3.1.1. Results of the Goal and Scope Definition Phase
This LCA study was carried out in accordance with the guidelines contained in standards [
24,
25,
26,
27]. The final goal of this LCA study, defined in accordance with the ISO 14040 standard, was to assess selected environmental aspects of lignite extraction using the HBM method from a single borehole. Moreover, another goal of this LCA study was to compare the formed lignite mining options of HBM technology usage, differentiated in terms of the amount of water and the amount of fuel used and, hence, differentiated in terms of extraction process duration and efficiency. The conducted assessment aimed to illustrate the parameters of the mining process in quantitative terms, taking into account the environmental aspects. The authors assume that the conducted analysis will contribute to the general development of the HBM method of lignite extraction. The results of the analysis will contribute to a more complete understanding of HBM technology, also from an environmental perspective, by quantifying the parameters of the mining process.
The production system to be studied together with the system boundaries defined for the system are presented in the figure below (
Figure 2).
At this point, it should be emphasized that for the purpose of this LCA study, the investigated system was limited only to the technological activities included directly in the lignite hydro borehole mining (HBM) process. Other related processes or systems, such as deposit exploration, HBM tool production together with its transportation and liquidation, infrastructure construction and liquidation, mining and transportation of backfilling material, surface processes of lignite separation from water and lignite drying processes, among others, were omitted from the scope of this study. Some of the omitted processes and systems, however, were additionally included and used only for the purpose of the land use category impact assessment. Moreover, it was assumed that the water used for the lignite hydro-cutting and lignite hydro-crushing unit processes would be put into a closed circuit using a cooperating system. The assessment of this system was beyond the scope of this LCA study; however, the amounts of water used in HBM production take into account its interaction. Due to the existence of the water closed circuit, the amount of water used will be much smaller than the total amount of water actually consumed. Furthermore, it should be emphasized that the system defined for this study can be interpreted in two ways: either as a product system within which the lignite mined and transported to surface is treated as a product, using a “cradle-to-gate” approach (i.e., what happens next within the product life cycle is being omitted) or interpreted simply as a process, specifically, the process of lignite HBM production from a single borehole. While interpreting the defined system in any of these ways, the lignite mined and transported to the surface is treated as a product. Thus, the function of the investigated system has been defined as mining and transporting of lignite to the surface. Additionally, the backfilling of the created cavern and liquidation of the production borehole are treated as additional functions of the system. Most importantly, 1 Mg (1 megagram) of lignite extracted and transported to the surface was assumed as the functional unit (the reference unit to which the inputs and outputs to the product system are related). This means that all data collected in the second phase of the LCA study, the LCI, will be then converted in relation to it (e.g., 4.01 kg of CO2 emitted per 1 Mg of lignite mined and transported to surface).
In order to describe the defined system and be able to assess it in the most sufficient way, some assumptions had to be made. The main assumptions completing the basic necessary knowledge of the defined system are presented in the table below (
Table 2).
Some of the important information describing the defined system can be obtained from simple calculations based on the above assumptions, as presented in the table below (
Table 3).
Based on the previous findings of the HydroCoal Plus project [
34,
35,
36,
37], it was assumed that the lignite HBM exploitation cavern would be cylindrical in shape, while its height would cover the total assumed lignite seam depth interval, i.e., it would be equal 25 m. Knowing the target radius of the production cavern, simple calculations allowed to obtain the lignite target volume and mass to be mined. However, these values increased by adding the amount of lignite obtained directly from the drilling process. Due to the technological solutions adapted to the hydro borehole mining technology, it was necessary to drill the production borehole down to the lignite seam bottom, from where the hydro borehole mining process commences [
11]. Therefore, an insignificant amount of lignite (5.9 m
3) must be drilled through. Furthermore, within the main unit processes, the amount of lignite cut was assumed to be equal to the amount of lignite crushed and equal to the amount of lignite transported to surface via “air-lift” [
35]. In other words, the amount of lignite was assumed to remain the same for all lignite-involved unit processes. Moreover, the unit processes of hydro-cutting and hydro-crushing were assumed to operate almost simultaneously, with similar process efficiencies.
For this non-site-specific assessment, the lignite deposit was assumed to be previously dewatered (via other system excluded from this LCA study). Moreover, the analyzed system cooperates with other system, which contributes to the amount of water to be used and consumed in a closed circuit. Taking into account that the water would be recovered and put into a closed circuit, the actual water consumption would be equal to the water losses from a closed circuit. This was estimated based on the sum of the amounts of water escaping through the bottom and sides of the exploitation cavern during unit processes related to the use of water, and the amounts of water absorbed into the mined lignite. For the defined system, this amount was estimated as 2%, taking into account a general geological conditions of lignite deposition and lignite material characteristics. It was also assumed that the lignite seam was homogeneous, i.e., the material parameters were the same throughout the deposit and there were no inserts of other material in the lignite to be mined [
38,
39,
40].
