Life-cycle energy production and emissions mitigation by comprehensive biogas–digestate utilization

Bioresource Technology 114 (2012) 357–364

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Life-cycle energy production and emissions mitigation by comprehensive biogas–digestate utilization Shaoqing Chen, Bin Chen ⇑, Dan Song State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, PR China

a r t i c l e

i n f o

Article history: Received 13 January 2012 Received in revised form 23 March 2012 Accepted 24 March 2012 Available online 2 April 2012 Keywords: Biogas Digestate Energy flow Emissions mitigation Life-cycle assessment

a b s t r a c t In the context of global energy shortages and climate change, developing biogas plants with links to agricultural system has become an important strategy for cleaner rural energy and renewable agriculture. In this study, a life-cycle energy and environmental assessment was performed for a biogas–digestate utilization system in China. The results suggest that biogas utilization (heating, illumination, and fuel) and comprehensive digestate reuse are of equal importance in the total energy production of the system, and they also play an important role in systemic greenhouse gas mitigation. Improvement can be achieved in both energy production and emissions mitigation when the ratio of the current three biogas utilization pathways is adjusted. Regarding digestate reuse, a tradeoff between energy and environmental performance can be obtained by focusing on the substitution for top-dressing, base fertilizers, and the application to seed soaking. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Anaerobic digestion and biogas production are promising ways to achieve energy and environmental benefits at both the local and global level (Börjesson and Berglund, 2006, 2007). Biogas plants can provide an alternative energy source for rural households and mitigate environmental emissions from agricultural activities (Chen and Chen, 2012; Prochnow et al., 2009). Biogas is different from traditional forms of rural energy in two main ways: first, it replaces fossil fuels with clean methane, which reduces not only the release of greenhouse gases, but also other detrimental emissions; second, the multiple utilization of digestates (i.e., substitution for such materials as fertilizers, pesticides, and feed additives) facilitates more efficient use of organic waste or plant nutrients in daily agricultural practice (Collet et al., 2011; Rehl and Müller, 2011). Such procedures are of particular importance in coping with the increasing pressure of problems related to global energy scarcity and climate change. To date, biogas projects have been implemented in many rural areas around the world, particularly in China, whose household biogas system has become a key program in the country’s renewable-energy construction efforts (Liu et al., 2008b). Household biogas construction and energy production have undergone a boost in rural China during the last three decades. As of 2009, about 35 ⇑ Corresponding author. Address: No. 19, Xinjiekouwai Street, Beijing 100875, PR China. Tel./fax: +86 10 58807368. E-mail address: [email protected] (B. Chen). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.084

million anaerobic digesters were put into use in China, which produced 2.73  1010 MJ of energy (Chen et al., 2010). Following years of practice, various comprehensive biogas-utilization modes have been established in different parts of the country, commonly linking biogas production with the agricultural industry (Chen et al., 2010; Wei et al., 2009). Among these modes, the so-called Six in One biogas system (SIOBS; the system comprises six components—pig breeding, anaerobic digester, cropping, fruit cultivation, vegetable growing, and agricultural processing) has recently become a widespread means of biogas–digestate utilization in South China thanks to its efficient, economical use of fermentation by-products. The technical overview of a typical SIOBS is provided in Fig. 1. The core of SIOBS is an 8-m3 fermenting reactor, surrounded by a set of supporting devices for raw-material treatment, methane utilization, and digestate recycling. The SIOBS has significant advantages over conventional agricultural practices and waste-handling procedures: it provides clean, cheap methane fuel for heating, illumination, or machinery; via the fermentation of household and agricultural waste, it is also effective in processing both liquid and solid digestates into substitutes for such materials as base fertilizer, top-dressings, and feed additives, which would otherwise demand high-emission chemicals. Life-cycle assessment (LCA) enables the evaluation of energy and environmental performance of a product or system at all stages. A handful of studies have evaluated household and industrial biogas projects in terms of their environmental benefits. The focus of these evaluations has included methane production from different types of feedstock (e.g., Al-Masri, 2001; El-Mashad and

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Raw materials

Biogas stove

as

G b

m

bo

Air reservoir 3 8m Fermenting reactor Biogas

Pretreatment tank

Desulfurization

Digestates Filter Acidification tank

Diesel machine

Liquid digestate

Purification

Sedimentation tank Solid digestate

Biogas lamp

Drying Pellet process process

Feed additive Pesticides Seed soaking Top dressing Base fertilizers

Anhydration tank

Fig. 1. Technical overview of SIOBS.

