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Article

Carbon Footprint Analysis for Biomass-Fueled Combined Heat and Power Station: A Case Study

by
Yingying Zheng
1,
Chang Liu
1,
Jie Zhu
1,
Yuanrui Sang
2,
Jinglong Wang
1,
Wenjing Zhao
1 and
Minghao Zhuang
3,*
1
College of Information and Electrical Engineering, China Agricultural University, Beijing 100081, China
2
Department of Electrical and Computer Engineering, The University of Texas at El Paso, El Paso, TX 79925, USA
3
College of Resources and Environmental Sciences, China Agricultural University, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Agriculture 2022, 12(8), 1146; https://doi.org/10.3390/agriculture12081146
Submission received: 21 June 2022 / Revised: 21 July 2022 / Accepted: 30 July 2022 / Published: 3 August 2022
(This article belongs to the Section Ecosystem, Environment and Climate Change in Agriculture)
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Abstract

:
Biomass could substitute fossil fuels in heat- and power-generation projects to reduce air pollution and greenhouse gas from many stages of the life cycle. The Nordjylland Power Station, one of Denmark’s largest power plants, is a 100% coal-fired combined heat and power plant. To reduce carbon dioxide emissions, this power plant is converting to be 100% biomass-fueled. However, biomass cannot be assumed as an emission-free energy source, even though it has certain advantages in terms of carbon sink capability. The environmental impacts among various biomass sources are unclear. Wheat straw and wood pellets are two of the most abundant biomass resources in Denmark. In this study, by conducting a screening life-cycle assessment, the expected savings of global warming potential emissions due to the substitution of coal with wheat straw or wood pellets are quantified. The life-cycle assessment’s results indicate that for producing 1 MJ of heat, the carbon dioxide equivalent from coal, wheat straw, and wood pellets are 117.01, 22.73, and 78.19 g, respectively. The combustion stage accounts for most of the carbon dioxide emissions. The recommendation is that wheat straw is preferred over wood pellets in terms of carbon emissions based on the current assumptions.

