Assessment of Platinum Catalyst in Rice Husk Combustion: A Comparative Life Cycle Analysis with Conventional Methods

Abstract

This study presents a novel approach to address these challenges by introducing automobile platinum honeycomb catalysts into biomass combustion systems. The study employed a dual methodology, combining experimental investigations and a Life Cycle Assessment (LCA) case study, to comprehensively evaluate the catalyst’s performance and environmental impacts. The catalyst’s ability to facilitate combustion without open flame formation and its operational efficiency throughout combustion phases position it as a promising avenue for reducing gaseous and particulate matter emissions. The LCA considers multiple impact categories, employing the ReCiPe 2008 Hierarchist midpoint and endpoint perspective to assess environmental effects. The experimental results show that the catalyst effectively reduced CO, SO₂, and particulate emissions. Temperatures below 400 °C diminished the catalyst’s performance. The catalyst achieved a 100% CO conversion rate at specific temperatures of 427.4–490.3 °C. The findings highlight the potential for a 34% reduction in environmental impacts when replacing conventional rice husk combustion with the catalyst-integrated system. Notably, the study emphasizes the significance of sustainable catalyst manufacturing processes and cleaner electricity sources in maximizing environmental benefits. In conclusion, the integration of platinum honeycomb catalysts into biomass combustion systems, exemplified by rice husk combustion, emerges as a promising strategy for achieving more sustainable and environmentally friendly bioenergy production.

1. Introduction

Bioenergy is expected to play a vital role in achieving a low-carbon future, with projections indicating a substantial increase in usage. It will likely comprise about 17% of the world’s energy by 2060 [1]. This shift towards bioenergy is expected to contribute to a combined carbon reduction of about 20% by the same year, as the International Energy Agency reported in 2017 [1]. Biomass combustion is recognized as one of the most straightforward methods to generate energy from biomass. Biomass combustion has been the dominant pathway for energy generation for many decades. However, its limitation remains its high pollutant emissions, which are harmful to human health and the environment. Rebryk et al. [2] recorded high emissions of volatile organic compounds during woody biomass combustion in Kenya. This emission poses a health risk [3]. Biomass fuels contain elements such as nitrogen, sulfur, and carbon, which are converted into gaseous emissions as flue gases during combustion [4,5].

In addressing this high-emission issue, many studies have used different approaches to reduce emissions from biomass combustion. These include methods such as using biomass for biodiesel production to reduce emissions [6], adding additives to capture emission initiators, and lowering fuel temperature to decrease combustion efficiency [7]. Additionally, other studied methods involve co-combustion with coal, examining its mechanistic aspects and properties. Research has also explored the use of non-isothermal thermogravimetry and tube-even techniques for co-combustion analysis [8]. Researchers have applied alkali removal technology with pretreatment for emission control [9]. They have also studied the effects of adding phosphoric acid-modified kaolin on thermal properties and NO emissions [10,11]. Furthermore, particle capture modeling has been used to design wet electrostatic scrubbers aimed at lowering particulate emissions [12]. Moreover, kaolin blending showed about 70% particulate matter reduction in wood combustion [9]. Medina et al. [13] used chemical simulation to study the effect of air excess ratio on biomass combustion efficiency and CO and NOx emissions. Studies have explored methods to reduce these gaseous emissions, including adjusting fuel particle size and density [14]. Efforts have been made to find ways to reduce particulate matter emissions, considering factors like combustion temperature, air-to-fuel ratio, the nature of the biomass feedstock, and particle morphology [15].

Furthermore, biomass additives have been used in coal briquettes to decrease pollutant emissions [16]; however, adding 15% biomass increased the emissions beyond the additive quantity. Also, the synergistic effect and volatile emission characteristics have been studied in biomass co-combustion, with only an 8.9% reduction in NO emissions [3]. A combination of methane gas and wood shavings has been integrated to increase the initial combustion temperature to minimize the high emissions from the initial combustion phase [17]. However, other combustion phases emit significant pollutants, rendering this method ineffective during the late combustion phases. Using a drop-tube furnace, Zheng et al. [3] introduced alkali and alkaline earth metal coal with ferric sludge to mitigate NOx emissions. While this method decreased NOx emissions, it increased ash deposition due to the formation of eutectics. Other studies have used amine blends in co-combustion to mitigate biomass emissions [18]. Recent research has integrated aerosol emissions from using forest-based biomass and fossil-based materials in assessing these emission impacts [19]. Wang et al. [20] concluded that biomass content is the leading cause of pollutant emissions. However, reaching the desired level of emission reduction remains a challenge, necessitating further research into emission control strategies and sustainable policies.

The urgent need to mitigate climate change and the potential risk of an energy crisis has increased interest in using biomass as an energy source. As such, continuous research and innovation in this field are essential to developing more sustainable and environmentally friendly methods for harnessing biomass’s potential for a low-carbon future. One of the significant sources of bioenergy is rice husk biomass. In 2020, it was estimated that global rice production amounted to approximately 499.3 million tons, and 1 kg of rice is nearly 0.28 kg of rice husk [14]. The growing need for biomass energy emphasizes the importance of finding innovative solutions to reduce its environmental impact and help combat climate change [21]. Metal honeycomb catalysts offer a promising way to enhance the sustainability of biomass combustion. They can significantly reduce environmental effects by causing the fuel to oxidize on the catalyst’s surface without an open flame. This differs from conventional combustion and relies on the interaction between fuel particles and the catalyst’s superficial oxygen. The catalyst’s surface is continuously regenerated through contact with oxygen from the gas phase [22].

Catalysts can completely oxidize fuel at relatively low temperatures (250–300 °C) in biomass combustion systems. Compared to alternative equipment like filters or electrostatic precipitators, catalysts reduce gaseous and particulate matter emissions during all combustion phases, even under suboptimal conditions. Using automobile platinum honeycomb catalysts in biomass combustion reduces emissions and enhances sustainability [23]. Life Cycle Assessment (LCA) evaluates environmental issues and resource utilization across a product’s entire lifecycle, from raw material extraction to manufacturing [24]. This study combines experimental and LCA methodologies to assess the applicability and environmental impacts of a platinum honeycomb catalyst in the exhaust chamber of a biomass combustion system to minimize pollutant emissions and environmental impacts.

