Environmental evaluation of a distributed-centralized biomass pyrolysis system: A case study in Shandong, China

Journal Pre-proof Environmental evaluation of a distributed-centralized biomass pyrolysis system: A case study in Shandong, China

Xiaoxiao Yang, Duoduo Han, Yuying Zhao, Rui Li, Yulong Wu PII:

S0048-9697(20)30425-3

DOI:

https://doi.org/10.1016/j.scitotenv.2020.136915

Reference:

STOTEN 136915

To appear in:

Science of the Total Environment

Received date:

25 October 2019

Revised date:

22 January 2020

Accepted date:

23 January 2020

Please cite this article as: X. Yang, D. Han, Y. Zhao, et al., Environmental evaluation of a distributed-centralized biomass pyrolysis system: A case study in Shandong, China, Science of the Total Environment (2020), https://doi.org/10.1016/j.scitotenv.2020.136915

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© 2020 Published by Elsevier.

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Environmental evaluation of a distributed-centralized biomass pyrolysis system: a case study in Shandong, China Xiaoxiao Yang, a Duoduo Han,a Yuying Zhao,a Rui Li a* and Yulong Wu b,c**

a

MOE Engineering Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry

University, Beijing 100083, China.

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c

Institute of Nuclear And New Energy Technology, Tsinghua University, Beijing 100084, China. Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education,

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b

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Tsinghua University, Beijing, 100084, China

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*Corresponding author: [email protected] (R, Li)

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** Corresponding author: [email protected] (Y, Wu)

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Environmental evaluation of a distributed-centralized biomass pyrolysis system: a case study in Shandong, China Abstract The limited quality of liquid product from the fast pyrolysis of biomass resources is a great obstacle to its large-scale application. Herein, a distributed–centralized agricultural straw pyrolysis

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(DCP) system with products of high market acceptance was implemented based on the design of a

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pilot plant and previous research. The system consisted of distributed pyrolysis workshops and a centralized upgrading factory that involved crude oil separation, hydrogen production, and

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hydrorefining. The crude fuel was separated by distillation before hydrotreatment, which avoids

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external hydrogen and energy consumption compared with direct hydrotreatment, and the

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hydrotreatment unit is independent of external hydrogen supply. The environmental impacts of a

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specific case designed for the northern region of Shandong province in China were evaluated using a life cycle assessment (LCA) method. LCA results indicated that abiotic depletion potential,

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acidification potential, global warming potential, ozone layer depletion potential, and human toxicity potential were primarily from fuel combustion, electricity, and N fertilizer. Eutrophication potential was primarily from the biomass production stage. Results demonstrated that the GWP of the system was -0.62 kg CO2,eq per kg crop straw. Comparison with the conventional straw incorporation method indicated the economic and social benefits of the DCP system, which is thus expected to be a promising option for crop residue disposal. Keywords: Aspen Plus; pyrolysis; biorefinery; life cycle assessment; biomass residues

1. Introduction Global warming has resulted in unpredictable and harmful impacts on humans and ecosystems.

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New technologies were exploited to produce sustainable energy in recent years. Bioenergy is considered to be one of the key technologies for reducing dependence on fossil fuels and minimizing greenhouse gas (GHG) emissions (Schade et al., 2011). Agriculture production generates approximately 4 billion metric tons of crop residue per year globally and 800 million tons per year in China (Clare et al., 2015; Wang and Luo, 2018). Nevertheless, 90% of straw

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residue is disposed as waste, which has a large impact on the cropland ecosystem (Qi and Hu,

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2015). In the past several years, the Chinese government forced farmers to return straw to the fields (Xu et al., 2017). However, most of the carbon in straw is unstable, and it is easily

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mineralized into CO2 shortly after being returned to the soil. In addition, straw incorporation

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increases the emissions of CH4 and N2O, two important GHGs. According to incomplete data,

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approximately 10-30% of CO2 in the atmosphere comes from the incorporation and incineration of

issue needing attention.

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biomass residues (Mendes et al., 2004). Therefore, how to handle biomass disposal is an important

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Fast pyrolysis is a method that operates at a moderate temperature of approximately 500 °C, with a short residence time (<2 s), and inert atmosphere (Bridgwater, 2012; Mettler et al., 2012). It converts biomass to bio-oil, non-condensable gas (NCG), and biochar (Demirbas et al., 2006). The goal of pyrolysis product utilization can be regarded as the integrated utilization of bio-oil, NCG, and biochar. Furthermore, pyrolysis is a suitable approach to fix most of the carbon in bio-oil and biochar and reducing the volume of GHG emissions, which are receiving increased research attention (Rezaei and Mehrpooya, 2018). Peters et al. (Peters et al., 2015) simulated the biofuel production process via fast pyrolysis and hydroupgrading. The crude bio-oil was hydrotreated, distillated, and hydrocracked in turn. Heng et al. (Heng et al., 2018) established a polyol fuel

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production system from corn stover pyrolysis, and separated the crude bio-oil to aqueous and non-aqueous phase for further hydrotreatment and hydrogen production, respectively., However, no effective fractionation of water-rich bio-oil was performed prior to upgrading in these studies. Moreover, the crude bio-oil contains a large amount of water, acids, and other components which are detrimental to catalytic activity, waste energy, and the market acceptance of products were low

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(Auersvald et al., 2019). To solve these problems, our previous work proposed the concept of

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resource-efficient biomass utilization system, where crude bio-oil is initially separated into several fractions through rectification operation. Then crude fuel, which consists primarily of ketones, is

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obtained (Han et al., 2019).

