There has been a rapid growth in using agricultural residues as an energy source to generate electricity in China. Biomass power generation (BPG) systems may vary significantly in technology, scale, and feedstock and consequently in their performances. A comparative evaluation of five typical BPG systems has been conducted in this study through a hybrid life cycle inventory (LCI) approach. Results show that requirements of fossil energy savings, and greenhouse gas (GHG) emission reductions, as well as emission reductions of SO2 and NO
With the rapid development of economy, electricity demand continues to grow in China. Significant attention has been focused on the high dependence of electricity generation on coal, including greenhouse gases (GHG) emissions and environmental pollution. Alternate approaches are being sought. Biomass is the only renewable fuel available for combustion-based electricity generation. Moreover, straw is no longer the primary fuel for cooking and space heating in many rural communities. A large amount of straw is abandoned or incinerated in the fields, resulting in environmental pollution and a waste of resources. For these reasons, biomass power generation (BPG) has gained significant attention in China. By the end of 2010, the overall installed capacity of biomass power had reached 5.5 GW. In addition, China has set the goals in State Plans for Medium and Long-Term Development of Renewable Energy to achieve 30 GW of biomass power capacity by 2020 [
There are currently three kinds of biomass power plants in China: biomass-only fired power plant, biomass-coal co-firing power plant, and biomass gasification power plant. These plants may vary significantly in technology, scale, and feedstock and consequently in their performances. In the short term, large-scale electricity generation based on biomass-only fired or cofiring is the most promising alternative to achieve expected biomass electricity market contribution and to fulfill GHG emissions targets, mainly due to better technological reliability and maturity [
Several studies have been undertaken using life cycle assessment (LCA) to analyze benefits and drawbacks of BPG systems in China because LCA considers all the processes involved in each alternative in a cradle-to-grave manner [
LCI analysis is one of the four phases of LCA involving the compilation and quantification of inputs and outputs. The two basic methods for compiling an LCI are the process-based analysis and the input-output (IO) analysis. Most LCIs have been conducted based on a process-based analysis where the physical quantities of energy and material use and environmental releases from the main production processes are assessed in detail, but nevertheless the process-based analysis suffers from a systematic truncation error due to the delineation of product system by a finite boundary and the omission of contributions outside the boundary [
The hybrid LCI method seeks to use advantages of both methods while mitigating their respective limitations. One of the practical examples has been presented by Inaba and his coworkers [
In a consequential LCA, the differences in environmental impact stemming from changes made to a reference system are quantified. Fossil fuel system substituted or most likely to be substituted by biomass energy system is usually chosen as the reference system. Compared with the reference system, biomass power plants’ performances may vary depending on other factors such as the normal routes of biomass disposal and local energy consumption structure. For the sake of comparison, some simplifications have been made in this study. Credits are not taken for the avoided operations of normal routes of biomass disposal such as field burning. Coal-fired plant before retrofitting for cofiring is chosen to be the reference system of biomass cofiring systems. It was assumed that the specific portion of coal cofired with biomass has the same electric efficiency and emissions as those before cofiring. The differences of electric output and emissions between cofiring and the specific portion of coal cofired with biomass were allocated to biomass. Biomass is assumed to substitute part of the coal without changing the performance of the rest part of coal in power generation. A simple and useful comparison can be carried out this way. For the biomass-only system and biomass gasification system, the reference system is the electricity sector in IO table (see Table
Two main objectives were pursued in this study: firstly, to determine the reduction of GHG emissions, the primary energy (PE) savings that could actually be attained when biomass power is compared to conventional electricity production, and secondly, to evaluate which BPG system is more beneficial on the same basis. We have established that the LCI is done from the production process of the biomass to the electric output suitable for consumption of power plants.
A consistent scope of system boundaries which mainly includes three life cycle stages is adopted to facilitate comparison between technologies, as shown in Figure
System boundaries of BPG system.
The previously described inventory data were summarized into an input-output format, and the energy and material flow between processes in the system was calculated. A make-use input-output framework was used to illustrate the BPG systems, as shown in Table
Coefficient matrices for the hybrid LCI analysis.
