With an increasing energy demand, exploration and utilization of new energy resources become more significant recently. Geothermal energy, characterized as a clean, renewable, and sustainable energy, has various engineering applications. Microbial-induced calcium carbonate precipitation (MICP) technique has a potential to improve soil thermal properties for geothermal applications. In this study, thermal conductivity of dry sands treated using MICP technique with different treatment cycles was investigated in laboratory. The results showed that thermal conductivity of MICP-treated sands was much higher than that of the untreated sand under dry condition and it increased with increasing treatment cycles. Based on the scanning electron microscopy (SEM) analyses, it is found MICP-induced CaCO3 crystals are being formed among sand particles functioned as “thermal bridge,” which provided more highly effective heat transfer path. It is concluded that the MICP technique could significantly improve the thermal conductivity of sands and the overall heat transfer efficiency. It is advantageous to use MICP-treated soils as enhanced grout materials for underground energy geostructures.
The current energy structure mainly relies on the coal; thereby, the exploration of new energy resources for energy conservation and reduction of CO2 emission is urgent in the world. Geothermal energy is a clean, renewable, and sustainable energy resource, which has various engineering applications, such as ground source heat pumps (GSHPs) and geothermal energy piles (GEPs) [
Microbial-induced calcium carbonate precipitation (MICP) technique aims to produce CaCO3 precipitation in the environment containing a large amount of Ca2+ through hydrolysis of urea by bacteria for in situ cementation of soils [
Soil thermal properties include thermal conductivity, thermal diffusivity, and specific heat capacity. Temperature distribution and efficiency of heat conduction in soils are mainly affected by its thermal conductivity [
In this study, thermal conductivity of MICP-treated sands under dry condition was first measured by a single thermal probe using the transient-state method. The effect of MICP treatment cycles on thermal conductivity of dry sands was also investigated. Additionally, the micromechanism of heat conduction in MICP-treated sands was presented based on the scanning electron microscopy (SEM) analyses.
Test sand was taken from a construction site and then sieved to remove any cobbles in laboratory. The sand is characterized as poorly graded fine sand (SP) according to the unified soil classification system (USCS). The properties of sand are shown in Table
Properties of test sand.
Name |
|
|
|
|
|
|
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Sand (SP) | 0.35 | 2.65 | 1.04 | 2.67 | 0.88 | 0.62 |
Gradation curve of test sand.
Nutrient solution contains nutrients needed for growth of microorganisms, and the amount of nutrients has a great impact on the quality of microorganisms. The procedures for preparing nutrient solution are as follows: (1) predetermined mass of each component (i.e., tryptone, soya peptone, NaCl, and urea, as shown in Table
Content of each component in nutrient solution.
Name | Purified water (ml) | Tryptone (g) | Soya peptone (g) | NaCl (g) | Urea (g) |
---|---|---|---|---|---|
Content | 100 | 1.5 | 0.5 | 0.5 | 6 |
The bacterial solution was obtained by the culture of bacteria in the nutrient solution.
The cementation solution provides sufficient Ca2+ and enough urea for microorganisms to produce CO32−, and Ca2+ reacts with CO32− to produce CaCO3 during MICP treatment. Previous test results showed that CaCO3 can be effectively generated by using cementation solution with a concentration of 1.0 mol/L. In this study, dry urea and anhydrous calcium chloride were first mixed in proportions and then dissolved in a certain amount of purified water to prepare cementation solution with the same concentration. Content of each component in cementation solution is presented in Table
Content of each component in cementation solution.
Name | Purified water (ml) | Urea (g) | Anhydrous CaCl2 (g) |
---|---|---|---|
Content | 100 | 6 | 11.1 |
Preparation of MICP-treated sands was performed in a plastic syringe with an inner diameter of 38 mm and a height of 100 mm. The schematic of experimental setup is shown in Figure
Schematic of experimental setup for preparation of MICP-treated sand (unit: mm).
