Two-Phase Anaerobic Digester Combined with Solar Thermal and

Feb 27, 2017 - ABSTRACT: Using solar energy as the heat source for biogas improvement of the anaerobic digester is an effective method. However, inter...
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Two-Phase Anaerobic Digester Combined with Solar Thermal and Phase Change Thermal Storage System in Winter Chuqiao Wang, Yong Lu, Feng Hong, Xianning Li,* Xueliang Zeng, and Haowei Lu School of Energy and Environment, Southeast University, Nanjing 210096, China S Supporting Information *

ABSTRACT: Using solar energy as the heat source for biogas improvement of the anaerobic digester is an effective method. However, intermittent solar radiation and low ambient temperature in the winter make it difficult to maintain a steady fermentation temperature. In this study, a pilot-scale two-phase anaerobic digester combined with solar thermal and phase change thermal storage (PCTS) system has been constructed. A set of comparative field studies were carried out during the winter in Maanshan City, Anhui Province, China; these periods of study included no heating mode (P1), heated with solar thermal and PCTS system (P2), and conventional solar heating mode (P3). The comparison shows that methane yields and the energy conversion rate of the substrates of P2 were 12.33 m3 and 48.6%, respectively, both of which increased by 5.65 and 1.01 times, compared with that of P1 and P3, respectively. The statistical coefficients of variation for the fermentation temperature were 2.5% in P2 and 6.6% in P3, which means that the PCTS system supplied heat with less temperature fluctuation when solar radiation was weak.

1. INTRODUCTION There are many predominantly agricultural provinces in China. The process of agriculture operations creates abundant biomass wastes, including roughly 700 million tons of straw and 3000 million tons of animal manure annually. The pollution of air, water, and soil is therefore caused by straw burning and untreated livestock and poultry dung due to improper management.1 Anaerobic digestion (AD) is one of an effective and environmentally friendly methods to convert biomass into energy and fuels, and there are many application practices in rural areas of China. Fermentation temperature is one of the most important parameters for AD, which can influence biogas production and substrate degradation rate. The anaerobic digesters under ambient temperature were widespread used in rural regions of China, with simple configuration, low initial investment, and requiring no additional heat sources.2 However, this type of anaerobic digester is restricted by ambient temperatures, resulting in low biogas production efficiency or even complete failure in cold winter. Recently, mesophilic fermentation has gained more and more attention, because of high biogas production rates and degradation ratios of organic substances. However, mesophilic fermentation requires more extra energy consumption to meet the heat demands of anaerobic digesters in a cold climate. There have been many studies conducted using various methods to improve or maintain the proper temperature for the anaerobic digester. In some practices, biogas projects, biogas boilers, ground-source heat pumps, and electric heaters were used to improve the fermentation temperature.3,4 Zeshan et al. maintained the fermentation temperature of a pilot-scale anaerobic dry digester by using an electric heater.5 Although these methods increased the inside temperature of the digester, one of the main problems is the requirement of extra conventional energy, which leads to excessive costs. Solar energy is a renewable, inexhaustible, and clean energy and © XXXX American Chemical Society

seems to be an attractive approach for heating anaerobic digester. Recently, many studies have shown that using solar energy to improve the fermentation temperature is an attractive approach for improving biogas production and biodegradation efficiency in the winter.6,7 Dong et al. built a solar thermal collector system to heat the river water and ultimately improve the inside temperature of digesters.8 Weatherford et al. simulated the thermal performance and validated it with a series of field experiments on a biodigester, and they recommended some parameters to the affordable biogas digesters in cold climates.9 These comparative studies showed that solar energy could be utilized to improve the fermentation temperature of biogas digesters in cold climates. However, the solar radiation in the winter is less than that in other seasons in central China, and only low-grade heat can be collected by solar thermal collectors. Meanwhile, maintaining the fermentation temperature within the mesophilic range (30−35 °C) is difficult, which ultimately leads to low biogas production rate. The rainy and cloudy weather in the winter can make solar thermal collectors unreliable, causing a deficit, with regard to the heating supply. Moreover, heat demand will further increase, as a result of low ambient temperature. The change and fluctuation of temperature could affect the growth and metabolism of methanogenic bacteria. Garba found that methanogenic bacteria was sensitive to drastic changes in the fermentation temperature.10 When the temperature fluctuated within 5 °C dramatically, the biogas yield obviously declined. Moreover, biogas production could stop, because of excessive fluctuations in temperature.11 It is very necessary to install a thermal storage system in a solar system, which was used to store the excess energy collected by solar collectors in autumn Received: December 19, 2016 Revised: February 25, 2017 Published: February 27, 2017 A

