Distributed Optical Fiber Temperature Sensor Applied in Underground Coal Gasification System

In a distributed fiber optic temperature sensing system, the intensity of Raman Stokes backscattering light serving as reference light increases with the increase of temperature, leading to measurement errors in the system. A novel method of dynamically calibrating Raman Stokes backscattering light intensity is proposed to improve temperature accuracy for distributed fiber optic temperature sensors. According to the real-time Stokes intensity distribution in the reference fiber, Stokes intensity curve of the whole fiber at a reference temperature is simulated, and the temperature response of Stokes light is corrected. The ratio of Raman anti-Stokes light intensity to the calculated Stokes light intensity is used to demodulate temperature along the fiber. The experimental results indicate that the temperature accuracy of the distributed optical fiber temperature sensor system after making the Stokes optical dynamic calibration is increased up to 4.3 ℃ compared with that from the conventional method. And the accuracy of temperature measurement is improved by 8.9 ℃ when combined with Rayleigh noise suppression method. This study provides a new solution for a distributed fiber optic temperature sensor system to monitor high temperature environment temperature.

A new sampling correct technology of distributed optical fiber temperature sensor (DOFTS) has been researched to eliminate the instability of DOFTS system that caused by the change of environment temperature, gain of amplifier and voltage of power supply. Put the optical sampling loop into a constant temperature thermostat. A computer automatically corrects signal voltage of every point. Thus the stability of the system is improved.

Underground coal gasification – A numerical approach to study the formation of syngas and its reactive transport in the surrounding strata

One of the most important performance indicators of a fiber optic distributed temperature sensing (DTS) system is its operational stability, which depends on the performance of each device making up the system. Such individual device performance itself is affected by temperature and other environmental conditions. As a result, ambient fluctuations invariably lead to measurement uncertainties and drifts, seriously degrading the system performance. To ameliorate such adverse effects of the environment, we propose a dynamic sampling-correction scheme, which we have implemented on our fiber optic Raman DTS system, resulting in an average temperature error within 1°C when the main chassis undergoes a temperature variation over the range -25°C-45°C. The drastically improved system stability foreshadows better performance and reliability in engineering applications.

In the underground coal gasification (UCG) process, cavity growth with crack extension inside the coal seam is an important phenomenon that directly influences gasification efficiency. An efficient and environmentally friendly UCG system also relies upon the precise control and evaluation of the gasification zone. This paper presents details of laboratory studies undertaken to evaluate structural changes that occur inside the coal under thermal stress and to evaluate underground coal-oxygen gasification simulated in an ex-situ reactor. The effects of feed temperature, the direction of the stratified plane, and the inherent microcracks on the coal fracture and crack extension were investigated using some heating experiments performed using plate-shaped and cylindrical coal specimens. To monitor the failure process and to measure the microcrack distribution inside the coal specimen before and after heating, acoustic emission (AE) analysis and X-ray CT were applied. We also introduce a laboratory-scale UCG model experiment conducted with set design and operating parameters. The temperature profiles, AE activities, product gas concentration as well as the gasifier weight lossess were measured successively during gasification. The product gas mainly comprised combustible components such as CO, CH4, and H2 (27.5, 5.5, and 17.2 vol% respectively), which produced a high average calorific value (9.1 MJ/m3).

This paper introduces the use of a Rayleigh backscatter-based distributed fiber optic sensor to map the temperature field in air flow for a thermal fatigue application. The experiment involves a pair of air jets at 22 and 70 °C discharging from 136 mm hexagonal channels into a 1 × 1 × 1.7 m tank at atmospheric pressure. A 40 m-long, ϕ155 µm fiber optic sensor was wound back and forth across the tank midplane to form 16 horizontal measurement sections with a vertical spacing of 51 mm. This configuration generated a 2D temperature map with 2800 data points over a 0.76 × 1.7 m plane. Fiber optic sensor readings were combined with PIV and infrared measurements to relate flow field characteristics to the thermal signature of the tank lid. The paper includes sensor stability data and notes issues encountered using the distributed temperature sensor in a flow field. Sensors are sensitive to strain and humidity, and so accuracy relies upon strict control of both.

Raman scattering in optical fibres provides a means of measuring continuous temperature distributions over extended distances. However, in many applications it has been found that the background transmission properties of the optical fibre either change after deployment of the fibre or vary over time due to ageing effects. Changes in the fibre transmission can be distinguished from thermal effects by measuring the Rayleigh backscatter, which is relatively insensitive to temperature. The combined use of Raman and Rayleigh data is discussed here in the context of a particularly simple and cost-effective sensor design, where a single, fixed optical filter and a single light source are used for both measurement modes. The Rayleigh backscatter measurement allows the accurate correction of background transmission changes in a probe arrangement that includes several splices between different fibres with different attenuations. In this way, the temperature in a 290 °C test region is accurately recovered from the anti-Stokes Raman signal. However, preliminary measurements of the fibre attenuation as a function of temperature and exposure to an accelerated ageing environment show that these two effects will be difficult to separate in practice. Therefore it remains challenging to perform accurate, unambiguous background corrections in situations where high-temperature ageing occurs.