BPG systems | IO sector |
Functional |
Total | ||||
---|---|---|---|---|---|---|---|
Process commodity |
Process activity |
Energy sector |
Nonenergy sector | ||||
BPG systems | Process commodity |
|
|
| |||
Process activity |
|
|
|
| |||
|
|||||||
IO sector | Energy sector |
|
|
|
| ||
Nonenergy sector |
|
|
|
|
Energy and materials in the system are defined as a process commodity sector
Energy and material inputs from IO sector are described as matrixes
Output of commodities from a process activity sector in the system (
A production equilibrium model for hybrid LCI analysis was established using a matrix representing the input-output relationship of materials and energy among the processes and sectors described above. The accounting framework is shown in Table
Accounting framework for BPG systems.
BPG systems | IO sector | Functional unit | Total output | ||||
---|---|---|---|---|---|---|---|
Process commodity |
Process activity |
Energy sector |
Nonenergy sector | ||||
BPG systems | Process commodity |
|
|
|
|
|
|
Process activity |
|
| |||||
|
|||||||
IO sector | Energy sector |
|
|
|
| ||
Nonenergy sector |
|
|
|
|
Classifications of IO sectors in hybrid LCI model.
Sector code | Sectors |
---|---|
1-1 | Raw coal |
1-2 | Cleaned coal |
1-3 | Other washed coal |
1-4 | Coke |
1-5 | Coke oven gas |
1-6 | Other gases |
1-7 | Other coking products |
1-8 | Crude oil |
1-9 | Gasoline |
1-10 | Kerosene |
1-11 | Diesel oil |
1-12 | Fuel oil |
1-13 | Liquefied petroleum gas |
1-14 | Refinery gas |
1-15 | Other petroleum products |
1-16 | Natural gas |
1-17 | Electricity |
1-18 | Heat |
2 | Farming, forestry, animal husbandry, fishery, and water conservancy (agriculture) |
3 | Ferrous and nonferrous metals mining and dressing |
4 | Nonmetal and other minerals mining and dressing |
5 | Food processing, food production, beverage production, and tobacco processing |
6 | Textile |
7 | Garments and other fiber products, leather, furs, and down and related products |
8 | Timber processing, bamboo, cane, palm and straw products, and furniture manufacturing |
9 | Papermaking and paper products, printing and record medium reproduction, and cultural, educational, and sports articles |
10 | Raw chemical materials and chemical products, medical and pharmaceutical products, and chemical fiber, rubber, and plastic products |
11 | Nonmetal mineral products |
12 | Smelting and pressing of ferrous and nonferrous metals |
13 | Metal products |
14 | Ordinary machinery, equipment for special purpose |
15 | Transport equipment |
16 | Electric equipment and machinery |
17 | Manufacture of communication equipment, computers, and other electronic equipment |
18 | Instruments, meters, and cultural and office machinery |
19 | Artwork and other manufacturing |
20 | Recycling and disposal of waste |
21 | Water production and supply |
22 | Construction |
23 | Transport, storage, postal, and telecommunications services |
24 | Wholesale, retail trade, hotels, and catering service |
25 | Other service activities |
The matrices
The matrixes
China’s 2007 monetary input-output table [
The energy consumptions of the 5 energy-related sectors in the 2008 China Energy Statistical Yearbook [
Allocation of energy consumptions for energy sectors.