Moving forward, the impact categories selected for this LCA study were narrowed down only for the impact categories relevant for the previously defined goal of the study. The main criterion for choosing the impact categories was the availability of data in the second phase of the LCA study (LCI). The selected categories are as follow:
The chosen methodology for the life cycle impact assessment included a third phase in the LCA study, i.e., an LCIA to present the results of the second phase of the LCA study, the LCI, in relation to the functional unit of the system, assign it to the selected impact categories, and subsequently describe it [
41,
42].
In this LCA study, a number of different lignite HBM production options were compared. First, variants were differentiated within the lignite hydro-cutting unit process. There are two variants that differ from each other in terms of the amount of energy used and process parameters applied (i.e., amount of water and pressures used) and, hence, differ from each other in terms of extraction process duration and efficiency. Second, due to the estimated character of the data collection process, in some cases, data were given as ranges of values. Therefore, it was decided to refer to these values throughout differentiating it for two scenarios:
Therefore, when considering that both lignite HBM production variants, differentiated within the hydro-cutting unit process, have a desired energy consumption and an undesired energy consumption scenario, four different options were compared in total (
Figure 3):
Option I—a desired energy consumption scenario for the first variant of process duration and efficiency;
Option II—an undesired energy consumption scenario for the first variant of process duration and efficiency;
Option III—a desired energy consumption scenario for the second variant of process duration and efficiency;
Option IV—an undesired energy consumption scenario for the second variant of process duration and efficiency.
3.1.2. Results of the LCI (Life Cycle Inventory) Phase
In this phase of the LCA study, the data collection and calculation procedures were performed in order to quantify the relevant inputs and outputs for the previously defined production system. The process of data collection was very time consuming. In this study, a technology for lignite mining that has not been recognized from an environmental point of view so far was used. Therefore, the data could not have been measured or read from the industry literature; thus, all of the data were ultimately collected based on assumptions, estimates and calculations. The previously mentioned survey constituted one of the tools for data collection. Relatively greater or smaller attention was paid to the distinguished unit processes of the defined production system, according to the decided relevance of each of the unit processes in terms of meeting the requirements of the defined goal and scope of this study and according to the data’s availability.
As an example, the details of the lignite hydro-cutting unit process are presented in the figure below (
Figure 4), including the main inputs and outputs as well as the direct inputs from the environment and direct emissions into the environment.
The results of the data collection for this unit process are presented in the figure below (
Figure 5).
The aggregate power was estimated to be in a range between 163 and 363 kW for different process efficiency variants. The lignite cutting efficiency for the first variant was estimated for 60 Mg/h, while the cutting efficiency for the second variant was estimated to be in a range between 6 and 12 Mg/h. Based on this, the total lignite cutting time in the cavern was calculated. It was equal to 28 h and 9 min for process efficiency variant I, and between 140 h 45 min (with 12 Mg/h process efficiency) and 281 h 30 min (with 6 Mg/h process efficiency) for variant II. Then, the total energy consumption for different options was possible to be calculated based on the aggregate power rating values and the estimated 70% power usage. Fuel (diesel) consumption data and noise emission data were estimated based on the experts’ knowledge and experience. The CO2 emissions data were estimated based on simple dependencies between diesel consumption and CO2 emissions, allowing to calculate total CO2 emissions from hydro-cutting process for all production options. The water pressures and water usage data (in a closed circuit) were assumed based on the knowledge gained so far within the HydroCoal Plus project. For both of the variants, the desired water pressure and water usage values were assigned to the desired energy consumption scenario (smaller values of the data range), while the undesired water pressure and water usage values were assigned to the undesired energy consumption scenario analogically (greater values of the data range). Then, based on the assumed 2% total water loss from a closed circuit, the water consumption values were calculated. The land use remained neglected at this moment, taking into account that it was assessed for the whole defined system separately.
For all of the other unit processes defined in the system, the collected data were presented in a similar manner. All of the collected data allow to conclude that for the assessment of the HBM extraction process from a single borehole within impact categories, such as energy consumption, fuel consumption and carbon footprint, the following unit processes will be of key importance:
Unit process of borehole drilling and casing;
Unit process of lignite hydro-cutting;
Unit process of lignite hydro-crushing.
