Mingyue Pang, Lixiao Zhang, Changbo Wang, Gengyuan Liu

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China

Emergy Analysis of a Biomass Direct-fired Power Plant in Inner Mongolia of China

Mingyue Pang, Lixiao Zhang?, Changbo Wang, Gengyuan Liu

State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of Environment, Beijing Normal University, Beijing 100875, China

Submission Info

Communicated by Zhifeng Yang

Biomass Direct-fired Power Plant

Emergy Analysis

Environmental Performance

Sustainability

Though encouraged by a series of policies and measures, the biomass direct-fired power generation industry in China has not achieved expected rapid development, like other renewable electricity industries such as small hydropower, photovoltaic power and wind power. In this paper, an emergy analysis was performed to evaluate the overall performance and relative environmental sustainability of biomass direct-fired power production systems, taking a plant in Inner Mongolia as an example. The evaluation results show that the solar transformity of biomass direct-fired electricity in 2011 is 1.14E+05 seJ/J, similar to that of typical renewable electricity technologies and much lower than that of fossil fuel power plants. The comparison between the emergy-based indicators with those of other electricity production systems also confirms the outstanding performance of the biomass direct-fired power system, indicating that biomass direct-fired power technology itself is relatively well established in China now. However, one of the key constraints to the healthy development of the biomass direct-fired power industry is the unstable supply of raw materials and the resulting increased collection radius, which have a large effect on the sustainability performance of biomass direct-fired power systems in terms of emergy indicators.

? 2013 L&H Scientific Publishing, LLC. All rights reserved.

1 Introduction

Today, climate change and the demand for energy security highlight the need for shifting the current energy structure from fossil fuels to environment-friendly and sustainable energy sources worldwide, especially in China, which already is the world’s largest energy consumer and largest emitter of greenhousegases, because it is mainly dependent on coal. In this regard, renewable energy can serve as a flexible and reliable energy source to replace fossil fuels and provide sustainable energy [1,2]. Biomass, a major source of energy throughout the world since the beginning of civilization, is considered to be an important renewable energy option because there are various modern technologies for its broad-scale use. Among the wide range of modern bioenergy conversion technologies, biomass direct-fired power generation is currently the most established technology to produce electricity from biomass. Since the first straw directfired power plant built in Denmark in 1988, this technology has been widely promoted and applied in Europe and North America; while it started fairly late in China with the first plant built in 2006 [3,4]. Being an agricultural country, China has relatively abundant biomass resources for energy use [5]. Favored by resource abundance and government policies, biomass direct-fired power generation is steadily expanding in China. By the end of 2012, the total installed capacity of biomass direct-fired power plants integrated into the grid had reached 3264 MW [6]. However unlike other renewable electricity industries, the biomass direct-fired power generation industry has not achieved rapid development as expected and has encountered many problems such as feedstock availability and unfair electricity prices [7]. Furthermore, the ecological impact and environmental sustainability of such technologies in China remains unknown. Therefore, a systematic evaluation for the biomass direct-fired power generation system is of particular importance to guarantee a sustainable future for China and the world.

Emergy analysis, capable of considering both natural and economic aspects of a system, can serve as a valid approach to evaluate the overall sustainability of systems. By definition, emergy is the available energy of one kind previously required directly and indirectly to make a product or service [8-10]. As solar energy is the most prevalent form of energy in all biochemical processes on the earth, emergy is commonly quantified in solar energy equivalents. The solar emergy required to make 1J of a product or service is defined as the transformity of the product or service. Its unit is solar emjoule per Joule, abbreviated as seJ/J. Other Unit Emergy Values (UEV) are the specific emergy, solar emjoules g-1(seJ/g) and the emergy to money ratio (seJ/$). The solar emergy of any resource input into a system can be calculated by multiplying its raw amount by its UEV [11,12]. Through accounting all forms of energy and materials on a common energy basis, a set of emergy indices can be calculated to characterize the environmental loading and sustainability of the whole system. Within a unified evaluation framework developed and further illustrated by Odum and his colleagues [8,9,13,14], emergy analysis has been successfully carried out worldwide for the evaluation of different systems such as agricultural systems, urban systems, and industrial systems on regional, national or global scales as well as renewable energy technologies including small hydropower, wind power, photovoltaic power [15-23]. In particular, Sha and Hurme [24] have conducted the research to evaluate the biomass and coal-based combined heat and power cogeneration processes by emergy analysis.

