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A publication of
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 39, 2014
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong
Copyright © 2014, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-30-3; ISSN 2283-9216
The Italian Association
of Chemical Engineering
www.aidic.it/cet
DOI: 10.3303/CET1439083
Carbon Footprint Assessment of Low-rank Coal-based
Acetylene System
Danxing Zheng*, Xiaohui Chen, Yue Mi, Peng Jin
Beijing University of Chemical Technology, Heping Street, Beijing, China
[email protected]
In this study, the multi-product carbon footprint evaluation method (MPCE), which was proposed by our
team previous work, has been applied to assess the carbon emission characteristic of the low-rank coalbased acetylene system. The system boundary was firstly determined. Then the data, such as mass and
energy balance, greenhouse gas (GHG) emission factors and global warming potential coefficients, were
extracted from the simulation results. Based on these data, the comprehensive carbon emissions of each
operation unit and the system were figured out. The comprehensive carbon emissions of each operation
unit are 6,796.5 kg-CO2e/h, 107.1 kg-CO2e/h, 3,825.7 kg-CO2e/h, 3,310.2 kg-CO2e/h, 155.1 kg-CO2e/h,
539.0 kg-CO2e/h, 88.0 kg-CO2e/h, 124.8 kg-CO2e/h, respectively, and that of the system is 14,946.4 kgCO2e/h. Besides, the distribution characteristics of direct and indirect carbon emissions among each unit
were investigated. The indirect carbon emission, which is mainly caused by material handling, accounts for
74.09 % of total carbon emission of the system. In addition, the carbon emissions of unit CO and C 2H2
were calculated which are 1.60 kg-CO2e/kg-CO and 16.04 kg-CO2e/kg-C2H2. Therefore, the multi-product
comprehensive carbon emission of the system is 2.31 kg-CO2e/kg-product.
1. Introduction
There are two methods to produce the low carbon hydrocarbons from the raw coal. The first method is to
produce methanol through the coal gasification, and the olefins are obtained by the methanol-to-olefin
(MTO) technology. The other method is to produce the calcium carbide through the oxygen-thermal
method process, and then the reaction of calcium carbide and water can produce the acetylene (Tang,
2009). Compared to the coal-based gasification MTO process, the coal-based calcium carbide acetylene
method is simpler, and the calcium carbide, as the intermediate product, is regarded as the important
chemicals for the organic synthesis. In the calcium carbide production system, the CO concentration in the
furnace gas is much higher than that in the syngas from coal gasification. In the conventional process of
“coal-coke-calcium carbide-acetylene”, the furnace temperature is maintained at 1,600-2,000 C through
the electrode. Then the coke and lime are conversed to the calcium carbide and furnace gas with high CO
concentration (Liu et al, 2011). The process to produce the acetylene was studied from the perspective of
thermodynamic analysis, which contained energy consumption and energy quality (Guo et al, 2012). The
relative research shows that the oxygen-thermal method to produce calcium carbide has lower multiproduct comprehensive energy and exergy consumptions than the electric-thermal method process (Mi,
2013). Kuppens (2014) assessed the pyrolysis char production and application from the techno-economic
viability, and the char could be used to produce calcium carbide. Therefore, the coal-based oxygenthermal calcium carbide acetylene system is a process to produce acetylene with the potentials of
efficiency and energy saving. As to the life cycle assessment (LCA), Cespi (2014) considered that LCA
could give the data to assess the environmental sustainability of an industrial. In the previous study about
the carbon footprint of the system, the comprehensive carbon emissions were investigated as the main
index (Yu et al, 2014). However, the carbon emission characteristic of each unit is rarely studied,
especially the coproduction process with two or more products.
This paper firstly describes the process of the low-rank coal-based oxygen-thermal calcium carbide to
produce the acetylene, and the system boundary is designated. Then the mass and energy balance and
Please cite this article as: Zheng D., Chen X., Mi Y., Jin P., 2014, Carbon footprint assessment of low-rank coal-based
acetylene system, Chemical Engineering Transactions, 39, 493-498 DOI:10.3303/CET1439083
494
the carbon emission data are collected from the simulation results and relative scientific reports. According
to the multi-product carbon footprint evaluation method, the comprehensive carbon emission of the system
and each unit are calculated. Besides, the multi-product carbon footprint is also investigated.