Energy-related sectors in IO table and energy statistical yearbook | Energy sectors in hybrid LCI model | Allocation method of energy consumptions |
---|---|---|
Coal mining and dressing | 1-1 Raw coal |
Ratio of energy consumptions of raw coal (1-1) and washed coal (1-2, 1-3) was assumed to be 25 : 9, based on Grade 3 of clean production standard of coal mining and processing industry [ |
|
||
Petroleum and natural gas extraction | 1-8 Crude oil |
Crude oil and refinery gas are consumed in Crude oil extraction. Natural gas is consumed in natural gas extraction |
|
||
Petroleum processing, coking, and processing of nuclear fuel | 1-4 Coke |
Coking products are consumed in coking. Crude oil and refinery gas are consumed in processing of petroleum. Ratios of refining efficiency of gasoline, kerosene, diesel, liquefied petroleum gas, and fuel oil are assumed to be 85% : 87% : 89% : 93.5% : 95% [ |
|
||
Electric power and steam production and supply | 1-17 Electricity |
The equivalent value of electricity to heat is assumed to be 2.78 [ |
|
||
Gas production and supply | 1-5 Coke oven gas |
Some of the energy is used as feedstock into different industrial processes. The 2008 China Energy Statistical Yearbook gives the total nonenergy use in the industrial sectors. It was assumed that all the nonenergy use is in the chemical sectors [
GHG emissions particularly CO2, CH4, and N2O expressed as CO2 equivalent (CO2-eq) have been assessed in this study. In addition, SO2 and NO
Chinese specific values for the carbon emission factor of each fuel and the fraction of carbon oxidized for each fuel in each sector from Peters et al. [
GHG emissions of nonenergy use from industrial processes.
Sector code | Sector category | Industrial processes | GHG emissions |
---|---|---|---|
10 | Raw chemical materials and chemical products, medical and pharmaceutical products, and chemical fiber, rubber, and plastic products | Manufacturing of ammonia, soda ash, and calcium carbide | 105.78 Mt CO2 |
|
|||
11 | Nonmetal mineral products | Manufacturing of cement and plain grass | 683.93 Mt CO2 |
|
|||
12 | Smelting and pressing of ferrous and nonferrous metals | Smelting and pressing of ferrochromium, silicon metal and ferro-unclassified, and |
873.59 Mt CO2 |
|
|||
2 | Farming, forestry, animal husbandry, fishery, and water conservancy (agriculture) | Enteric fermentation, manure management, rice cultivation, and field burning of agricultural residues | 18.44 Mt CH4 |
|
|||
1-1 | Raw coal | Coal mining | 19409.97 kt CH4 |
|
|||
1-8, 1-16 | Crude oil, natural gas | Oil and natural gas systems | 258.31 kt CH4 |
|
|||
2 | Farming, forestry, animal husbandry, fishery, and water conservancy (agriculture) | Manure management, cropland, and field burning of agricultural residues | 614.97 kt N2O |
|
|||
10 | Raw chemical materials and chemical products, medical and pharmaceutical products, and chemical fiber, rubber, and plastic products | Nitric acid, adipic acid | 74.55 kt N2O |
Based on the country specific values of sectoral NO
Category | Industrial processes | Quantity (Mt) |
|
|
---|---|---|---|---|
10 raw chemical materials and chemical products, medical and pharmaceutical products, and chemical fiber, rubber, and plastic products | Nitric acid | 2.009 | 0.012 | 0.0241 |
Adipic acid | 0.215 | 0.0081 | 0.0017 | |
|
||||
12 smelting and pressing of ferrous and nonferrous metals | Iron | 494.889 | 0.000076 | 0.3761 |
Ferrochromium-silicon | 0.043 | 0.0117 | 0.0005 | |
Silicon metal | 0.81 | 0.0117 | 0.0095 | |
Aluminum | 9.358 | 0.00215 | 0.0201 | |
|
||||
13 metal products | Steel rolling | 60.927 | 0.00004 | 0.2437 |
Biomass power plants’ main features vary depending on several factors: amount of available resources and their properties, pretreatments required, and generation technology employed. For the sake of comparison, some representative average characteristics in biomass production and supply had to be selected in this case.
Corn stover is considered as feedstock for biomass power plants in this study. Life cycle data for the production, collection, and transportation of the feedstock include the energy and emissions associated with fertilizers, herbicides, and fuel to operate harvesting equipment. Data for the agricultural phase for corn originate mainly from national statistics [
Inputs and allocation in agricultural phrase.