1. Introduction

There is a growing trend in the utility industry to convert existing coal-fired power plants to burn other fuel types, such as agricultural residues or natural gas, although whether such conversions are environmentally beneficial remains controversial. The heavy use of wood-based fuel in power plants is criticized for the rapid deforestation and loss of biodiversity [1]. Moreover, compared with coal ash, biomass ash is more prone to forming slagging and fouling deposits within the combustor, and the chemicals required for manual cleaning may lead to water pollution [2]. Therefore, the trend of converting the existing coal-fired power plants to biomass-fueled ones is driven by several factors, including state-level renewable portfolio standards, federal incentives and looming environmental regulations, consumer demand and environmental awareness, and the economic climate that is making coal less attractive.
Biomass resources are characterized as green, low-carbon, clean, and renewable. Typical biomass fuels for power generation include agricultural residues such as rice straw, wheat straw, corn stover, nut shells, and rice husks; wood-based fuels such as wood pellets, wood chips, sawdust bark, tree trimmings, and cardboard; sludges from paper mills and municipal sources; and energy crops specially grown for use as biofuels such as willow, switchgrass, eucalyptus, and poplar trees [3], as shown in Figure 1. The use of agricultural wastes for power generation varies from country to country. Demark utilizes the greatest proportion of agricultural wastes for power generation at 16.8%, followed by Finland (15.6%), Brazil (8.4%), and Germany (8.8%) [4].
The Danish National Allocation Plan states that the country aims to be fossil-fuel-free and 100% carbon-neutral by 2050. As an incremental step, the use of coal in electricity and heat production is going to be phased out in Denmark by 2030. The transition of the Danish energy system to 100% renewable carries many challenges. To become independent of fossil energy sources, offshore and onshore wind farms are expected to provide the majority of electricity, and agricultural residues are expected to serve as the main source for heating in Denmark [5].
Combined heat and power (CHP) is the simultaneous production of electricity and thermal energy with a total system efficiency of 80% or higher. Biomass is generally more flexible and dispatchable as an energy option than wind, solar, and hydro energy. Therefore, agricultural residue-fueled CHP power plants have been a common source of electricity and heating supply around the world [6].
The major biomass for power and heat generation utilized in Denmark include wood pellets, wood chips, and wheat straw (Table 1). The wood pellets in Denmark are heavily dependent on imports, accounting for over 90% of the annual demand, while straw is 100% domestically produced [7]. Denmark has an annual cultivation of straw-producing crops of about 16,000 km2 and annual use of straw for the energy of about 17.5 PJ [8].
Developing a sustainable biomass supply chain depends on many factors, including biomass composition, harvesting and pretreatment techniques, transportation methods and distances, energy conversion technology, and waste recycling and management. It is important to compare the economic and environmental performance of different agricultural residues to determine the optimal resource mix for each plant.
Life-cycle assessments (LCA) are a common technique for assessing the potential environmental impacts linked with any given industrial activity from the initial collecting of raw materials from the farmland until the point at which all residuals are sent back to the field [10]. To evaluate the tradeoffs for biomass use in energy and power production, LCA approaches have been widely adopted [11]. The LCA of biomass-fueled CHP systems involves a variety of biomass types, including energy crops [12], forest residues [13], and agricultural residues [14]. The agricultural residues include rice straw, wheat straw, rice husk, and corn stover, which are