Effects Of Moisture Content On Coal Analysis Using Laser Induced-breakdown Spectroscopy

Coal occupies a major position in our country's energy production and consumption structure,so the clean and efficient coal utilization is a long-term problem need to be solved in China.Effective on-line analysis of coal is useful for the realization of the efficient and clean utilization of coal,however there is no on-line coal analysis technology of large-scale commercial application.Laser induced breakdown spectroscopy(LIBS)technique,shows a great potential in coal on-line analysis due to its inherent advantages.However the accuracy and precision of LIBS are influenced by many factors,and needs to be improved.The moisture content is a factor which cannot be neglected in LIBS coal analysis.Therefore,in the article we mainly focus on the influence of the moisture content on LIBS analysis.In the present work,the effects of moisture content variations on the properties of the laser-induced plasma and its spectral signals were investigated using the same coal samples mixed with different moisture contents.Plasma morphology and spatially resolved spectra were also obtained to further investigate the effects,and the mechanism of the effects was also discussed.Results showed that most spectral line intensities decreased as the moisture content increased,even including those of H and O.The relative standard deviations(RSDs)of the line intensities also increased.Based on the spatially resolved spectra,the profile of H intensity over the height of plasma showed a higher peak position with increasing moisture content,and the height of the peak of the H intensity profile was slightly higher than those of C and Al.The plasma images demonstrated that the plasma from drier coal sample was slightly larger than that from wetter coal sample.Based on the experimental results,the laser ablation process and mechanism underlying the effects of moisture content were determined.Part of the moisture was evaporated first under laser radiation and ablated earlier than those of other coal materials because of its water properties;therefore,the outside layer of the laser-induced plasma was mostly from the dissociation of moisture.Given the earlier evaporation of moisture,part of the energy was used to dissociate and ionize the surrounding water vapor or even reflected by the splashed particle generated by fast expanding moisture,thereby shielding the further absorption of laser energy to the solid coal material.This phenomenon resulted in less coal mass being ablated into the plasma with increasing moisture content,emitting smaller line intensities.On the basis of this mechanism,traditional spectrum normalization method was used to correct the data.Results showed that the channel normalization method can correct the matrix effect brought by the water well.Furthermore,we used the channel normalization based partial least squares(PLS)model to do quantitative analysis of samples and obtained satisfactory results.On the basis of the mechanism,we first adopt the channel normalization based partial least squares(PLS)model to do quantitative analysis of 6 kinds of samples and obtained satisfactory results.Then we adopt the spectrum standardization based PLS model to do quantitative analysis of 91 kinds of samples.We studied the influence of moisture changes on the deviation degree and then determined that to gain accurate coal analysis results the water content change acceptable range was about 3%.