Ground source heat pump (GSHP) systems have been widely used in the field of shallow geothermal heating and cooling because of their high thermal efficiency and environmental friendliness. A borehole heat exchanger (BHE) is the key part of a ground source heat pump system, and its performance and investment cost have a direct and significant impact on the performance and cost of the whole system. The ground temperature gradient, air temperature, seepage flow rate, and injection flow rate affect the heat exchange performance of BHEs, but most of the research on BHEs lacks field test verification. Therefore, this study relied on the results of a field thermal response test (TRT) based on a distributed optical fiber temperature sensor (DOFTS) and site hydrological, geological, and geothermal data to establish a corrected numerical model of buried pipe heat transfer and carry out the heat transfer performance analysis of a buried pipe in the heating season. The results showed that the ground temperature gradient of the test site was about 3.0 °C/100 m, and the temperature of the constant-temperature layer was about 9.17 °C. Increasing the air temperature could improve the heat transfer performance. The temperature of the surrounding rock and soil mass of the single pipe spread uniformly, and the closer it was to the buried pipe, the lower the temperature. When there is groundwater seepage, the seepage carries the cold energy generated by a buried pipe’s heat transfer through heat convection to form a plume zone, which can effectively alleviate the phenomenon of cold accumulation. With an increase in seepage velocity, the heat transfer of the buried pipe increases nonlinearly. The heat transfer performance can be improved by appropriately reducing the temperature and velocity of the injected fluid. Selecting a backfill material with higher thermal conductivity than the ground body can improve the heat transfer performance. These research results can provide support for the optimization of the heat transfer performance of a buried tube heat exchanger.

Geothermal energy exhibits considerable development potential in space heating. Shallow geothermal energy stored in the soil in the form of low-grade energy is mainly extracted via the ground source heat pump (GSHP) system. GSHP systems use the subsoil as a heat source, typically involving a vertical borehole heat exchanger (BHE) to extract heat from the formation. Accurate measurement of the thermal properties of the formation is very important for the design of BHEs. At present, the most common and effective method to measure the thermal conductivity of the formation in the field is the thermal response test (TRT). However, the test conditions (heat load, test time) during the thermal response test can impact the test results. Therefore, in this study, a borehole with a depth of 130 m was evaluated in the field. The TRT module and the distributed thermal response test (DTRT) module based on distributed optical fiber temperature sensor (DOFTS) technology were used to monitor the test with different working conditions in real-time. In the field tests, geothermal conditions and the evolution of the formation temperature with time and depth were determined. Based on the test results under different heat loads and test times, the influence of the test conditions on the thermal conductivity results was analyzed and described. A constant temperature zone was located at a depth from 25 m to 50 m, and an increasing temperature zone was located at a depth from 50 m to 130 m, with a geothermal gradient of 3 °C/100 m. The results showed that the heat load slightly influenced the thermal conductivity test results. At the initial stage of the test, the temperature significantly increased from 0 to 12 h. After reaching the quasi-stable state, the test time slightly influenced the thermal conductivity test results. The characteristics of the formation thermal recovery stage after the test stage were studied. The heat load decreased, which could shorten the time for the formation to recover the initial temperature. The results could provide a basis for the optimization of thermal response test conditions.

The use of fiber optic distributed temperature sensing (DTS) to detect and locate leaks is still in its infancy in water infrastructure, despite its promising capabilities. Only few experiments tested this technology, and none of these studies focused on small but persistent leaks, like background leakages, which are ubiquitous and generally go undetected with the technology currently available, thus posing a serious threat to the available water resource. To test the feasibility of detecting and locating background leakages with fiber optic DTS, this study provides a detailed analysis on flow and temperature alterations around leaking water pipelines in presence of small leaks (5, 25, and 125 L/d) with small to moderate temperature differences with the surrounding soil, under 3 different pipe defect configurations, either in absence or in presence of pipe thermal insulation. Transient 3D heat transfer-unsaturated flow numerical simulations showed that there is potential to use temperature alterations to detect and locate incredibly small leaks with fiber optic DTS, like background leakages, despite the influence of pipe temperature on the surrounding soil. The analysis showed that extent, distribution, and magnitude of these alterations are convection dominated at a given temperature difference between leaked water and undisturbed soil, and that it may not be strictly necessary to place the optical fiber directly below the pipe. Indeed, optical fibers located within the utility trench at the sides of the pipe and below its bottom showed comparable or even better performance, thus giving new opportunities to retrofit existing pipelines as well.