mostly left on the fields after harvests and used for fodder and landfill material or burnt in many places. Forestry residues consist of branches, leaves, bark, and other portions of wood [15]. The global warming potential (GWP), acidification potential, eutrophication potential, photochemical oxidation potential, abiotic depletion potential, and energy use are the common impact categories that have been considered in biomass-related LCA studies [16,17]. The LCA study for electricity generation by coal alone, biomass alone, and biomass and coal co-firing showed that the co-firing scenario is better than the others in terms of the chosen LCA impact categories [18]. The environmental impact of energy production change on the coal-fired CHP boiler after low-share wood pellet co-combustion was quantified and the results confirmed that biomass and coal co-firing is environmentally better than sole coal or biomass combustion [19]. The environmental impact of willow biomass power generation was compared with other renewable and nonrenewable resources. The LCA study of willow-fueled power generation achieved an over 70% reduction in greenhouse gas (GHG) emissions compared to U.S. grid-generated electricity [20]. A scenario analysis compared the impacts of the biomass plant against fossil alternatives and identified which renewable energy sources should be prioritized for development and investment [21].
When substituting one type of fuel with another one, it is important to identify the emissions-related hotspots from their supply chain. Biomass exploitation results in emissions hazardous to human health and the environment, both at the local and global scale. When considering the effect on local air quality, the fine particulates from biomass combustion have been identified as a relevant air pollution source in the U.S. [22]. Therefore, there is an interest in identifying the environmental impacts and benefits of biomass use via combustion in CHP units. The carbon footprint analysis of a product refers to a quantification of the GHG emissions during the LCA of the product. The development of an accurate methodology in carbon footprint analysis assists the consumers in the selection of products serving their best interests.
This study aims to conduct a carbon footprint analysis (screening LCA) to: (a) identify and pinpoint the emission hotspots from the whole supply chain of biomass used in CHP production; (b) compare the use of wheat straw and wood pellets as fuel for the CHP station in terms of GWP to the power station’s current production using coal.

2. Materials and Methods

As a simplified version of LCA, screening LCA has a limited scope, narrowed-down system boundary, and lower level of precision. However, for the sake of brevity, it helps to identify the main environmental issues, trends, and hotspots of the studied product system, which is especially valuable in the early stage of research [23]. This study calculated the emissions by doing a carbon footprint analysis based on the screening LCA process for coal, wheat straw, and wood pellets. The guiding standards for model development and calculations are ISO 14040 and ISO 14044 [24], the inventory reported by this model is developed in Microsoft Excel. The assessment consists of four components, as depicted in Figure 2.

2.1. Case Study Overview

The Nordjylland Power Station is a coal-fired CHP plant located in Vodskov, Denmark. It supplies district heating to more than 200,000 households and electricity to meet the annual consumption of 1.3 million households in the greater Aalborg area. The station is a turbine group about 50 m long, where the energy in the bituminous coal is converted into electricity and district heat in the form of steam. Currently, the station consumes 223 t/h of coal at full load, which yields a gross electricity capacity of 716 MW and a maximum district heat capacity of 420 MJ/h. The city of Aalborg is investigating its possibilities of alignment with the national vision by switching coal to agricultural residues and creating more financial benefits for the local farmers.

