Design, construction and performance evaluation of a temperature-controlled underground fixed dome biodigester with an automatic stirrer
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Date
2024-05
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University of Fort Hare
Abstract
The negative impacts of temperature fluctuations on biogas production in anaerobic digesters are now a fact. Cold temperatures can completely paralyze bacterial activity in a digester. In uncontrolled digesters, biogas production varies throughout the day following environmental temperature variations and throughout the year following seasonal temperature variations. The challenge in household digesters is that they are generally constructed without heating mechanisms because of cost. Constant biogas supply throughout the year is thus not feasible in such digesters. Further, not all energy available in each substrate sample can be extracted without controlling operating conditions. Some of these process conditions include temperature, pH, total solids (TS), volatile solids (VS), stirring mode (SM), and chemical oxygen demand (COD). For a reliable biogas supply throughout the year, a robust heating system to heat the substrate from the centre of the biogas reactor tank and maintain a constant optimum operating temperature is key in biogas production.
The present empirical dissertation used a multidisciplinary approach to automatically maintain an operating temperature of 35 +/- 0.5 Degrees Celsius and automatically stir the substrate intermittently. An Arduino-aided electronic circuit was used to control three electrical circuits, which operated three mechanical devices: the water pump, the electric ball valve, and the stirrer. The slurry temperature was controlled by flowing hot water through the heat exchanger anchored at the centre of the temperature-controlled reactor. The other digester had no temperature control system but was stirred at the same rate and was used as a control.
This study aimed to design, construct, and assess the efficiency of a temperature-controlled and automatically stirred underground bio-digester. This is achieved by heating the slurry from the centre of the digester for uniform heat distribution. The novelty of this approach lies in the combination of the insulative properties of the soil and the automatic temperature control. Most temperature- controlled digesters studied in literature so far are surface plants. The idea of heating the digester slurry from the centre has been explored but for above ground digesters. Automation of the intermittent stirring system was another novel idea. The hypothesis of this research is: “ What is the relationship between quantity and quantity of biogas and constant digester temperature when the stirring is intermittent?”
Two underground fixed dome digesters with curved bases, each with a volume of 6 cubic meters, were designed, dug, and constructed at the University of Fort Hare research site. One was fitted with a heat exchanger secured at the centre of the digester and a stirring mechanism. The heat exchanger was designed using 22 mm diameter copper pipes of length 1m, separation of 0.07 m, and joined with 22 mm copper elbows and soldered. The other digester was fitted with a stirring mechanism only. The heat exchanger was connected to the solar geyser using Pex-AlPex tubing via an electric ball valve (to control the hot water flow), and a solar circulation pump (to pump the water from the heat exchanger back to the geyser for re-heating). The slurry in the temperature-controlled digester was heated using solar-heated water from the geyser. The electric ball valve was controlled by an Arduino circuit which was programmed to open the electric ball valve when the average slurry temperature was below 35 Degrees Celsius and to stop hot water flow when the average slurry temperature went above 35 Degrees Celsius. The water circulation pump was synchronized to the flow of hot water such that it pumped water only when the hot water was flowing and stopped when the ball valve closed. Thus, the electric ball valve and the water pump were operated by the same signal from the Arduino circuit via the same relay.
The stirrers in each digester were controlled by an Arduino code which was programmed to stir the slurry for 10 minutes after every four hours. A signal from the Arduino switched the stirrers per the programme via an automotive relay (12V, 60-80 A).
Three temperature sensors were deployed inside each digester under the slurry, along the diameter to measure slurry temperature at the centre of the digesters and 0.3 m from either wall. Two more temperature sensors were deployed to the temperature-controlled digester, one at the hot water inlet to the exchanger and the other at its outlet. The ambient temperature was also measured. All these sensors were connected to an Arduino-aided data logger. The two digesters were fed with cow dung slurry the same day in a batch mode for a retention time of 30 days. A constant temperature of 35+/-0.5 Degrees Celsius was maintained in the temperature-controlled
digester by automatically flowing hot water when the digester temperature fell below 35 Degrees Celsius and stopping when the temperature exceeded 35 Degrees Celsius. The other digester was operating under natural conditions but stirred at the same rate as the temperature-controlled one. The amount of gas produced in the two digesters per day was measured, tabulated, and analysed in terms of quantity and quality. The digester temperatures and ambient temperature were recorded every 10 minutes in the SD card of the data logging shield, but the methane concentration and amount of gas were recorded daily at 1800hrs.