Zhang, 2010), biogas utilization (e.g., Börjesson and Berglund, 2006; Patterson et al., 2011), and by-products with reuse potential (e.g., Pöschl et al., 2010, 2011; Rehl and Müller, 2011). Still, there is huge uncertainty in defining the benefits of bioenergy utilization and digestate reuse (Gerin et al., 2008; Meyer-Aurich et al., 2012; Tilman et al., 2009), and few studies have addressed the overall energy efficiency and emissions mitigation of biogas production from agricultural or household resources, especially when the by-products of a biogas system are utilized via diverse pathways. Furthermore, the possibility of a tradeoff between energy production and environmental performance has rarely been discussed with respect to the various stages of complex biogas systems, even though such knowledge is highly important in achieving both an energy-productive and environmentally-friendly bioenergy-implementation capability. The purpose of this paper is to undertake an energy and environmental assessment of a typical SIOBS, with the focus on energy production and emissions mitigation by comprehensive biogas– digestate utilization. In light of the ongoing promotion of biogas projects in rural areas, it is crucial to probe into their performance in terms of energy output and effects on the environment. Future system optimization can be implemented based on a life-cycle examination of the advantages in utilizing biogas and digestate. The life-cycle inventory analysis and impact assessment were carried out using the International Standards Organization (ISO) 14040 framework. The method employed to assess the potential environmental impact was that of Centrum voor Milieukunde Leiden (CML), which was elucidated by Guinée et al., 2002.

2. Methods 2.1. Goal and scope definition The goal of this study is to appraise the energy and environmental performance of the comprehensive utilization of biogas and digestate at the household scale. The considered functional unit is 1 MJ produced by the combustion of biogas (via diverse pathways) within an SIOBS. It should be noted that the current LCA is not based on the complete infrastructure of the SIOBS, i.e., the contribution of agricultural industries are not included. Instead, the energy and environmental performance of the current system is compared with a reference system, whereby the additive infrastructure and the resultant energy production and environmental benefit of SIOBS are calculated and demonstrated. The reference system is a traditional agrosystem conducting household and farm work without biogas facilities, which employs the in situ composting to dispose of food residues and waste and uses chemical fertilizers and pesticides for crop cultivation (i.e., rice) and fruit and vegetable growing (e.g., persimmons). The functions and charac-

teristics of the SIOBS are detailed in Section 2.2, where the reference system is also further specified. Fig. 2 illustrates the system boundary of the study. In light of the principles of LCA, the inventory includes biogas plant construction, biogas plant maintenance, feedstock supply, biogas energy utilization, and digestate processing. The energy production or consumption and emission release or mitigation are emphasized at these stages. Initial observation suggests that construction and maintenance, feedstock pretreatment, and desulfurization are energy-consuming and emission-releasing processes, while feedstock supply, biogas utilization, and digestate reuse are the major energy-producing and emission-mitigating processes. Within an SIOBS, the biogas project serves as an important component that links agricultural industries, such as cropping, vegetation and fruit growing, pig breeding, and agricultural processing. The environmental impact of these industries is mitigated in SIOBS owing to the substitution for chemical fertilizers and pesticides by fermentation by-products. To take into account the impact generated by these by-products, the method for substitution estimation entails an expansion of the system boundary. The information for the inventory was derived from multiple sources, including a literature review, communications with local users, and inventories conducted on similar functional units and processes in the Ecoinvent database (Ecoinvent, 2006). 2.2. Life-cycle inventory 2.2.1. Biogas plant construction stage The extraction and transportation of building materials for the biogas plant project are covered in this stage. An 8-m3 cylindrical hydrostatic anaerobic digester is employed in SIOBS, which encompasses equipments such as pretreatment tank, acidification tank, fermenting reactor air reservoir, sedimentation tank and anhydration tank (shown in Fig. 1). The application of the project has a 20-year lifespan before the materials are buried as landfill. For the extraction process, 1.15 ton cement, 10 kg rebar, 2000 red bricks, 2.5 ton coarse and fine sand, and 1 ton gravel are required. In making the rebar, only the steelmaking process is considered in this study. The pipelines used in the biogas plant are all PVC pipes, whose length is assumed to be 27 m. For transportation of the above building materials, a low-speed van capable of a 12-ton load is employed; its diesel consumption is 0.45 L/km. And the distance between the extraction factory and farmers is assumed to be 50 km. The background figures for the system construction were gathered from field investigation and domestic databases, while the data of energy consumption and air emissions (i.e., emission factors and coefficients) in the extraction and transportation processes were calculated from IPCC (IPCC, 2006) and various research studies (Huijbregts et al., 2000; Streets et al., 2003).

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P P

Energy consumption (down) or production (up)

E

Emissions mitigation (down) or release (up)

E

P

Biogas plant construction & maintenance

E

Building materials Extraction

Transportation E

Basic infrastructure P

E

Feedstock supply

P

Collection &Pretreatment

Anaerobic digester

P

Electricity Substitution of coal

E

Home scraps Organic wastes (Excrement & manure)

Biogas energy utilization

Heat

Desulphurization

Biofuel

Substitution of diesel E

P

Digestate processing

Crop straws

E

P

Base fertilizers

Recycling

Top dressing Pesticides Feed additive

Substitution of chemical fertilizers, etc.