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In addition, waste water from pyrolysis contains certain amount of phenolic compounds,

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which are detrimental to the environment and difficult to handle. Thus, an emission-reducing and

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resource-efficient pyrolysis system is urgently required to achieve sustainable human development. The main disadvantages in many current bio-oil upgrading processes include intensive energy

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requirement, hydrogen supply, and sustainable catalysts. In addition, the high oxygen content of crude fuel attributes to a higher consumption of hydrogen and a lower yield of fuel product. On the basis of the material and energy balance, we found that the RBU system is energy self-sufficient by burning NCG. In addition, biochar is redundant for subsequent hydrotreatment. Thus, the biomass conversion system becomes more economic and independent of external supply. However, even if an efficient straw pyrolysis system can be obtained, it has to offer environmental benefits in comparison with incorporation. The environmental impacts of the system can be analyzed by conducting life cycle assessment (LCA). LCA is a potent tool that uses a cradle-to-grave approach to evaluate the potential

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environmental impacts of a system (Peters et al., 2015). It has been widely employed to assessing biofuels systems such as bioethanol (Cherubini and Jungmeier, 2010), biodiesel (Kiwjaroun et al., 2009), and dimethyl ether (Higo and Dowaki, 2010). In many studies, owing to the insufficiency and low accuracy of operating data from commercial-scale plants, Aspen Plus was often used to simulate chemical engineering processes to provide inventory data for LCA (Carrasco et al., 2017;

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Han et al., 2017; Ismail et al., 2017).

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On the basis of a pilot-scale pyrolysis plant (20 t/d) in Hebei Province in China and our previous work, a system consisting of distributed pyrolysis workshops and a centralized upgrade

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factory for crude oil separation, hydrogen production, and hydro-refining was proposed. This

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study aims to analyze the environmental impacts and benefits of the distributed-centralized straw

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pyrolysis (DCP) system and compared with that of incorporation. Thus, a more informed

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comparison can be made to guide future research and development (R & D) on crop residue disposal. Furthermore, in terms of the industrial distribution and crop yield of Shandong province,

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a detailed plan was designed. The energy and material balance were simulated by Aspen plus, and the life cycle performance was evaluated by SimaPro 9.0 software.

2. Methodology

2.1 Process design based on regional characteristics This work was conducted on selected areas in Shandong Province, which is the major crop production area in China. The northern region of Shandong includes Weifang, Jinan, Binzhou, Dezhou, Dongying, and Liaocheng, representing the majority of granaries in Shandong. We selected this region for two reasons, as follows: first, Binzhou has many biochar production workshops, and the large number of liquid byproducts have become a burden to deal with and

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brought environmental pressures. However, these liquid products can be inputted as an intermediate product to our process for further refining. Second, as an oil exploration base, Dongying has convenient conditions for oil storage, sales, and transportation, facilitating the utilization of refined oil. On the basis of these considerations, a concept of “n+1” (distributed-centralized pyrolysis system) is adopted, representing the “decentralized pyrolysis

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workshop and concentrate refining factory”. As shown in the map in Fig.1, the refining plant is

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located in Zibo for its convenient transportation.

The configuration of pyrolysis workshop and refining plant, and mass transportation scheme

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should be determined. The designed biomass pyrolysis workshop here processes 15 t/h wet straw,

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which could be designed as mobile plant and make the whole process more flexible The run time

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of pyrolysis workshops was assumed to be 7200 h/a, and therefore the biomass plant requires an

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input of 108000 t straw each year. The average yield of grain was 6200 kg/ha according to the Shandong Statistical Yearbook. On the basis of the mass ratio of straw to grain (1.09) and the

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collection coefficient (0.2) (Heng et al., 2018; Zuo et al., 2015), the land area of 79905 ha could be calculated for each pyrolysis plant (land area=108000×1000/(6200×1.09×0.2)=79905 ha). The collection region is assumed to be a circular area, considering the pyrolysis plant as the center. On the basis of the estimated cultivation area ratio (0.8), the average collection radius is calculated as 20 km; it can be transported by the farmer’s small trucks, saving on collection cost and improving the process feasibility. Approximately 40 workshops should be set up within the selected region.

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Fig. 1. Zoning and traffic map of distributed-centralized pyrolysis system in northern Shandong province.

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On the basis of the traffic map, the 40 workshops were divided into seven routes as follows,

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and the detailed transportation distance can be found in Fig. 2.

Zibo (W23);

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Route 1: W1, W2, W7, W12, W13, road transport to Dezhou (W6), and then railway transport to

Zibo;

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Route 2: W26-W28, W30, W31, road transport to Weifang (W26), and then railway transport to

Route 3: W3-W5, W8-W11, road transport to Binzhou (W10), and then highway transport to Zibo; Route 4: W14-W18, road transport to Boxing (W16), and then highway transport to Zibo; Route 5: W32, W34-W40, road transport to Liaocheng (W34), and then railway transport to Zibo; Route 6: W19-W21, W33, road transport to Jinan, and then railway transport to Zibo; Route 7: W22-W25, W29, highway transport to Zibo.

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2. 2. Process description

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Fig. 2. Transportation distance of distributed-centralized pyrolysis system in northern Shandong province.