2. Results and Discussion

2.1. Experimental Results of the Conventional and Catalyst Systems

The Pt/Al₂O₃ catalyst exhibits thermal stability up to 800 °C due to its Al₂O₃ support and Pt particles averaging 2.5 nm. It maintains porosity, as indicated by a pore volume of 0.45 cm³/g, after exposure to 800 °C, which suggests resistance to sintering, a common deactivation mechanism. This stability is crucial for hydrogen combustion under the studied conditions.

Table 1. Summary of the experimental results of the conventional and catalyst combustion systems.

Temperature °C	HC temperature	380.1	481.3	490.3	534.1
	Combustion temp	600	700	800	900
RH_CO (ppm)	HC	23,941	0	0	56
	Conventional	72,453	5043	1095	4345
RH_SO2 (ppm)	HC	40	162	188	170
	Conventional	416	711	4	468
RB_CO (ppm)	HC	124,953	0	0	0
	Conventional	433	774	265	141
RB_SO2 (ppm)	HC	74	0	0	2
	Conventional	138	140	98	65
RH_PM2.5 (mg/m3)	HC	22.50	43.39	128.16	165.04
	Conventional	354.58	372.05	629.60	826.59
RB_PM2.5 (mg/m3)	HC	18.95	170.07	265.75	856.05
	Conventional	1169.91	1572.22	1103.21	1209.56

Note: RH = rice husk; RB = rice husk Briquette; HC = honeycomb catalyst.

summarizes the experimentation results of the platinum-iron alumina (Pt-Fe) honeycomb catalyst flue oxidation, surface reactions, and the impact of temperature on emission conversion on the catalyst surface. The limited catalytic efficiency below 400 °C for the catalyst is primarily due to kinetic limitations, as the oxidation of biomass volatiles over platinum sites (with an average particle size of 2.5 nm) requires sufficient activation energy, which is inadequately supplied at these temperatures. Below 400 °C, surface-mediated reactions proceed at a rate too slow for complete conversion of volatiles, despite thermodynamic feasibility, as evidenced by the catalyst’s stable mesoporous structure and high surface area [25]. Additionally, the adsorption of intermediate species may temporarily block active Pt sites, reducing reactivity. Above 400 °C, enhanced thermodynamic favorability, coupled with increased reaction rates, drives improved oxidation performance, aligning with the observed efficiency in the 600–1000 °C fuel combustion range. As a next step, a detailed analysis of these kinetic barriers and thermodynamic drivers, supported by the catalyst’s thermal stability data (e.g., >95% activity retention after 48 h at 800 °C), would clarify the temperature-dependent behavior and guide optimization of operational conditions.

As hypothesized, treating the catalyst with heat resulted in lower gaseous and PM_2.5 emissions. Due to the additional heat, the catalyst’s molecules and particles reacted faster with absorbed emissions, forming new products. This is consistent with Arrhenius’s collision theory (Equation (1)). CO reacts with chemisorbed oxygen to form CO₂. The catalyst chemisorbs CO on its active sites, which enhances CO oxidation to CO₂ at preheated temperatures. However, chemisorbed CO inhibits CO oxidation at low temperatures. At low catalyst temperatures, almost the entire surface of the catalyst is covered by CO, and oxidation occurs between the molecular oxygen physically absorbed on top of an absorbed CO layer. As the catalyst temperature increases, CO oxidation proceeds through the reaction between absorbed oxygen atoms and gas phase CO, according to the Eley–Rideal reaction [26]. As CO oxidation proceeds, active sites initially occupied by chemisorbed CO become available for oxygen and SO₂ adsorption. Therefore, the CO conversion rate increases because of the improved balance of adsorbed CO and oxygen, considering the stoichiometry of the surface reaction.

The catalyst’s temperature intensifies flue combustion and increases the likelihood of CO on the catalyst’s surface reacting with oxygen and getting oxidized. However, raising the catalyst temperature further results in higher CO emissions due to CO accumulation and an imbalance between oxygen desorption and CO and SO₂ adsorption. CO and adsorbed oxygen molecule levels depend on temperature and excess air. In the current experiment, with constant excess air, an increase in temperature beyond the equilibrium point leads to a higher CO concentration because of its inhibitory effects caused by overcrowding of active sites on the catalyst. As the CO produced by the rapid combustion of the rice husk in the combustion chamber reaches the preheated catalyst, it undergoes slow oxidation to form CO₂. A high combustion temperature consumes more oxygen, and low excess oxygen increases CO formation. When CO covers more than one-third of the surface, it prevents the subsequent adsorption of O₂. Therefore, at lower CO coverages, dissociative O₂ adsorption occurs. However, these two groups form distinct domains on the catalyst surface. Oxidation can then proceed at the boundaries between the domains at a relatively low rate. A mixed domain, adsorbed CO and O₂, immediately encounters oxygen atoms at twice the surface concentration, probably even in the absence of CO. The hot-catalyzed combustion produced the lowest PM₂.₅ levels at 600 °C for RH and RB samples (22.5 mg/m³ and 19.0 mg/m³, respectively). This is significant, as previous studies recorded higher concentrations at 600 °C for JPN and RB samples [15]. The PM emission increased with higher combustion temperatures due to inadequate excess air, leading to insufficient combustion. Integrating the catalyst into a biomass combustion system allows for lower combustion temperatures with minimal particulate emissions. The catalyst can be easily maintained and cleaned to prevent clogging. Heat treatment of the catalyst ensures that unburnt carbon and gases from the combustion chamber are combusted as they move through the exhaust.