Inputs | Plantation inputs (yuan/mu) | Assigned input of corn stover (yuan/GJ) |
---|---|---|
Seed | 26.92 | 0.308 |
Chemical fertilizersa | 88.43 | 1.013 |
Farmyard manure | 8.66 | 0.099 |
Pesticide | 7.96 | 0.091 |
Agricultural film | 2.62 | 0.030 |
Field machinery, irrigation, and animal power | 55.64 | 0.637 |
Fuels | 0.03 | 0.393 |
Technical service | 0.03 | 0.143 |
Tools and materials | 2.1 | 0.061 |
Maintenance | 1.27 | 0.101 |
Others | 0.12 | 0.000 |
Note: athe amount of N-fertilizer applied in physical unit is 10.27 kg N/mu [
Energy allocation was rejected as grain has been considered an alimentary product and not a fuel. It has been considered that agricultural residues resources would not be collected without an energy demand and no economic value would be obtained. For this reason, partitioning on an economic basis using the share in revenues (grain and straw) was the method finally chosen.
The agricultural production is mainly carried out based on households in China, which results in a small average planted area and thus scattered straw resources. Thus the feedstock supply has become a bottleneck for large-scale use. There are mainly two patterns for biomass supply in China, which can be referred to as centralized pattern and distributed pattern. The centralized pattern involves a centralized storage site by the plant which can receive straw and ensure the plant’s operation. Straw is mainly collected by farmers manually and then delivered to storage site by tractors. The distributed pattern involves a bunch of straw-receiving stations, which also serve as intermediate storage sites where biomass is baled and stacked [
Major parameters for biomass feedstock supply.
Items | Value |
---|---|
Straw/grain ratio | 0.75 |
Corn production (kg/mu) | 422.4 |
Lower heating value (LHV) of corn stover (MJ/kg, dry basis) | 15.6 |
Moisture content of corn stover (wt%) | 10 |
Sulfur content of corn stovera (wt%) | 0.21 |
Distribution density of biomassb (t/km) | 103.3 |
Average transport distance from straw-receiving station to the plantc (km) | 30 |
CO2 emission factor of dieseld (g/GJ) | 74100 |
SO2 emission factor of diesele (g/GJ) | 93.78 |
NO |
643.19 |
Note: aan average value of sulfur content of corn stover from Cuiping et al.
bThe transport distance for centralized pattern was calculated using a farmland coverage rate of 0.7 and a availability factor of 0.4 in the model [
cThe average transport distance is used for the distributed pattern in the cases of 25 MW biomass-only fired and 140 MW cofiring.
dCO2, CH4, and N2O emission factors for diesel utilization were adopted from IPCC road transport default values, the latter two of which are 3.9 g/GJ [
eThe Chinese specific value for sulfur content of diesel was taken from Song [
fThe Chinese specific value of
The combustion characteristics of biomass are well-understood and already wildly used in biomass applications worldwide. However, the development of biomass-only fired technologies starts fairly late in China. Advanced oversea technology and equipment have been employed in most cases. In recent years, some domestically developed boilers have met the basic operating requirements and have been put into operation but the performance still needs to be confirmed. A 25 MW biomass power plant in Anhui province was taken as an example in this study, which mainly consists of a 130 t/h high-temperature and high-pressure steam boiler with vibrating grate and a condensing steam turbine generator unit.
On the other hand, the R&D activity of BPG technology started since the 1960s, characterized by rice hull gasification and power generation system with sizes from 60 to 200 kW. A number of demonstration plants have been erected over the past few decades and some of them have been in operation for several thousands of hours. A demonstration project of 1 MW circulating fluidized bed biomass power plant which was established by the Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences in Putian, Fujian Province, in 1998, was the first project of a MW-scale biomass power plant in China. Many improvements had been made in the 1 MW system as compared to the former 200 kW system. However, the overall efficiency of 1 MW system is still less than 20% [
Compared to biomass-only fired plants, cofiring offers two important advantages. Firstly, cofiring can take advantage of the higher efficiency of large-scale coal-fired power plants, even though boiler efficiency may decrease. Secondly, investment costs required to achieve bioelectricity production might be greatly reduced. However, there is still no subsidiary policy to support biomass cofiring in China. Only a few demonstration cases exist, like the Shiliquan Power Plant and several others. In addition, biomass price keeps increasing rapidly, leading to enormous fuel costs, and therefore the scheme has not been adopted in other power plants in China. The Shiliquan Power Plant is the first cofiring plant in China. Essential facilities were commissioned in a 140 MW generator set and crushed biomass is pneumatically conveyed into two cyclone burners in the boiler. The Shiliquan Power Plant and a 25 MW cofiring plant were taken as an example in this study. Data of cofiring plants was mainly collected from a report [
The major parameters of biomass power plants are shown in Table
Major parameters for biomass power plant.