For the impact category of water consumption, only the unit processes of lignite hydro-cutting and lignite hydro-crushing are of key importance. Based on the data collected, the four options of lignite HBM production to be compared were formed (
Table 4). The further comparison of the production options concerned only the impact categories mentioned above. During the data collection, the rest of the unit processes of the system were decided to be of very low significance concerning the impact categories mentioned above.
Moving forward to the remaining impact categories, only the unit process of borehole drilling and casing can be indicated as one with importance for the solid waste generation impact category. Taking into account all the assumptions made within the scope of the study, the only solid waste resulting from the system’s operation would be generated from drilling. The material generated from drilling would be 10.7 m3 in total, including 4.8 m3 of waste material (overburden) and 5.9 m3 of lignite.
The abiotic resource depletion impact category concerns the whole defined system, and specifically, the main function of the defined system, which is mining and transporting of lignite to the surface. Moreover, this impact category can be addressed to the unit process of cavern backfilling, when treating the backfilling material as an abiotic resource as well. Due to the fact that lignite is being classified as an abiotic resource, the function of the defined system itself contributes to the abiotic resource depletion impact category. As it has been indicated, the total amount of lignite produced by the defined system was equal to 1697 Mg (1257 m3).
As established during the goal and scope definition, the land use impact category was assessed separately for extended boundaries of the defined system (
Figure 2). With regard to the land use impact category, it would be illogical to consider the HBM process itself (in such a case the land use would be minimal, i.e., equal to the cross-sectional area of the production borehole). Therefore, other systems cooperating with the defined system and the associated infrastructure around the production borehole were taken into account. According to [
43], the area of land development for the purposes of HBM lignite extraction consists of the following elements (
Figure 6):
A borehole head;
Pipe station (column modules and HBM tool head);
Clean water tanks;
Clean water filters for hydraulic aggregates (pumps);
Settling tank;
Pump station;
Compressor station;
Operator’s cabin (container);
Warehouse and workshop;
Access roads and maneuvering area;
Energy substation.
The required minimum area around the production borehole should be approximately 2000 m
2 (40 × 50 m), while the comfort area may have the dimensions of 80 × 100 m (8000 m
2) [
43].
3.1.3. Results of the LCIA (Life Cycle Impact Assessment) Phase
In this phase of the performed LCA study, all of the selected impact categories were assessed. Where possible, the previous results were recalculated in relation to the functional unit of the defined system, which was 1 Mg of lignite mined and transported to the surface. Taking into account all the assumptions describing the defined system, the total amount of lignite mined and transported to the surface (including the lignite obtained from the drilling process) for each of the described mining options was equal to 1697 Mg. This value was used to recalculate the process parameters in relation to the functional unit. To simplify, the “1 Mg of lignite mined and transported to surface” will be hereinafter referred to as “1 Mg lignite”.
The energy, fuel and water consumptions were assessed for three most relevant unit processes recognized, i.e., borehole drilling and casing; lignite hydro-cutting; lignite-hydro crushing. All previous results on the energy, fuel and water consumption and CO
2 emissions were recalculated in relation to the functional unit of the defined system (
Table 5).
A comparison of the energy, fuel and water consumption for individual unit processes for different options is presented in figures below (
Figure 7).
In addition, the values for the remaining selected impact categories, collected during the LCI phase, were recalculated at this stage in relation to the functional unit. This concerned the impact categories of solid waste generation and abiotic resource depletion. Taking into account all the assumptions made for the defined system, the obtained results are presented in the table below (
Table 6).
The only solid waste resulting from the system’s operation would be generated from drilling (drilled through overburden), in the amount of 4.8 m3 per 1697 Mg of lignite produced from a single cavern.
Due to the fact that lignite is being classified as an abiotic resource, the function of the defined system itself contributes to the abiotic resource depletion impact category. The total amount of lignite produced by the defined system was equal to 1697 Mg.
As it has been indicated during the scope definition, the land use impact category was assessed separately for an extended system. While treating the required minimum area and the comfort area around the production borehole as two scenarios (i.e., desired and undesired) in terms of the land use and recalculating the estimated areas in accordance with the reference unit, then:
The interpretation of the results obtained in the LCIA phase is presented in the following.