Pt/Fe–Al honeycomb catalysts, such as Pt/Al₂O₃, used in biomass combustion face deactivation issues, including coke buildup, which blocks active sites and reduces efficiency [27]. High temperatures, particularly exceeding the tested stability limit of 800 °C for 48 h [25], promote Pt nanoparticle sintering, resulting in a decrease in surface area and performance. Impurities, such as alkali metals and sulfur, chemically poison the catalyst by damaging Pt sites and the support, thereby accelerating deactivation [28]. These issues hinder the long-term stability of catalysts in harsh conditions. Effective regeneration strategies are crucial for restoring catalyst performance and extending its lifespan. Oxidative regeneration, where air or oxygen at high temperatures burns off coke and reactivates Pt sites [29], is a common process. Steam-assisted gasification is a gentler alternative, converting coke into CO and CO₂ at moderate temperatures, thereby reducing thermal damage, such as sintering [30]. Redox-active Fe–Al₂O₃ supports enhanced coking resistance by facilitating the in situ oxidation of carbon deposits through oxygen storage and release cycles [31,32]. To evaluate catalyst durability under real conditions, tests such as time-on-stream (TOS), thermogravimetric analysis (TGA), and surface techniques like XPS and TEM are essential. These assessments provide insights into deactivation mechanisms and support the development of regeneration protocols for biomass combustion.

2.2. Scenario 1: LCA Comparison of Characterized Results of Rice Husk Combustion

Table 2. Characterized LCIA results of conventional and catalyst-integrated rice husk combustion for generation of 1 MJ.

Impact Category	Unit	Catalyst Integrated Combustion	Conventional Combustion
Climate change	kg CO2 eq	9.7255	9.6952
Ozone depletion	kg CFC-11 eq	1.25 × 10−6	1.21 × 10−6
Terrestrial acidification	kg SO2 eq	0.3978	1.0400
Freshwater eutrophication	kg P eq	0.0004	0.0004
Marine eutrophication	kg N eq	0.0321	0.0389
Human toxicity	kg 1,4-DB eq	0.3277	0.2834
Photochemical oxidant formation	kg NMVOC	0.3679	0.8068
Particulate matter formation	kg PM10 eq	0.1255	0.2755
Terrestrial ecotoxicity	kg 1,4-DB eq	0.0009	0.0009
Freshwater ecotoxicity	kg 1,4-DB eq	0.0032	0.0022
Marine ecotoxicity	kg 1,4-DB eq	0.0051	0.0045
Ionizing radiation	kBq U235 eq	0.5064	0.4986
Agricultural land occupation	m2a	0.6011	0.5867
Urban land occupation	m2a	0.0559	0.0537
Natural land transformation	m2	0.0015	0.0014
Water depletion	m3	0.0749	0.0728
Metal depletion	kg Fe eq	0.1780	0.0909
Fossil depletion	kg oil eq	3.0553	3.0102

shows the characterized midpoint environmental impact assessment results of conventional and catalyst-integrated rice husk combustion. In a comparative analysis of rice husk combustion for generating 1 MJ, the following table summarizes the characterized Life Cycle Impact Assessment (LCIA) results for both catalyst-integrated and conventional combustion systems. These results highlight the environmental performance of the catalyst-integrated combustion system compared to the conventional system across various impact categories, offering valuable insights into the sustainability and ecological implications of rice husk combustion.

The comparison of both systems reveals that they have similar impacts on several impact categories, including freshwater eutrophication and terrestrial ecotoxicity. However, there are distinctions in their performance in other categories. The catalyst-integrated system shows comparatively higher impact on some impact categories such as climate change, ozone depletion, human toxicity, freshwater ecotoxicity, marine ecotoxicity, ionizing radiation, agricultural and urban land occupations, natural land transformation, water depletion, metal depletion, and fossil depletion. Conversely, this system demonstrates comparatively lower impacts in other categories. Climate Change is characterized by the equivalent amount of carbon dioxide (CO₂) emissions in kilograms. The catalyst-integrated system emits 9.7255 kg CO₂ eq, while the conventional system emits 9.6952 kg CO₂ eq, indicating a slightly higher carbon footprint for the integrated system. Terrestrial Acidification shows the impact on terrestrial ecosystems due to acidification. The catalyst-integrated system significantly outperforms the conventional system with emissions of 0.3978 kg SO₂ eq, compared to the conventional system’s 1.0400 kg SO₂ eq. This represents a notable reduction in terrestrial acidification potential.

Human Toxicity measures the potential toxicity to humans, and the catalyst-integrated system emits 0.3277 kg 1,4-DB eq, slightly higher than the conventional system’s 0.2834 kg 1,4-DB eq. Photochemical Oxidant Formation quantifies the potential for forming ground-level ozone and smog. The catalyst-integrated system is more favorable, emitting 0.3679 kg NMVOC than the conventional system’s 0.8068 kg NMVOC. Particulate Matter Formation evaluates the formation of fine particulate matter, which can have health impacts. The integrated system emits 0.1255 kg PM10 eq, whereas the conventional system emits 0.2755 kg PM10 eq, indicating a lower potential for particulate matter formation in the integrated system. Metal Depletion assesses the potential depletion of metal resources. The catalyst-integrated system depletes 0.1780 kg Fe eq, significantly higher than the conventional system’s 0.0909 kg Fe eq. This suggests a higher impact in terms of metal resource depletion for the integrated system. Fossil Depletion measures the potential depletion of fossil fuel resources. The integrated system depletes 3.0553 kg oil eq, slightly more than the conventional system’s 3.0102 kg oil eq. The difference is relatively small, indicating a comparable impact on fossil resource depletion.

The comparison between the Integrated Catalyst and conventional combustion systems reveals that the former has a slightly higher carbon footprint, potentially limiting its ability to address climate change on a global scale effectively. Both systems have similar impacts on ozone depletion, although the integrated combustion system shows a slightly higher impact, indicating a potentially somewhat greater contribution to ozone depletion. On the positive side, the catalyst-integrated combustion system significantly reduces terrestrial acidification, demonstrating a lower environmental impact and improved overall performance. Both systems have equivalent impacts on freshwater eutrophication emission, suggesting neither has a distinct advantage in this area. However, the integrated combustion system has a lower impact on marine eutrophication, indicating better environmental performance in this category.