This paper presents an emergy assessment of the overall performance of a case of biomass direct-fired power system in Inner Mongolia of China. The objectives of this paper are: (1) to analyze the environmental sustainability of the system compared with other renewable energy generation technologies, (2) to understand the factors that influence the performance of the system; (3) to explore the implications for the sustainable development of the biomass direct-fired power generation industry.

2 Materials and methods

2.1 The description of the case plant

The biomass direct-fired power plant, as a case study, is located in the Mu Us Sandland of Wushen Banner in Erdos, Inner Mongolia. There are relatively abundant and widely distributed biomass resources especially Salix psammophila (Calorific value =16.72 MJ/kg) in Mu Us Sandland. In recent years, the Erdos government issued a series of policies to promote vegetation planting to improve the environment, thus the plantation area ofSalix psammophilahas been largely extended. The biomass direct-fired power plant is designed to consume the residues ofSalix psammophilato further promote planting vegetation. The annual production ofSalix psammophilain Wushen Banner is estimated to be 71.3E+04t. The power plant is designed to consume about 16.6E+04 t ofSalix psammophilawith 40% moisture. The biomass is transported to the plant by trucks using diesel fuel with an average transportation distance of 100 km.

Fig.1 Process diagram of biomass direct-fired power plant.

The power plant is comprised of two steam turbines each with a generating capacity of 12 MW. The steam produced by direct biomass combustion in the boiler is then used to generate electricity via steam turbines. The steam returns to the liquid state in the condenser by circulating cooling water around the condenser tubes. And then the condensed water is returned to the boiler to be heated again. The electricity produced is integrated into the local 110 kV grid after transforming the voltage (see Fig.1).

The designed annual operating time is 6000 h. However, in such a sparsely populated area, the harvest and delivery of the biomass resource is not always ensured as planned during actual operation. Because of the limited availability of Salix psammophila, the plant produced a total of 80 GWh of electricity in 2011, of which 72 GWh was integrated into the grid, significantly different from the designed annual electricity output of 168 GWh.

2.2 Emergy analysis

The emergy method is a ‘donor-side’ evaluation approach, and it looks at the system in a holistic way, by underlining the need for the system to adjust its behavior to the performance of the surrounding ecosystem, instead of analyzing local efficiency and local flows in an analytical and deterministic way [8]. As mentioned above, Odum and his colleagues give detailed explanations of the application of emergy accounting procedures for a variety of systems [8,11,14]. For the biomass direct-fired power generation system, the different inputs to the plant might be aggregated into four types: local renewable resources directly falling on the plant (R1), including sunlight, rain, wind; local renewable resources supplied by the local ecosystem and used by the plant for the production process (R2), such as oxygen and biomass used for the combustion in the boiler; local nonrenewable resources (N), i.e., ground water used by the production system; purchased inputs (P) from economic society that are used to construct, operate and maintain the power plant. Purchased inputs can be further divided into renewable purchased input (PR) and nonrenewable purchased input (PN). The yield of the process is the electricity output, for which the total emergy input to the system is required. Table 1 presents the definitions and descriptions of different resources and emergy indices that are used to indicate the various performances of the system.

Fig. 2 Diagram of the emergy flows in the biomass direct-fired power system. All emergy flows are 1016seJ/yr.

Table 1 Different inputs and emergy indices calculated for the biomass direct-fired power system

Since the designed operating lifetime of the plant is 11 years and 1 year is taken as the time scale for the present analysis, all up-front inputs into the plant including construction materials and equipment are scaled to a 1-year cycle. Based on the standard methods of emergy analysis, an emergy diagram of the biomass direct-fired power plant is shown in Fig. 2 and the detailed emergy flows of the actual production system in 2011 are shown in Table 2. Data for the plant was obtained from the design report and through local investigations, and it was processed by transforming mass quantities (kg) and energy quantities (joule) into emergy units by multiplying the raw amounts by appropriate UEVs. The global emergy baseline used in this study is 15.83E+24 seJ/yr [25]. The transformities used in this study include labor and services required to produce economic goods.