Seed soaking

System boundary Agricultural industries

Cropping

Vegetable/fruit growing

Breeding

Agricultural processing

Fig. 2. System boundary of the study.

2.2.2. Biogas plant maintenance stage The biogas plant in SIOBS needs to be maintained to eliminate problems during the equipment’s operation, such as water or steam leakage from the reactor or treatment tanks. Normally, faults occur about twice per year, and materials consumed in the maintenance step include 50 kg cement, 10 kg concrete, and 300 kg feedstock materials. The calculation of transporting these materials was similar to that in the construction stage.

Pretreatment and transportation of feedstock are energy-consuming processes. The pretreatment of raw materials should be conducted before their fermentation in the biogas plant; 24 kWh/ ton electricity and 80.6 MJ/ton energy consumption are required (Börjesson and Berglund, 2007). The distance between household and farmland was assumed to be 5 km. The background data for the feedstock reuse and transportation were collected from relevant case studies in China (Liu et al., 2008a; Wang et al., 2007).

2.2.3. Feedstock supply stage In the conventional agrosystem in South China (the reference system), crop straw from rice farming (two crops a year) is disposed of through direct combustion, while organic waste (i.e., pig manure and human excrement) is exposed to the atmosphere as in situ composting. In rural households, home scraps are often discarded without any reuse. With the application of SIOBS, environmental emissions are mitigated by mixing organic waste, crop straw, and home scraps for airtight fermentation instead of direct combustion or exposure to the atmosphere. It is assumed that 800 kg straw and 1600 kg manure are needed per year to support the operation of the biogas plant in the SIOBS. The emissions of methane (CH4) and ammonia (NH3) during in situ composting are 3.48 kg and 7.43 kg, respectively (Jenkins et al., 1996), whereas in the biogas system, the emission of CH4 reaches zero and the emission of NH3 is reduced to 4.84 kg. The total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) of the organic waste are 5.25%, 5.34%, and 5.58%, respectively. The reuse of food residues (from the household or vegetable-growing system) is not considered in this study since it is usually used as pig feed in the reference system. Inoculum, which is usually activated sludge, is also required in the process of biogas plant operation; For one year’s operation, 300 kg inoculum is required. Since landfill is at present the main treatment measure for activated sludge, the reduction of the relevant emissions by reusing activated sludge in SIOBS equals the magnitude of emissions caused by landfill disposal. The energy cost by landfill treatment is 65.36 kJ/kg.

2.2.4. Biogas energy utilization stage The utilization of biogas not only produces efficient energy for households and other agricultural industries, but also mitigates detrimental air emissions by its substitution for coal and diesel. The annual output of biogas is about 450 m3, as the biogas plant operates 300 days a year and produces 1.5 m3 gas per day on average. There are three utilization pathways of biogas—heating for biogas stoves, illumination for biogas lamps, and alternative fuel for diesel engines—which are responsible for the substitution for coal, electricity, and diesel, respectively. In a typical SIOBS, the diesel engine is used in a stationary machine for farm work and is employed for agricultural processing activities, such as flour grinding, bean-curd making, and fruit and vegetable processing. Since diesel machines are not incorporated in the calculation, the purification of biogas and upgrading of the engine (to operate with methane) are not considered. According to the implementation situation in most regions, the ratio of the three utilization pathways—heating: illumination: fuel—was set at 2:1:1, while a sensitivity analysis later explores the influence of this ratio on the mitigation of greenhouse gases (GHGs) and net energy gain (NEG). The environmental benefits of biogas utilization were analyzed based on energy production and emission reduction caused by the replacement of traditional agricultural or domestic practices. Biogas should be desulfurized before any application. The desulfurization devices were considered in the emission and energyconsumption calculation; they contain 1.27 kg iron and have an assumed lifetime of 5 years (Brentrup et al., 2004). In the heating process, the emission release during biogas combustion was also

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considered for its environmental impact. The illumination provided by a biogas lamp is similar to that of a 100-W incandescent bulb. The biogas stove and lamp used 30 kg steel in total and are assumed to have a lifetime of 10 years (Jury et al., 2010). The energy content of biogas is 20,000 kJ/m3, and that of diesel is 46,900 kJ/kg (IPCC, 2006).