Additionally, the catalyst combustion system significantly reduces the impact on photochemical oxidant formation for integrated combustion, thereby showcasing improved performance. There is also a substantial reduction in particulate matter formation, indicating a cleaner emissions profile for integrated combustion. On the other hand, the integrated combustion system is associated with higher human toxicity, raising concerns regarding its safety and health impacts. Moreover, there was slightly higher marine ecotoxicity for integrated combustion, indicating potential ecological concerns. The analysis also reveals a significant increase in metal depletion for the catalyst-integrated combustion, suggesting a higher impact on resource extraction.

Furthermore, the catalyst combustion system has slightly higher fossil fuel depletion for integrated combustion, highlighting concerns about resource sustainability. Overall, the catalyst-integrated combustion system performs better in several critical environmental categories, such as terrestrial acidification, marine eutrophication, and particulate matter formation. However, it also shows increased impacts on human toxicity, freshwater ecotoxicity, and metal depletion. While it may offer advantages in reducing specific pollutants and impacts, the toxicity and resource depletion trade-offs warrant careful consideration. This highlights the need for future evaluations to focus on optimizing the benefits of the catalyst-integrated system while addressing its negative impacts and engaging all stakeholders in the process. Figure 1 presents environmental impact scores (in percentage terms) for different combustion, transport, and electricity categories in conventional rice husk combustion.

The combustion stage has a notable impact on climate change (1.47%), primarily due to emissions from the burning process. In contrast, electricity production contributes significantly to climate change (98.52%) due to greenhouse gas emissions. Electricity production (99.99%) also impacts marine eutrophication, indicating potential nutrient pollution in marine ecosystems. The impact of terrestrial acidification is mainly attributed to combustion (94.70%) due to emissions generated during burning. Combustion (95.86%) significantly impacts photochemical oxidant formation, reflecting the potential for ground-level ozone creation and particulate matter formation (92.45%) due to the release of fine particles during burning. Electricity production and transport have relatively low impacts. Most of the impact categories, other than terrestrial acidification, marine eutrophication, photochemical oxidant formation, and particulate matter formation, are affected by electricity production. Figure 2 gives a breakdown of environmental impact assessments for different life cycle stages, including combustion, transport, electricity, and catalyst manufacturing in catalyst-integrated rice husk combustion. Electricity production has been identified as the primary source of environmental impacts across a wide range of categories, particularly concerning climate change, ozone depletion, and human toxicity. In contrast, combustion processes significantly influence terrestrial acidification and marine eutrophication. However, they fare better in terms of human toxicity and ecotoxicity measures. Notably, transport’s impact is minimal in most categories, indicating that it could be a more sustainable option in certain circumstances. To mitigate environmental impacts, it is crucial to concentrate efforts on enhancing electricity generation methods and reducing emissions from combustion technologies.

Combustion contributes 0.037% to climate change, reflecting emissions from the burning process. Transport has a minor impact (0.014%), while electricity production has a substantial impact (98.22%) due to greenhouse gas emissions. Catalyst manufacturing contributes 1.73% to climate change. Catalyst manufacturing contributes significantly to metal depletion, marine ecotoxicity, freshwater ecotoxicity, human toxicity, and freshwater eutrophication while showing relatively low impacts on other impacts. Electricity production contributes significantly to many impact categories like conventional rice husk combustion. The use of electricity in rice husk combustion has contributed considerably to the impacts of several categories of conventional and catalyst-integrated rice husk combustion. One of the significant contributors to carbon dioxide (CO₂) emissions, which significantly contribute to climate change, is burning fossil fuels for electricity generation. Fossil fuels, such as coal, oil, and natural gas, release CO₂ when combusted to produce energy [32]. It also produces sulfur dioxide (SO₂) and nitrogen oxides (NO₂), which can lead to acid rain and pose health risks to humans. Electric generation technologies’ life-cycle emissions encompass direct (operational phase) and indirect (entire life span) emissions. Indirect emissions come from the extraction, processing, transportation, and disposal of resources and the manufacturing and decommissioning of power plants. Different energy sources have varying life-cycle emissions, with renewable energy sources generally having lower emissions compared to fossil fuels [33]. Generation of 1 kWh with coal, biomass-co-firring, natural gas, and biomass emits 820, 740, 490, and 230 gCO₂ equivalent, respectively.

Solar power, geothermal, nuclear, hydropower, and wind power account for less than 50 gCO₂ equivalent when producing 1 kWh of electricity [34]. The composition of electricity generation varies by country and region, and it often involves a mix of different energy sources. This combination typically includes renewable power sources, fossil-fueled thermal power, and, in some cases, nuclear power. Addressing the environmental impact of electricity generation is a complex challenge, requiring a combination of technological, policy, and behavioral interventions. Transitioning to renewable energy is one of the environmental impact-mitigating strategies. Improving the efficiency of energy use and adopting advanced technologies are other practical options that help reduce overall emissions. The process of platinum treatment of automobile catalysts was obtained from this study’s Ecoinvent version 2 dataset. In this process, dismantling the catalysts and subsequently engaging in pyrometallurgical processing, followed by a hydrometallurgical purification step, have been considered. The inventory accounts for smelter slag disposal, categorized as the disposal of inert material, with potential heavy metal emissions needing to be addressed due to their unknown nature. Platinum’s primary function in catalytic converters is the oxidation of carbon monoxide (CO) and hydrocarbons. Platinum exhibits exceptional effectiveness in environments with excess oxygen, making it a preferred metal for diesel applications. For three-way catalysts utilized in petrol vehicles, the capability to oxidize CO and hydrocarbons and reduce NOx to nitrogen is essential. Consequently, rhodium is commonly incorporated in addition to platinum or palladium to fulfill this comprehensive functionality [35,36]. The manufacturing processes of automobile platinum catalysts can generate various emissions.

These emissions may include greenhouse gases, volatile organic compounds, and particulate matter. Greenhouse gas emissions like carbon dioxide (CO₂) can result from energy-intensive manufacturing steps. Volatile organic compounds are released during the production and application of coatings or adhesives [37,38]. Particulate matter emissions can occur during processes like machining or grinding of materials. Other air pollutants, such as nitrogen oxides (NO₂) and sulfur dioxide (SO₂), might be produced depending on the specific manufacturing techniques and materials used. The emissions can vary based on factors like raw material extraction, energy sources, and production methods employed in manufacturing automobile platinum catalysts. The data illustrates that electricity generation substantially impacts the environment, particularly in its contributions to climate change, ozone depletion, and freshwater eutrophication. It primarily contributes to greenhouse gas emissions, accounting for 98.21 kg CO₂ eq in climate change impact and over 97% in ozone depletion and freshwater eutrophication. This emphasizes the critical importance of addressing the sources and technologies used for electricity production to minimize environmental and public health risks.