2.2.1. Biomass pretreatment and pyrolysis

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The process diagram of pyrolysis unit is shown in Fig.3, including biomass pretreatment, fast pyrolysis, cyclones, condensation, and combustion. Briefly, straw is grounded in a crasher and dried in a roller dryer, and then fed into a rotary kiln for pyrolysis. The NCG acquired from quench tower burn outside of the kiln for heat supplement. The combusted exhaust gases then flow through a drum dryer to take away moisture from feedstock. Biochar entrained in the gaseous pyrolysis products is separated by cyclone. The gaseous mixture is cooled in a quench tower and then liquid product is transported to the central plant for further refinery. Product distribution and bio-oil composition were adapted from literature so that mass balance was obtained for simulation (Han et al., 2019; Xu, 2016). Bio-oil composition is presented in Table S1.

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Biomass

Quench tower

Crasher Cyclone Furnace Rotary kiln

Exhaust gases

Cool water

Dryer

Storage tank

Chilling water

Crude Bio-oil

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Material Waste material Gas

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Fig. 3. Process flow diagram of pyrolysis plant.

The simulation scheme is shown in Fig. S1, wet ground biomass (WETSTRAW) with 15 wt%

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M (moisture content) is fed into a roller dryer (DRYER). The heat for drying is provided by hot

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exhausted gases (EX-GAS) from the NCG combustion (NCG-COMB) and biochar, then the

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biomass moisture content is reduced to the desired 7.5 wt%. The dry biomass particles

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(DRYSTRAW) are introduced into the rotating kiln (REACTOR) for pyrolysis where they are decomposed at 550 °C into crude bio-oil (65%), gas (20%) and char (15%). The heat requirement

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pyrolysis (1.27 MJ kg-1) is consistent with that reported by Yang et al. (1.1-1.6 MJ kg-1) (Yang et al., 2013). The pyrolysis products (PY-OUT) enter the cyclone (CYCLON) to separate the biochar (CHAR), and the remaining vaporous mixture (VAPOR) is then cooled to 55 °C with cold oil in the primary condenser (COOLER1). The circulating oil is cooled by a naturally circulating water cooling system. In the secondary condenser (COOLER2), a chilling water system is needed to further cool the mixture to 20 °C. The inlet air flow rate is determined to ensure that the NCG is completely burned. The inlet air flow rate is controlled by the design specification modules. The recovered liquids (OIL1 and OIL2) are collected and transported to the bio-refining plant for upgrading. The hot biochar is cooled for

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hydrogen production, and field application. 2.2.2. Bio-oil refinery As shown in Fig. 4, the refining part consists of three parts, namely, crude-oil rectification unit, water–gas shift unit and hydrotreatment unit. The rectification process consists of three rectification columns, which exports wood vinegar, mixed acid, crude–fuel, and phenolic oil,

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respectively. Wood vinegar has no phenol and low toxicity, which can be used as odor remover

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and agricultural uses such as an insect repellent, and soil or foliar fertilizer (Mohan et al., 2006). Specifically, the low molecular acids and alcohols have sterilization effects, a research conducted

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by an animal husbandry and veterinary workstation indicated that wood vinegar can be directly

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used as deodorant and antivirus in the livestock breeding industry (Si et al., 2014). The water–free

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crude–fuel is fed into the hydrotreating reactors for upgrading, while H2 is produced by a water–

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gas shift unit. The hydrotreated vapor mixture is cooled and decompressed in a quench tower, then

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the aqueous phase is separated.

Journal Pre-proof Biochar from n pyrolysis plants

Crude Bio-oil From n pyrolysis plants

Wood vinegar Mixed acids

Crude fuel

WGS reactor Phenol oil PSA

Crude-oil rectification

Water-gas shift

Storage tank

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Quench tower

Hydrotreating reactors

Oil phase

Waste water

Crude fuel hydrotreatment Material Product Waste material Gas

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Light fuel tank

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Water

Fig. 4. Process flow diagram of bio-refining plant.

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Fig. S2 presents the flowsheet of the bio-oil refining plant. After rectification, approximately

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60.2% wood vinegar, 11.2% mixed acids, 13.3% crude fuel, and 15.3% phenolic oil were obtained. The crude bio-oil (CRUDEOIL) is fed into T101 to remove most of the water. To avoid harmful impacts humans, the phenolic content of the produced wood vinegar (VINEGAR) is controlled within 0.6%. Wood vinegar is composed of 81.5% water and 7.1% acids. The bottom stream enters the second column (T102), separating the residual water and acids (MIX-ACID) in which contains 90.9% acid. Then, the bottom stream of T102 is fed into T103 where aldehydes (7.8%), ketones (74.5%), and acids (9.4%) are separated as crude-fuel (FUEL). The remaining bottom stream is treated as phenolic oil (PHEN-OIL) which contains 79.9% phenols and 4.5% ketones. The composition of each product is shown in Fig.5. Heat for reboilers is supplied by the water-gas shift unit. Approximately 79.5% of the total consumption is in the first tower (T101) because a

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large volume of water is distillated, which makes the load of the tower fairly large. The NRTL (nonrandom two-liquid) property method in Aspen Plus was selected in simulation which is suitable for strong polar system containing water. 100 Water

Acids

Ketones

Aldehydes

Phenols

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40

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Relative content (%)

80

Wood vinegar

Mixed acid

Crude fuel

Phenolic oil

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20

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Fig. 5. Composition of each products in rectification unit.Due to the complexity of the catalytic

hydrotreating reaction, related experimental was not carried out in this study. Ru/C is considered

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as an excellent catalyst for deoxygenation and has been employed in many works, thus was chosen