2. Method of carbon footprint assessment
2.1 Calculation method of carbon footprint (Suh et al, 2004)
The calculation method of the product carbon footprint contains the following five steps.
1) The definition of system boundary is performed.
2) The data, which include system flexibility level, emission factors and GWP coefficients, are collected
from the technical report (Sinden, 2008).
3) The quality of extracted data is assured, which is to make sure that the data meet consistency,
accuracy and veracity.
4) The mass and energy balances of the system are carried out. The masses of feedstock and products
and the utility required by each unit are calculated according to the balance results.
5) The carbon footprints of each unit and the system are figured out, which mainly are direct and indirect
carbon emission.
2.2 Evaluation indexes of product carbon footprint
According to the definition of system boundary, the carbon emission of the system is composed of the
direct and indirect carbon emissions, which can be expressed as
C  Cd  Ci
(kg-CO2e/h) (1)
d
i
where C is the carbon emission of the system, C and C indicate direct carbon emission and indirect
d
i
carbon emission respectively. During the calculation of C and C , many other factors should be taken into
d
consideration. For example, C can be calculated by the addition of CO2 equivalent quantity of each unit,
and CO2 equivalent quantity is obtained from different greenhouse gas (GHG) factors. Therefore, the direct
carbon emission is calculated as
C d   Ckd,g  GWPg
k
(kg-CO2e/h) (2)
g
where k and GWPg indicate the unit k and the global warming potential of green gas g.
i
u
m
C is generally decomposed into utility carbon emission (C ) and material handling carbon emission (C ).
u
m
C is caused by the utility consumption, such as electric, steam and cooling water. C is caused by
m
preparing the feedstock and handling the byproduct, and C belongs to the whole system and not the
i
certain unit. So C is calculated as
C i   Cku,g  GWPg   Cnm,g  GWPg
k
g
n
(kg-CO2e/h) (3)
g
where n donates material n. Combining Eqs(1), (2) and (3), the carbon emission of the system is then
written as


C   Ckd, g  GWPg    Cku, g  GWPg   Cnm,g  GWPg 
k
g
n g
 k g

Based on the GHG emission factors and the utility consumption,
Cku, g   f k , g ,i  Ek ,i
i
Cku,g
(kg-CO2e/h) (4)
can be calculated as
(kg-CO2e/h) (5)
where fk,g,i is the emission factor of GHG g in utility i of unit k, Ek,i is utility i consumption in unit k. Similarly,
Cnm, g
is calculated as
Cnm,g  f n,g  M n
( kg-GHG/h) (6)
where fn,g is the emission factor of GHG g for material n, and Mn is the mass flow of material n.
The product carbon emission should be quantified based on a certain unit. Because of the variety of
chemical process, most processes contain two or more products. To consider the effects of different
495
products’ distribution on the comparison between the carbon footprints of different processes, the multiproduct carbon emission is defined as (Mi et al, 2013)
c   x jc j
(kg-CO2e/kg-product) (7)
j
where xj is the carbon conversion ratio of product j and it can be written as
xj 
amount of carbon atom in product j
amount of carbon atom in feedstock
(8)
Beside, cj is unit carbon emission of product j in Eq(7), and it can also be calculated as
cj 

C
 c dj  c uj  c mj
Pj

(kg-CO2e/kg-productj) (9)
where pj is the mass flow of product j. Therefore, the multi-product carbon emission of the system can be
calculated as


c   x j c dj  c uj  c mj

(kg-CO2e/kg-product) (10)
j
3. Carbon footprint of low-rank coal-based acetylene manufacturing process
3.1 Description of coproduction system and definition of system boundary(Guo et al, 2012)
The low-rank coal-based acetylene coproduction system is decomposed into acetylene production system
and utility system. As shown in Figure 1, acetylene production system has 8 units (Chen et al, 2013):
drying unit (U1), pyrolysis unit (U2), cooling unit (U3), calcium carbide production system (U4), gas-solid
separation unit (U5), calcium carbide pre-handling unit (U6), acetylene production unit (U7) and lime
recycling unit (U8). The utility system, which is not presented in the flow chart, provides electric, heat and
cooling water. Heat is required in U1, U2 and U8. Electric is required in U5, U6 and U7. The energy
consumption of calcium carbide reaction is provided by the combustion of the portion of coke, and small
amount of electricity is needed in the system.
3.2 Data Collection
The mass and energy balances of the acetylene system are performed, and the results of mass balance
are listed in Table 1. Because of the complex system, the energy consumption of each unit is not listed.
