1. Introduction
Agricultural systems are significant contributors to global climate change and ecosystems degradation [
1]. Local, regional, and global agreements are increasingly mandating legislative and regulatory actions to restrict emissions to mitigate the short-term and long-term environmental degradation. However, both legislative and regulatory efforts to reduce environmental impacts, especially greenhouse gas (GHG) emissions, will eventually put a more significant burden on agricultural and industrial sectors as well as increase the cost of production. Livestock production, in particular, has been recognized as a significant source of GHG emissions and a driver of both freshwater and marine water eutrophication [
2,
3]. Therefore, the vulnerability of livestock production and the agriculture sector to climate change further incentivizes the search for and adoption of sustainable agricultural practices [
4].
In livestock production, manure management is a significant source of direct GHG emissions, such as methane (CH
4) and nitrous oxide (N
2O) [
5]. Land application is the most common practice to handle swine manure to use available nutrients for crop production. However, applying swine manure to crop and grass fields where nutrients are available more than agronomic crop needs or where fields have historically received large volumes of manure application increase environmental risks to surrounding ecosystems. Liquid manure management systems, relevant to swine production, are also a significant source of gaseous emissions. Liquid manure storage promotes anaerobic conditions, which transform organic matter into CH
4 and ammonia (NH
3). Besides, uncontrolled anaerobic and aerobic conditions initiate nitrification-denitrification processes, which convert a share of manure nitrogen to N
2O, which is a potent GHG. Solid-liquid separation of swine manure has been recognized as an emission mitigation strategy. However, increased N
2O and CH
4 emissions have been reported during storage of manure-separated solids [
6]. Transforming separated solids into a gas fuel (syngas) and a stable nutrient-rich co-product (biochar), via gasification, can potentially reduce emissions associated with manure-separated solids and generate value-added products. Furthermore, gasification-derived biochars have been shown to have adsorbing characteristics for various organic contaminants such as
p-Cresol [
7,
8].
Evaluating emissions and impacts associated with this conversion strategy, i.e., gasification of swine manure solids, can facilitate adoption and expand the set of technologies available to livestock producers for manure management. Life cycle assessment (LCA) is an essential tool to assist decision-makers by evaluating the environmental performance of proposed management strategies. According to ISO standard 14040 [
9], LCA considers the various input and output flow, and the corresponding environmental burdens, resulting from production, consumption, and disposal of associated product systems. Energy recovery from swine manure incineration was found to be a promising pathway to reduce GHG emissions associated with manure management [
10]. Several LCA studies have reported on the performance on anaerobic digestion as manure management and energy recovery as a sole feedstock or in combination with other biomass streams [
11,
12,
13]. Wu et al. [
10] performed an LCA comparing GHG emissions between land application and gasification as manure management practices. The study showed that gasification has high potential to reduce GHG emissions due to the environmental benefits of syngas production and biochar application to crop field. Biochar also has been used for carbon sequestration, soil amendment and biomass waste management [
14].
In a comprehensive review of swine manure conversion technologies, Sharara and Sadaka [
15] highlighted the scarcity of research studies that investigated swine manure solids gasification and pyrolysis. Accordingly, it was recommended to develop an LCA of swine manure management systems. Therefore, the objective of this study is to evaluate potential environmental impacts of a manure management scenario that utilizes thermal gasification of swine manure solids as a disposal/energy retrieval strategy using consequential life cycle assessment (CLCA) methodology.
3. Results and Discussions
3.1. Impact Assessment
Table 6 presents a summary of the cumulative potential environmental impacts of the swine manure management scenario according to selected categories. Positive impact characterization values indicate an added environmental burden, while negative values represent avoided burden. Detailed descriptions are addressed in the following sections.
3.2. Global Warming Potential (GWP)
The proposed manure management scenario has net emissions of 166 kg CO
2-eq emitted per ton of swine manure slurry treatment. A detailed representation of the contribution of each stage to the cumulative GHG emissions is shown in
Figure 2. Emissions during manure storage under slatted floors in the house and during external storage represented the majority of the GHG emissions, with the two stages contributing 42.1% and 35.1% respectively of the total emissions. This significant contribution is attributed to the high levels of N
2O and CH
4 emissions during these two steps, with both gases having a significantly higher impact on global warming potential. Similarly, the third-largest contributing stage to GHG emissions is post-separation slurry storage, i.e., 22.4% of scenarios of GWP.
Manure solids gasification and syngas combustion (in a boiler), represented as one coupled process (
Figure 2), contribute - 3.54% of the total GWP. The net negative contribution here indicates that the avoided GHG emissions by syngas combustion, instead of natural gas, completely offset the combined emissions from syngas combustion and those associated with gasifier electricity consumption. Even though the low hot gas efficiency, 70%, and the low boiler efficiency, 78%, was used in this model, the overall ratio of avoided natural gas use resulted in a net negative GWP.
GWP for drying manure solids, 6.68 kg CO2-eq, represented 4.02% of overall GWP emissions. Despite being an energy-intensive process, the low GWP contribution here for drying is attributed to the fact that the process heat is recycled from the gasification-boiler output heat, which reduces the overall energy requirement for drying and thus the impact. The following stages: pumping, stirring, separation, and transportation cumulatively contributed 3.49% of the total GHG emissions. Land application of liquid slurry and biochar, which is a co-product of thermal gasification, contributed net negative GHG emissions (- 5.92 kg CO2-eq) due to the credit of displacing synthetic fertilizer. One thing to note is that CO2 emission during land application is accounted for as biogenic CO2 emission. The land application represents a 3.57% reduction of total GHG emissions.
3.3. Fossil Fuel Use
Cumulative fossil fuel energy use in this scenario was - 58.0 MJ per functional unit.