Items | 25 MW biomass-only |
140 MW |
25 MW |
1 MW |
5.5 MW |
---|---|---|---|---|---|
Electric efficiencya | 25.6% | 35.4% | 27.6% | 18.0% | 27.0% |
Electric efficiency before cofiring | 36.1% | 28.0% | |||
Cofiring ratio by energy | 20% | 15% | |||
Auxiliary power consumption rateb | 8% | 10% | 8% | 10% | 10% |
Annual operating hours (h) | 6500 | 7000 | 7000 | 6000 | 6000 |
Annual power supplyc (GJ) | 538200 | 571536 | 78246 | 19440 | 106920 |
Life expectancy (year) | 15 | 10 | 10 | 15 | 15 |
Capital investment (104 yuan)d | 24145 | 8413 | 1155 | 428 | 3350 |
Annual cost (104 yuan)e | 3255 | 1148 | 230 | 92 | 572 |
CH4 emissionf (kg/GJ biomass) | 0.0037 | 0.0037 | 0.0037 | 0.0037 | 0.0037 |
N2O emissionf (kg/GJ biomass) | 0.0105 | 0.0105 | 0.0105 | 0.0105 | 0.0105 |
SO2 emissiong (kg/GJ biomass) | 0.2393 | 0.0299 | 0.0299 | 0.0008 | 0.0008 |
NO |
0.0590 | 0.3291 | 0.3300 | 0.1733 | 0.1733 |
aUtility boiler efficiency decrease: 1% for each 10% of coal replaced by biomass (on an energy basis) [
bEnergy consumption of biomass crushing is included.
cThe energy inputs of the boiler in cofiring plant remain the same as before. The biomass-related power output was listed as the annual power supply of cofiring plant.
dInvestment on feedstock supply system is not included.
eThe annual cost consists of depreciation cost, maintenance, materials, and personnel. The cost of feedstock supply is not included.
fSince all the carbon in the biomass is recycled, it has been assumed that biomass fuel combustion does not produce GHG emissions due to CO2. The CH4 and N2O emission factors of biomass-only fired plant and biomass gasification system were taken from Wang [
gFor biomass gasification power plant, the SO2 emission may vary significantly from one to another. The data of the 5.5 MW gasification system adopted here is converted from Jia [
hThe NO
Inputs of BPG system from background economy consist of fuels, machinery, and other essential facilities. Corresponding IO sectors in the LCI model mainly include diesel, electricity, transport equipment, and ordinary machinery, equipment for special purpose, as indicated in Table All inputs with clear category were classified according to the standards of classification of national economic industries [ Inputs without clear category were classified into the most related sectors based on evaluations of engineers. Maintenance inputs were classified into sectors of corresponding equipment and services. Purchase cost of equipment consists of prime cost and freight and miscellaneous charges. The freight and miscellaneous charges were classified into sector “transport, storage, postal, and telecommunications services.”
Major expenditures and corresponding IO sectors.
Inputs | Sector category |
---|---|
Diesel | 1-11 diesel |
Electricity | 1-17 electricity |
Boiler, steam turbine, internal gas engine, handling equipment, air blower, drying equipment, and auxiliary equipment | 14 ordinary machinery, equipment for special purpose |
Transport vehicles | 15 transport equipment |
Generator, electricity transmission, and distribution equipment | 16 electric equipment and machinery |
Construction engineering, wiring, piping, and installation of electric equipment | 22 construction |
Transportation of equipment and materialsa | 23 transport, storage, postal, and telecommunications services |
Technical service, insurance | 25 other service activities |
aThe freight and miscellaneous charges of boiler, internal engine, steam turbine, and generator set were evaluated to be 0.6% of the purchase cost. The freight and miscellaneous charges of other equipment and materials were evaluated to be 7%.