The totally inputted energy including the enthalpies of feedstock and utility is 94,307.73 kW, and the
enthalpy of all the outputted streams is 94,306.12 kW. The calculation shows that the system energy is
balanced. Besides, the type and amount of utilities are also listed in Table 1 for each unit.
According to the calculation steps of carbon footprint, the collected data contains GHG factor and GWP
coefficient. GHG factor can be obtained from IPCC report (IPCC, 2007) and other resources. The GHG
factors involved with low-rank coal-based acetylene process are listed in Table 2. The GHG factor and
GWP coefficient belong to the secondary activity level data which is the average data obtained from the
databank (Sinden, 2008). What’s more, the consistency and accuracy of the data should be maintained.
3.3 Results and Discussion
In this study, only the raw material handling process, product production and waste disposal are
considered, and other processes, such as product transportation, are ignored. The definition of the system
boundary can refer to Figure 1. The coke and oxygen are consumed to produce calcium carbide in the
oxygen-thermal method process. A large amount of CO is discharged from the calcium carbide furnace.
Because CO can be utilized proficiently, the furnace gas enriched in CO is also considered as a product.
The products of low-rank coal-based acetylene process are furnace gas and acetylene, and the
distribution of product carbon emission should be considered in the calculation of carbon footprint.
Therefore, the multi-product carbon footprint evaluation method is used to calculate the carbon emission of
the acetylene system. Based on the calculation results, the direct and indirect carbon emissions of each
unit are listed in Figure 2. The indirect carbon emission contains the utility consumption carbon emission
and material handling carbon emission.
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Figure 1: Low-rank coal-based acetylene coproduction system
Table 1: Mass balance and utility consumption of coproduction system
Material balance
Feedstock
Lignite
Lime
Oxygen
Water
Total
Input
Mass flow (kg/h)
13,473.90
3,140.00
5,090.00
3,393.56
21,957.54
Products
Acetylene
Off-gas
Dust
Slag
Syngas
Coal tar
Water
Total
Utility Consumption
Output
Mass flow (kg/h)
931.92
916.08
10,196.64
3,278.02
1,157.51
1,347.92
4,129.45
21,957.54
Unit
U1
U2
U3
U5
U6
U6
U7
U8
Type of Unit
Heat
Heat
Water
Electric
Electric
Water
Electric
Heat
Energy (kW)
4,886.00
2,078.75
648.96
10.00
540.00
1,280.00
110.00
1,560.00
Table 2: GHG factors of process system
Industrial
processes
Waste
Utilities
Type
Raw coal mining
Lime production
GHG factor
0.017
0.73
Unit
kgCH4/kg
kgCO2/kg-lime
Coke production
Oxygen-enriched gas
0.56
0.20
kgCO2/kg
kgCO2/kg
Ash
Electricity
Cooling water
Heat power
3.66
0.80
0.084
0.08
kg-CO2e/kg-ash
kgCO2/kWh
kgCO2/kWh
kgCO2/ kWh
In Figure 2, the brown, grey and black colors indicate the material handling carbon emission, the utility
consumption carbon emission and the direct carbon emission of each unit. The figure at the upper right
corner of Figure 2 is the amplification of U2, U5, U7 and U8. As is shown in Figure 2, the comprehensive
carbon emissions for U1, U2, U3, U4, U5, U6, U7 and U8 are 6,796.5 kg-CO2e/h, 107.1 kg-CO2e/h,
3,825.7 kg-CO2e/h, 3,310.2 kg-CO2e/h, 155.1 kg-CO2e/h, 539.0 kg-CO2e/h, 88.0 kg-CO2e/h and 124.8 kgCO2e/h, respectively. And the total carbon emission of the low-rank coal-based acetylene system is
14,946.4 kg-CO2e/h.
497
Figure 2: Diagram of carbon emission for each unit
At the same time, the material handling carbon emission, the utility consumption carbon emission and the
direct carbon emission of the whole system can also be calculated from Figure 2, which are 3,872.1 kgCO2e/h, 1,166.0 kg-CO2e/h and 9,908.3 kg-CO2e/h. As to the direct carbon emission, it accounts for 25.91
% of the total carbon emission. The causes for the direct carbon emission are the CO2 discharge from the
syngas produced by coal pyrolysis (U3) and the furnace gas from calcium carbide production (U4). The
direct carbon emission of U3 is 3,778.2 kg-CO2e/h which takes up 25.27 % of the total carbon emission.