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in our simulation. Besides, Elliott et al. (Elliott et al., 2009) chose guaiacol, furfural, and acetic acid as model compounds to study the chemical mechanisms of catalytic hydroprocess in bio-oil, and give us some reference of hydrotreating mechanisms. Based on previous studies, the reactions and specifications in RStoic reactors were modeled (Benés et al., 2019; Bridgwater, 2012; Elliott et al., 2009). A two-stage catalytic hydrotreatment process is employed to convert the bio-oil into a nearly oxygen-free product. The bio-oil from the pyrolysis plant, along with hydrogen, is pressurized up to 170 bar, heated to 250 °C, and fed into the first reactor (HYD-R1). Then the stabilized mixture (HYD1-OUT) has an oxygen content of approximately 30% was obtained. The second reactor (HYD-R2) operates at 150 bar and 350 °C to deoxygenate the oil to the required oxygen content (HYD2-OUT). The chemical reaction

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equations are listed in Table S2. A reduction in the O content from 38% to 15% after two stages hydrotreatment, which is close to Benes (34 to 13%) (Benés et al., 2019). The hydrotreatment product contains a small amount of unreacted hydrogen. Therefore, the mixture is cooled in three steps and then separated into a hydrogen-rich gas (H2MIXED) and a liquid (HYLIQUID). This step releases heat and is used to heat the bio-oil (H1) and the stabilized mixture (H2) to the

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required temperatures. The gas is further cooled by chilling water. Most of this gas is recycled to

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the reactor (H2RECYCL), and the rest is exported (GASOUT). The liquid is decompressed to 17 bar to release NCG (HY-NCG) and is then separated into an aqueous fraction (WATER) and an

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organic fraction (LF) in a water separate drum (SEPDRUM). The product of the hydrotreatment

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unit contains 69.3% biofuel, 28.3% water, and 2.4% NCG. The hydrogen compressor requires the

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majority of the electricity consumption, which accounts for 93.2% of the total. The remaining

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electricity is primarily used for pressurizing the crude fuel and pumping the circulating water. The hydrogen required by the hydrotreatment unit is produced by a water-gas shift reactor

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(WGS-R) using the char from the pyrolysis section and water. Although the hydrogen production from char has not been industrialized in this field, the reaction of the water–gas shift is a relatively mature technology (Holladay et al., 2009). The operating conditions are based on those of Yan et al. (Yan et al., 2010). In summary, steam passes through the hot char bed, and heat is provided by the combustion outside the reactor, producing H2, CO, and CO2. The gaseous components lead to a PSA unit which can provide high purity of 99%, at a hydrogen recovery rate in the high 80% range. The comparison between the simulation and literature is listed in Table S3, and similar results demonstrate the reliability of the simulation. In addition, in the unit of the water–gas shift, the quantities of biochar, water, and air were determined by the hydrogen requirement and the heat

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required by the rectification unit. The reactor is modeled as RStoic reactor. The produced stream (WATERGAS) is cooled in two steps. Then, a PSA unit separates hydrogen (WGS-H2) from the product stream, which is delivered to the hydrotreatment reactors. The other gases are mainly CO, which is treated as gas to nearby farmers. The carbon flow of the DCP system is presented in Fig. S3, and the stream table is shown in Table S4.2.3. LCA framework

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2.3.1. Goal and scope definition

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The goal of this study was to identify the environmental impact of the distributed-centralized pyrolysis system. Conventional straw incorporation system is selected as the comparative case.

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The process carried out by LCA helps identify which of the parameters are likely to be significant

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to the GHG emission. In order to make comparable LCA results, the functional unit (FU) is

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defined as 1 kg of crop straw, with an assumed 15% moisture content.

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The scope of two systems, from an overall perspective, includes two main life cycle stages: straw production and disposal. In order to investigate the effects of two disposal approaches, the

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system boundary for this work starts from harvest to the next harvest. The LCA boundary of pyrolysis system is illustrated in Fig. 6. It includes 6 sub-stages: straw collection, transportation, pretreatment and pyrolysis, bio-oil refinery, product utilization and straw production.

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Fertilizer, water...

Straw production & collection

Straw production

Biomass transportation Pretreatment & pyrolysis

Biochar collection

Electricity

System boundary

Burning

NCG

Fast pyrolysis plant

Bio-oil recovery

Exhaust gas, water, waste

Bio-oil transportation

H2

Crude fuel hydrotreating

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Crude fuel

Electricity, heat

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Bio-oil rectification

Water-gas shift

Wood vinegar Mixed acid Phenolic oil

Light fuel

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Bio-refining plant

Energy flow

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(b) Fertilizer, water...

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Material flow

Straw production & collection

Exhaust gas, water, waste

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Electricity, diesel

Fig. 6. Process flow diagram and system boundary of the (a) DCP system and (b) straw corporation.