2.2.5. Digestate-processing stage The digestate-processing stage embraces the recycling of both liquid and solid fermentation digestates. Compared with the reference system, the environmental benefits of digestate processing lie in the substitution for chemical feed additives, pesticides, topdressings, base fertilizers, and application to seed soaking. A digester produces 16 ton biogas slurry each year. In an SIOBS, filtered biogas slurry in the middle level of the reactor is used as a feed additive for pig keeping owing to its highly nutritious and aseptic character. According to local investigation, 0.66 ton of biogas slurry is utilized as feed additive. Two pigs, which are marketed every year, are kept by every household. A pig usually gains 21.7% in weight as a result of the feed additive. For pig keeping, 1.00 ton forage is needed per year. Seed soaking is a pre-sowing process and is generally conducted to augment the germinating capacity of crop seeds. Unlike the reference system (in which pure water is used for seed soaking), liquid digestate is used to complete this process in an SIOBS. It is assumed that 2.50 ton liquid digestate is used for seed soaking, and the seed product from this can be utilized for a 0.65-ha cropping field. Compared with the seed-soaking treatment using pure water, crop production is increased by 0.44 ton per ha, thereby reducing certain emissions and energy consumption from various cropping activities. It has recently been established that filtered liquid digestate can be employed for disinsection in cropping or fruit and vegetable growing, and such use has proved to be both effective and relatively clean. Another 0.30 ton liquid digestate is used to substitute for conventional pesticides, and the environmental benefits are calculated from the reduction in emissions and energy consumption caused by the chemical compounds (Geisler et al., 2005). Another function of biogas slurry is top-dressing, in which liquid digestate is spread over the field as foliar fertilizer. Another 12.54 ton liquid digestate is used for this purpose, which is equivalent to 118.02 kg urea, 150.00 kg potash fertilizers, and 78.68 kg phosphoric fertilizers. In addition, 5 ton biogas residue (solid digestate) produced by fermentation is commonly used as base fertilizers for fruit and vegetable growing (instead of chemical fertilizers in the reference system), which is equivalent to 30.00 kg urea, 30.00 kg potash fertilizers, and 20.00 kg phosphoric fertilizers. The processing activities of digestates include separation of liquid and solid digestates, granulation, and other specific treatments. The electricity and heating energy consumed in the different pathways are as follows: 210 MJ/ton and 300 MJ/ton for feed-additive substitution; 145 MJ/ton and 180 MJ/ton for seed soaking (Charles et al., 2006); 230 MJ/ton and 387 MJ/ton for base-fertilizer substitution (Börjesson and Berglund, 2006); 300 MJ/ton and 510 MJ/ton for top-dressing substitution (Pöschl et al., 2010); and 410 MJ/ton and 210 MJ/ton for pesticide substitution (Geisler et al., 2005). The energy consumption in transporting digestate is 5.99 MJ/ton km (small truck) (Streets et al., 2003), and the distance is assumed to be 10 km. With the joint operation of all five digestate-processing pathways outlined as the base case, four scenarios are developed to determine the optimal energy and environmental performance for the SIOBS: Scenario 1—substitution for base fertilizer and topdressing; Scenario 2—substitution for base fertilizer, top-dressing, and forage; Scenario 3—substitution for base fertilizer and topdressing and application of biogas slurry to seed soaking; Scenario

4—substitution for base fertilizer, top-dressing, and fertilizer and application to seed soaking. 2.3. Life-cycle energy analysis This section aims to assess the life-cycle energy performance of the comprehensive biogas–digestate utilization in the SIOBS, including all the stages during the system’s implementation. Two components constitute the energy input or output in this study. For the direct energy input, the energy consumption consists of the sum of the heating value of diesel oil or coal and electricity supply; the indirect energy input represents the energy required for biogas–digestate production, e.g., fuel extraction, equipment, and transportation. With regard to direct energy output, the energy production includes the steam from biogas energy as a substitution for coal, electricity, and diesel; the indirect energy production synthesizes the energy saved by the construction of SIOBS, e.g., the energy savings by substituting for chemical fertilizers, pesticides, and feed additives. Based on the total energy input and output of the SIOBS, NEG can be calculated from Eqs. (1)–(3). NEG is a key indicator for the energy performance of the SIOBS to identify whether the system is gaining or losing energy.

Eti ¼ Ei þ E0i

ð1Þ

Eto ¼ Eo þ E00

ð2Þ

NEG ¼ Epo  Epi

ð3Þ

where the total energy input (Eti) is determined by the direct energy input (Ei) and the indirect energy input (E’i); the total energy output (Eto) is formulated by the direct energy output (Eo) and the indirect energy output (E’o). 2.4. Life-cycle environmental assessment The inventory data for each stage were complied in SimaPro 7.1 (PRè Consultants BV) to assess the environmental performance of the SIOBS. Due to the relevance to the biogas system studied, the following potential impact categories were considered: global warming (GWP), human toxicity (HTP), photochemical oxidation (POCP), acidification (AP), eutrophication (EP), abiotic depletion (ADP), and ozone-layer depletion (ODP). Traditional impact assessment methods, such as CML (Guinée et al., 2002), restrict the modeling of the cause–effect chain to an early stage to limit uncertainties and group life-cycle inventory results into midpoint categories. Damage-oriented methods, such as Eco-indicator 99 (Goedkoop and Spriensma, 2000), simulate the cause–effect chain up to the end points, but do so sometimes with considerable uncertainties (Jolliet et al., 2003). The SIOBS is a complex biogas system, which has its inherent uncertainty factors and assumptions. Also, the substitution effects generated by the by-products need to be taken into account in the system. According to the scope and purpose of the current LCA, the CML 2000 method was selected to quantify these impact categories. 3. Results and discussion 3.1. Energy flows and emissions inventory The results of the energy flows and emissions inventory on a per-year basis are shown in Table 1. The energy flows (both energy consumption and production) and environmental emissions are tracked within five stages (biogas plant construction, biogas plant maintenance, feedstock supply, biogas energy utilization, and digestate processing). Based on these calculations, the situation of the