PE consumption represents the sum of direct and indirect consumptions of fossil fuel energy associated with unit output of electricity from biomass. The PE consumption of BPG systems can also be defined as the fossil fuel energy consumed within the system per electric energy delivered to the utility grid. The results expressed in GJ/GJ are shown in Figure
Comparison of PE consumptions of BPG systems.
The 25 MW cofiring system exhibits a lower PE consumption, followed by the 140 MW cofiring system, the 1 MW gasification system, the 5.5 MW gasification system, and the 25 MW biomass-only fired in an ascending sequence: 0.11 < 0.15 < 0.17 < 0.19 < 0.28 GJ/GJ, respectively. Without the agricultural inputs, BPG systems appear to be of the same sequence of PE consumption, ranging from 0.09 GJ/GJ to 0.25 GJ/GJ. The major reason may be that cofiring systems avoid inputs of plant construction that are found in intensively invested system such as biomass-only fired system. Another reason is that depreciation of original coal power plant property has not been allocated to biomass power. The agricultural inputs play a noticeable role, especially in the 1 MW gasification system. PE consumption in Figure
The PE consumption of feedstock supply accounts for a significant portion in the case of the 25 MW biomass-only fired system and the 140 MW cofiring system (see Figure
In addition to PE consumption, two other measures for assessing energy use can be defined:
The energy savings represent the amount of PE saved when unit electric energy from biomass is delivered to the utility grid. The cost of energy savings measures the amount of investment for every unit of PE saved by the BPG system. These two indicators may provide a better means of assessing the BPG systems. Comparing the results presented in Table
Energy saving performance of BPG systems.
Items | 25 MW biomass-only fired | 140 MW cofiring | 25 MW cofiring | 1 MW gasification | 5.5 MW gasification |
---|---|---|---|---|---|
Energy savingsa (GJ/GJ) | 2.57 | 3.44 | 4.42 | 2.68 | 2.66 |
Cost of energy saving (yuan/GJ) | 29.5 | 10.5 | 8.3 | 21.8 | 22.3 |
Cost of energy saving (GJ biomass/GJ) | 1.66 | 0.99 | 0.98 | 2.30 | 1.55 |
aIn the case of cofiring systems, coal transportation in reference system was not taken into consideration.
The GHG emission intensity of BPG systems is defined as the GHG emission by the system per electric energy from biomass delivered to the utility grid, which includes the direct and indirect GHG emissions. By the commonly referred IPCC global warming potentials (CO2 : CH4 : N2O = 1 : 21 : 310), the GHG emissions of each BPG system assessed are shown in Figure
Comparison of GHG emission intensities of BPG systems.
For all systems, the majority of GHG emission comes from the life stage of power generation. N2O emission, specifically, from combustion of biomass fuels has significant contributions. In the cofiring cases, the results might differ widely from that obtained by the other researchers. According to Sebastián et al., biomass pretreatments account for more than 50% of biomass-related GHG emissions [
Similar to the energy saving indicators, two other measures for assessing GHG emission reduction can be defined:
The GHG emission reductions represent the amount of GHG emission avoided per electric energy from biomass delivered to the utility grid. The cost of GHG emission reductions measures the amount of investment for every unit of GHG emission avoided by the BPG system. The cofiring plant performs better than the biomass-only fired plant and the biomass gasification plants in GHG emission reductions, as indicated in Table
GHG emission reduction of BPG systems.
Items | 25 MW biomass-only fired | 140 MW cofiring | 25 MW cofiring | 1 MW gasification | 5.5 MW gasification |
---|---|---|---|---|---|
GHG emission reductions |
220 | 404 | 517 | 224 | 229 |
Cost of GHG emission reductions |
0.34 | 0.09 | 0.07 | 0.26 | 0.26 |
Cost of GHG emission reductions |
0.019 | 0.008 | 0.008 | 0.028 | 0.018 |
The SO2 (or NO
Comparison of SO2 emission intensities of BPG systems.
Comparison of NO
The sulfur content of biomass is much lower than that of coal. On the other hand, desulphurization involves a large amount of investment. For these two reasons, desulphurization device is not usually equipped in the biomass-only fired system in China. Desulphurization device of the coal-fired power plant can effectively remove the SO2 for the cofiring system, just as the water scrubber does for the biomass gasification systems. As expected, the worst SO2 emission performance is that of the 25 MW biomass-only system.
