Formula derivation
The concept of carbon neutrality is essentially a balance between carbon sources and carbon sinks. The carbon source is the process of releasing CO2 into the atmosphere, and the carbon sink is the process of absorbing CO2 from the atmosphere. Investigating the atmospheric system within a certain period of time, when the amount of CO2 emitted by all carbon sources into the system is equal to the amount of CO2 absorbed by all carbon sinks from the system, carbon neutrality can be achieved. There are various carbon sources and sinks in nature with complicated relationships. Among them, the most important carbon source related to human activities is energy production. In 2020, China’s power sector emitted about 4 billion tons of CO2 per year, which is close to 40% of the total emissions. Thus, the power sector is one of the key areas in emission reduction to achieve the carbon-neutral goal. To this end, this article focuses on the issues of carbon neutrality in the power industry.
According to the amount and sources of carbon elements, energy can be divided into three types: Carbonaceous energy [11], Carbon free energy and Carbon-neutral energy [12]. Carbonaceous energy mainly refers to fossil fuel, which contains carbon elements mined underground (the carbon elements in the ancient atmosphere are transformed into biomass through photosynthesis, and then become fossil fuels through long geological years). And in the process of utilization, the carbon element emitted to the atmosphere in the form of CO2. In addition, carbon free energy refers to renewable energy such as solar energy and wind energy, which contain no carbon elements and does not directly emit CO2 during utilization. Moreover, carbon-neutral energy refers to biomass, whose carbon elements come from the atmosphere. During the short period of growth and utilization, the carbon element is absorbed by biomass from the atmosphere and transferred to the atmosphere. Thus, the net CO2 content in the atmosphere is not changed by carbon-neutral energy.
Carbonaceous and carbon-neutral energy are the most important carbon sources in the power industry. And the CO2 cycle in the atmosphere will be affected by both of them during the utilization processes. According to characteristics of different energy and diverse energy utilization technologies, the carbon emissions during the utilization process can be obtained by the following equation [13]:
$$E=\frac{44}{12} FCO$$
(1)
E is the amount of CO2 emissions; F is the total energy of the fuel consumed, depending on the amount of fuel consumed and the heating value of the fuel; C is the carbon content per unit of fuel energy, depending on the type of fuel; O is oxidant factor, indicating the percentage of carbon in the fuel that is eventually converted to CO2, depending on whether the fuel is burned adequately. Considering the professional technology of boilers and with the advancement of combustion technology, the oxidation factor of most energy conversion processes have approached to 1 [14]. Thus, the oxidation factor will be assumed to be 1 in the following discussion [13].
The net carbon emissions of power system can be expressed as Eq. (2) based on carbon balance:
$$\mathrm{NE}=\mathrm{Source}+\mathrm{Sink}$$
(2)
Source refers to the amount of carbon emissions from carbon sources in the system, sink represents the carbon absorption from carbon sinks in the system.
Define K as the carbon recovery ratio, representing the proportion of CO2 emitted by carbon sources that is absorbed by carbon sinks. According to the elemental balance, the carbon-balanced equation for different types of energy can be established as follows. The subscripts C, CN, and CF represent carbonaceous energy, carbon-neutral, and carbon free energy, respectively:
As for carbonaceous energy:
$${\mathrm{NE}}_{\mathrm{C}}=\frac{44}{12}{F}_C{C}_C+\left(-\frac{44}{12}{F}_C{C}_C{K}_C\right)$$
(3)
The carbon recovery ratio KC < 1 and NCEC > 0 due to the technical limitation;
For carbon neutral energy:
$${\mathrm{NE}}_{\mathrm{CN}}=\frac{44}{12}{F}_{\mathrm{CN}}\kern0.1em {C}_{\mathrm{CN}}+\left(-\frac{44}{12}{F}_{\mathrm{CN}}\kern0.1em {C}_{\mathrm{CN}}\kern0.1em {K}_{\mathrm{CN}}\right)+\left(-\frac{44}{12}{F}_{\mathrm{CN}}\kern0.1em {C}_{\mathrm{CN}}\right)$$
(4)
Considering that the amount of CO2 emitted (\({F}_{CN}\cdot {C}_{CN}\cdot \frac{44}{12}\)) and absorbed (\(-{F}_{CN}\cdot {C}_{CN}\cdot \frac{44}{12}\)) by carbon-neutral energy are equal. Thus, Eq. (4) can be simplified as:
$${\mathrm{NE}}_{\mathrm{CN}}=-\frac{44}{12}{F}_{\mathrm{CN}}\kern0.1em {C}_{\mathrm{CN}}\kern0.1em {K}_{\mathrm{CN}}$$
(5)
when the carbon recovery ratio KCN > 0, the carbon recovery technology is equipped in biomass power plants and the negative emission can be obtained (NECN < 0).
