| The energy structure of "dominated on coal" determines power plants of our country using coal as the main fuel for electricity generation. Due to the difference of coal forming ages, coal rakes and diging areas between different kinds of coals, coals are composed of various elements. The majar elements in coal are carbon (c), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S), which determined the calorific value of coal, the content of N and S were also related to pollution control The secondary elements are iron (Fe), calcium (Ca), magnesium (mg), sodium (Na), potassium (K), etc. Fe, Ca and Mg affect the coal ash fusion temperature and the contents of Na, K in coal are the main reason of slagging and corrosion of the boiler. Mercury (Hg), lead (Pb) and other trace heavy metals seriously threaten the safety of crops, animals and human beings. Because of the economic benefits, power plants changed burning coal frequently inside of design coal, which resulted that on-line coal analysis technique is urgently to develop in order to resolve the problem of real-time combustion adjustment and pollution control.Laser induced breakdown spectroscopy (LIBS) having the advantages of simple experimental setup and fast online multiple elements analysis, which makes this technique becomes a great potential on-line coal analysis technique. In this paper, the producing process and several basic parameters of laser plasma were firstly introduced. Based on the foundermental background knowledge, LIBS experimental setup was constructed by relevant instruments. The elements such as C, Al and Mg were using to research the affections of different laser wavelength, and the rearchs were about plasma time evolution characteristics, laser power and calibration. Other paraments were also inculed in this reasch, such as len focusing postion. sample preparation pressure, etc. The results showed the optimal laser waveleghthto generate laser plasma was 532 nm, while 1064 nm was the worst. After the optimizing of the experimental paraments, delay time was set as 250 ns, len focusing position was-2 mm, sample preparation pressure was 30 t and the method of reducing laser frequency and dynamic mode were using in the LIBS experiment.The content of the majar elements C, H and O are the main basis for calculating the calorific value of coal. 17 diferent kinds of coal were using in this paper to do the LIBS experiment. The characteristic spectral lines were C 247.9 nm, H 656.3 nm and O 777.5 nm. Firstly the form of data preprocessing is discussed. And then direct calibration method was introduced to resolve the calibration of C, H and O, but due to the severe matrix effect between different coals, the results didn’t show correct trend of calibration lines. To resolve this problem, an internal standard line was choosing to modify different matrix. Si 288.2 nm was used as the internal standard line to calibrate C, H and O. The correlation coefficient (R2) were 0.916,0.917,0.901, prediction are the root mean square error of prediction (RMSEP) were 4.34%,7.39% and 8.40%. The adequate calibration result of O may due to the error of known concentration, because the known element O of coal comes from elemental analysis, moisture analysis and ash analysis. Its own large error casused laeger inaccuracies. But generally speaking, the results came from the the internal standard method were good enoughThe secondary elements Na and K, on on the one hand, help to reduce the pulverized coal ignition temperature and improve the combustion characteristics of coal, on the other hand, are the main reasons of slagging and corrosion of boiler heating surface. Zhun-dong coal, which contain lagere amount of alkali elements, was used in this paper to do the LIBS experiment. Alkali metal in coal can be divided into water soluble, ammonium acetate soluble, hydrochloric acid soluble and insoluble sodium, different forms of alkali mental can be deionized by water washing, ammonium acetate washing and wash hydrochloric acid washing. Various kinds of coal samples with different alkali metals were made by above methods. Firstly, the raw coal and the hydrochloric acid washing 180 min samples were measured by different laser energy and 160 mJ/pulse was determined as the optimal laser energy. Then standard sample of Na, k elements were made and used for calibration, the error between ICP was only 0.87%. Limits of detection of Na, K were also caluated and the results were 133.1 μg/g and 34.6 μg/g. Only in 5 minutes, different forms ofNa, K were removed mostly. After the washing process,53.6% of sodium and 13.7% of potassium were removed. Therefore, water washing can effectively remove alkali metal in Zhun-dong coal. The samples were also calibrated by themselves and the results were not good as above results. RSD were calualted and the results showed RSD was high when the concentration of Na, K were less, and RSD became lower when the concentration reaches a certain threshold.In summary, calibration effect of majar elements C, H, O and secondary elements Na, K using laser induced breakdown spectroscopy are good. Due to its characteristics of fast multi elements online analysis, LIBS has enormous potential in the field of on- line coal analysis. |
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