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S. Chen et al. / Bioresource Technology 114 (2012) 357–364 Table 1 Life-cycle energy and emissions annual inventory of an SIOBS.

Eti/(MJ) Eto/(MJ) CO2/(104 kg) CH4/(104 kg) CO/(104 kg) SO2/(104 kg) NOX/(104 kg) NH3/(104 kg) TN/(104 kg) TP/(104 kg) COD/(104 kg) VOC/(104 kg) PM10/(104 kg)

Biogas plant construction

Biogas plant maintenance

Feedstock supply

Biogas energy utilization

Digestate processing

Whole system

0.052 0.000 70.055 0.008 0.126 0.167 0.136 0.000 0.000 0.000 0.003 0.003 0.239

0.016 0.000 47.215 0.001 0.038 0.013 0.065 0.000 0.000 0.000 0.002 0.001 0.086

0.004 0.002 1027.240 11.041 61.843 0.276 1.705 5.228 0.847 0.718 13.024 10.554 3.879

0.015 1.000 841.827 0.960 0.998 3.796 6.015 0.000 0.000 0.000 0.001 0.021 5.639

1.145 1.126 874.708 3.345 0.974 6.838 3.785 0.986 0.000 0.000 0.000 0.288 2.722

1.233 2.128 2626.505 15.338 63.651 10.730 11.305 6.215 0.847 0.718 13.020 10.817 11.916

‘0.000’ signifies no or negligible emission/energy. Negative values indicate emissions release or energy consumption, while positive ones indicate emissions mitigation or energy production. SIOBS: ‘Six in One’ biogas system. Eti: total energy input; Eto: total energy output; TN: total nitrogen; TP: total phosphorus; COD: chemical oxygen demand; VOC: volatile organic compound; PM10: particles on the order of 10 lm or less.

whole system was also assessed. To obtain 1 MJ biogas energy, the energy consumption of the SIOBS is 1.233 MJ in total, whereas the total energy production is 2.128 MJ. Digestate processing is the most energy-consuming process, much higher than construction and maintenance of the anaerobic digester. The most energy-productive processes are biogas-energy utilization and digestate processing, which supply energy directly for household use and save indirect energy by substituting for conventional chemicals or activities, respectively. The emission releases in these stages are relatively small compared with the magnitude of emissions mitigation. The total mitigation of CO2 alone amounts to 2626.505  104kg/MJ. In terms of the three major greenhouse gases (CO2, CH4, and N2O), the feedstock-supply stage contributes the most to emissions mitigation, followed by the stages of digestate processing and biogas-energy utilization. Feedstock supply is also the biggest contributor to the mitigation of NH3 and COD, while digestate processing benefits the system most by reducing the emissions of NOX (nitrogen oxides) and PM10 (particles of the order of 10 lm or less). 3.2. Overall energy and environmental evaluation Table 2 displays the improvement in energy and environmental performance by SIOBS, and the contribution of the different stages is also presented. The improvement in environmental condition at each stage of the SIOBS is estimated though various impact categories (i.e., GWP, AP, EP, POCP, HTP, ADP, and ODP), while the energy performance of the system is indexed by the NEG. Evidently, the improvement in environmental conditions from feedstock supply,

biogas utilization, and digestate processing are significant and can be attributed to the indirect benefits of these processes (Börjesson and Berglund, 2007). In comparison, the impact of the construction and maintenance of the biogas project on the environment is relatively small. The total reduction of GHGs in the SIOBS is 3036.809  104 kg CO2-eq/MJ. In particular, the benefits in mitigating global warming from the three stages during the system’s operation—1382.785  104 kg in the feedstock-supply stage, 850.660  104 kg in the biogas-utilization stage, and 921.135  104 kg in the digestate-processing stage—contribute 45.53%, 28.01%, and 30.33% to the total effect, respectively. Feedstock supply is the largest contributor to the mitigation of almost all categories—45.82% to AP, 79.85% to EP, 97.38% to POCP, and 34.83% to ODP—the exceptions being HTP and ADP, where the major contribution comes from the utilization terminals of the system (i.e., the biogas-utilization and digestate-processing stages). According to the NEG, construction and maintenance, feedstock supply, and digestate processing overall are energy-consuming, whereas biogas utilization is energy-producing. With all the lifecycle stages taken into account, the NEG of the entire system is 0.894 MJ. Biogas utilization is the dominant process affecting final energy supply, contributing 110.28% to the NEG of the SIOBS. On the other hand, the construction process decreases the systemic NEG by 5.94%, while the recycling of liquid and solid digestates reduces it by 2.16%. It is important to note that the figures are not the absolute emissions specifying the energy or environmental performance; rather, they designate the improvement represented by an SIOBS over conventional agricultural activities or waste treatment by adding a biogas project to a household.