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2.3.2. Inventory data

2.3.2.1. Inventory data of distributed-centralized pyrolysis system In the straw production stage, diesel energy of 216.3 MJ per hectare is consumed for farm machinery such as broadcaster and combine harvester (Yu and Tao, 2012). The biomass production inputs are based on the Shandong Statistical Yearbook: 20 kg seeds, 600 kWh for irrigation, 4.6 kg herbicides, and 4.4 kg pesticides are consumed during the crop growth. Nitrogen (urea), phosphate (P2O5), and potassium (K2O) fertilizer at the rates of 168 kg, 81 kg, and 255 kg per hectare are utilized per hectare to replenish the soil nutrient content. The straw is harvested and packed, then it will be transported to the onsite pyrolysis workshop by a truck with a distance

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of 20 km. The inventory data of pretreatment and pyrolysis, bio-oil refinery stages such as electricity consumption, are obtained primarily from Aspen Plus. During simulation, the heat transmission losses are assumed to be approximately 10%. Biochar is distributed to the bio-refining plant for hydrogen production or returned to the farmland for soil amendment. Crude bio-oil is transported

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to the bio-refining plant by a freight train or truck for further upgrading. The average transport

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distance of the train and truck from farmland to Zibo is assumed to be 160 and 112 km, respectively. Finally, the refined oil is transported to Dongying by train with a distance of 170 km.

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Wood vinegar, mixed acid, and phenolic oil are assumed to be sold locally nearby the refining

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pyrolysis workshop and refining plant.

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plant in Zibo. In addition, the transport distance of biochar is set on the basis of the distance of

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Biochar has been widely accepted for use as soil amendment tools (Lee et al., 2017; Mungkunkamchao et al., 2013; Zhu et al., 2014). Positive effects from biochar addition was

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reported by Sohi and Renner et al. (Renner, 2007; Sohi et al., 2010).

(Sohi et al., 2010; Yang et

al., 2017). In field experiments in Colombia, N2O emissions was reduced by 80% and methane emission was completely suppressed with biochar additions (Renner, 2007). The fertilizer efficiency was set to be increased by 7.2% with biochar, and the crop yield was raised by 20%. In addition, the irrigation water was 18% lower due to the enhanced water–holding capacity. The inventory data are shown in Table 1. Table 1 Inventory data for pyrolysis system. Parameter

Item

Amount per FU

Unit

Land use

Land use

1.75

m2

Material

Seed

0.0030

kg

Input

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Electricity

Fertilizer & Pesticide

1.23×10-5

Steam for water-gas shift

0.1054 0.5119 (0.6243)

Diesel for farming

0.0300

MJ

Diesel for transportation

0.0962

MJ

Irrigation

0.0746 (0.0910)

KWh

Pretreatment & pyrolysis

0.0224

KWh

Rectification

0.0056

KWh

Hydrotreatment

0.0252

KWh

Urea (46% N)

0.0237 (0.0255)

kg

Singlesuperphosphate (21% P2O5)

0.0114 (0.0123)

kg

Potassium chloride (60% K2O)

0.0359 (0.0387)

kg

Pyretroid

0.0007

kg kg

0.0895

kg

0.0916

kg

0.0670

kg

0.3608

kg

0.0932

kg

1.1009 (0.9174)

kg

0.1206

kg

CO2 from Pretreatment & pyrolysis

0.1388

kg

CO2 from diesel combustion (farming)

0.0023

kg

CO2 from diesel combustion (transportation)

0.0072

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Light fuel Phenolic oil

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Mixed acid Wood vinegar

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Biochar Crops

CH4

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N2O

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Water gas

kg -6

-5

8.20×10 (4.10×10 )

kg

0 (3.16×10-7)

kg

The values in brackets represent the current agricultural data.

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a

kg

0.0007

Output

Emissions

kg a

Irrigation water

Atrazine Products & Byproducts

kg

of

Fuel

Catalyst for hydrotreatment

2.3.2.2. Inventory data of straw incorporation The advantages of incorporation, a strongly promoted method of crop residue disposal, are its simple operation and reduction in labor requirements. To further evaluate the appropriate approach of straw disposal, the pyrolysis system is compared with straw incorporation. In general, the incorporation of crop residue may affect soil moisture, microbial activity, anaerobic microsites, inorganic N content, and soil temperature, thereby regulating N2O emissions (Liu et al., 2011). Straw incorporation significantly enhanced N2O efflux by 28.4%, and tended to reduce CH4 absorption (9.5%) (Xu et al., 2017). N could not satisfy microbial requirements when the C/N of

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straw residues were larger than 40(Jiang et al., 2017). It is suggested that approximately 0.03 kg urea/kg straw should be applied to complete the decomposition process; 1 kg returned straw could be treated as 1×10-7 kg P and1×10-6 kg K; and irrigation water could be reduced by 22.8%-27.8% (Wang). The inventory data for straw incorporation is shown in Table 2. Table 2 Inventory data for incorporation system. Parameter

Item

Amount per FU

Unit

Land use

Land use

1.75

m2

Material

Seed

0.0030

kg

0.7229 (1.0012)

kg

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Input

Diesel for farming

Electricity

Irrigation

Fertilizer & Pesticide

Urea (46% N)

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Fuel

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Irrigation water

0.0300

MJ

0.0657 (0.0910)

KWh kg

0.0123

kg

Potassium chloride (60% K2O)

0.0387

kg

Pyretroid

0.0007

kg

0.0007

kg

0.9999 (0.9174)

kg

0.0023

kg

re

0.0555 (0.0255)

Singlesuperphosphate (21% P2O5)

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Atrazine Output Crops

Emissions

CO2 from diesel combustion (farming) N2O CH4

5.30×10-5 (4.10×10-5) -7

-7

3.46×10 (3.16×10 )

kg kg

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2.3.2.3. GHG emissions

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Products & Byproducts

GHG emissions can be classified into direct and indirect GHG emissions. Direct GHG emissions include the GHG emitted from the combustion of NCG and char and from the combustion of liquid fuels in an engine, assuming all fuels are fully and completely combusted and the exhaust gases are emitted in the form of CO2. Indirect GHG emissions include the emissions from the material production chain when fertilizer, pesticides, and electricity are produced, transported and distributed.

