A quantitative sustainable comparative study of two biogas systems based on energy, emergy and entropy methods in China

Abstract

In China, as an important source of renewable energy, the standard biogas production module (SBPM) plays a key role and accounts for the majority of biogas production. Given its greater environmental pollution, it is particularly important to assess its quantitative sustainability status for optimizing system design and policy-making. This paper conducted the sustainability assessments of the open-loop standard biogas production module (OL-SBPM) and closed-loop standard biogas production module (CL-SBPM). Compared with the OL-SBPM system, the CL-SBPM system integrates a power generation subsystem. To evaluate two types of systems quantitatively, energy, emergy and entropy methods have been utilized in this study. The results illustrated that only partial indicators are more sustainable in the CL-SBP system rather than all indicators by comparing with the OL-SBPM system, which is contrary to the prevailing view. Taking the information entropy as an example, the equivalent information entropy flow values of the OL-SBPM and CL-SBPM systems are 2.04 × 10²⁵ bits and 1.47 × 10²⁵ bits, respectively, which also demonstrate that the OL-SBPM system has more useful information resulting in higher degree of sustainability. Therefore, these two systems need to be selected according to the existing conditions, rather than the subjective ideation.

Appendices

Appendix 1

All calculated details from 1 to 17 in OL-SBPM system (Table 3).

• 1. Sunlight energy = plant area (2 × 10⁴m²)*Avg. annual solar radiation (5.01 × 10⁹ J/m²)*Albedo(0.13). UEVs of sunlight = 1.00 sej/J by definition. Sunlight emergy = 1.3 × 10¹³sej.

• 2. Rain chemical potential energy = plant area(2 × 10⁴m²)*annual rainfall (0.59 m)*Evaporation rate (60.0%)*water density (1000 kg/m³)*Gibbs free energy (4.94 × 10³ kJ/kg); UEV = 2.35 × 10⁴sej/J (Jae & William, 2017). Rain chemical potential emergy = 3.5 × 10¹⁰ × 2.35 × 10⁴ = 8.23 × 10¹⁴sej.

• 3. Rain geopotential energy = plant area (2 × 10⁴m²)*annual rainfall (0.59 m)*Runoff rate (40%)*water density (1000 kg/m³)*average elevation (4 m)*gravity (9.8 m/s²); UEV = 2.79 × 10⁴sej/J (Jae & William, 2017). Rain geopotential emergy = 1.85 × 10⁸ × 2.79 × 10⁴sej = 5.16 × 10¹²sej.

• 4. Wind kinetic energy = plant area (2 × 10⁴m²)*air density (1.29 kg/m³) *drag coefficient (0.001) * velocity of geostrophic wind³(27)* (3.15 × 10⁷ s/year). UEV = 1.9 × 10³sej/J (Jae & William, 2017). Wind kinetic emergy = 2.2 × 10¹⁰ × 1.9 × 10³sej = 4.18 × 10¹³sej.

• 5. Geothermal energy = plant area (2 × 10⁴m²)*heat flow (1.45 × 10⁶ J/(M²*year). UEV = 3.44 × 10⁴sej/J (Brown & Bardi, 2001). Geothermal emergy = 2.9 × 10¹⁰ × 3.44 × 10⁴sej = 9.98 × 10¹⁴sej.

• 6. Dairy wastewater emergy

  As the major effluent, COD of dairy wastewater needs to be calculated in the open loop SBPM process. Therein, 75% of water is responsible for washing project and 25% of others for chemical reaction of dairy wastewater. The average input flow is about 25 m³/d and COD amount load is 15.8 kg/m³.

  $${\text{The amount of COD}} = {\text{25 m}}^{{3}} /{\text{d}} \times {15}.{\text{8 kg}}/{\text{m}}^{{3}} \times {\text{365d}} = {1}.{44} \times {1}0^{{5}} {\text{kg}}/{\text{y}}.$$

  Lactic acid molecular mass = 90 g.mol⁻¹. The free enthalpy of lactic acid:

  $$\Delta H_{f}^{0} ({\text{C}}_{3} {\text{H}}_{6} {\text{O}}_{3} ) = - 672\;{\text{KJ}} \cdot {\text{mol}}^{ - 1} = 7.47 \times 10^{6} {\text{J/kg}}$$

  The energy of lactic acid in the dairy wastewater = 1.44 × 10⁵ kg/y × 7.47 × 10⁶ J/kg = 1.08 × 10¹² J/y.

  Unit emergy value of lactic acid is 4.83 × 10⁵seJ/J (Zhou et al., 2009).

  Emergy of lactic acid in the dairy wastewater = 1.08 × 10¹² J/y × 4.83 × 10⁵seJ/J = 5.22 × 10¹⁷sej/y.

• 7. Concrete amount calculation

  See Table

  Table 8 Concrete content calculation in open loop SBPM system

  8.

• 8. Reinforced fibers amount calculation

  See Table

  Table 9 Reinforced fibers amount calculation in open loop SBPM system

  9

• 9. Steel amount calculation

  See Table

  Table 10 Steel amount calculation in open loop SBPM system

  10.