Table 2 Improvement in energy and environmental performance at different stages in SIOBS. Stage

GWP CO2-eq/ (104 kg)

AP SO2-eq/ (104 kg)

EP PO4-eq/ (104 kg)

POCP C2H4-eq/ (104 kg)

HTP 1,4-DCB-eq/ (104 kg)

ADP Sb-eq/ (104 kg)

ODP CFC-11-eq (104 kg)

NEG /(MJ)

Biogas plant construction Biogas plant maintenance Feedstock supply

70.472 (2.32%)a 47.300 (1.56%) 1382.785 (45.53%) 850.660 (28.01%) 921.135 (30.33%) 3036.809

0.263 (1.06%) 0.058 (0.24%) 11.351 (45.82%) 7.731(31.21%)

0.018 (0.24%) 0.008 (0.12%) 5.905 (79.85%) 0.756 (10.23%) 0.760 (10.29%) 7.396

0.039 (0.15%)

0.076 (1.06%)

0.012 (0.05%)

0.013 (0.18%)

24.962 (97.38%)

0.474 (6.61%)

3.50E-07 (0.24%) 1.98E-07 (0.14%) 5.03E-05 (34.83%)

0.055 (0.21%)

1.617 (22.57%)

0.223 (3.33%) 0.071 (1.05%) 0.008 (0.12%) 4.200 (62.82%)

0.669 (2.61%)

5.161 (72.05%)

2.770 (41.43%)

25.634

7.163

6.685

0.052 (5.84%) 0.016 (1.84%) 0.003 (0.29%) 0.985 (110.12%) 0.019 (2.13%) 0.894

Biogas energy utilization Digestate processing Whole system a

6.014 (24.27%) 24.775

3.36E05 (23.25%) 1.11E-04 (76.82%) 1.45E-04

The value in the bracket denotes the contribution of each stage to the whole system. SIOBS: ‘Six in One’ biogas system. GWP: global warming; AP: acidification; EP: eutrophication; POCP: photochemical oxidation; HTP: human toxicity; ADP: abiotic depletion; ODP: ozone layer depletion; NEG: net energy gain.

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3.3. Biogas-energy utilization The contribution to the improvement in energy and environmental conditions by the comprehensive utilization of biogas is shown in Table 3. The allocation among heat and power has been discussed for their different roles in improving environmental conditions (Börjesson and Berglund, 2006; Patterson et al., 2011). In this study, overall, the substitutions for diesel and coal benefit the system by their great contribution in reducing environmental emissions. With regard to the relative impact, the substitution of diesel plays an important role in mitigating GWP, EP, POCP, and ADP, contributing 62.06%, 69.61%, 108.29%, and 41.04% in this stage, respectively. The substitution for coal is responsible for diminishing AP, HTP, and ODP, making a contribution of 57.17%, 93.33%, and 84.30%, respectively. Furthermore, these substitution processes increase energy production by 101.57% in total. The biggest contributor comes from the substitution for diesel, followed by the substitution for coal and the substitution for electricity. With a focus on mitigating global warming, the substitution for diesel and coal results in a reduction of 536.155  104 kg CO2-eq and 341.144  104 kg CO2-eq, respectively; the mitigation effect from the substitution for electricity is not included owing to the inconsistent sources of power generation. Concerning total GHG mitigation (which accounts for 28.33% of the whole system), substituting diesel with biogas is ideal since it makes the highest contribution (contributing 62.06% in this stage), followed by the substitution for coal (39.49%). The carbon-trading prices of these different items were also estimated and amount to 11.192  104 US$. The NEG of this stage is 0.985 MJ. The results of energy flows indicate that in terms of biogas transformation, the highest efficiency is in the fuel supply for diesel machines, followed by the substitution for electricity in biogas lamps. 3.4. Sensitivity analysis Taking the utilization of biogas for direct combustion (substitution for coal), powering biogas lamps (substitution for electricity), and fueling machines (substitution for diesel) in the ratio of 2:1:1 as the base case, six valuations were considered in this sensitivity analysis: (1–1) decreasing direct combustion by 1/6 (i.e., a ratio of 1:1:1 among the three utilization pathways); (1–2) increasing direct combustion by 1/6 (a ratio of 4:1:1); (2–1) decreasing the powering of biogas lamps by 1/6 (a ratio of 7:1:4); (2–2) increasing the powering of biogas lamps by 1/6 (a ratio of 5:5:2); (3–1) decreasing the fueling of machines by 1/6 (a ratio of 7:4:1); (3– 2) increasing the fueling of machines by 1/6 (a ratio of 5:2:5); (4–1) reducing the cost of biogas equipment by 1/2; (4–2) increasing the cost of biogas equipment by 1/2. The results of the sensitivity analysis of GHG mitigation and energy production are shown in Fig. 3. It is evident that the influences of these activities on GHG mitigation and NEG are disparate