3. Results and discussion 3.1. Environmental impacts

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The most popular categories used in the bioenergy system are abiotic depletion potential (ADP), acidification potential (AP), eutrophication potential (EP), global warming potential (GWP), ozone layer depletion potential (ODP), and human toxicity potential (HTP). In this section, the environmental impact of the DCP system and straw incorporation system are evaluated based on the CML 2 baseline 2000 method. Table 3 shows the characterization of the impacts of DCP

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system per FU. Table 3 Characterization results of environmental impacts for 1 kg crop straw. AP

EP

GWP100

𝑃𝑂4−3

(kg Sb eq)

(kg SO2 eq)

(kg

2.09×10-3

2.04×10-3

1.48×10-3

-4

-4

-6

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Biomass production

ADP

eq)

ODP

HTP

(kg CO2 eq)

(kg CF-11 eq)

(kg 1,4-DB eq)

-1.05

4.96×10-8

0.21

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Impact category

1.92×10

1.36×10

8.76×10

0.16

5.43×10-11

4.87×10-3

Bio-oil refinery

2.64×10-4

1.88×10-4

1.21×10-5

0.25

7.47×10-11

6.70×10-3

-4

-4

-5

1.58×10

1.22×10

Total

2.70×10-3

2.49×10-3

3.1.1 GHG emission

-9

2.49×10

0.02

3.49×10

1.53×10-3

-0.62

5.43×10-8

5.14×10-3 0.22

lP

Material transportation

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Biomass pyrolysis

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The three main GHGs, namely, CO2, CH4, and N2O, are displayed in terms of their CO2

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equivalent (CO2,eq) GWP, according to the IPCC guidelines of CO2,eq, that is, 25 for CH4 and 298 for N2O. As shown in Table. 3, the GWP impact category shows a negative value that indicates CO2 removal from the atmosphere. This finding is attributed mostly to the large amount of CO2 capture during the biomass growth stage. To identify the processes with the highest environmental impact, the contributions of individual impact categories are broken down in Fig.7. The absorbed CO2 credit can offset direct and indirect GHG emissions in the biomass production process. Direct emissions include agriculture activities, maize growing, and the combustion of diesel in farm machines, such as a combine harvester, broadcaster, and baler. Indirect emissions are derived from the electricity for irrigation and agricultural chemicals, in which irrigation makes up approximately half of the

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contribution. The main contribution to positive GWP is obtained from the hydrotreatment stage (20.1%) because the combustion of water gas in the unit of the water–gas shift is considered. Biomass pretreatment and pyrolysis process (13.6%) and the indirect emission of biomass production (12.4%) also contribute remarkably. Electricity consumption and NCG combustion were responsible for a significant contribution to GHG emissions. Material transportation

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contributes only a small amount to the overall GHG emissions. The overall GHG emission was

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calculated at -0.62 kg CO2,eq given that approximately 67.3% of carbon is fixed in the products. In the system as a whole, GHG emissions can be reduced by improving the thermal efficiency of the

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combustion processes in the plants or by using a spray and dripping irrigation system for

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electricity reduction. In addition, compared with conventional plants, the described system can

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decreases the GHG emissions.

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produce energy self-sufficiently without the input of external natural gas, which significantly

HTP ODP GWP EP AP ADP

-100

-80 -60 -40 -20 biomass production-direct emissions biomass production-indirect emissions

0 20 transportation bio-oil rectification

40 60 80 100 biomass pretreatment & pyrolysis crude-fuel hydrotreatment

Fig. 7. Contribution of the process to the potential environmental impacts.

3.1.2. Other impact categories The ADP impact category is produced primarily by the biomass production process, where direct and indirect emissions account for 28.2% and 49.1% of the total, respectively. Significant

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contributors to indirect emissions are electricity consumption (15.7% of the total) and the application of nitrogen fertilizer (27.4% of the total), but direct emissions are primarily caused by the combustion of diesel for the farming machine. The next largest contributors are electricity consumption by pretreatment and pyrolysis (7.1%) and crude-fuel hydrotreatment (8.0%). As in the ADP category, the biomass production process is identified as the main source of AP.

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The direct and indirect emissions of biomass production are 41.6% and 40.5%, respectively. The

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primary source of direct emissions from biomass production is from the growth of biomass (26.6% of the total), while P (20.5% of the total) and N (12.1% of the total) fertilizer contribute the largest

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share of indirect emissions. Therefore, the ADP and AP impacts can be optimized by increasing

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the proportion of clean power, such as wind power, solar power, and hydropower, over coal power,

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as well as decreasing the proportion of chemical fertilizer.

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ODP is dominated by biomass production which makes up 87.0% of the total share. The contribution of indirect emissions during biomass production makes up 51.9% and the use of

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nitrogen fertilizer (29.1% of the total) contributes half of the total share. Direct emissions (35.1%) primarily derive from the combustion of diesel in the farming machine (22.2% of the total). Aside from the biomass production process, material transportation also has a significant effect (9.5%) on ODP. The impacts to ODP can be optimized by applying organic fertilizer and decreasing the proportion of chemical fertilizer or by promoting the application of biochar and wood vinegar in farmland, as well as by the application of clean energy such as hydrogen to reduce carbohydrate combustion. In addition, the direct emissions from biomass production, primarily biomass growth, are responsible for the EP, reaching up to 84.8% of the total share. HTP, which involves the effects of

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toxic chemicals on humans, is also governed by the biomass producing process (92.5%). The primary contributors to HTP are the direct emissions of biomass growth (54.1% of the total) and the indirect emissions from fertilizer application (33.8% of the total). On the basis of these discussions, the main contributors to six categories are concluded in Fig. 8. In addition, the breakdown contribution networks of six categories are illustrated in Fig. S4,

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which strongly supports the discussion. The following approaches are beneficial for alleviating the

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environmental impacts: a) improving the thermal efficiency of the combustion processes; b)

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reducing the consumption of farm chemicals.