In contrast, while still impactful, combustion processes have lower contributions to human toxicity and ecotoxicity. For instance, the impact of combustion on terrestrial acidification (85.83 kg SO₂ eq) is notable. However, electricity generation still has a more significant overall negative environmental impact. Additionally, the manufacturing of catalysts also leaves a significant environmental footprint, particularly in freshwater ecotoxicity and metal depletion, accounting for 30.85% and 48.89%, respectively. This underscores the need to scrutinize catalyst production to mitigate its environmental impact despite its benefits in reducing emissions in combustion processes. In summary, a comprehensive strategy is essential to transition toward cleaner energy sources and improve the sustainability of catalyst manufacturing processes.

2.3. Scenario 2: LCA Endpoint Evaluation in Terms of Damage Done to Human Health (DALY)

As presented in the system boundary, scenario two’s assessment considered emissions from the catalyst as indirect emissions, with the viewpoint that this is already covered in the automobile industry if obtained as a reused product. Figure 3 presents the damage to human health (DALY) from the endpoint and Eco-indicator method required to produce 1.0 MJ heat energy from the conventional and integrated catalyst combustion systems.

Using a catalyst for energy production (HC energy) has been found to have a lower negative impact on life-saving potential when compared directly, according to both the ReCiPe and Eco-indicator methodologies. The eco-indicator method considers impact categories such as acidification potential/eutrophication potential (AP/EP), respiratory inorganics, climate change, and carcinogens. On the other hand, the ReCiPe method is primarily influenced by PM formation, photochemical oxidation formation, and terrestrial acidification. The catalyst-integrated combustion system shows lower PM emissions than the conventional rice husk combustion system, positively impacting life-saving potential. Figure 3 illustrates the contribution of the initial heat supplied to the two combustion systems, revealing that the initial heat supply to the conventional combustion system exceeds that of the catalyst-integrated combustion system. Although both systems provide the same initial heat, the catalyst-integrated system significantly reduces emissions at the exhaust point. However, it is important to note that there are indirect emissions from the catalyst production process and the additional heat energy needed for preheating, which results in the catalyst causing more loss of life than it saves. Therefore, when considering the impact of this extra process, the catalyst’s life-saving potential is deemed insignificant. However, if the catalyst production process is considered in the automobile industry, then the life-saving potential of the catalyst becomes significant.

The findings from Eco-indicator 99 suggest that incorporating honeycomb catalysts (HC) into heat energy processes substantially improves environmental and health outcomes. Specifically, introducing HC leads to a noteworthy decrease in carcinogenic impact, with the potential health risks associated with carcinogen exposure dropping from 2.996 × 10⁻⁹ DALY to 6.748 × 10⁻¹⁰ DALY. This reduction is significant in mitigating the adverse health effects linked to carcinogens. Moreover, the integration of HC decreases respiratory inorganic impacts from 6.032 × 10⁻⁶ DALY to 3.449 × 10⁻⁶ DALY, positively influencing air quality and respiratory health. Notably, no visible impacts are associated with climate change, radiation, or ozone layer depletion in either scenario. However, the reduction in acidification potential (AP/EP) from 0.282 (conventional) to 0.198 (catalyst) underscores the environmental benefits of incorporating HC in heat energy applications, indicating a reduced environmental footprint. The evaluation of ReCiPe endpoints reveals that using honeycomb catalysts offers numerous advantages. For instance, the reduction in photochemical oxidant formation from 2.249 × 10⁻⁹ DALY to 1.317 × 10⁻⁹ DALY indicates a lower risk of smog and related respiratory health issues. Additionally, there is a notable decrease in particulate matter formation impacts, from 4.461 × 10⁻⁶ DALY to 2.355 × 10⁻⁶ DALY, emphasizing the positive impact on air quality. The decline in terrestrial acidification, from 3.558 × 10⁻¹⁰ species per year to 1.571 × 10⁻¹⁰ species per year, signifies a reduced impact on ecosystems. Overall, the ReCiPe endpoint findings closely correspond with the Eco-indicator, suggesting that integrating honeycomb catalysts mitigates health risks and improves environmental sustainability in heat energy systems.

The process contributions for the Ecoindicator results indicate substantial decreases in health and environmental impacts when the honeycomb catalysts (HCs) are utilized in heat control processes. For instance, the use of HC leads to a notable reduction in carcinogenic impact, with the effect decreasing from 1.06 × 10⁻⁸ DALY without HC to 2.25 × 10⁻⁹ DALY with HC, highlighting a significant improvement in potential health risks. Furthermore, the decline in respiratory inorganic pollutants from 4.95 × 10⁻⁸ DALY to 1.77 × 10⁻⁸ DALY underscores the positive effect of HC on air quality and respiratory health. The decrease in climate change impact from 4.31 × 10⁻⁸ to 1.64 × 10⁻⁸ DALY further underscores the environmental benefits, showcasing that HC can substantially reduce greenhouse gas emissions. Other categories, such as ecotoxicity and acidification potential, also demonstrate noteworthy declines, reinforcing the overall effectiveness of hydrophobic chemicals in enhancing health and environmental outcomes. The process contribution analysis from the ReCiPe endpoint aligns with the results from the Ecoindicator, showing consistent patterns in reducing various environmental impacts through the utilization of honeycomb catalysts (HCs). Climate change’s impact on human health decreases from 2.92 × 10⁻⁷ DALY to 1.11 × 10⁻⁷ DALY, substantially enhancing public health outcomes. Moreover, the decrease in ozone depletion impacts from 6.90 × 10⁻¹¹ DALY to 1.19 × 10⁻¹¹ DALY suggests that HC can play a role in mitigating risks to the ozone layer. Additionally, there is a significant reduction in particulate matter formation, dropping from 3.12 × 10⁻⁸ DALY to 1.45 × 10⁻⁸ DALY, which highlights improvements in air quality. The data from both the Ecoindicator and ReCiPe emphasize the effectiveness of HC in improving the sustainability of heat control technologies, indicating their potential to diminish health risks and ecological impacts significantly.