• 10. PVC amount calculation

  PVC pipe is a vital part in open-loop SBPM system and the collected data are about 110 m lengths (60 mm diameter pipe and 100 diameter pipe), with 25 MWh/m3 and the lifespan is designed to be 15 years.

  $${\text{PVC}}_{{{\text{amount}}}} = {{(\pi } \mathord{\left/ {\vphantom {{(\pi } {4) \times (D_{{{\text{outside}}}}^{2} - D_{{{\text{inside}}}}^{2} ) \times L \times \rho (1.4 \times 10^{3} {\text{kg/m}}^{3} }}} \right. \kern-\nulldelimiterspace} {4) \times (D_{{{\text{outside}}}}^{2} - D_{{{\text{inside}}}}^{2} ) \times L \times \rho (1.4 \times 10^{3} {\text{kg/m}}^{3} }}) = 1.1\;{\text{t}}$$

  Unit emergy values of PVC = 7.46 × 10¹⁵ sej/t (Tilley et al., 2020)

  $${\text{PVC emergy}} = {1}.{1} \times {7}.{46} \times {1}0^{{{15}}} {\text{sej}} = {8}.{21} \times {1}0^{{{15}}} {\text{sej}}$$

• 11. Transportation amount calculation (material and device)

  In Table

  Table 11 Transportation amount calculation in the open-loop SBPM system

  11, the material and devices transport data have been listed in open-loop SBPM system. Meanwhile, an assumption is made: 12L fuel per 50km for the transport (35MJ/L).

  Unit emergy values of transportation = 8.4 × 10⁴ sej/j (Tilley et al., 2020)

  $${\text{Transportation emergy}} = {2}.{16} \times {1}0^{{9}} \times {8}.{4} \times {1}0^{{4}} {\text{sej}} = {1}.{81} \times {1}0^{{{14}}} {\text{sej}}$$

• 12. Power consumption amount calculation (electricity)

  See Table

  Table 12 Power consumption amount calculation in open-loop SBPM system

  12.

• 13. Service technician emergy

  In China, the unit emergy value of service is 7.96×10⁰⁹sej/$. The average cost of the machine service is 5×10⁰⁵$/y and the working time is about 40 h.

  $${\text{Emergy}}_{t} = 7.96 \times 10^{9} \times 5 \times 10^{5} = 3.98 \times 10^{15} {\text{Sej/year}}$$

• 14. Service engineer emergy

  In China, the unit emergy value of service is 7.96×10⁰⁹sej/$. The average cost of the engineering service is 6.3×10⁰⁵$/y and the working time is about 40 h.

  $$E_{e} = 7.96 \times 10^{9} \times 6.3 \times 10^{5} = 5.01 \times 10^{15} {\text{sej/year}}$$

• 15. Biogas emergy

  The biogas amount can be collected by the flow meter and was 75 m³/J. According to the standard biogas energy (2.1×10⁷ J/m³) and transformity (1.68×10⁶ sej/J) (Merlin et al., 2012), the biogas emergy can be obtained by using the following equation.

  $$E_{{{\text{biogas}}}} = 75 \times 365 \times 2.1 \times 10^{7} \times 1.68 \times 10^{6} {\text{sej/year}} = 9.66 \times 10^{17} {\text{sej/year}}$$

• 16. Effluent and sludge emergy

There are two types of residues in the outflows, including effluent and sludge, respectively. The inflow loss rate of COD is 95% and the transformity is 4.83×10⁵sej/J (Zhou et al., 2009). Effluent and sludge emergy can be got by as the following calculation.

$$E_{{{\text{outflow}}}} = 1.08 \times 10^{12} \times (1 - 95\% ) \times 4.83 \times 10^{5} {\text{sej/year}} = 2.61 \times 10^{16} {\text{sej/year}}$$

Appendix 2

All calculated details from 1 to 28 in CL-SBPM system (Table 4).

• 1. Sunlight emergy has been shown in “Appendix 1”.

• 2. Rain chemical potential emergy has been shown in “Appendix 1”.

• 3. Rain geopotential emergy has been shown in “Appendix 1”.

• 4. Wind kinetic emergy has been shown in “Appendix 1”.

• 5. Geothermal emergy has been shown in “Appendix 1”.

• 6. Dairy wastewater emergy has been shown in “Appendix 1”.

• 7. Crops residues emergy (Tilley et al., 2020):

  $$E_{{{\text{crop}}}} = 5.73 \times 10^{11} \times 1.9 \times 10^{5} = 1.09 \times 10^{17} {\text{sej/year}}$$

• 8. Concrete emergy has been shown in “Appendix 1”.

• 9. Reinforced fibers emergy has been shown in “Appendix 1”.

• 10. Steel emergy has been shown in “Appendix 1”.

• 11. PVC emergy has been shown in “Appendix 1”.

