Research On The Detection System Of Ammonia Concentration In Livestock Based On TDLAS

Intensive and large-scale farms have high feeding densities,which leads to the great influence of ammonia concentration on the growth of livestock and poultry.Ammonia is a stressor that can affect the healthy growth of livestock and poultry and reduce animal welfare.The distribution of ammonia’s concentration is usually affected by many factors.such as,the building structure of the livestock house,the ventilation mode,the type of livestock and poultry,the breeding process,the growth stage,the season,etc.Therefore,the single-point and non-continuous monitoring cannot accurately reflect the real gas environment.The long-term and continuous concentration monitoring of livestock and poultry houses is helpful to understand the law of ammonia emission and provide technical support for scientifically regulating the gas environment.At present,the electrochemical sensors are often used to monitor the concentration of harmful gases in livestock and poultry houses.But the electrochemical sensors have fatal problem in the long-term continuous monitoring process,such as low resistance to the complex environment,serious interference to other gases,poor accuracy and short life span.In order to solve the above problems,this paper designs an online monitoring system for ammonia concentration based on tunable absorption spectroscopy technology to meet the long-term,continuous and stable monitoring requirements for ammonia concentration in livestock and poultry houses.The main research contents and work of this article are as follows:Chapter 1:It analyzed advantages and disadvantages of the common methods for detecting ammonia’s concentration in livestock and poultry houses,and explained the long-term and continuous monitoring of ammonia’s concentration to control the environmental gas regulation of livestock and poultry and the necessity of understanding the ammonia emission rules of livestock and poultry houses.By analyzing the problems in the continuous monitoring of ammonia concentration,the advantages and needs of the tunable absorption spectroscopy technique in the long-term continuous monitoring of ammonia’s concentration are clarified.Chapter 2:It describes the principles of spectral absorption technology and wavelength modulation technology,and simulats the absorption of ammonia,water,carbon dioxide and other livestock houses by HITRAN data.By this simulation,we determine the appropriate absorption spectrμm of ammonia in the near-infrared region.Chapter 3:It designed an online monitoring system for NH₃ concentration in livestock and poultry houses based on tunable absorption spectroscopy technology,and designed related host computer software.The whole monitoring system is divided into two parts based on PLC multi-channel gas collection system and TDLAS-based ammonia concentration detection system.The collection system is composed of PLC,grouping solenoid valve,sensor,vacuμm pμmp and pipeline,which realizes the function of collecting and transmitting the gas to be measured to the gas chamber for concentration detection.The detection system is composed of laser emission unit,optical path and gas path unit,photoelectric signal receiving unit and harmonic signal processing unit,which realizes the function of detecting the concentration of the gas to be measured.Chapter 4:In order to reduce system noise and improve the system signal-to-noise ratio,multi-cycle averaging and wavelet denoising are used for the harmonic signals to reduce noise.Considering the factors of system stability time,the average nμmber of multi-cycles be set to 5 times.And It optimized the wavelet noise reduction parameters.such as the wavelet base with DB4,the nμmber of decomposition layers with 5 and the threshold type with Universal Threshold Soft.Finally,we verify the adaptability of noise parameters to different concentrations.Chapter 5:The optimal harmonic parameters were determined by optimizing the modulation parameters of the TDLAS detection system:detection system sinusoidal modulation signal frequency1)_sin=9 k Hz,sinusoidal modulation signal amplitude V_sin=30 m V,sawtooth scanning signal frequency1)_saw=1 Hz,Sawtooth scanning range170～215 m V,harmonic analysis input signal and reference signal phase difference?=50°.In order to obtain the best system parameters,the system’s optimal heating temperature optimization experiment,response time parameter optimization,and Allan’s variance experiment were performed on the system.Through optimization experiments,it can be seen that the heating temperature of the gas chamber is set to 403K,which can significantly reduce the impact of ammonia adsorption on the gas chamber.the detection system has a rise lag time of about 3.8 s at a flow rate of 500 m L/min.The rise response time is 28.2 s,the fall lag time is 3 s,the fall response time is 17 s,the 90%response time is 32 s,and the 99%response time is 60 s.under the test conditions with a sampling time of 2 s,the TDLAS system integrates time is 10 s,the system reaches the minimμm detection limit,which is 0.054 ppm.Chapter 6:It completed the concentration calibration of the detection system and conducted the system performance test.According to the calibration experiment,we know the linear symmetry of the second harmonic curve under different NH₃concentrations is good.The fitting formula obtained by two-square fitting is Y=0.089?X-0.065,and the linear fitting coefficient R=0.996.The maximμm deviation error of single-point fitting is 0.37 V.Through the performance test of the multi-channel collection system,it is known that the collection system has good airtightness,and the gas can flow smoothly in the pipeline.Under the test conditions of the collection pipeline length of 1 m,10 m,and 20 m,the detection error of carbon dioxide is 0.42%,0.78%and 1.03%in sequence,the detection error of ammonia is 1.44%,4.44%and7.33%in sequence.The gas detection error increases with the length of the collection pipelinein the analysis loop.In the case of accompanying heat treatment,the ammonia gas detection errors are 1.19%,3.67%,and 7.11%,indicating that the accompanying heat treatment has a certain effect on reducing the absorption of ammonia gas in the pipeline,but the effect is not obvious.Through the performance test of the TDLAS detection system,it can be known that under four different concentration test conditions,the INNOVA acousto-optic spectrμm is similar to the concentration curve of the TDLAS system,and the curve fluctuation is small,but the electrochemical sensor fluctuation is large;through further analysis,TDLAS The linearity error with INNOVA is around 1%（standard is≤5%）,the repeatability error is less than 2%（standard is≤5%）,the electrochemical repeatability error is 5.98%.Therefore,the detection accuracy,and stability of the TDLAS system can meet the needs of ammonia concentration detection in livestock houses.