The production of catalysts results in indirect emissions that considerably impact the environment and human health (

Table 3. Indirect contributions to the catalyst production process.

	Impact Category	HC Catalyst	Electricity (Co Product)
Eco-indicator 99 Indirect contributions	Carcinogens (DALY)	8.18969 × 10−6	8.50956 × 10−13
	Resp. organics (DALY)	2.29701 × 10−8	2.09766 × 10−11
	Resp. inorganics (DALY)	2.5282 × 10−5	2.45066 × 10−10
	Climate change (DALY)	6.31224 × 10−6	1.98898 × 10−10
	Radiation (DALY)	4.86307 × 10−8	5.41186 × 10−14
	Ozone layer (DALY)	5.77073 × 10−9	6.49188 × 10−15
	Ecotoxicity (PAF∙m2∙yr)	46.38897098	3.21522 × 10−6
	AP/EP (PDF∙m2∙yr)	0.745041133	1.53112 × 10−5
ReCiPe Indirect contributions	Climate c. Hum. Health (DALY)	4.21338 × 10−5	1.32426 × 10−9
	OD (DALY)	1.44176 × 10−8	1.60624 × 10−14
	Human toxicity (DALY)	5.76533 × 10−6	2.8175 × 10−11
	Photochem. Oxi. Format. (DALY)	5.07189 × 10−9	2.72641 × 10−13
	PM formation (DALY)	1.79374 × 10−5	1.6064 × 10−10
	Ionising radiation (DALY)	3.79754 × 10−8	4.25363 × 10−14
	Climate c. Ecosyst. (species.year)	2.38651 × 10−7	7.50086 × 10−12
	T. acidification (species.year)	1.34701 × 10−9	9.47752 × 10−15

). A comparison between HC catalysts and electricity as a co-product reveals significant disparities in their environmental and health effects. HC catalysts show much higher adverse effects across various categories. For example, the carcinogenic impact of HC catalysts is measured at 8.19 × 10⁻⁶ DALY, whereas electricity has a negligible impact of 8.51 × 10⁻¹³ DALY. Similarly, HC catalysts contribute significantly more to respiratory inorganics at 2.53 × 10⁻⁵ DALY compared to electricity’s contribution of 2.45 × 10⁻¹⁰ DALY. The climate change impact is also notably higher for HC catalysts at 6.31 × 10⁻⁶ DALY compared to 1.99 × 10⁻¹⁰ DALY for electricity. Furthermore, regarding ecotoxicity, HC catalysts contribute 46.39 PAF·m²·yr, while electricity only contributes 3.22 × 10⁻⁶ PAF·m²·yr. These findings highlight that while HC catalysts are effective for specific applications, they present a significantly higher risk to health and the environment compared to electricity generated as a co-product. This indicates a crucial need for careful consideration in their use and development.

2.4. Normalized Results

Figure 4 compares normalized environmental impact assessments between catalyst and conventional combustion for various impact categories. Normalized results are crucial for understanding the relative magnitude of the impact in each category, as they provide a standard unit of measurement, facilitating meaningful comparisons [39].

These three impact categories, namely terrestrial acidification, photochemical oxidant formation, and particulate matter formation, are notably affected by conventional rice husk combustion. The total environmental impact of catalyst combustion is reported as 0.05, while conventional combustion has a slightly higher total impact of 0.076. The comparison suggests that by replacing conventional rice husk combustion with catalyst-integrated rice husk combustion, there is a potential reduction of nearly 34% in environmental impacts. Figure 5 shows the normalized impact of obtaining 1 MJ of heat with catalyst combustion and conventional combustion systems without considering the manufacturing of the catalyst. The graph compares the normalized environmental impact of catalyst combustion and conventional combustion across various categories by neglecting the impact of catalyst manufacturing. Catalyst combustion consistently demonstrates lower environmental impacts across these categories compared to conventional combustion. The total environmental impact of catalyst combustion has been reduced to 0.036, so catalyst-integrated rice husk combustion can reduce the environmental impact by 52.6% when neglecting the effect of catalyst manufacturing.

3. Materials and Methods

3.1. Experimental Setup and Data Collection

This study used the automobile catalyst originally developed by Tomita et al. [25] for reducing vehicle emissions and adapted it for biomass combustion. The Pt-based catalyst, supported on a metallic substrate, was supplied by the National Agriculture and Food Research Organization, Agricultural Technology Innovation Research Center, Japan. Access was granted through a collaborative effort led by the director of strategy promotion. The catalyst arrived as a fabricated honeycomb structure and was then integrated into the biomass exhaust system as shown in Figure 6. The originality of this study lies in the method of application and integration of the catalyst for biomass combustion. Moreover, it is unique in its determination of the optimum performance of the catalyst under different biomass combustion scenarios. The decision to use the catalyst was based on its thin outer layer, high surface area, and durability as a monolith. Its cost-effectiveness and high cross-sectional aperture ratio allow for the suppression of emission gases and minimal pressure loss. Due to its metal composition, it boasts high physical strength and can be easily regenerated and cleaned.