DOI: 10.1021/acs.energyfuels.6b03376 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels or on successive sunny days.12 The stored thermal energy can be released to resist the intermittent solar radiation in cloudy days, creating a steady heating condition. These studies were designed to collect and store solar energy as the supplement of anaerobic fermentation in the winter or on cloudy days, as well as some evaluations and simulations. However, there were not many specific practical applications. In some practical studies, a certain fermentation temperature increase was obtained by using solar energy under cold conditions.8,13 To the best of our knowledge, there have been few practical applications using solar energy to raise the fermentation temperature and achieve steady fermentation in the winter. Our previous study revealed that the water temperature of the solar thermal collector was 56 °C) and electricity generated by PV. The temperature of hot water stored in a PCTS tank and the temperature of paraffin wax phase change insulation on Jan. 14 (in fermentation period P2) can be seen in Figure S2 in the Supporting Information. The overall variation of water temperature in the PCTS tank and temperature of paraffin wax phase change insulation were both in a relatively stable state. The temperatures of paraffin wax were maintained around the melting point (55 °C) in 24 h, which can be considered to be under phase transition most of the time. The temperature of the water was slightly higher than that of the paraffin wax throughout the P2 fermentation period. This might be due to the 50 mm rigid polyurethane foam sandwiched between the water tank and the paraffin wax can form a temperature gradient from inside to outside. In addition, the water temperature inside the storage tank fluctuated within a small range (56−57.5 °C), because of the fact that the variation range of the paraffin wax temperature was extremely small. This could be the reason why the hot water stored in the PCTS tank has a relatively constant temperature. There is a certain temperature gradient between the wall of the PCTS tank and the ambient air (Figure S3 in the Supporting Information). This indicated that the convective heat transfer between the PCTS tank and the surroundings is constant. Figure S3 indicates that the ambient temperature has little effect on the temperature of the stored hot water in the PCTS tank. However, the requirement for heat of the paraffin wax and the working time of the heating pump will be improved with the decrease of ambient temperature. As can be seen in Figure 5, the daily electricity consumption during P2 generally is inversely proportional to the ambient temperature, but complete correspondence is not observed. The specific

Figure 5. Influence of the electricity consumption on solar radiation and ambient temperature during P2.

electricity consumption has more significant relationships with the solar radiation than ambient temperature, because of the fact that the temperature of the solar collector tank must be increased to 56 °C when solar radiation was insufficient, and part of latent heat of paraffin wax can be supplemented by hot water (>56 °C) in sunny days. The electricity consumption data during P2 is used to calculate the peak power of the solar PV power generation device. The daily average electricity consumption was 3.48 kWh in fermentation period P2. The daily average solar radiation of the experimental site in the winter was 9.8 MJ/m2, and the corresponding peak number of hours of sunshine was 2.72 h. Based on eq 1, the peak power of the solar PV power generation device was calculated as 1422 W. The finished products of the PV panel in the market are generally 250 W per panel. Therefore, a 1.5-kW solar PV power generation device should be installed for follow-up study. 3.6. Economic Analysis. The economy of the solar−biogas system in this study was analyzed using the net present value (NPV) method (see section 2.6). The initial investment is 22 200 yuan (Table S1 in the Supporting Information), the deposit interest rate (i) is 3%, and the estimated useful life (N) is 20 years. It is worth mentioning that the straw, duck manure, and food waste used in this study were freely available at the experimental site and most rural areas in China.2 In addition, the solar−biogas plant is located close to where agriculture residue produced, and the local farmers are willing to collect these wastes spontaneously. Based on the above, logistics expenses and the costs of raw substrates are assumed to be free in this study. The substances produced by this system can reinforce and influence each other. The combination of solar and biogas techniques converts the traditional single agricultural production model to a recycling production model. The total economic benefit of this solar−biogas system includes direct benefits and indirect benefits. The substance circulation graph in Figure S4 in the Supporting Information shows that biogas, hot water, and electricity were the direct benefits. The daily average biogas production of TPAD was 1.03 m3, and the average methane content of biogas was ∼67%. The price for biogas is 1.9 yuan per cubic meter (calculate based on the local price of natural gas). The annual benefit of the biogas is M1 = 1.03 × 1.9 × 365 = 714.3 yuan

A rate of 308 L of hot water per day was used to maintain the fermentation temperature when the ambient temperature was H

DOI: 10.1021/acs.energyfuels.6b03376 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels 3.39 ± 0.18 °C. The average temperature of water inside the PCTS tank was maintained at ∼56.7 °C. The hot water savings benefits, compared with natural gas heating of the water from 15 °C to 56.7 °C is M 2 = 4.2 × (430 − 308) ×

were 5.65 times greater than that of TPAD without heating. This implies that the cost of biomass waste treatment in rural area was reduced, and the utilization of solar energy was improved. Net present value (NPV) analysis shows that investment in the recommended solar−biogas system is economically feasible. The introduced system can effectively solve some defects of conventional heating methods, and has good prospects for future application in rural areas of China.