2.2. Goal and Scope

The goal of this LCA is to conduct a unit-based analysis of the life-cycle environmental impacts due to generating 1 MJ of thermal energy for the end users. The model accounts for energy and material contributions at every phase in the energy-conversion process, from biomass cultivation to combustion, and the upstream environmental burdens associated with these inputs. The functional unit of the LCA is 1 MJ of heat produced from the CHP unit [25].
The system boundary and the process flow diagram are presented in Figure 3. It illustrates a cradle-to-gate LCA of biomass (wheat straw or wood pellets) production and its conversion in a CHP plant to produce heat and power. The system boundary is comprised of the key processes, starting from biomass acquisition, harvesting, collection pre-processing (including field transportation and chopping), delivery to the CHP storage site, energy production (heat), and the marginal electricity substitution (as a co-product of the system). The slag has fertilizer value in the form of P2O5 and K2O [26]. Those nutrient values of slag return to farmland are assumed as ‘avoided product’, which carries a negative contribution to carbon emissions. Therefore, the slag from biomass combustion is included in the system boundary.

2.3. Life Cycle Inventory

The inventory reported by this model is developed in Microsoft Excel. The majority of datasets used in this study for material and energy inputs are derived from the US EPA’s Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI v2.1). Other data are taken from the literature.
  • Acquisition: Currently, the Nordjylland Power Station burns bituminous coal, which is found near the Earth’s surface. The most common extraction method of bituminous coal is open-cast or strip mining. This acquisition stage involves heavy machinery such as power shovels, wheel excavators, dragline conveyors, and heavy-duty trucks. Other emissions contributors to the mining process are explosives to break through the surface. Wheat straw is assumed to be a byproduct of wheat production, and the raw material for wood-pellet production is considered to be collected from a sustainably managed natural forest.
Impacts of biomass acquisition have mainly been involved with the reduction in soil fertility and nitrate leaching. It is also found that if biomass has to be harvested from the field for fuel purposes, an equivalent amount of fertilizers should be added [13]. The wheat straw is considered a byproduct of growing and cultivating wheat grain. The wood-pellet production is treated as collecting forest residuals. Therefore, the agricultural treatments including sowing, harvesting, and the addition of fertilizers and herbicides are excluded from the LCA inventory. For both biomass feedstocks, the time required for regrowing the biomass to sequester the initial CO2 pulse released is assumed to be approximately one year [27].
  • Pretreatment: For the wheat straw to be ready and suitable burned in the supercritical boiler, it must be piled and organized into a manageable size for the boiler. This process involves grouping the wheat straw into bales and handling the bales into a machine that feeds the boiler.
For wood pellets, the main stages of production include pretreatment, drying, sizing reduction, pelletizing, and cooling. The raw materials for the wood-pellet production in Denmark are primary by-products of sawmills and the wood-processing industry. The pretreatment stage of wood pellets in this study is assumed to include impurity separation, drying, and pelleting. The large pieces of sawdust or wood chips are produced on-site and transported to a preparation site using trucks. At the preparation site, large pieces of sawdust or wood chips are fed into chipping machines, and these machines take and break them down into a uniform smaller size. The wood pellets have to be dried to reduce the moisture content for easy and complete combustion in the CHP units.
There are three types of energy used and included in the preparation-stage emission calculation: diesel (for vehicles in factory such as forklift trucks and tractors), firewood (for heat production in the pellets drying process), and electricity (used in machineries and factory lighting). The use of diesel and firewood was collected from factory surveys and site visits. Electricity consumption was estimated by the utility bills. The carbon emissions are calculated by multiplying the emission factors of diesel and grid electricity by the fuel consumption.
  • Transportation: The coal and wheat straw are assumed to be transported over a distance of 200 km, from the pretreatment plant to the CHP plant. The transportation is assumed to start from the pretreatment/storage site by 10-wheel truck, with 15 tonnes of carrying capacity (100% loading) and 0% loading on the return trip. The average freight truck is assumed to emit 0.1618 kg of carbon dioxide equivalent (CO2e) per tonne-mile [28]. The wood pellets are assumed to be imported by freight ship from Latvia, which is 525 nautical miles away from Denmark. The commercial ship is assumed to emit 0.0418 kg of CO2e per tonne-nautical mile [29]. The total required coal or biomass mass is calculated based on the CHP plant capacity, fuel heating value, working hours, and operating efficiency. The total emissions are calculated by multiplying the emission factors by the distance and the weight.
  • Combustion: Prior to combustion, coal is crushed into fine dust at silos on site. Then a mixture of air and coal is blown into the burner, where the combustion takes place in the boiler furnace at a temperature as high as 1800˚C. The biomass is fed into the boiler, and the steam from the incinerated boiling water is used for district heating. The coal and biomass-powered CHP plant efficiency is approximately 90% [30], indicating that 90% of the primary energy supplied from coal or biomass feedstock is converted to electricity and heat. The power-to-heat ratio in existing CHP systems is 0.75.
In this paper, wheat straw is assumed to have a moisture content of 6.8%. With this moisture content, the lower heating value (LHV) of wheat straw is 14 MJ/kg. Wood pellets are assumed to have a moisture content of 5.9% and an LHV of 17.6 MJ/kg. Based on the LHVs, energy-conversion efficiency, and the power-to-heat ratio, one tonne of wheat straw produces 827.05 kWh of electricity and 7100 MJ of heat, and one tonne of wood pellets produces 1094.63 kWh of electricity and 9310 MJ of heat. Bituminous coal has the highest LHV of 25.2 MJ/kg, as shown in Table 2.
The carbon dioxide (CO2), nitrogen oxides, and sulfur oxides from the combustion stage are calculated based on the carbon, nitrogen, and sulfur content in the fuel.
  • Slag back to the field: Slag, a glass-like by-product left over after combustion, is assumed to be recycled and reused as fertilizer because of its nutrient value. The presence of heavy and toxic metals makes ash valueless. Slag from the burning of wheat straw and wood pellets is found to be 54 kg/t and 67 kg/t in the dry base, respectively [32].