The catalyst comprises platinum as the primary catalyst and iron oxide as the co-catalyst. Additionally, water activation treatment was performed to serve as an interface between the platinum and the co-catalyst. The catalyst was adopted following the findings of Tomita et al. [25]. The characterization and kinetic parameters of the catalyst was reported by Tomita et al. [25]. When a catalyst is used for the oxidative decomposition reaction, oxygen and organic substances are absorbed into the catalyst’s active sites. This leads to the combustion of combustible substances at moderate temperatures, a process known as oxidative decomposition. The active site for O₂ is found at the interface between platinum (Pt) metal substrates and ferric oxide (Fe₂O₃), where the dissociated atomic oxygen atom exhibits moderate adsorption energy and high reactivity, resulting in the expected carbon monoxide (CO) conversion. According to Arrhenius’s collision theory:

$$\mathit{K}=\mathit{A}{\mathit{e}}^{{\displaystyle \raisebox{1ex}{$-{\mathit{E}}_{\mathit{a}}$}\!\left/ \!\raisebox{-1ex}{$\mathit{R}\mathit{T}$}\right.}}$$

(1)

where K is the rate constant, Ea is the activation energy (J/mol), R is the ideal gas constant, T is the temperature (Kelvin), and a is the pre-exponential factor. Therefore, we hypothesized that the heat-treated catalyzed combustion would emit lower gaseous and particulate matter (PM_2.5) emissions from the viewpoint of the collision theory on reaction rates. As a result, the combustible substances are combusted at a moderate temperature in an oxidative decomposition process. The following is a typical illustration, where ${\mathit{C}}_{\mathit{x}}{\mathit{H}}_{\mathit{y}}$ represent particulate matter.

$${\mathit{C}}_{\mathit{x}}{\mathit{H}}_{\mathit{y}}+{\mathit{O}}_2=\mathit{C}{\mathit{O}}_2+{\mathit{H}}_2\mathit{O}+\mathit{H}\mathit{e}\mathit{a}\mathit{t} \mathit{o}\mathit{f} \mathit{r}\mathit{e}\mathit{a}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}$$

(2)

The gaseous emission reduction is obtained at the complete oxidation reaction, and the resulting products are CO₂ and H₂O. The experimental setup (Figure 7) for assessment comprises a Yamato F100 fixed-bed electric furnace attached with a detachable exhaust pipe housing the metal honeycomb catalyst for the catalytic combustion [23]. For the PM_2.5, CO, and SO₂ emission measurements, a Dust Track II aerosol analyzer and a Testo 350 flue gas analyzer were mounted at the top of the exhaust pipe. The Dust Track II aerosol analyzer is a real-time particulate matter counter with a standard air flow rate of 3 L/min and uses size-selective cascade impactors [15]. The Testo 350 flue gas analyzer is suitable for a wide range of professional measurements of industrial emissions. It comprises two key components: a control unit and an analyzer box. The control unit and the analysis box control the emission measurement. The analyzer box includes sensors and electronics for emission measurements [14]. Combustion experiments were performed at furnace temperatures ranging from 600 °C to 1000 °C.

The combustion experiments used Japanese rice husk samples JPN, specifically from the Koshihikari variety. These husks are derived from Japonica husk particles of the Oryza Sativa species, exhibiting a standard size range of 4.0 to 5.5 mm. The rice husk briquette samples RB underwent grinding and compression procedures at temperatures nearing 300 °C. The elemental analysis (

Table 4. Elemental analysis of the biomass fuel used for this study.

Element/Parameter	JPN	RB
C (wt.% db)	37.53	39.37
H (wt.% db)	5.05	5.41
N (wt.% db)	0.18	0.34
S (wt.% db)	0.23	0.11

Note: JPN refers to Japanese rice husk and RB refers to rice husk briquette.

) shows that the rice husk briquette (RB) has slightly higher carbon (39.37 wt.%) and hydrogen (5.41 wt.%) contents than Japanese rice husk (JPN) at 37.53 wt.% and 5.05 wt.%, indicating potentially higher energy content and better combustion. RB also has more nitrogen (0.34 wt.%) than JPN (0.18 wt.%), possibly increasing NOx emissions. Conversely, JPN has higher sulfur (0.23 wt.%) than RB (0.11 wt.%), implying a greater risk of SOx emissions and related issues during combustion.

Table 5. Experimental conditions and catalyst specifics. For the catalyst technical data, please see Tomital et al. [25].

Catalyst Type	Metal Honeycomb
Material composition	Pt/Al2O3
Platinum loading	1 wt%
BET surface area (m2/g)	180 (support)
Pore volume (cm3/g)	0.45
Thermal stability	Stable up to 800 °C (confirmed by TGA and XRD data)
Pt particle size	2.5 nm (average, from TEM analysis)
Heating temperature range of the catalyst (°C)	100–600
Combusted fuel samples	Rice husk and briquette
Fuel sample weight (g)	3.0
Fuel combustion temperature range (°C)	600–1000
Combustor air intake (m/s)	1.5
Sample combustion duration (min)	3.0
Number of repetitions for each data point	3.0

provides general information on the biomass fuel combustion conditions and the technical data of the catalyst as obtained from Tomita et al. [25]. Figure 1 and Figure 2 illustrate the experimental setup, including the catalyst, particulate, and gaseous sampling. The energy required to heat the catalyst was determined by calculating the specific heat capacity of the constituent elements and using the following equation, where m is the mass of the catalyst, c is the heat capacity of the catalyst, and dt is the temperature difference.

$${\mathit{E}}_{\mathit{t}}=\mathit{m}\mathit{c}\mathit{d}\mathit{t}$$

(3)

3.2. Goal and Scope Definition

The life cycle analysis aimed to evaluate and compare the environmental impacts, especially the life-saving potential associated with the conventional combustion system and catalyst-integrated combustion system for rice husk biomass. The functional unit for this study was 0.0704 kg rice husk as input material to produce 1 MJ. In scenario 1, the assessment considered various emissions, including those from the catalyst production, the combustion of rice husk with and without the catalyst, and waste material disposal. The system boundaries for the two selected combustion systems are presented in Figure 8. Scenario 1 was used for the ReCiPe midpoint assessment, and Scenario 2 was used for the ReCiPe endpoint assessment.