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increasing the proportion of clean power and clean energy; c) using electricity-saving measures; d)

AP

ODP

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EP

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Fuel burning

Electricity

N fertilizer

Biomass growing

Fig. 8. Main contributors to six categories.

3.1.3. Comparison with straw incorporation The total values of six categories are compared between the two systems as shown in Fig. 9. The results show that the carbon sink capacity of the incorporation system is superior to that of pyrolysis due to the large quantity of CO2 emissions during the biomass pyrolysis and water-gas combustion processes. As expected, the values of ADP, EP, ODP, and HTP in pyrolysis are significantly lower than those in the incorporation. This finding can be attributed to the higher input of nitrogen fertilizer and the higher emissions of N2O during incorporation. On the contrary,

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the return of biochar decreases the quantity of these chemicals. Incorporation

Pyrolysis

Relative impact (%)

100

50

0 ADP

AP

EP

GWP

ODP

HTP

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-50

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-100

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Fig. 9. Comparison of environmental impacts of fast pyrolysis and incorporation.

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3.2. Economic estimation

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Despite this analysis, identifying a superior method for straw disposal remains difficult. Therefore, we estimated the net profit of the pyrolysis system, as shown in Table 4. On the basis of

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the investment of the pilot plant, the total investment (15 t/h) is approximately 25.8 million RMB,

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with a service life of 15 years. The results show that approximately 1.07 RMB/ FU will be obtained by the pyrolysis system. In Table 5, the case of farmers’ profit, we assumed that the traditional faming data is the baseline, and only the changed items are calculated; the related data are shown in Tables 1–2. In addition, we supposed that the amount of biochar returned to the field is proportional to the amount of collection (approximately 0.63 t/ha). The results show that the profit of pyrolysis system (0.47 RMB/FU) is much higher than that of incorporation (0.08 RMB/FU). Moreover, pyrolysis system can increase the employment rate in rural areas and improve farmers’ economic source structure. Table 4 Economic estimation for pyrolysis system. Item

Amount (per FU)

Unit

Unit price (RMB)

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0.0532

kWh

0.5

Straw

1

kg

0.25

Transportation

0.0897

Infrastructure

0.0159

Output 0.0670

kg

2.4

Phenolic oil

0.0916

kg

3.2

Wood vinegar

0.3608

kg

0.13

Biochar

0.0932

kg

1.0

Water gas

0.1206

kg

0.7

light fuel

0.0895

kg

4.0

Profit= Output-Input+Save

1.07 RMB

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Mixed acid

Table 5 Economic comparison for farmers of two systems. Amount (per FU) Pyrolysis system

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0.0932

Output 0.1835

Straw

1

0.0825

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Crops Save

0.0018

P fertilizer

0.0009

K fertilizer

0.0028

Electricity

0.0164

Profit= Output-Input+Save

0.47 RMB

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0.0253

kg

1.0

kg

1.6

kg

0.25

kg

2.0

kg

2.6

kg

2.4

kWh

0.5

0.08 RMB

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3.3. Sensitivity analysis

-0.0300

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N fertilizer

Unit price (RMB)

Incorporation

Input Biochar

Unit

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Item

In addition to the base case analysis, the sensitivities of 6 contributor variables to the GWP category of the DCP system, including the yield of crop straw, the total application volume of nitrogen fertilizer, bio-oil transportation distance, the electricity consumption of biomass production, bio-oil production, and bio-oil upgrading, are analyzed by varying ±20% of their baseline data. The sensitivity analysis can determine how different values for these characteristic factors impact GWP and consequently can help the project developer identify strategies for reducing negative impacts. The results are shown in Fig. 10 in the form of relative deviation. It should be noted that the negative deviation of GWP represents a carbon sink effect, due to the

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GWP baseline being a negative value in this study. The greatest impact on GHG emissions is derived from the electricity consumption for bio-oil production. A deviation of ±20% electricity results in ±11.91% in GWP, and the electricity consumed in bio-oil upgrading and irrigation leads to deviations of ±6.73% and ±3.93%, respectively. In reality, substantially increasing the electrical efficiency in an established plant is

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impractical. However, optimizing the power structure or reducing electricity during agricultural

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activities is still possible. The yield of crop straw is the second most sensitive variable in the GWP, which varies with climate in different years, thereby resulting in a collection radius change and the

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variation of inventory data. A decrease in crop straw yield results in a 10.23% reduction in GWP,

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and an increase of 20% in crop straw yield causes a 6.82% rise in GWP. The application of

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nitrogen fertilizer produces a GWP change of ±2.33%. In addition, the distance of bio-oil

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transportation has the smallest impact on net GWP, deviating from -1.27% to 1.55%. The sensitive analysis results indicate that the uncertainties of crop straw yield and electricity consumption have

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clear impacts on the GWP of the DCP system. The sensitive analysis results of other categories are shown in Fig. S5. In summary, crop straw yield has the largest impact on ADP, AP, EP, ODP, and HTP. EP is only slightly affected by the fluctuation of all contributor variables. +20% -20%

Crop straw yield N fertilizer Elec-irrigation Elec-oil production Elec-oil upgrading Oil transportation -10

-5

0

GWP change (%)

5

10

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Fig. 10. Sensitive analysis for calculated GWP.