3.1.4. Key Insights of the Interpretation Phase
During the data collection phase, it was determined that the unit processes of borehole drilling and casing, lignite hydro-cutting and lignite hydro-crushing were the key processes for the environmental impact categories such as energy consumption, fuel consumption and carbon footprint. During the study, it was shown that the energy consumption differed for the four resulting mining options. Differences in energy consumption for individual variants resulted from the adopted different efficiencies of individual unit processes and associated different individual duration times. Additionally, the differences in energy consumption result from different aggregate power adopted for individual extraction options. The analysis showed that options I and II (energy consumption in relation to the functional unit equal to 4.38 and 6.95 kWh/1 Mg lignite, respectively) will be much less energy consuming than options III and IV (energy consumption in relation to the functional unit equal to 37.09 and 30.77 kWh/1 Mg lignite, respectively). In addition, the energy consumption of the individual unit processes will vary from option to option (
Figure 7a). Each time (i.e., for all of the options) borehole drilling and casing turned out to be the least energy-consuming unit process among the processes identified as having a significant impact on energy consumption. Nevertheless, for HBM mining technology, the total energy consumption of the borehole drilling and casing process will depend directly on the depth of the lignite seam. As the depth of lignite deposition increased, the proportion of energy consumption by the drilling process in relation to other processes increased. Similar results were obtained for other impact categories such as fuel consumption and carbon footprint, which are directly related to energy consumption. Additional differentiation of the results resulted from the established ranges of fuel consumption for a unit process of lignite hydro-cutting. This range was set at 27–90 L/h, where the value equal to the lower limit of the range was assigned to the desired variants, and the value equal to the upper limit of the range was assigned to the undesirable variants. The analysis showed that options I and II (fuel consumption in relation to the functional unit equal to 1.04 and 2.14 L diesel/1 Mg lignite, respectively) consumed significantly less fuel than options III and IV (fuel consumption for the functional unit equal to 8.80 and 9.77 L diesel/1 Mg lignite, respectively). Moreover, similar to energy consumption, the fuel consumption by individual unit processes varied for each of the options (
Figure 7b). Each time (i.e., for all options), among the processes identified as those significantly affecting fuel consumption, the least amount of fuel was used for the borehole drilling and casing unit process. CO
2 emissivity within individual unit processes and for individually defined production options resulted directly from fuel consumption, and the obtained results were directly proportional. The analysis showed that options I and II (CO
2 emissivity in relation to the functional unit equal to 2.79 and 5.76 kg CO
2/1 Mg lignite, respectively) were characterized by a significantly lower emissivity than options III and IV (emissivity CO
2 in relation to the functional unit equal to 23.65 and 26.26 kg CO
2/1 Mg lignite, respectively). Moreover, as for energy and fuel consumption, the CO
2 emissivity through individual unit processes varied for each option (
Figure 7c). Each time (i.e., for all options), among the processes identified as those significantly affecting CO
2 emissions, the lowest CO
2 emissions were for the borehole drilling and casing unit process. Mining options I and II seemed to be the most likely ones. Lower energy consumption values, together with the resulting fuel consumption values and CO
2 emissions values, resulted primarily from the high efficiency of the process adopted for these options (60 Mg/h). However, the degree of recognition of the process did not allow for an unequivocal assumption of the efficiency of the process, which will depend on many parameters describing a specific lignite deposit to which a defined mining system can be adapted. Hence, in options III and IV, significantly lower values of the process efficiency were assigned (6–12 Mg/h).
For the water consumption category, unit processes such as lignite hydro-cutting and lignite hydro-crushing were examined, since only for these processes is water the supplied medium. The water consumption values resulted from the estimated water usage (in a closed circuit) and the assumed losses of the closed circuit of water. The other systems whose functions will complement each other will cooperate with the defined system. It is assumed that much of the water in use within the operation of the studied system can be recovered through the other system of lignite separation from the water, and then reused in the unit processes of the system analyzed in this LCA study. Thus, water could be treated both as an input into the system and an output from the system (as a coproduct) that is being fed into a closed circuit (
Figure 2). The variation in the total water consumption resulted from different values of water usage estimated for individual unit processes for the different options as well as different duration times of the individual options, which resulted from the efficiency of the processes. It was found that for each of the four production options, the water consumption of the lignite hydro cutting process will be significantly greater (three to four times greater) than the water consumption of the lignite hydro crushing process. The total water consumption for the various options will be much lower for options I and II than for options III and IV. This is mainly due to the much lower efficiency of the unit processes for options III and IV, and the longer duration of the processes associated with it (
Figure 7d). For the lignite “air-lift” vertical transport unit process, it was assumed that the amount of water supplied to the system during the two abovementioned processes would be sufficient. Moreover, the authors of this article see an opportunity to use water from the complementary drainage processes (other system) of a hypothetical lignite deposit and put this water into the aforementioned closed circuit.