However, for carbon free energy, the net carbon emission NECF always equal to 0 due to its carbon free characteristics (CF = 0), as shown in Eq. (6):
$${\mathrm{NE}}_{\mathrm{CF}}=\frac{44}{12}{F}_{\mathrm{CF}}\kern0.1em {C}_{\mathrm{CF}}\kern0.4em {\displaystyle \begin{array}{c}\underline {\underline {\kern0.6em {C}_{\mathrm{CF}}=0\kern0.7em }}\\ {}\end{array}}\kern0.4em 0$$
(6)
Based on Eqs. (3), (5) and (6), the total net carbon emission of the power system composed of fossil energy (carbonaceous energy), carbon free energy (non-biomass renewable energy) and carbon neutral energy (biomass energy) is:
$$\mathrm{NE}=\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}{\mathrm{NE}}_i=\frac{44}{12}{F}_{\mathrm{C}}\kern0.1em {C}_{\mathrm{C}}+\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left(-\frac{44}{12}{F}_i\kern0.1em {C}_i\kern0.1em {K}_i\right)$$
(7)
NE is the net carbon emission of power system. If the dimensionless parameter R is defined as the proportion of carbon contained in carbonaceous energy among the total carbon input of the system:
$$R=\frac{F_{\mathrm{C}}\kern0.1em {C}_{\mathrm{C}}}{F_{\mathrm{C}}\kern0.2em {C}_{\mathrm{C}}+{F}_{\mathrm{C}\mathrm{N}}\kern0.1em {C}_{\mathrm{C}\mathrm{N}}}$$
(8)
Equation (7) can be simplified as:
$$\mathrm{NE}=\frac{44}{12}\left[R\left(1-{K}_{\mathrm{C}}\right)-\left(1-R\right){K}_{\mathrm{C}\mathrm{N}}\right]\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\kern0.1em {C}_i\right)$$
(9)
Obviously, the power system can achieve carbon neutrality when NE ≤ 0. But in fact, the realization of carbon neutrality must meet the demand for energy products. In other words, carbon neutrality is a carbon balance under the premise of a certain power output. Therefore, carbon emission intensity is required to be adopted to characterize the carbon emission of the power system:
$${I}_{\mathrm{c}}=\mathrm{NE}/\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\kern0.1em {\eta}_i\right)=\frac{44}{12}\left[R\left(1-{K}_{\mathrm{C}}\right)-\left(1-R\right){K}_{\mathrm{C}\mathrm{N}}\right]\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\kern0.1em {C}_i\right)/\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\kern0.1em {\eta}_i\right)$$
(10)
where Ic is the carbon emission intensity of the power system, which represents the amount of CO2 emitted per unit of power generated by the power system. ηi represents the power supply efficiency, \(\sum \limits_i{F}_i\cdot {\eta}_i\) is the total power generation of the power system.
The carbon emission intensity Ic ≤ 0 must be met by power systems achieving carbon neutrality. Thus, the formula (6) can be further simplified, and the following carbon-neutral formula can be obtained:
$$\left[R\left(1-{K}_{\mathrm{C}}\right)-\left(1-R\right){K}_{\mathrm{C}\mathrm{N}}\right]\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\kern0.1em {C}_i\right)/\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\kern0.2em {\eta}_i\right)\le 0$$
(11)
For power systems with multiple energy inputs, the above carbon neutrality equation must be satisfied to achieve the goal of carbon neutrality.
The different effects of emission reduction approaches
As a major carbon emitter, the power industry will play a key role in achieving carbon neutrality. According to the criterial equation of carbon neutrality derived above, there are three major ways to establish a carbon-neutral power system: (1) Improve energy efficiency thus to save the consumption of carbonaceous energy; (2) Shift the energy structure and reduce the proportion of carbonaceous energy (3) Rebuild the balance of sources and sinks by adopting CO2 capture, utilization and storage technology to achieve low-carbon utilization of carbonaceous energy.
