## 4 Carbon Footprint Calculation and Analysis  
The life cycle approach (LCA) was used to initially study the carbon emissions of samples CGBS1 and CGBS4. The system boundary of CGBS in this study is shown in Figure 13. The LCA includes the entire life cycle of raw material extraction and transportation, product production, product transportation, solid waste recycling and reuse [36]. Based on the above life cycle system, the calculation model of CGBS carbon emissions is established, as shown in Equations (3)-(8):  
\[\mathrm{C_{e}}=\mathrm{M_{1}}\times\mathrm{CEF_{e1}}+\mathrm{M_{2}}\times \mathrm{CEF_{e2}}+\mathrm{M_{3}}\times\mathrm{CEF_{e3}}+\mathrm{M_{4}}\times \mathrm{CEF_{e4}} \tag{3}\]  
\[\mathrm{C_{t}}=(\mathrm{M_{1}}+\mathrm{M_{2}}+\mathrm{M_{3}}+\mathrm{M_{4}}) \times\mathrm{S_{t1}}\times\mathrm{CEF_{t1}}+\mathrm{M_{s}}\times\mathrm{S_{t2 }}\times\mathrm{CEF_{t2}} \tag{4}\]  
\[\mathrm{C_{p}}=\mathrm{M_{s}}\times\mathrm{CEF_{p}} \tag{5}\]  
\[\mathrm{C_{r}}=\mathrm{M_{1}}\times\mathrm{CEF_{r1}}+\mathrm{M_{2}}\times \mathrm{CEF_{r2}} \tag{6}\]  
\[\mathrm{C_{a}}=\mathrm{M_{s}}\times\mathrm{CO_{2 uptake}}(\%) \tag{7}\]  
\[\mathrm{C}=\mathrm{C_{e}}+\mathrm{C_{t}}+\mathrm{C_{p}}+\mathrm{C_{r}} \tag{8}\]  
Figure 12: SEM images of CGBS at different carbonization pressures. (**a**) CGBS1, (**b**) CGBS2, (**c**) CGBS3, (**d**) CGBS4, (**e**) CGBS5 and (**f**) CGBS6. where C\({}_{\rm e}\) is the carbon emission during the introduction of raw materials, CO\({}_{2}\) equivalence; M\({}_{1}\), M\({}_{2}\), M\({}_{3}\) and M\({}_{4}\) are the masses of FA, CG, NaOH and water in the functional unit AGBS, t; CEF\({}_{\rm e1}\), CEF\({}_{\rm e2}\), CEF\({}_{\rm e3}\) and CEF\({}_{\rm e4}\) are the carbon emission factors of FA, CG, NaOH and water extraction stages, KgCO\({}_{2}\)eq\(\cdot\)t\({}^{-1}\); C\({}_{\rm t}\) is the total carbon emission of various raw materials, products and energy transportation, CO\({}_{2}\) equivalence; CEF\({}_{\rm t1}\) is the carbon emission factor of the raw material transportation stage, KgCO\({}_{2}\)eq\(\cdot\)t\({}^{-1}\); CEF\({}_{\rm t2}\) is the carbon emission factor in the CGBS transportation stage, KgCO\({}_{2}\)eq\(\cdot\)t\({}^{-1}\); S\({}_{\rm t1}\) and S\({}_{\rm t2}\) are the raw material transportation distance and CGBS transportation distance, respectively, km; M\({}_{\rm s}\) is the mass of the functional unit CGBS, t; C\({}_{\rm p}\) denotes the total carbon emissions generated during the product production process, CO\({}_{2}\) equivalence; CEF\({}_{\rm p}\) is the carbon emission factor in the CGBS production stage, KgCO\({}_{2}\)eq\(\cdot\)t\({}^{-1}\); C\({}_{\rm r}\) is the carbon emission reduced by recycling industrial solid waste, and its value is negative, CO\({}_{2}\) equivalence; CEF\({}_{\rm r1}\) and CEF\({}_{\rm t2}\) are the carbon emission factors of CG and FA recycling, and their values are negative, KgCO\({}_{2}\)eq\(\cdot\)t\({}^{-1}\); and C\({}_{\rm a}\) denotes the carbon emissions reduced by functional unit CO\({}_{2}\) sequestration, and its value is negative, CO\({}_{2}\) equivalence. C is the total carbon emission of functional unit CGBS, CO\({}_{2}\) equivalence. Table 5 shows the results of the carbon emission calculation. As shown in Table 5, the CGBS's primary source of carbon emissions is the transportation of raw materials and products. Consequently, it is essential to enhance transportation efficiency, prioritize the local sourcing of materials, and minimize long-distance transport. The total carbon emissions of CGBS1 and CGBS4 are 33.21 kg and \(-\)0.64 kg, respectively. It can be seen that after adding carbonization pressure, negative carbon filling can be achieved throughout the life cycle. In 2023, China's carbon dioxide emissions amounted 12.6 billion tons, and the goaf space generated by coal mining is 3.3 billion m\({}^{3}\). If CGBS is used to store CO\({}_{2}\) in the goaf, carbon emissions reductions of 2.0588 million tons can be achieved throughout the life cycle.