(56.7 − 15) × 2.88 × 365 37590

= 597.5 yuan

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The daily average solar radiation for the year at the experimental site was 12.4 MJ/m2 (data based on Meteorological Administration, China), and based on eq 1, the corresponding electricity generated by PV is 4.65 kWh per day. The daily average electricity consumption of the recommended solar−biogas system was 3.48 kWh. The local price of electricity is 0.58 yuan per kWh, and the annual saving benefits by PV is

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03376. Photograph of pilot-scale two-phase biogas plant combined with solar thermal and phase change thermal storage system (Figure S1); methane yield of TPAD heated by solar collector and solar thermal−PCTS system in different seasons (Figure S2); change curves of temperatures of PCTS tank and ambient temperature on Jan. 14, where the temperature data are presented as averages of the corresponding three measuring points (Figure S3); the graph of the substance circulation (Figure S4); Initial investment of the solar−biogas system (Table S1) (PDF)

M3 = 0.58 × (4.65 − 3.48) × 365 = 239.1 yuan

Meanwhile, the biogas residue can be used as fertilizer to reduce the consumption of chemical fertilizer, which could be regarded as indirect benefits. The nitrogen and phosphorus contained in biogas residue per year are ∼0.9 and 0.7 kg, respectively, which could equate to urea (1900 yuan per ton) and 10% superphosphate (35.3 and 126 kg, respectively). Assuming the biogas residue utilization is 50%, the annual indirect benefits are given as

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M4 = (1900 × 0.0353 + 600 × 0.126) × 50%

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 137 76650963. Fax: +86 025 83795618. E-mail: [email protected].

= 71.3 yuan

The total saving benefit is 1622.2 yuan per year, and the corresponding present discounted value (X) can be calculated as follows:

ORCID

Xianning Li: 0000-0003-3177-7114

⎡ 1 − (1 + i)−N ⎤ ⎡ 1 − (1 + 0.03)−20 ⎤ X = M⎢ ⎥ = 1622.2 × ⎢ ⎥ i 0.03 ⎣ ⎦ ⎣ ⎦

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was funded by the National Key Technology R&D Program of China (No. 2013BAJ10B12-02). This pilot scale study was conducted at a field laboratory located in a village in Anhui Province, and some of the study was conducted at Southeast University.

= 24 134 yuan

It can be seen that the investment for the recommended solar−biogas system is economically feasible, according to the positive NPV (24 134 − 22 200 = 1934). In addition, there exists the possibility of further increases in economic benefits. This is mainly because (i) the direct benefits calculated in this study were based on data obtained in the winter, and the improvement of direct benefits is predictable in other seasons; (ii) if the production scale ia properly enlarged, the average unit cost will decrease; and (iii) an energy crisis could encourage subsidies for clean technology investments.24 Moreover, the solar energy and biogas bringing notable environmental benefits and society makes this system more attractive.

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4. CONCLUSIONS Using the solar thermal and phase change thermal storage (PCTS) system to collect and store solar energy for heating of the two-phase anaerobic digester (TPAD) in the winter was determined to be feasible. The PCTS system reduced the effects of the intermittent solar energy and the low ambient temperature on the heating temperature and fermentation temperature. The biogas yield and energy conversion rate of the feedstock were improved 87% and 101%, respectively, compared with conventional solar heating mode. In addition, the biodegradability and energy conversion rate of the substrate I

ABBREVIATIONS AD = anaerobic digestion Ccor = Pearson correlation coefficient Cvar = statistical coefficients of variation HAR = hydrolytic−acidification reactor MC = moisture content MR = methanogenic reactor NPV = net present value PCTS = phase change thermal storage PV = photovoltaic sCOD = soluble chemical oxygen demand SD = standard deviation TMP = theoretical methane potential TMY = total methane yield TPAD = two-phase anaerobic digester TS = total solids VFAs = volatile fatty acids VS = volatile solids DOI: 10.1021/acs.energyfuels.6b03376 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.6b03376 Energy Fuels XXXX, XXX, XXX−XXX