Energy efficiency improvement - potential
According to the Eq. (10), the consumption of carbonaceous energy can be reduced by improving the utilization efficiency, thereby reducing carbon emissions from the power system. Taking coal power plants as an example, Fig. 1 shows the trend of average standard coal consumption and its corresponding carbon emission intensity in the past 20 years in China. It can be seen from the figure that with the development of power generation technology focusing on thermal cycle upgrading, such as the improvement of initial steam parameters and the introduction of reheating and recuperation, the power generation efficiency of coal power plants has risen from ~ 20% in the middle of the twentieth century to the current ~ 47% power generation efficiency. At the same time, the average standard coal consumption of electricity has been declining year by year, dropping to 304.7 g/kWh by 2020. The reduction in coal consumption has also significantly reduced the carbon emission intensity of coal power plants. As shown in the green curve in Fig. 1, in the past 20 years, the carbon emission intensity of coal power plants in China has dropped from 1078 gCO2/kWh to 838 gCO2/kWh (the carbon content of coal is assumed to be 75 wt.% [12] and the oxidant factor as 1.0). However, it can also be found that the declining trend of power generation energy consumption has gradually flattened. The reason for this trend comes from the following reality, super-critical or ultra-super-critical power plants had already accounted around 70% of installed capacity, while “650 °C” or “700 °C” technologies are still in research due to material limitation. Thus, the potential for continuing to reduce carbon emissions through energy saving is confined.
Shifting the energy structure - reliability
Coal, natural gas and oil are the most widely used carbonaceous energy at present. Due to composition and energy characteristics of fossil fuels and technical utilization efficiency, different fossil fuels have various CO2 emission characteristics. Among fossil fuels, coal has the highest carbon content ranging from 0.024 kg /MJ to 0.026 kg /MJ [13], which means that 0.08 kg ~ 0.1 kg CO2 will be produced by direct combustion of coal with every 1 MJ energy released. CH4 is the main component of natural gas with the carbon content about 0.015 kg/MJ, which is only half of that of coal. Obviously, natural gas is lower carbon than coal from the perspective of CO2 emission characteristics. On the other hand, the average efficiency of coal-based power generation technology is only about 40%, while combined cycle burning natural gas can reach higher than 60%. Correspondingly, about 1 kg of CO2 will be emitted into the atmosphere for every 1 kWh of electricity output by traditional coal power plants, and natural gas power plants adopting combined cycle only emit about 0.35 kg of CO2 to the environment for every 1 kWh of electricity output, which is about 1/3 of that of coal power plants. Figure 2 shows the carbon emission intensity of different fossil fuels with different power efficiency, and renewable energy. Peat and coking coal represent two typical coals with low and high carbon content per calorific value, respectively. When power generation efficiency (η) equals to 100%, the value of carbon emission intensity equals to the carbon emissions per unit of fuel energy (44/12C). It’s naturally that the net carbon emission intensity are both 0 gCO2/kWh for carbon-neutral and carbon free renewable energy. Although biomass will produce a certain amount of CO2 emissions during collection, processing (~ 20 gCO2/kWh emissions) and transportation (about 3 gCO2/kWh emissions for 50 km) processes [15], the emission amount is very small and can be ignored. Thus, carbon emission reduction can be achieved by increasing the proportion of renewable energy in the power system.
From 2000 to 2020, the electricity installed capacity and generation ratio of renewable energy in China increased from 24.9% and 16.4% to 26.8% and 41.2%, respectively. This change in the energy structure has reduced the carbon emission intensity of the power system by about 30-35%. The temperature control plan of the 13th Five-Year Plan mentioned that CO2 emissions intensity need to be controlled within 550 g/kWh, mainly by increasing the proportion of renewable energy [16]. At present, most strategic studies suggest that in a carbon-neutral scenario by 2060, renewables will be the main source of electricity accounted for more than 80% electricity generation ratio [17, 18].