• 12. Asphalt emergy (Tilley et al., 2020):

  $$E_{{{\text{asphalt}}}} = 200 \times 6.02 \times 10^{14} = 1.2 \times 10^{17} {\text{sej/year}}$$

• 13. Polyane emergy (Tilley et al., 2020):

  $$E_{{{\text{polyane}}}} = 0.67 \times 3.44 \times 10^{15} = 2.3 \times 10^{15} {\text{sej/year}}$$

• 14. Plastic emergy (Tilley et al., 2020):

  $$E_{{{\text{plastic}}}} = 2.41 \times 1.25 \times 10^{15} = 3.01 \times 10^{16} {\text{sej/year}}$$

• 15. Waterproofing emergy (Tilley et al., 2020):

  $$E_{{{\text{crop}}}} = 3.59 \times 1.25 \times 10^{15} = 4.49 \times 10^{16} {\text{sej/year}}$$

• 16. Compactor emergy:

  The power of compactor is 250kw/h and the operating time is 130 h. UEVs of compactor are 1.43×10⁵ sej/J (Tilley et al., 2020).

  $$E_{{{\text{compactor}}}} = 250 \times 130 \times 3.6 \times 10^{6} {\text{J}} \times 1.43 \times 10^{5} {\text{sej/J}} = 1.67 \times 10^{16} {\text{sej/year}}$$

• 17. Mobile crane emergy:

  The power of mobile crane is 200kw/h and the operating time is 24 h. UEVs of mobile crane are 1.43×10⁵ sej/J (Tilley et al., 2020).

  $$E_{{\text{mobile - crane}}} = 200 \times 24 \times 3.6 \times 10^{6} J \times 1.43 \times 10^{5} {\text{sej/J}} = 2.47 \times 10^{15} {\text{sej/year}}$$

• 18. Perforator emergy:

  The power of perforator is 2.5 kw/h and the operating time is 100 h. UEVs of perforator are 2.54×10⁵ sej/J (Derovil et al., 2020).

  $$E_{{{\text{perforator}}}} = 2.5 \times 100 \times 3.6 \times 10^{6} {\text{J}} \times 2.54 \times 10^{5} {\text{sej/J}} = 2.29 \times 10^{14} {\text{sej/year}}$$

• 19. Vibrator emergy:

  The power of vibrator is 1.1 kw/h and the operating time is 120 h. UEVs of vibrator are 2.54×10⁵ sej/J (Derovil et al., 2020).

  $$E_{{{\text{vibrator}}}} = 1.1 \times 120 \times 3.6 \times 10^{6} {\text{J}} \times 2.54 \times 10^{5} {\text{sej/J}} = 1.21 \times 10^{14} {\text{sej/year}}$$

• 20. Air compressor emergy:

  The power of air compressor is 3.5 kw/h and the operating time is 110 h. UEVs of air compressor are 2.54×10⁵ sej/J (Derovil et al., 2020).

  $$E_{{\text{air - compressor}}} = 3.5 \times 110 \times 3.6 \times 10^{6} {\text{J}} \times 2.54 \times 10^{5} {\text{sej/J}} = 3.52 \times 10^{14} {\text{sej/year}}$$

• 21. Transportation emergy has been shown in “Appendix 1”.

• 22. Power consumption has been shown in “Appendix 1”.

• 23. Biogas generator emergy:

  The Biogas generator amount is 3.5×10⁴ $. UEVs are 2.45×10¹² sej/$ (Tilley et al., 2020).

  $$E_{{\text{Biogas - generator}}} = 3.5 \times 10^{4} \$ \times 2.45 \times 10^{12} {\text{sej/}}\$ = 8.58 \times 10^{16} {\text{sej}}$$

• 24. Oil emergy:

  The oil amount is 4.5×10¹⁰ J. UEVs are 8.39×10⁴ sej/J (Tilley et al., 2020).

  $$E_{{{\text{oil}}}} = 4.5 \times 10^{10} {\text{J}} \times 8.39 \times 10^{4} {\text{sej/J}} = 3.78 \times 10^{15} {\text{sej}}$$

• 25. Maintenance emergy:

  The maintenance cost is 5.18×10⁵ $. UEVs are 3.38×10¹² sej/$ (Tilley et al., 2020).

  $$E_{{{\text{Maintenance}}}} = 5.18 \times 10^{4} \$ \times 3.38 \times 10^{12} {\text{sej}}/\$ = 1.75 \times 10^{17} {\text{sej}}$$

• 26. Biogas emergy has been shown in “Appendix 1”.

• 27. Effluent and sludge emergy has been shown in “Appendix 1”.

• 28. Power generation emergy:

The power of generation is 2.8 kw/h and the operating time is 10×10⁶ hours. UEVs of power generation are 1.59×10⁵ sej/J (Xi and Qin, 2006).

$$E_{{\text{power - generation}}} = 2.8 \times 10 \times 10^{6} \times 3.6 \times 10^{6} J \times 1.59 \times 10^{5} {\text{sej/J}} = 1.6 \times 10^{18} {\text{sej/year}}$$