2.4. Impact Assessment and Interpretation

The impact category guiding the calculation in the study is the 100-year GWP to the three primary GHG emissions: CO2, methane (CH4), and nitrous oxide (N2O), in g CO2-equivalent (CO2e) [33]. The GWPs are based on the IPCC’s Fourth Assessment Report and equate to 25 for CH4 and 298 for N2O [34]. The results are interpreted and compared to the GWP values reported in other LCA studies.

2.5. Assumptions and Limitations

The key assumptions that are made in the LCA process include that there is unlimited biomass feedstock in the area; transportation distance is identical for coal and wheat straw; the combustion process is stoichiometric combustion, which is an ideal combustion process where the fuel is burned completely, and all the C is converted to CO2 [35]; the transportation distance of slag is not included. All facilities involved in this supply chain produce multiple products, and in some cases, were initially constructed and employed for other purposes. Therefore, the impacts of facility construction, land use, and capital equipment are eliminated from the system boundaries. Results from this study should be interpreted with these assumptions and uncertainties in mind.

3. Results

The impact assessment results for each step considered in the LCA are shown in Table 3. The results show that the coal-powered CHP station has the highest CO2e value of 117.01 g CO2e/MJ heat. The wheat-straw- and the wood-pellet-fueled stations produce less CO2e emissions than coal, with 22.73 and 78.19 g CO2e/MJ heat, respectively, since wheat straw is 100% domestically produced, but wood pellets are not produced in Denmark. With the assumptions made in this case study, wheat straw is preferred over wood pellets in terms of environmental impact and resource availability.
The negative CO2e emission value in the biomass acquisition stage indicates that the biomass feedstock draws down CO2 from the atmosphere through photosynthesis as the plants grow. Moreover, it is found that if biomass is cut off from the field for energy-generation purposes, an equivalent amount of fertilizers should be added to the biomass removal because of the reduction in soil fertility and nitrate leaching. For coal, the acquisition stage only considers the surface coal mining, and the emissions mainly come from the machinery fuel assumption that is required for crushing, stripping, drilling, blasting, excavating, and shipping the coal. Although coal is formed by dead plant matter decaying into peat, the full conversion takes millions of years. Therefore, the carbon sequestration from the soil is not included in the coal-acquisition stage. The growth of wood products absorbs a portion of the CO2 from the atmosphere and stores the carbon for longer periods than agricultural products. The net sequestration of CO2 from the atmosphere into wheat straw takes a couple of months or even less.
Although the wheat straw has a much higher LHV than the wood pellets, it has a lower CO2e emission value. That is expected, because wheat straw has a shorter harvesting period than wood pellets and can be replaced at a faster rate. It can also grow domestically in Denmark, which decreases transportation emissions.
The CO2e emission distributions for wheat straw and wood pellets show that the hotspot in the carbon emissions is from the combustion stage. At the end of the combustion, CO2e (mainly CO2 and CH4) is released into the atmosphere immediately. However, the combustion of biomass could be considered “carbon neutral” because the carbon discharged during the combustion can be leveled by the carbon absorbed by plants during growth (Figure 4).
The CO2e emissions caused by the transportation of wood pellets is about 11.89 g CO2e/MJ heat. The high value is due to the fact that Denmark is heavily reliant on importing wood pellets from overseas. It is important to notice that the pretreatment stage is responsible for 29.07 g CO2e for wood pellets, almost 13.44% of the gross GWP of the entire energy conversion, indicating drying as a hotspot for generating GWP emissions. Large dryer drums are powered by natural gas, propane, or sawdust burners, driving off the moisture in the raw wood materials, which causes an increase in emissions. The impact from the slag back to the field is minor. The output (MJ) per amount (kg) of coal, wheat straw, and wood pellets is 25.2, 14.0, and 17.6, respectively. The results are subject to the emission factors applied in this study, and most of them were collected from peer-reviewed articles. Custom emission factors are not considered because standard factor sets cover the calculation needs.

4. Discussions

Comparing our CO2e calculations with other literature, the GWP results are partially in agreement with the results in [36], and this study reported 59–66 g CO2/MJ heat with wood pellets. The other LCA study for electricity generation with forest residues presented that GHG emissions range from 39.6 to 50.4 g CO2e/MJ heat [37]. The carbon footprint analysis reported the GWP related to the straw-fired CHP plant is 4.31 g CO2e/MJ heat [11], which is lower than our results. The emissions related to straw transportation are calculated as 2.45 g CO2e/MJ heat, which is close to the value reported in our study. Moreover, an LCA study of a biomass-based CHP plant in Norway claimed that most of the emissions are contributed by the biomass transport and cogeneration stages [38], which is identical to our findings and conclusions.
Our study showed opportunities for reducing the GWP by adopting wheat straw as the fuel for the studied CHP station. However, converting the coal-fired CHP station to an agricultural residue-fueled station carries many challenges. From an economic point of view, the biggest challenge is the high equipment cost. The Nordjylland Power Station currently consumes 118 t/h of coal. If the power plant uses 100% of either wheat straw or wood pellets, the feedstock consumption rate would be 236 t/h and 189 t/h, respectively. Because of the relatively low energy content, agricultural residue needs a larger storage space. There is a high facility cost associated with the bulk silos’ construction. In the transportation chain, the energy content of coal almost doubles the energy content of wheat straw, resulting in the transportation cost being higher for delivering biomass from rural areas than coal.

5. Conclusions

Examining whether biomass energy is carbon neutral should consider many factors, including the biomass type, processing technology, and the time frame examined. The screening LCA results claim that wheat straw is better than wood pellets due to the short harvest cycle and resource availability. Markets for biomass are always in flux and setting up the chain of supply is very important. The employment of diesel-fueled vehicles for long-distance transport to transport biomass increases the endpoint environmental impacts. The impact from the slag back to the field sector is not significant.
For future work, the availability of agricultural residues and wood-based fuels in the regains needs to be considered. In the short term, it is possible to collect the local biomass feedstock. However, it is necessary to improve forest management, logistics, and technology to avoid rapid deforestation and loss of biodiversity. The biomass and coal share different pretreatment techniques before the combustion step. The equipment modifications and upgrades are needed for the power plant. Moreover, an accurate and detailed model is needed to quantify the carbon footprint of the transportation sector.