As to U4, its direct carbon emission is 93.88 kg-CO2e/h which takes up only 0.63 % of the total carbon
emission. The direct carbon emission is caused by the discharge of material not required by the system.
The coupling with other processes to utilize the waste can reduce the direct carbon emission.
The indirect carbon emission of the system is 1,1074.3 kg-CO2e/h which accounts for 74.09 % of the total
carbon emission. As is mentioned before, the utility consumption carbon emission and material handling
carbon emission belong to the indirect carbon emission. From Figure 2, the material handling, taking up
66.29 % of the total carbon emission, is the main cause of the indirect carbon emission. The utility
consumption carbon emission accounts for 7.80 % of the system.
Based on the unit product carbon footprint, the carbon emission characteristics of the material handling
and utility consumption are analyzed as follows. Figure 2 shows that the carbon emission caused by the
material handling is the main carbon emission of the whole system. The material handling contains raw
coal, lime, oxygen-enriched gas and ash in the system, and their unit product carbon emissions are 1.005
kg-CO2e/kg-product, 0.353 kg-CO2e/kg-product, 0.156 kg-CO2e/kg-product and 0.008 kg-CO2e/kg-product,
respectively. The proportions for these four material handling carbon emissions are 43.50 %, 15.28 %,
6.75 % and 0.35 % of the total carbon emission of the system.
The utility of the acetylene system contains cooling water, heat power and electric, and their carbon
emissions are 0.024 kg-CO2e/kg-product (1.04 % of total carbon emission)、0.084 kg-CO2e/kg-product
(3.64 % of total carbon emission) and 0.086 kg-CO2e/kg- product (3.72 % of total carbon emission). As to
the utility carbon emission of each unit, Figure 2 shows that the carbon emission caused by utility
consumption of U6 is the greatest in these 8 units. That is because large amounts of furnace gas is in high
temperature, and the cooling of furnace gas needs much cooling water. Through the above analysis, the
solution to reduce the carbon emission of the system should consider the indirect carbon emission,
especially that caused by the material handling. For example, the present method of dealing with the raw
coal and lime is replaced by other advanced exploitation methods. Besides, the utility carbon emission can
be reduced through the heat integration of energy system and the energy coupling with other processes.
In the system, both the furnace gas and acetylene are products, so the unit product carbon emission of
each product can be calculated by Eq(9) which are 1.60 kg-CO2e/kg-CO and 16.04 kg-CO2e /kg-C2H2. The
carbon conversion ratios of CO and C2H2 are 0.456 and 0.098 respectively. Similarly, the multi-product
carbon footprint of the low-rank coal-based acetylene system is performed by Eq(10) which is 2.31 kgCO2e /kg-product.
498
4. Conclusions
Based on our previous work on the multi-product carbon footprint and the process simulation, the carbon
footprint characteristic of the low-rank coal-based acetylene process was studied in this paper. The above
research shows that:
1) the totally comprehensive carbon emission of the system is 14,946.4 kg-CO2e/h, and those of each
unit are 6,796.5kg-CO2e/h, 107.1 kg-CO2e/h, 3,825.7 kg-CO2e/h, 3,310.2 kg-CO2e/h, 155.1 kgCO2e/h, 539.0 kg-CO2e/h, 88.0 kg-CO2e/h and 124.8 kg-CO2e/h, respectively.
2) The direct and indirect carbon emissions of the system are 3,872.1 kg-CO2e/h and 11,074.3 kgCO2e/h, which accounts for 25.91 % and 74.09 % of the total carbon emission of the system. In the
indirect carbon emission, the carbon emission caused by the material handling takes up 84.97 %, and
the main cause is the exploitations of raw coal and lime. In order to reduce the indirect carbon
emission, the exploration of advanced material handling method and the clean utilization of the waste
should be considered in the latter optimization of the system.
3) Both the calcium carbide furnace gas and acetylene are the products of the system. The carbon
footprints of unit CO and C2H2 are 1.60 kg-CO2e/kg-CO and 16.04 kg-CO2e /kg-C2H2. Therefore, the
multi-product comprehensive carbon footprint of the system is 2.31 kg-CO2e /kg-product.
Acknowledgement
This work is supported by the National Basic Research Program of China (2011CB201306).
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