A novel fiber optic distributed temperature and strain sensor for building applications

Modelling and energy analysis of an integrated coal gasification and pyrolysis system for synthetic natural gas

Global and micro/macro structural models simulating the Longwall generator concept for underground coal gasification (UCG) are formulated for several geometric and loading conditions. Dynamic thermal and stress response solutions are computed for axisymmetric boundary value problems represented by a stationary circular cavity model and an expanding radially propagating circular cavity. Static thermoelastic response solutions are presented for circular and elliptical cylindrical presented for circular and elliptical cylindrical cavities in a homogeneous, isotropic medium and layered media. Considerations pertaining to roof collapse and surface subsidence are discussed. The micro/macro modeling investigations include thermorheological representation of Pittsburgh coal at elevated temperature and studies on thermal crack propagation of coal fissures. Finally, the relevance of stress and temperature profiles, fracture permeabilities, and fissure response is discussed in relation to the propagation and stability of the UCG process. Introduction Fossil fuels are currently the main energy resource for the U.S. with coal reserve estimates ranging from 3 to 4 trillion tons. Only one-sixth of this coal can be extracted by state-of-the-art mining techniques. It is anticipated that about one-third of the U.S. total coal energy reserves, particularly thin coal beds and thick-seam deep coal, can be recovered by underground coal gasification (UCG). This process entails ignition of a coal seam and circulation of gas through a controlled path. The resulting chemical reactions produce a gas with calorific value, depending on several process parameters and variables. Their optimization parameters and variables. Their optimization requires sophisticated analytical simulations of the structural, heat transfer, fluid flow, and reaction kinetics aspects along with controlled laboratory experiments and field tests. Descriptions and appraisals of various underground coal pregasification schemes and gasification processes mad a discussion of significant problems, processes mad a discussion of significant problems, such as roof collapse, gas leakage, water control, and surface subsidence, etc., have been given in a detailed report by Arthur D. Little, Inc.. Reports on UCG experiments in the Soviet Union, Great Britian, and other countries have been summarized in excellent monographs. The outlook for UCG in the U.S. was discussed recently at the First Annual UCG Symposium conducted by the Laramie Energy Research Center in Wyo. It appears that several candidate concepts and solutions exist, depending (on the type of coal, coal seam thickness, overburden, and environmental requirements. The concepts currently being investigated by ERDA's research centers and laboratories includethe vertical well to well linking experiments at Hanna, Wyo., for medium seams (Laramie Energy Research Center),the packed-bed concept for thick coal seams (Lawrence Livermore Laboratory), andthe Longwall generator concept for thin seams (Morgantown Energy Research Center). This paper presents results obtained from analytical structural and thermal simulations associated moth the Longwall generator UCG concept. Temperature and stress response solutions are presented for different boundary value problem representations. The results of this study provide fundamental insight to the interpretation of UCG global and micro/macro mechanisms. UCG LONGWALL GENERATOR CONCEPT AND MECHANISMS Morgantown Energy Research Center presently is developing the Longwall generator concept to gasify thin-seam eastern coals (Fig. 1). In this concept, directional holes (6-in. diameter) are drilled from the ground surface with horizontal holes, 500 ft in length, entering through the 6-ft-thick coal seam (1,000-ft overburden) and returning vertically back to the coal surface. The gasification reaction zone propagates horizontally in the coal propagates horizontally in the coal maximum-permeability direction between the parallel horizontal holes drilled approximately in the coal butt-cleat direction. JPT P. 413

In this paper, the temperature performance of Raman scattering spectrum in a data fiber was studied in theory and in experiment. The optimum system parameters were decided and a distributed fiber-optic temperature sensor (DFTS) system was developed. Theoretical and experimental analyses showed that the temperature response function of the DFTS system is much different from the actual one. A novel signal process method was put forward to improve the system precision. The results of the measurements show an apparent improvement in the performances of DFTS system. The temperature accuracy and the distance resolution reached to +/- 1 and +/- 1 m, respectively. The system is stable and adequate for commercial usage, such as the power industry, the underground tunnel, the subway, the pipe laying, and also for the mission applications, such as the warship and the airplane.

A special optical fiber is described which can measure local temperature from 0 to 150/spl deg/C in electrical power equipment with an accuracy of /spl plusmn/5/spl deg/C. Changes in Rayleigh backscattered light from the fiber cladding indicates changes in local temperature at points along the fiber using an optical time domain reflectometer approach. This distributed fiber optic temperature sensor (DFOTS) had a spatial resolution of 10 cm using fiber sensors up to 100 m long. Field test data were successfully obtained for several electrical power system applications. Switchgear finger cluster, stator winding, and rotor shaft temperatures were all measured in operating power plants. Results show that the Rayleigh DFOTS approach can provide valuable information about thermal performance of operating electrical equipment. >