diameter will appear open fire on the condition of sufficient oxygen.The maximum temperature in the top space of inernal smoldering furnace is up to about 200℃, and the temperature maintains at 100℃for long time. Experiment results show that the lower intake could not only help to extend combustion time, but also help to maintain the temperature for long time.The smoke temperature of smoldering furnace becomes lower from below,and temperature gradient is large,the temperature drop is about 90℃/m.The average smoldering velocity of smoldering furnace is 0.155mm/min, which is 4-5 times of velocity of burning cave.The density of biomass fuel compressed into particle fuel becomes large, the density of the combustion ashes is also very large.Finally, this study put forward the basic idea of the design the of burning cave.Volume method is used to determine the volume of burning cave,and then the calculated area of burning cave must be checked according to heat flow rate per unit area, until meeting the requirements; this study put forward the basic idea of using the temperature control panel to control the operation of burning cave, and takes a burning cave with area of 7m2 for example, when the outdoor temperature is-14℃, the angle between the flue axis and the temperature control panel should be adjusted to 68°.Research shows that, burning cave for heating can makes full use of large abandoned biomass in rural areas.Although burning cave forl heating still has many problems to be improved, burning cave is a low cost, resources recycling and local conditions way of improving the living environment quality for rural areas.