Table 2 Emergy evaluation table of the biomass direct-fired power generation system (on a yearly basis)

3 Results and Discussion

3.1 Emergy evaluation of the case plant

The results show that the total emergy use (U) of the whole production system in 2011 is 2.96 E+19 seJfor the generation of 2.59E+14 J electricity, with 51.45% of the emergy cost being renewable. Thus, the transformity of electricity produced by the plant is calculated to be 1.14 E+05 seJ/J, i.e., the plant needs 1.14 E+05 seJ of solar emergy to produce 1 J of electricity. The major input into the system is biomass (1.43 E+19 seJ/yr), accounting for 48.31% of the total emergy use. Among local renewable resources directly falling on the plant, the largest contributor to emergy inputs is rain and taken as total amount of R1in the evaluation table to avoid double accounting. As to the local nonrenewable resources, ground water (5.77 E+17 seJ/yr) used for the production process is the only input. Total emergy of purchased inputs amounts to 1.46 E+19 seJ/yr and takes up 49.32% of the total emergy use.

3.2 Emergy-based indices of the case study plant

Based on emergy accounting, indices such as transformity, emergy yield ratio (EYR), environmental loading ratio (ELR) and emergy sustainability index (ESI) can be calculated to evaluate the performance of the biomass direct-fired power plant system in 2011 [8,9,36]. To have a better insight into the relative performance of the production system, the calculated emergy indices are compared with other renewable energy power generation technologies including photovoltaic power, wind power, small hydropower and biomass combined heat and power (CHP) cogeneration processes [20,22,24,37] and oil-based thermoelectric plants [31]. The emergy indices for those power systems are listed in Table 3.

Transformity is an important indicator to measure the overall biological efficiency of natural production systems. When comparing two or more processes with the same output, a lower transformity can be seen as a measure of higher ecological efficiency because the same amount of emergy input can produce more products or the same amount products need less emergy input [8,18]. With regard to the case plant considered, the transformity for the biomass-electricity is 1.14 E+05 seJ/J, similar to that of other renewable energy power plants in the control group whose transformity varies from 1.74 E+04 to 1.03 E+05 seJ/J and much smaller than that of the oil fired plant (2.00 E+05 seJ/J), indicating that the studied system has a relatively high thermodynamic efficiency.

The EYR measures the ability of a process to exploit locally available resources by investing economic resources from outside. The higher the EYR, the less the use of external inputs for a given output [9,25]. In this study, the EYR of the biomass power production system is 2.03 which falls in the middle (1.13-4.21) of the electricity production systems examined, suggesting that the biomass direct-fired power plant is relatively efficient in converting local resources to electricity and providing net benefits to society.

With regard to ELR, the value of the production system under study is 0.94, much smaller than that of oil plant (14.2). The ELR is a measure of the pressure on the environment due to the production system. The higher the value, the higher the stress on the environment [18,38]. The comparison of ELR values indicates that electricity generated using biomass direct-fired, solar (0.39), wind (4.00), small hydropower (0.92) and biomass CHP plant (0.62) have the lower environmental impacts, while oil fired plants higher, as might be expected.

The ESI is a very important integrated indicator of the sustainability of the system [18,36,39]. Generally, if the value of ESI is less than 1, the production system is a resource-depleting unsustainable system with high environmental stress; if it is greater than 10, the system would be termed undeveloped; when it falls between 1 and 10, the system has good economic vitality as well as good sustainability [9]. In our study, the ESI of the biomass production system is 2.15, while the ESI of small hydropower and biomass CHP plants are 4.77 and 4.27, respectively. These three plants show a better ecological performance and sustainability than wind power and the oil plant.