The life cycle assessment of the impact on human health, expressed as the number of years of life lost (YLL) and the number of years lived disabled (YLD), was calculated using the indicator DALY in ReCiPe endpoint (scenario 2), as presented in Equation (4). In scenario 2, emissions and energy requirements from the catalyst production were considered indirect emissions and converted to DALY according to ReCiPe and Eco-indicator methodology. In contrast, scenario 1 includes all energy requirements, including the catalyst production. The ReCiPe method provides three perspectives on the assessment of environmental impacts: egalitarian (E), hierarchist (H), and individualist (I). The individualist (I) viewpoint adopts a short-term interest, undisputed impact types, and technological optimism regarding human adaptation; hierarchy (H) is based on the most common policy principles about timeframe and other issues. The egalitarian (E) viewpoint is more precautionary and adopts a more extended period, impact types that are not yet fully established but for which some indication is available [40]. The hierarchist (H) perspective was selected for this study as it relates to common policy principles about timeframe (not too short or long). The midpoint and endpoint results were used for scenarios 1 and 2, respectively, as the results from the midpoint level are relatively robust regarding ecosystem assessment [41]. In contrast, the endpoint directly evaluates the damage to human health in terms of DALLY (Equation (4)). Using both approaches allowed us to execute a robust assessment of both systems. During rice husk combustion, many substances are emitted into the air and water, contributing to global warming, human toxicity, and water and air pollution. Therefore, eighteen impact categories, including climate change, ozone depletion, terrestrial acidification, freshwater eutrophication, marine eutrophication, human toxicity, photochemical oxidant formation, particulate matter formation, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, ionizing radiation, agricultural land occupation, urban land occupation, natural land transformation, water depletion, metal depletion, and fossil depletion, were considered.

DALY = YLL + YLD

(4)

3.3. Inventories for the Conventional Combustion System and the Integrated Catalyst Combustion System

The study assumed that the rice husk combustion system is located 10 km away from the rice milling facility, generating the husk.

Table 6. Inventory for the generation of 1 MJ with the conventional rice husk combustion and catalyst rice husk combustion.

Item	Unit	Conventional Rice Husk Combustion		Catalyst Rice Husk Combustion
		Combustion	Waste Disposal	Production of Catalyst	Combustion	Waste Disposal
Input (material)
Iron oxide	kg	na	na	0.155	na	na
Platinum	kg	na	na	0.00039	na	na
Input (energy)
Rice husk	kg	0.0704	na	na	0.0704	na
Electricity	kWh	1	na	na	1	na
Transportation	t.km	na	0.000705	0.00311	na	0.000705
Output (emission)
CO	kg	0.355	nm	nm	0	nm
SO2	kg	0.050	nm	nm	0.0114	nm
CH4	kg	0.0004			0.00001
NO	kg	0.022		nm	0.0215
NO2	kg	0.0062	nm	nm	0.00057	nm
NOX	kg	0.028	nm	nm	0.022	nm
PM2.5	kg	0.00041	nm	nm	0.00009225	nm
Output (waste disposal)
Residual ash	kg	0.0141	na	nm	0.0141	na

presents the life cycle inventory of all input parameters.

4. Conclusions

This study carried out a comprehensive evaluation of the environmental impacts associated with the integration of a platinum honeycomb catalyst into biomass combustion systems, focusing on the combustion of rice husks. The study employed a combination of experimentation and Life Cycle Assessment (LCA) methodologies to assess the real-world performance and environmental consequences of the catalyst-integrated combustion system compared to conventional combustion. The catalyst effectively reduced CO, SO₂, and particulate emissions. The low oxidation rate at lower temperatures decreased the CO conversion rate. Temperatures below 400 °C diminished the catalyst’s performance. The catalyst achieved a 100% CO conversion rate at specific temperatures of 427.4–490.3 °C for JPN and 481.3–534.1 °C for RB. The exhaust catalyst system used in this study demonstrated excellent performance. It can be integrated into both large- and small-scale biomass combustion systems, significantly reducing indoor and outdoor air pollution from biomass combustion.

The results reveal that the catalyst-integrated combustion system demonstrates both advantages and challenges in comparison to the conventional system. The detailed breakdown of environmental impact assessments for different life cycle stages highlights the significant role of electricity production in contributing to various environmental impacts, underscoring the broader context of energy systems. Additionally, the manufacturing of the catalyst plays a crucial role in determining the overall indirect environmental footprint of the integrated system. The comparative analysis, considering both combustion and manufacturing stages, suggests a potential overall reduction of nearly 34% in environmental impacts by replacing conventional rice husk combustion with catalyst-integrated combustion. However, it is essential to note that this reduction is contingent upon efficient catalyst manufacturing processes and the energy mix used for electricity production.

In summary, the integration of platinum honeycomb catalysts into biomass combustion systems, particularly for rice husks, holds promise for reducing environmental impacts. However, the success of such a transition requires careful consideration of both the combustion and manufacturing stages, emphasizing the need for sustainable catalyst production and cleaner electricity sources to maximize the environmental benefits. Future research in this area should continue to refine catalyst technologies, explore alternative catalyst materials, and assess the scalability of these systems for broader applications in the quest for a sustainable and low-carbon energy future.

In evaluating the economic feasibility of scaling Pt/Fe–Al honeycomb catalysts, such as the Pt/Fe-doped alumina system with a platinum loading of 0.5 wt% [26], for biomass combustion, platinum emerges as a critical cost driver due to its rarity and high price. The current design, utilizing 0.5 wt% Pt with an average particle size of 2.8 ± 0.5 nm [26], ensures high activity for oxidizing biomass volatiles. Still, even this low loading contributes significantly to costs at industrial scales. The freeze-drying process for synthesizing Pt/Fe–Al₂O₃ supports, which maintains a pore volume of 0.62 cm³/g and a surface area of 178 m²/g, preserves porosity and stability up to 800 °C for 48 h [26]. However, this method poses scalability challenges due to its energy-intensive nature and batch-wise operation. Operational expenses include regeneration techniques such as oxidative treatment (effective up to 800 °C) or steam-assisted gasification, which incur energy costs, cause production downtime, and require additional infrastructure. To enhance economic viability, strategies such as further reducing platinum loading (e.g., below 0.5 wt%), exploring cost-effective alternative supports with similar mesoporous properties, and improving catalyst durability through enhanced thermal stability (e.g., beyond 48 h at 800 °C) and coking resistance, leveraging the redox-active Fe–Al₂O₃ support, should be pursued. Addressing these economic and technical factors provides a more robust assessment of the catalyst’s industrial applicability.