CO2 capture, utilization and storage technology – energy penalty
CO2 Capture, Utilization and Storage (CCUS) is the only technology that can achieve low-carbon utilization of high-carbon fuel and realize negative emissions. As shown in Fig. 3, for carbonaceous energy, CCUS constructs a carbon cycle, capturing CO2 released from carbonaceous energy and storing it in the ground. For carbon-neutral energy, CCUS breaks the original carbon cycle by storing the CO2 absorbed from the atmosphere into the ground, creating a negative emission effect. The more fossil energy power plants or biomass power plants equipped with CCUS technology, the higher the recovery ratio KC or KCN in the carbon neutral formula. For a power plant, KC or KCN represents the CO2 recovery rate of the specific power plant, or it represents the proportion of the power plants equipped with CCUS in a regional energy system after comprehensive consideration the CCUS deployment conditions.
At present, the average carbon emission intensity of 600 MW newly-built power generation plants are about 770 gCO2/kWh and 350 gCO2/kWh for ultra-supercritical power plants and gas-fired power plants, respectively. The carbon emission intensity is close to or even better than that in the United States and the United Kingdom [19]. Under the carbon neutral scenario, the role of CCUS has to be shifted or significantly enhanced. After installing CCUS, the average carbon emission intensity of coal power plants can be reduced to about 80 gCO2/kWh when the carbon capture rate reaches 90%. However, it’s inevitable that some coal fired power plants are un-retrofitable, due to practical reasons like lacking of space for CO2 separation unit, or no matched storage sites. So, it will be difficult to install CCUS for all power plants in a specific region, which also means that KC for a regional power system could not reach 100%. For biomass power plants, the average carbon emission intensity is about 1200 gCO2/kWh without considering the carbon source of the biomass itself [20, 21]. If CCUS (90% carbon capture rate) is further introduced, the average carbon emissions will drop to about − 1080 gCO2/kWh under 90% capture rate, creating so called negative emission effect. Correspondingly, an capture rate K for an energy system with both KC and KCN can be defined, indicating the ratio of the total amount of CO2 captured of the power system with CCUS to the total CO2 emissions of the power system without CCUS. Similarly, K can be derived from Eq. (10) as follows:
$$K=R-\left[R\left(1-{K}_{\mathrm{C}}\right)-\left(1-R\right){K}_{\mathrm{C}\mathrm{N}}\right]$$
(12)
By defining K, the overall contribution and the importance of CCUS to carbon neutral system can be reflected. Thus, Eq. (9) can be further written as:
$${I}_{\mathrm{c}}=\frac{44}{12}\left(R-K\right)\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\kern0.1em {C}_i\right)/\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\kern0.1em {\eta}_i\right)$$
(13)
R represents the ratio of system carbon emissions to the total carbon input, and K represents the ratio of captured carbon to input carbon. When R = K, the energy system achieves carbon neutrality.
It is worth noting that although increasing capture rate can directly reduce carbon emissions, the capture rate and ηi are not independent of each other. As CO2 capture requires additional energy consumption, the power generation efficiency of power plants equipped with CCUS technology will be significantly reduced, which is currently one of the key obstacles hindering the large-scale promotion of CCUS technology. Therefore, the influence of the capture rate K on ηi should be further considered for Ic. Assuming that the capture energy consumption per CO2 (ECCO2) is the same for carbon neutral and carbonaceous energy, the carbon emission intensity Ic after modifying ηi can be written as follows:
$${\displaystyle \begin{array}{l}{I_{\mathrm{c}}}^{'}=\frac{44}{12}\left(R-K\right)\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\cdot {C}_i\right)/\left[\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\cdot {\eta}_i\right)-\frac{44}{12}\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\cdot {C}_i\cdot K\cdot {EC}_{\mathrm{CO}2}\right)\right]\\ {}\kern1.3em =\frac{44}{12}\left(R-K\right)\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\cdot {C}_i\right)/\left[\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}{F}_i\cdot \left({\eta}_i-\frac{44}{12}{C}_i\cdot K\cdot {EC}_{\mathrm{CO}2}\right)\right]\\ {}\kern1.3em =\frac{44}{12}\left(R-K\right)\sum \limits_{i=\mathrm{C},\mathrm{CN}}\left({F}_i\cdot {C}_i\right)/\left[\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}{F}_i\cdot \left({\eta}_i-{\eta_i}^{\hbox{'}}\right)\right]\end{array}}$$
(14)
\(\frac{44}{12}\sum \limits_{i=\mathrm{C},\mathrm{CN},\mathrm{CF}}\left({F}_i\cdot {C}_i\cdot K\cdot {EC}_{\mathrm{CO}2}\right)\) represents the total capture energy consumption of CCUS, ηi’ is the reduction in power generation efficiency of the original system caused by the adoption of CCUS.