4. Conclusions An LCA study based on a crop straw fast pyrolysis and bio-oil upgrading system was conducted. In view of the complexity of crude bio-oil, an integrated utilization concept for products was proposed, from which fuel substitute, chemical raw materials. In this system, wood

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vinegar (contains 0.6% phenol), mixed acid (contains 90.9% acid), and phenolic oil (contains 79.9%

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phenol) were produced. All the products from the system have high market acceptance. Wood vinegar can be used as odor remover, insect repellent, soil or foliar fertilizer, and livestock

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breeding industry. Phenolic oil is equal to that from dry distillation of coal, thus can partly replace

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phenol for phenolic resin production, which is applied in many factories. The mixed acid can be

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easily distillated to get acetic acid products. Notably, the hydrogen used in the upgrading unit is

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produced by biochar, a byproduct of the pyrolysis through the reaction of water–gas and no longer needs to be obtained externally. The crude fuel was distillated before hydrotreatment, which

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avoids external hydrogen and energy consumption compared with direct hydrotreatment. The distributed-centralized system can achieve energy self-sufficiency. In summary, the straw residue fast pyrolysis is beneficial to the GWP. The biomass production process contributes the largest impact values to all categories. The biomass growing activities primarily impact the AP and EP, and the application of nitrogen fertilizer primarily impacts the ADP, ODP, and HTP. Electricity consumption significantly influences GWP, ADP, AP, and HTP, and the combustion of liquid or gas fuel greatly influences the GWP, ADP, and ODP categories. A sensitive analysis indicated that the uncertainties of electricity consumption and crop straw yield remarkably affect the GWP categories. In terms of the overall carbon flow, 67.3% carbon is

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sequestrated and the other is completely combusted and released to the air in the form of CO2. In accordance with the comparative analysis, the carbon sink capacity of incorporation is 1.25 times that of pyrolysis due to diesel and NCG combustion. However, incorporation is unsustainable and has long-term effects on soil and farming. Furthermore, the negative GWP does not indicate that this process is neutral. By comparing the economic profit of two systems, we

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found that the pyrolysis system has economic and social benefits, but incorporation has less

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benefits.

Further research should be undertaken to reduce the harmful environmental impacts of fast

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pyrolysis, particularly in the areas of developing clean agriculture, CO2 capture and reuse, and

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efficient utilization of energy. Furthermore, due to the harsh conditions of hydrotreatment,

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catalytic fast pyrolysis system maybe a more effective approach for biomass disposal. Therefore,

Acknowledgements

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relevant experiments and LCA studies should be performed in the future.

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This work was supported by the Beijing Forestry University hot-spot tracking project (No. 200-121701284),

National Natural Science Foundation of China (No. 21838006 and No.

21776159) and National Key R&D Program of China (No. 2018YFC1902101). References Auersvald M, Shumeiko B, Vrtiška D, Straka P, Staš M, Šimáček P, et al. Hydrotreatment of straw bio-oil from ablative fast pyrolysis to produce suitable refinery intermediates. Fuel 2019; 238: 98-110. Benés M, Bilbao R, Santos JM, Alves Melo J, Wisniewski A, Fonts I. Hydrodeoxygenation of Lignocellulosic Fast Pyrolysis Bio-Oil: Characterization of the Products and Effect of the Catalyst Loading Ratio. Energy & Fuels 2019; 33: 4272-4286. Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading. Biomass and Bioenergy 2012; 38: 68-94. Carrasco JL, Gunukula S, Boateng AA, Mullen CA, DeSisto WJ, Wheeler MC. Pyrolysis of forest residues: An approach to techno-economics for bio-fuel production. Fuel 2017; 193: 477-484.

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Lee J, Kim K-H, Kwon EE. Biochar as a Catalyst. Renewable and Sustainable Energy Reviews 2017; 77: 70-79. Liu C, Wang K, Meng S, Zheng X, Zhou Z, Han S, et al. Effects of irrigation, fertilization and crop straw management on nitrous oxide and nitric oxide emissions from a wheat–maize rotation field in northern China. Agriculture, Ecosystems & Environment 2011; 140: 226-233. Mendes MR, Aramaki T, Hanaki K. Comparison of the environmental impact of incineration and landfilling in São Paulo City as determined by LCA. Resources, Conservation and Recycling 2004; 41: 47-63. Mettler MS, Mushrif SH, Paulsen AD, Javadekar AD, Vlachos DG, Dauenhauer PJ. Revealing pyrolysis chemistry for biofuels production: Conversion of cellulose to furans and small oxygenates. Energy Environ. Sci. 2012; 5: 5414-5424. Mohan D, Pittman, Charles U., Steele PH. Pyrolysis of Wood/Biomass for Bio-oil:? A Critical Review.

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical abstract

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

A case study of DCP system in Shandong Province was assessed. All products from the system have high market acceptance. The system shows strong positive environmental benefits by GHG analysis. Biomass production process contributes a primary share to environmental impacts. Compared to straw incorporation, DCP system has both social and economic potential.

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