The procedures of sample preparation are as follows: (1) A 4 mm thick porous stone was first placed at the bottom of the syringe, and a 5 mm thick gauze was placed above it to prevent sand from flowing into the grouting tube; (2) test sand was poured into the syringe in four equal layers and slightly compacted to a total height of 80 mm. The initial dry density of dry sand sample was 1.50 g/cm3; (3) another 5 mm thick gauze was placed on top of the sand sample. A rubber stopple was plugged into the syringe from the top to be in contact with the gauze to make sure the syringe was sealed. Then, the syringe was fixed vertically on a test frame; (4) 100 mL of purified water was continuously injected into the sand sample at a rate of 5 mL/min using the pump to remove the trapped air among sand particles; (5) 100 mL of bacterial solution was then injected into the sand sample at the same rate. The valve was closed for 2 hours to allow bacteria to be uniformly distributed and adhered on sand particles; (6) the valve was opened to remove the residual solution in the tube. 100 mL of cementation solution was then injected into the sand sample at the same rate. The valve was again closed for 6 hours to ensure the biochemical reaction was completed; (7) the valve was again opened to remove the residual solution. This entire process represents one MICP treatment cycle. The sand sample was then cured at room temperature (i.e., 30 ± 2°C) for 20 days; (8) the porous stone, gauze, and rubber stopple were removed, and the MICP-treated sand sample along with the syringe was transferred into an oven at a temperature of 108°C for 8 hours. The dry MICP-treated sand sample was ready for the thermal conductivity measurements; (9) procedures (5), (6), (7), and (8) were repeated to prepare MICP-treated sand samples with other treatment cycles (i.e., 2, 3, and 4).
The transient-state method had been widely used to measure thermal properties of soils in literature [
Schematic of the single thermal probe (unit: mm).
Figure
Schematic of experiment setup for thermal conductivity measurement (unit: mm).
Test procedures of thermal conductivity measurement are described below: (1) due to the relatively high stiffness of the MICP-treated sands, a hole with a diameter of 0.18 mm was predrilled vertically at the center of the top surface of the sand sample; (2) the thermal grease was spread on the surface of the thermal probe, and then, the probe was vertically inserted into the hole for measurements of sand thermal conductivity; (3) a current of 0.1 amp was applied for 300 s to heat the sand sample, and the temperature variation was recorded in the heating and cooling processes. It is noted that thermal conductivity of the untreated dry sand sample with the same initial dry density (i.e., 1.50 g/cm3) was also measured for comparison.
According to the line heat source theory, the temperature rise of the probe (Δ
Figure
Variation of temperature with heating time of the single thermal probe [
Thermal conductivity and dry density of MICP-treated sand under dry condition with different treatment cycles are depicted in Figure
Measured thermal conductivity, dry density of MICP-treated sand, and MICP treatment cycles.
In addition, dry density of MICP-treated sands also increased gradually with the treatment cycles as shown in Figure
Figure
Measured thermal conductivity of MICP-treated sand and dry density.
Cote and Konrad [
As reported by Cote and Konrad [
Measured soil thermal conductivity and porosity (comparison between natural soils and MICP-treated sand).
Scanning electron microscopy (SEM) technique was adopted to further study the micromechanism of heat conduction in MICP-treated sands. Figure
SEM images of pure sands and MICP-treated sand. (a) Untreated sand at magnification of 300x. (b) MICP-treated sand at magnification of 500x. (c) MICP-treated sand at magnification of 1000x. (d) MICP-treated sand at magnification of 3000x.
Schematic of “thermal bridge” in MICP-treated sand.
In this study, thermal conductivity of MICP-treated sands under dry condition was investigated using a single thermal probe based on the transient state method. The effect of MICP treatment cycles on measured thermal conductivity was also discussed. It is concluded that thermal conductivity of MICP-treated sands was much higher than that of the untreated sand sample under dry condition, and it was increased by 95%, 100%, 107.5%, and 120% as compared to the untreated sand sample for the four different treatment cycles. The dry density of the MICP-treated sands with one to four treatment cycles was increased by 4.7%, 7.2%, 8.2%, and 9.6% as compared to the untreated sand. Based on the SEM analyses, the “thermal bridge” effect caused by the produced CaCO3 is the fundamental micromechanism for the improvement of sand thermal conductivity. The use of MICP technique to enhance heat transfer efficiency in sands at low saturation degree or nearly dry condition is feasible, and the MICP-treated soils has a great potential to be used as enhanced grout materials in GSHPs and GEPs for geothermal applications.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was financially supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20161311), the Six Major Talent Peak in Jiangsu Province in China (Grant no. 2015-JZ-011), the Joint Technology Transfer Center of Yancheng Vocational Institute of Industry Technology, Yancheng Polytechnic College (Grant no. YGKF-201705), and the Innovation of Science and Technology of Institution of Higher Education in Jiangsu Province (Grant no. 2017-51).