The relationship and trend of Ic and Ic’ are shown in Fig. 4. Considering the deterioration of system performance due to CCUS, the carbon emission intensity Ic’ of the system after the introduction of CCUS will be higher than that under the ideal situation Ic (the impact of CCUS on system performance is not considered). When R = K, the energy system achieves carbon neutrality.
As discussed above, the contribution of each technical approach on carbon neutrality can be further evaluated by Eq. (10). From 2000 to 2020, the total CO2 emission intensity of China’s power sector decreased by 317.3 gCO2/kWh (from 878.6 gCO2/kWh to 561.3 gCO2/kWh), whose calculation is based on generation data of China Electricity Council (CEC) and fuel emission factor data from Intergovernmental Panel on Climate Change (IPCC) [13, 22] with the oxidant factor being assumed to be 1.0. The share of power generation from non-fossil fuel had increased from 19% to 32.2%, contributing 157 gCO2/kWh of CO2 emission reduction. Meanwhile, average coal consumption of fossil fuel power plants had been decreased from 392 g/kWh to 305 g/kWh with the power generation proportion of that decreased to 67.8%, which leads to 160.3 g CO2/kWh of CO2 mitigation. It can be seen that the increment of renewable share and energy saving of fossil fuel power plants play the similar role (nearly half to half contribution for total CO2 emission reduction) for decarbonization of power sector in China in the past two decades. Due to the immaturity of CCUS technology, the contribution of it is negligible in the absence of domestic CCUS demonstration projects.
In addition, as renewable energy is expected to account for about 80% electricity generation ratio in 2060 [17, 18], which will definitely become the main contributor. At the same time, if the average coal consumption of thermal power plant declines to about 200-250 g/kWh in 2060 with around 20% electricity generation ratio, about 10-20% of the carbon emission intensity reduction of power system can be anticipated by improving energy efficiency. Finally, the left part of carbon emissions will be covered by CCUS technology on the scenario of carbon neutrality.
Indispensable role of CCUS in carbon neutral target
Due to the high reliance of carbonaceous energy in China and the instability and unsafety of high portion of renewable energy, it’s really difficult to imagine an energy system totally phasing out the utilization of carbonaceous energy over a long period of time, which means R will not be 0. Correspondingly, according to Eq. (13), it’s apparently that Ic could not reach 0 when R is not 0, only if K equals R, and naturally, the CCUS technology for both carbonaceous energy and carbon neutral energy is indispensable in achieving carbon neutral target. However, KC and KCN play the different role in power system reaching carbon neutral target.
Based on Eq. (11), the relationship of capture rates between fossil fuel (KC) and biomass (KCN) can be derived as follows in the carbon neutral scenario:
$${K}_{CN}=\frac{R}{1-R}-\frac{R}{1-R}{K}_C$$
(15)
K
C and KCN represents the capture rates of carbonaceous energy and carbon neutral energy for an area or an energy system. On the basis of Eq. (15), Fig. 5 depicts the relationship between energy inputs and capture rates of different CO2 capture technologies in the carbon neutral scenario. Considering that the typical or the optimized value of KC for fossil fuel power plants is around 0.7 ~ 0.9 [23,24,25], KC is adopted as a variable parameter with specific value of 0.7, 0.8 and 0.9 in the analysis. As shown in Fig. 5, with the increase of R, the ratio of KCN / KC increases dramatically as its growth rate increases under a specific KC, which implies a much higher demand for negative emission technology if more coal fired power plants are deployed. The left side of Fig. 5 indicates the optimum area for CCUS retrofit under a comparatively lower capture rates. However, carbon neutrality is difficult to be achieved even with negative emission technology when R increases to a certain value (as shown in the shaded areas in Fig. 5), such as R > 0.9 under KC = 0.9 condition, due to the unreachable value of KCN (KCN ≥ 0.9). Similarly, when KC increases (less than 0.9), the demand of carbon neutral system for negative emission technology declines, and a higher proportion of carbonaceous energy can be input into energy system under carbon neutral scenario.