Figure 3 details the individual contribution of manure management stages to overall fuel consumption. Manure storage steps, from an energy perspective, were all-passive and therefore had no fossil fuel expenditure or saving. The maximum energy burden was associated with the drying stage, which represented 62.0% of total fossil fuel energy input, followed by the slurry transportation stage, which represented 24.6% of total fossil fuel energy input. The gasification-boiler stage was attributed with the net negative energy use of - 95.7 MJ, by offsetting natural gas firing to produce the credited amount of thermal energy. The energy demand for the drying process, 107 MJ, represents the electricity demand in the dryer, which cannot be met through the gasification-boiler stage supply. The primary energy saving in this scenario, - 135 MJ, was attributed to the consequences of slurry-biochar land application. This savings is from the avoided synthetic fertilizers and the fossil fuel energy used in their production. For illustration, production of 1 kg N fertilizer requires 88.0 MJ of energy using global unit process of ecoinvent v3.4 database [
16]. Similarly, production of 1 kg of P
2O
5 and K
2O require 20.1 and 18.4 MJ of fossil fuel energy in their production.
3.4. Water Depletion
This category indicates the total water use from different water sources: lakes, rivers, and wells. In this study, total water depletion was a process credit, i.e., avoided water depletion of 0.015 m3 per functional unit. This credit is an indirect water-saving resulting from displacing synthetic fertilizers with the slurry-biochar mixture. The difference between land application impacts on water depletion, - 0.112 m3, and total impact, 0.111 m3, is attributed to all the energy-positive stages in the scenario. However, the savings accrued by displaced fertilizers outweighed the combined water depletion potential for these stages.
3.5. Marine Eutrophication
This mid-point impact category expresses the amounts of nutrients emitted, expressed in units of kg N equivalent, which potentially reach marine water causing eutrophic conditions. The studied scenario had a net positive (a burden) of marine eutrophication, 88.5% of which is attributed to the slurry-biochar land application. This results from nitrate (NO3) leaching, and NH3 emissions following land application. Considering the full lifecycle, 80% of marine eutrophication potential is attributed to NO3 leaching, while the remainder is due to NH3 emissions. The eutrophying effect of NH3 occurs through the formation of acid rains that deposit back in water bodies causing N enrichment. Swine houses and pre-separation storage are together responsible for 5.7% of total marine eutrophication potential due to their NH3 emissions.
3.6. Freshwater Eutrophication
Given that P is the limiting nutrient for most freshwater bodies, introducing P to rivers and lakes results in eutrophying conditions. In this study, 98.5% of total freshwater eutrophication potential is attributed to the impacts of slurry-biochar application. The leaching of 10% of P from the slurry is responsible for this impact.
3.7. Model Sensitivity to Thermochemical Conversion Parameters
To improve understanding of the implications of the proposed thermochemical conversion system (drying-gasification-boiler) on swine manure treatment, the conversion parameters, i.e., hot-gas efficiency (HGE) and boiler efficiency was varied to represent two additional alternatives. The first set represents low-efficiency conditions: HGE and boiler efficiencies at 60% and 68%, respectively. The second, a high-efficiency scenario, shows HGE and boiler efficiency at 80% and 88%, respectively.
Figure 4 shows the impacts of the performance levels on the gasification-boiler stage. A 10% increase in the performance of both the gasifier and the boiler yielded a decrease in the GWP for this stage by 2.6 kg CO
2-eq (from 2.4 kg CO
2-eq to -0.2 kg CO
2-eq), and a corresponding increase in fossil fuel energy saving by 40.5 MJ (from -11.9 MJ to - 52.4 MJ). A 10% drop in the efficiencies increased GWP, from 2.4 to 4.6 kg CO
2-eq, and a change from a fossil fuel energy use of -11.9 MJ to an energy expenditure of 23.5 MJ. The non-linear response in the efficiency scenarios is because the overall efficiency for the gasification-boiler is the product of the conversion and the boiler efficiencies. No noticeable changes were observed in the other impact categories with changes in the efficiencies.
For the full treatment system, increasing the thermochemical conversion efficiency by 10% relative to the baseline led to a 1.5% decrease in GWP and an increase of the fossil fuel savings of 52.4 MJ. These findings suggest that the range of sensitivity for the thermal conversion system has a marginal impact on the GWP for the entire management scenario. It is worth noting, however, that the combined GWP for the separation, drying, and gasification-boiler stages, 0.89 kg CO2-eq, is lower than the difference in GWP between pre-separation storage, 58.2 kg CO2-eq, and post-separation storage, 37.3 kg CO2-eq. The separation-drying-gasification-boiler combination can be considered an emission reduction measure for manure storage. From an energy perspective, the gasification system has a beneficial impact on the total energy use in manure management, notwithstanding high energy requirements for drying. Improvements to thermal conversion efficiency (gasification and syngas firing) combined with improvements to the drying technology can significantly improve the overall environmental performance for swine manure management via thermochemical conversion.
4. Implications of the Study
The findings in this investigation contribute to the ongoing discussion on manure management best practices. Given the swine manure composition and the management techniques practiced on the farm, the thermochemical conversion is a challenging technique to dispose of wet swine manure. Improvements to the solid-liquid separation system that reduces moisture content in solid fraction can potentially improve the environmental process of the proposed system.
From GHG emissions and energy use perspectives, the land application step of swine manure management appears beneficial due to the credits from replacing synthetic fertilizer consumption. However, in regions of intensive swine production where manure land application regulations are strict, thermochemical conversion can be an alternative approach to utilize manure. Adopting innovative sludge drying technologies, i.e., biodrying technology [
16], can significantly reduce the drying energy demand, and consequently, the GHG emissions. Also, more studies towards a better understanding of biochar agronomic value could potentially help in incentivizing the thermochemical conversion of swine manure solids.