Comparing the SO2 emission intensities of 1.79 kg SO2/GJ of the electricity sector obtained by the hybrid LCI model, BPG systems can anyhow provide a significant reduction in SO2 emissions, due to the very low sulfur content of biomass. Despite the relatively low SO2 emissions intensities, the cofiring systems exhibit a small capacity for reducing SO2 emissions (Table
SO2 and
Items | 25 MW biomass-only fired | 140 MW cofiring | 25 MW co-firing | 1 MW gasification | 5.5 MW gasification |
---|---|---|---|---|---|
SO2 emission reductions (kg SO2/GJ) | 0.59 | 0.46 | 0.62 | 1.67 | 1.64 |
|
0.80 | 0.21 | 0.30 | 0.04 | 0.39 |
NO
Methodological constraints of process-based life-cycle analysis, particularly a problem associated system boundary selection, may lead to some uncertainties in the LCI results. A hybrid LCI framework can ensure the completeness of system boundary and provide a desirable method for quantifying a system’s environmental footprint.
There are currently a number of biomass power plants in China. The government has not offered any guidance on preferred type, leaving the market open. A comparative study is necessarily important. In this paper, a hybrid LCI model is used to comparatively evaluate five BPG systems, which may represent the present state-of-the-art in China. A preliminary feasibility estimation of the biomass power in China is provided in terms of primary energy (fossil energy) savings, GHG emission reductions, and avoided emissions of SO2 and NO
To get 1 GJ electricity from corn stovers, only 0.11–0.28 GJ of primary energy (PE) is consumed by BPG systems, whereas primary energy as much as 4.42 GJ can be saved by substituting conventional electricity. At the same time, the BPG systems only contribute 26–44 kg CO2 eq of GHG emissions, while up to 517 kg CO2 eq of GHG missions can be avoided. The cofiring systems, especially the 1 MW cofiring system, can achieve the highest PE savings and GHG emission reductions per electric energy from biomass delivered to the utility grid. Moreover, the PE savings and GHG emission reductions are accomplished at a lower cost of biomass resource and monetary investment. Thus the cofiring systems give better behavior than the biomass-only fired system and the biomass gasification systems. For all systems, the life stage of power generation is responsible for the largest share of PE consumptions and GHG emissions. N2O emission from combustion of biomass fuels has made a significant contribution to GHG emission. Another important aspect that should be addressed is the significant contributions of infrastructure, equipment, and maintenance of the plant, which may be easily ignored in a process-based LCI. Inputs of various types of fossil fuels, materials, and services are required in construction and operation of a biomass plant. And the consequent PE consumptions and GHG emissions should be taken into consideration.
The emission intensities of SO2 and NO
The innovative base of comparison between BPG systems has allowed assessment and comparison of the five electricity production system alternatives with agricultural residues. From the case presented, it is shown that BPG systems could be a high-potential alternative for electricity generation. In addition to the environmental benefits quantified in this LCI, BGP systems will provide other benefits as they are deployed in China, such as rural economic development through the creation of new markets and jobs. A specific LCI study should take into consideration local conditions such as the normal routes of biomass disposal and energy consumption structure, which may have significant effects on results. Moreover, it has to be noted that, by expanding the scope of analysis in hybrid LCA, the level of precision is lost due to the use of highly coarse and aggregated data in input-output table that involved significant amounts of uncertainties and assumptions. The choice of process parameters and allocation procedures can have significant effects on results as well. Although much work is currently being undertaken to determine several values used in this analysis in a more precise way, the main conclusion that can be highlighted is that, based on the values and assumptions used, the cofiring system is more beneficial than biomass-only fired power plant and biomass gasification system, when the PE savings and GHG emission reductions are taken into account.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study has been supported by the National Key Technology R&D Program in the 12th Five-Year Plan of China (Grant no. 2012BAA09B03), the National Natural Science Foundation (Grant no. 51176194), and in part by the Strategic Emerging Industries of Guangdong (Grant no. 2012A032300019).