Author Contributions

Conceptualization, Y.Z.; methodology, Y.Z.; software, Y.S.; validation, W.Z., C.L. and M.Z.; formal analysis, Y.Z.; investigation, M.Z.; resources, Y.Z.; data curation, Y.S.; writing—original draft preparation, M.Z.; writing—review and editing, Y.Z.; visualization, J.Z.; supervision, C.L.; project administration, J.W.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Chinese Universities Scientific Fund under grant number 15052002.

Acknowledgments

Stephanie Owyang and Yini Xu are acknowledged for proofreading this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Agriculture 12 01146 g001 550
Figure 1. Types of biomass feasible for heat and power generation [3].
Figure 1. Types of biomass feasible for heat and power generation [3].
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Figure 2. The frameworks of LCA according to ISO 14040 [24].
Figure 2. The frameworks of LCA according to ISO 14040 [24].
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Agriculture 12 01146 g003 550
Figure 3. The system boundary of the LCA process.
Figure 3. The system boundary of the LCA process.
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Agriculture 12 01146 g004 550
Figure 4. CO2e emissions distribution for cases of wheat straw and wood pellets.
Figure 4. CO2e emissions distribution for cases of wheat straw and wood pellets.
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Table
Table 1. Biomass consumption (PJ) in 2012 [9].
Table 1. Biomass consumption (PJ) in 2012 [9].
BiomassDomestic ProductionImportsTotal
Wood pellets1.731.833.5
Wood chips126.218.2
Straw17.5017.5
Biofuels2.579.5
Biogas4.404.4
Total79.248.3127.5
Table
Table 2. Composition (% DM) and lower heating value (LHV, MJ/kg) of coal and biomass [31].
Table 2. Composition (% DM) and lower heating value (LHV, MJ/kg) of coal and biomass [31].
CoalWheat StrawWood Pellets
Moisture8.06.85.9
Ash14.211.71.0
Carbon66.540.746.6
Hydrogen3.85.55.8
Nitrogen1.60.60.2
Sulfur0.50.10.8
Oxygen5.534.639.7
LHV25.214.017.6
Table
Table 3. The carbon footprint analysis for 1 MJ of heat production from coal, wheat straw, and wood pellets.
Table 3. The carbon footprint analysis for 1 MJ of heat production from coal, wheat straw, and wood pellets.
Processg CO2e/MJ Heat
Coal
mining21.78
transportation 1.73
combustion93.5
Total117.01 (CO2: 80.95 g, CH4: 0.11 g, N2O: 0.09 g)
Wheat Straw
acquisition−187.46
baling and handling0.73
chopping0.08
transportation3.11
combustion206.31
slag back to the field−0.04
Total22.73 (CO2: 9.15 g, CH4: 0.04 g, N2O: 0.03 g)
Wood Pellets
acquisition−138.07
drying29.04
chopping0.03
transportation11.89
combustion175.35
slag back to the field−0.05
Total78.19 (CO2: 44.70 g, CH4: 0.16 g, N2O: 0.07 g)
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Zheng, Y.; Liu, C.; Zhu, J.; Sang, Y.; Wang, J.; Zhao, W.; Zhuang, M. Carbon Footprint Analysis for Biomass-Fueled Combined Heat and Power Station: A Case Study. Agriculture 2022, 12, 1146. https://doi.org/10.3390/agriculture12081146

AMA Style

Zheng Y, Liu C, Zhu J, Sang Y, Wang J, Zhao W, Zhuang M. Carbon Footprint Analysis for Biomass-Fueled Combined Heat and Power Station: A Case Study. Agriculture. 2022; 12(8):1146. https://doi.org/10.3390/agriculture12081146

Chicago/Turabian Style

Zheng, Yingying, Chang Liu, Jie Zhu, Yuanrui Sang, Jinglong Wang, Wenjing Zhao, and Minghao Zhuang. 2022. "Carbon Footprint Analysis for Biomass-Fueled Combined Heat and Power Station: A Case Study" Agriculture 12, no. 8: 1146. https://doi.org/10.3390/agriculture12081146

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