To assess the performance of the temperature-controlled digester, the amount of biogas produced, and the methane content were compared with those of the uncontrolled digester. The percentage removals of TS, VS, and COD for both digesters were also compared to see how effective they were at removing these compounds. Additionally, the ability of the temperature-controlled digester to maintain the required slurry temperature was assessed. It was discovered that the temperature-control system managed to maintain slurry temperature in the required range for 82.76 % of the time. The temperature sensor at the centre of the heated digester was predominantly registering higher temperatures than those located 0.3 m from the walls creating some temperature gradient between the centre and the wall. A maximum temperature gradient of 7 °C per meter was observed. This points to the fact that the 30-rpm stirrer speed could not create enough vortex needed to sufficiently distribute the heat in the digester. A stirrer motor
with a rotational speed of 100 rpm is recommended. The temperature-controlled digester produced 33% more cumulative biogas than the uncontrolled digester during the 30-day retention time, with normalized values of 26.77 cubic meters and 18.05 cubic meters, respectively. The
temperature-controlled digester produced biogas at a daily rate of 1.72 m3
, which is 90% of the theoretical value of 1.92 cubic meters. The uncontrolled digester produced biogas at a daily rate of 1.21 cubic meters, which is 63% of the theoretical value. In the temperature-controlled scenario, the rate of
biogas production increased by 42% compared to the uncontrolled scenario. The percentage of methane concentration in the produced biogas increased by 14%, while the concentration of carbon dioxide decreased by 10% from the uncontrolled scenario to the temperature-controlled one. The results showed that the temperature-controlled digester produced a much larger amount of
biogas with higher methane content when compared to the uncontrolled digester. This indicates that maintaining a constant temperature in a bio-digester improves both the quantity and quality of the biogas produced. The study also found a significant improvement in the percentage removals of total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) when moving from an uncontrolled to a temperature-controlled situation. The percentage removals of TS, VS, and COD increased by 19.26%, 16.44%, and 16.88%, respectively. The results of this research demonstrate that much quality biogas can be produced if the household digesters are fitted with a temperature control facility. Much biogas can be produced from the same
quantity of substrate if the temperature is maintained constant at the mesophilic optimum level.
The efficiency of biogas production has been enhanced by 42% with the use of temperature-controlled bio-digesters. The cost of heating in this process has been significantly reduced by utilizing solar energy. In areas where the intensity of sunlight is not strong enough, a hybrid system can be employed by combining recovered heat from electricity-generating plants with solar energy.
A mathematical model was developed for biogas yield using pH and slurry temperature as input variables. The developed model was of the form:
𝑉 (𝑝𝐻, 𝑇𝑠) = 𝑎 + 𝑏 (𝑝𝐻) + 𝑐 (𝑇𝑠)
where, a is the y-intercept, b, and c are scaling coefficients and V is the biogas yield.
𝑉 = −10.480 + 1,139 𝑝𝐻 + 0,087 𝑇𝑠𝑙𝑢𝑟𝑟�
It was observed that both the slurry temperature and pH positively impacted biogas yield, with slurry temperature having a higher contribution by weight (0.1594) and pH the least contribution (0.0957) to the daily volume of biogas produced. The derived model gave a very good determination coefficient and adjusted determination coefficient values of 0.9587 and 0.9551, respectively showing its predictive power.
Description
PhD thesis
Keywords
heat exchangers, Arduino (Programmable controller), Solar heating, Biogas, Sewage -- Purification -- Anaerobic treatment
Citation
Makamure, F. (2024) Design, construction, and performance evaluation of a temperature-controlled underground fixed dome biodigester with an automatic stirrer. PhD thesis. Alice, South Africa: University of Fort Hare.