in the biogas-utilization stage. Substituting for electricity and coal would significantly mitigate global warming. Specifically, if the biogas utilized in lighting biogas lamps is reduced by 1/6, GHG mitigation will amount to 937.088 kg  104CO2-eq. In comparison, biogas equipment and substitution for diesel are insignificant factors in this context. However, with respect to NEG, the substitution for diesel plays an important role by its high energy-transforming efficiency. The influence of substituting for electricity is also notable, but adjusting the proportion of direct combustion does not significantly change energy production. Despite different trends in influencing energy production and emissions mitigation, a tradeoff can still be attained between these two indicators. For example, both GHG mitigation and NEG reach a maximum value (937.088 kg CO2-eq and 12060.65 MJ) when the ratio of the three pathways is 7:1:4, whereas the minimum values for both (10725.122 kg CO2-eq and 1.013 MJ) are attained from the ratio of 5:2:5. These results can be used as a guideline to choose effective routes to improve the energy and environmental performance of household biogas projects, especially when a tradeoff between energy production and GHG mitigation has to be taken into account.

3.5. Digestate reuse The contributions of various activities to improving energy and environmental conditions by utilizing digestate in multiple pathways are presented in Table 4. The results show that substitution of top-dressing with liquid digestate contributes over half of the total mitigation of GWP, AP, EP, and ADP (59.32%, 63.33%, 58.72%, and 90.61%, respectively). In mitigating POCP and ODP, using digestate for seed soaking is revealed to be the most important means (the contribution is 46.16% and 45.35%, respectively). In mitigating HTP, substitution for pesticide is a major contributor in the entire stage (56.98%). Evidently, with current utilization patterns, the replacement of the chemical fertilizers plays an important role in improving environmental conditions. GHG reduction in this stage is 921.135  104 kg, which represents 30.20% of the total mitigation in the whole system. Substitution for top-dressing contributes most to the GHG reduction in this stage (59.32%), followed by the substitution for base fertilizer (22.71%) and seed soaking (10.22%). The carbon-trading prices of these GHG reductions are also estimated to be 11.932  104 US$. However, the utilization of digestate is less likely to augment overall energy production. The substitution for top-dressing and base fertilizers produces a significant increase in indirect energy: the energy saving due to this process is 1.126 MJ. The energy consumed for processing and transporting digestate in all the reuse pathways amounts to 1.145 MJ. As a result, a negative NEG is evident in this stage, with an energy loss of 0.019 MJ.

Table 3 Contribution of different processes in improving energy and environmental conditions through biogas energy utilization.

a

Item

AP

EP

POCP

HTP

ADP

ODP

GWP

% (%a)

104 $b

EF/(MJ)

Combustion of methane Desulphurization Biogas-fueled cooker/lamp Substitution of coal Substitution of electricity Substitution of diesel Total

0.106 0.013 0.157 4.577 0.000 3.706 8.007

0.020 0.000 0.005 0.263 0.000 0.544 0.782

0.149 0.007 0.084 0.215 0.000 0.318 0.294

0.000 0.010 0.114 1.625 0.000 0.240 1.741

0.000 0.005 0.061 1.614 0.929 1.723 4.200

0.00E + 00 1.34E-07 1.59E-06 2.83E-05 0.00E + 00 7.06E-06 3.36E-05

8.213 0.397 4.709 341.144 0.000 536.155 863.979

0.95(5.89) 0.05(0.28) 0.55(3.37) 39.49(10.70) – 62.06(16.81) 100.00(28.33)

0.109 0.008 0.059 4.420 – 6.940 11.192

– 0.001 0.014 0.378 0.218 0.404 0.985

The values without and within the brackets are the proportional contributions of each item to the GHG mitigation of this stage and that of the system, respectively. The economic benefits of carbon mitigation were transformed by the carbon trading price. The carbon trading price of renewable energy project is 12.70 $/ton (UNFCCC CDM project database). GWP: global warming; AP: acidification; EP: eutrophication; POCP: photochemical oxidation; HTP: human toxicity; ADP: abiotic depletion; ODP: ozone layer depletion; EF: energy flow (negative value: energy input; positive value: energy output). b

