| Environmental pollution is currently one of the toughest issues that the whole world has to tackle. From the perspective of historical development, the more developed the economy of a region is, the more serious the environmental pollution of the region will be, and the principle of "governance after pollution" has all the more rendered a precipitous decline to the quality of environment. People’s awareness of environmental protection has been reinforced in recent years, and the state has made more and more investments in environmental protection, however, driven by economic interests, people are still playing havoc with the environment every day through a large number of pollution sources. As indicated in "2010 Report of The First National General Investigation on Pollution Sources", there were over 200 billion tons of effluent discharged through all kinds of pollution sources, and 63720.369 billion cubic meters of total exhaust emissions; in terms of the total emissions of major pollutants, chemical oxygen demand surpassed 30 million tons, the amount of ammonia and nitrogen surpassed 1.07 million tons, heavy metals amounted to 900 tons, total nitrogen amounted to 4.7289 million tons, total phosphorus amounted to 0.4232 million tons, carbon dioxide amounted to 23.2 million tons, and compounds of nitrogen and oxygen amounted to 17.977 million tons. Facing these enormous figures of the pollutant emissions, only by making governance over the pollution sources and by realizing "governance before monitoring", can we gradually improve the quality of the environment.By applying Prompt Gamma-ray Neutron Activation Analysis( PGNAA for short), this paper makes an analysis on multiple elements, and carries out a simulated study on the feasibility of detecting carbon, hydrogen, oxygen, nitrogen, sulfur, cadmium, arsenic, chromium and lead. In order to verify the findings of the simulated study, a set of quick-detection experimental devices for measuring the contents of multiple elements in coal was devised. This set of devices can quickly analyze the contents of all elements in coal, which coincide with the findings of the simulated study. If it is applied to quickly analyzing the quality of coal in coal-fired enterprises, it will be constructive for the enterprises to take corresponding measures to reduce emissions of polluted gas and to process waste and so forth, therefore, it plays a significant role for guiding enterprises in improving the quality of air and in recovering the environment. The main steps that the author has taken are as follows:( 1) By utilizing MCNP, the author carried out a simulated study on the feasibility of applying PGNAA to detecting carbon, hydrogen, oxygen, nitrogen and sulfur in coal, and cadmium, arsenic, chromium, lead and other heavy metals in soil and water. As indicated by the findings of the simulated study, PGNAA has a good effect on detecting carbon, hydrogen, oxygen, nitrogen and sulfur in coal, and heavy metals in soil.( 2) The author devises a set of quick-detection experimental devices for the multiple elements in coal, which has realized a quick detection on the contents of the multiple elements in coal. The specific process is as follows:(a)Making an optimized design of neutron shield for the experimental devices. Because neutrons don’t have electric charges, they are not subject to the action of atomic internal electric field, and can be very close to the atomic nuclei. When neutrons enter a human body, they can produce rays through the reaction with the atomic nuclei in the body, which will cause huge damage to the body. As shown in the findings of MCNP simulation, when polyethylene is used as the material of a neutron shield and is over 61 cm in thickness, the neutronic radiation dose in the surrounding of the experimental devices can meet the requirements of the protection standard against radiation.(b)Effectively improving the accuracy of detection by utilizing lead screen to interfere gamma rays. Since there are large amounts of carbon, hydrogen, oxygen and other elements in the surrounding of the BGO detector and inside the neutron shield( made of polyethylene), the gamma rays that are generated from the fast and hot neutronic reaction between these elements and the neutron generator, will be mixed in the characteristic gamma rays that are generated from the reaction between the coal samples and the neutrons, which will influence the accuracy of detection on the contents of elements. Therefore, in the structure of the experimental devices, a lead bucket whose inner diameter, outer diameter, and height were 5cm, 23 cm, and 20 cm respectively was devised for the BGO detector. This lead bucket can not only screen the interfering gamma rays that have entered the BGO detector, but also can quickly reduce the energy of the 14 Me V neutrons, so that most of the 14 Me V neutrons will see a decline in energy upon entering the BGO detector, lower than the threshold value( 6.4 Me V) of the non-elastic scattering reaction between the element oxygen and the fast neutrons. In this way, the background of the energy spectrum of the oxygen-characteristic gamma rays which is detected by the BGO detector will be reduced, and the accuracy of detection will be enhanced.(c)Enhancing the accuracy of data processing. In the course of experimental data processing, it is found that the two elements of carbon and oxygen can be worked out by applying the characteristic-gamma count in full spectrum, however, the accuracy of such detection is rather low, resulting from the large number of gamma rays generated from radiation capture in full spectrum, therefore, this method doesn’t meet the requirements for industrial coal. Through a great number of simulated calculations, this paper finally chooses utilizing non-elastic spectrum which is to be acquired from the difference between a full spectrum and a capture spectrum as the standard for working out the contents of carbon and oxygen, which has enhanced the accuracy of detection on these two elements. The approach of utilizing the sum of hydrogen-characteristic gamma rays in full spectrum and capture spectrum to work out the content of hydrogen has improved the accuracy of detection on hydrogen.(d)Manufacturing 60 groups of standard coal samples. As the set of devices carries out relative detections on the contents of elements, it is necessary to revise the formulae by utilizing the standard coal samples whose contents of elements are known. In order to cut down the cost, certain amounts of raw materials that have stable chemical properties, such as graphite, polypropylene, sulfur, silicon dioxide, aluminium oxide, ferric oxide, calcium carbonate, titanium dioxide, sodium carbonate, potassium carbonate, melamine and ethylene glycol, are added into the coal whose contents of elements are known, and then 60 groups of standard coal samples whose contents of the elements of carbon, hydrogen, oxygen, nitrogen, sulfur, silicon, aluminum, iron, calcium, sodium, potassium, titanium etc. are known are compounded.Based on the aforementioned work, we devised this set of quick-detection experimental devices for detecting the contents of multiple elements in coal. As it can quickly and accurately detect the contents of the main elements in coal, and both the devised neutron shield and the gamma-ray shield can meet the requirements of the protection standard against radiation, it can be installed in large-scaled coal-fired units for practical utilization. We can make refinement on this set of devices by using the same principle, and apply it to the detection on heavy metals in effluent and in soil, etc. In this way, we can provide a fast and necessary data support for environmental protection, which will play a significant role for guiding enterprises in improving work procedures and in advancing the techniques of processing polluted gas and effluent. |
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