Table 3 Comparison of emergy indices between the biomass-fired power plant and several other energy power plants

3.3 Extended discussion

From the accounting results and comparisons of emergy-based indicators with other electricity generation systems, it can be easily concluded that the biomass direct-fired power plant presented a relatively outstanding performance and good sustainability, although the plant did not operate as planned because of the lack of raw material. If the supply of raw material is sufficient, the performance of the production system would be better. It reveals that the biomass direct-fired power generation technology in China itself is well established now. However after several years of development, the biomass direct-fired power generation industry in China is still in an initial stage of development, which is significantly different from wind power and solar power that have experienced an astonishing growth in a short time [23]. Most existing biomass direct-fired power plants are still demonstration projects and large scale development has not yet begun. The problem might lie in the availability of raw materials, which has been mentioned in the literature and many reports [4,7]. The dispersed distribution and high collection cost of biomass resources cause high operation costs for the power plants. In the case of the plant considered in this paper, although the Salix psammophila is widely distributed in the Mu Us Sandland, it is difficult to collect it and transport it to the plant in this vast desert area which would require a huge amount of labor and transportation fuels. To make this situation worse, there is a competition between the plant and other factories that use the same raw material.

According to our investigation of the biomass direct-fired power plants, raw material is a key constraint to the operation of the plants. To biomass direct-fired power plants, an optimal radius for collecting raw materials exists. However, many plants have to look to quite remote areas for sufficient raw materials, which consequentially increases the operation cost of the plants. Further research is needed to quantify the economic and energetic impacts of the radius of biomass collection on the environmental performance of the biomass direct-fired plants of the region.

4 Conclusions

This paper presented a comprehensive assessment of the environmental sustainability of biomass directfired power generation systems based on an emergy analysis of a plant in Inner Mongolia, P.R. China, as well as providing comparative analysis with several other power generation technologies including solar, wind, small hydro, biomass CHP and oil-fired power plants. The evaluation results show that in 2011 the biomass direct-fired power system is supported by a total emergy of 2.96E+19seJ with 51.45% of the emergy inputs being renewable. Biomass is the largest emergy inflow (1.43E+19seJ/yr), accounting for 48.31% of total inputs. The comparison of emergy-based indicators suggests that the overall biomass direct-fired power plant has a relatively modest impact on the environment and good sustainability as indicated by ELR and ESI respectively.

The evaluation results of emergy analysis confirm good performance of the overall biomass direct-fired power production system, indicating that this technology is environmentally sound. The real problem for the success of this technology might lie in the feedstock supply, due to the large collection radius and high trade cost derived from bargaining with diversified local farmers, which deserves our further research.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant No. 41371521) and National Science Foundation for Innovative Research Group (Grant No. 51121003).

Appendix

1. Sunlight (The plant’s design report)

2. Rain (The plant’s design report)

3. Wind (The plant’s design report)

4. Biomass (Local investigation)

5. Oxygen (Reaction stoichiometry)

6. Ground water (Local investigation)

7. Building, concrete (The plant’s design report)

8. Building, brick (The plant’s design report)

9. Building, steel (The plant’s design report)

10. Pipes (steel) (The plant’s design report)

11. Electronic scale (steel) (The plant’s design report)

12. Forklift (steel) (The plant’s design report)

13. Boiler (steel) (The plant’s design report)

14. Stream turbine (steel) (The plant’s design report)

15. Electronic generator (steel) (The plant’s design report)

16. Chimney (concrete) (The plant’s design report)

17. Cooling tower (The plant’s design report) Concrete

18. Transformer (The plant’s design report)

19. Bag filter (The plant’s design report)

20. Building (The plant’s design report)

21. Construction services (The plant’s design report)

22. Diesel for transportation (Local investigation)

22. Labor

Labor for transportation (Local investigation)

Labor for plant operation (Local investigation)

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21 September 2013

?Corresponding author.

Email address: zhanglixiao@bnu.edu.cn (L.X. Zhang).

ISSN 2325-6192, eISSN 2325-6206/$- see front materials ? 2013 L&H Scientific Publishing, LLC. All rights reserved.

10.5890/JEAM.2013.11.002

Accepted 1 November 2013

Available online 1 January 2014