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NEG

GHGs mitigation 863.979 Base case 950.000 857.320 (4-2)

851.802

0.977 (4-2)

(1-1)

900.000

0.973

0.990

(1-1)

0.960

850.000

0.930

800.000

870.639 (4-1)

0.985 Base case 1.020

0.992

876.157

750.000

(4-1)

(1-2)

700.000

0.900

0.996

0.870

(1-2)

0.840

(2-1)

(3-2) 790.871

(3-2) 0.901

937.088 (3-1)

(2-1) 1.013

(2-2)

803.049

(3-1)

790.871

(2-2) 0.956

0.967

Unit:10-4kg

Unit: MJ

Fig. 3. Sensitivity analysis of GHG mitigation and NEG. (1-1): decreasing direct combustion by 1/6; (1-2): increasing direct combustion by 1/6; (2-1): decreasing the lightening of biogas lamp by 1/6; (2-2): increasing the lightening of biogas lamp by 1/6; (3-1): decreasing the fueling of a machine by 1/6; (3-2): increasing the fueling of a machine by 1/6; (4-1): reducing the cost of biogas equipments by 1/2; (4-2): adding the cost of biogas equipments by 1/2.

Table 4 Contribution of different processes in improving energy and environmental conditions through digestate processing. Item

AP

EP

POCP

HTP

ADP

ODP

GWP

% (%a)

104 $b

EF/(MJ)

Combustion of methane Desulphurization Biogas-fueled cooker/lamp Substitution of coal Substitution of electricity Substitution of diesel Total

0.036 0.692 3.808 1.480 0.000 0.002 6.013

0.007 0.134 0.447 0.173 0.000 0.000 0.761

0.000 0.309 0.251 0.096 0.022 0.009 0.669

0.000 1.565 0.473 0.183 2.941 0.001 5.161

0.613 0.358 2.510 1.020 0.301 2.032 2.770

0.00E + 00 5.38E-05 4.28E-05 1.67E-05 3.80E-06 1.51E-06 1.19E-04

546.435 209.195 79.824 – 94.120 8.439 921.135

59.32(17.13) 22.71(6.56) 8.67(2.50) – 10.22(2.95) 0.92(6.05) 100.00(30.20)

7.075 2.706 1.034 – 1.218 0.109 11.932

0.588 0.239 0.071 0.144 0.084 1.145 0.019

GWP: global warming; AP: acidification; EP: eutrophication; POCP: photochemical oxidation; HTP: human toxicity; ADP: abiotic depletion; ODP: ozone layer depletion; EF: energy flow.

3.6. Environmental benefits under different scenarios

GWP 90.00% 60.00%

NEG (×10)

30.00%

AP

0.00% -30.00%

-60.00% -90.00%

ODP

EP

-120.00%

ADP

POCP

HTP Scenario 1: STD + SBF Scenario 3: STD + SBF + BSS

Scenario 2: STD + SBF + SF Scenario 4: STD + SBF + SF + BSS

Fig. 4. Environmental condition improvements under different digestate-reuse scenarios. STD: Substitution of top dressing; SBF: Substitution of base fertilizer; SP: Substitution of pesticide; SF: Substitution of forage; BSS: Biogas slurry for seed soaking. The proportions indicate the deviation rates from the values in base case (negative proportion denotes a decrement, and positive proportion denotes an increment).

Fig. 4 shows the energy and environmental performance of the SOIBS under different scenarios compared with the base case. Considering the benefits of digestate reuse and the processing cost, it is difficult to judge the energy efficiency and environmental effects of a complex biogas system in terms of diverse utilization pathways (Börjesson and Berglund, 2007; Pöschl et al., 2010). That is, no one option may be better than any other in every respect. The best energy-saving solution is the base case (i.e., combining all five utilization pathways) owing to the significant falloff in NEG in other scenarios by comparison (from 29.89% to 109.38%). However, if the emphasis is on environmental performance, Scenario 3 (combining the substitution for top dressing and base fertilizer and the application of biogas to seed soaking) and Scenario 2 (combining the substitution for top-dressing, base fertilizer, and forage) are competitive candidates for system optimization owing to the notable reduction in emissions in most impact categories. In particular, GHG reduction can be promoted by 6.56% and 4.77%, respectively, in scenarios 3 and 2 compared with the base case. In its construction, Scenario 4 is the scenario most similar to the base case, though it appears to be less effective in improving the system’s overall performance despite the fact that it is more beneficial in mitigating ADP (18.17%). Monotonic treatment of digestate for top-dressing and base fertilizer (Scenario 1) was ineffective in enhancing the system’s performance, both in terms of emissions mitigation and indirect energy production. It is suggested that the joint processing of digestate by multiple household units be promoted in future to lower energy costs and the resultant emissions. Only then will the integration of diverse utilization

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