Journal of Thermodynamics & Catalysis
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Co-gasification of Waste Biomass and Plastic for Syngas Production with CO₂ Capture and Utilization: Thermodynamic Investigation

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Highlights

• Thermodynamic modelling for co-gasification of Rice Husk (RH) and polyethylene.
•CO₂ and steam used as gasifying agent fors yngas production with H₂/CO ratio of 2.
• Process steam and CO₂ demand decreases with increase in RH wt.% in feed mixture.
• Syngas production and efficiency decreases with increase in RH wt.% in feed mixture.

Introduction

Plastic waste is one of the major causes of environmental pollution, as it is not biodegradable and it can end up in landfills, oceans, and other natural environments, causing harm to wildlife and ecosystems. Moreover, plastic waste breaks down into micro plastics, it can enter the air, water and food chain leading to severe health hazards to humans and animals. Globally, more than 400 Million Tons per Annum (MTA) of plastic waste is generated and India ranks as the second-highest annual producer of plastic waste (27 MTA), behind the United States (34 MTA) [1]. Among the generated waste globally, only 9% is recycled, 12% is incinerated, and the remaining 79% ends up in landfills and dumps which becomes a major threat to the environment [2]. High Density Polyethylene (HDPE) accounts for most of the plastic waste generated globally, weighing in at ~40%, compared to India, where it makes up 35%-38%, as shown in Figure 1A. Another important class of waste generated in large quantities is agricultural waste such as rice husks, wheat straw, and corn stover which is estimated to be around 1400 MTA globally [3]. Among the various agricultural waste, rice husk is the most prevalent type as shown in Figure 1B. India and China together generates ~72 MTA i.e., almost 50% of the world’s rice husk [4]. According to statistics, ~0.30-0.70 kg of fuel (equivalent to petroleum) can be produced from 1 kg of biomass waste [5,6]. Since these abundant agricultural wastes are carbonaceous, they can be used as a replacement for fossil fuels and as a large-scale source of clean energy production, reducing the nation's reliance on imported gasoline and diesel (Figure 1).

Figure 1: Indian (a) Plastic waste composition and (b) Agricultural waste.

However, most of these biomass/plastic wastes are either burned openly or disposed in landfills, resulting in release of greenhouse gases (CO₂ and CH₄) along with other hazardous gases (SO_x and NO_x) and particulate matters [7]. Utilization and conversion of waste to useful products are essential for addressing environmental, social, and economic challenges associated with plastic waste. As a result, over the past decade the attention has shifted to thermochemical conversion of plastic and biomass via pyrolysis, gasification, or combustion because of their potential to generate heat and power. Among those methods gasification has drawn the greatest interest because of its wide adaptability towards various feed and capability to produce different products depending upon the type and amount of the gasifying agents i.e., air, O₂, steam or CO₂ used. Lately, co-gasification of biomass and plastic waste has emerged as one of the most effective way to manage waste while also producing value added syngas [8]. It is also useful for waste reduction, makes energy recovery easier, helps to mitigate climate change, promotes resource conservation, and has several economic benefits by generating new jobs in waste management and energy production and lowering the cost of waste disposal and energy production. Through literature review on cogasification of biomass and plastic waste, through Web of Science with keywords “Co-gasification”, “plastic” and “biomass” in all fields yielded 133 results of which just 14 articles have performed thermodynamic modelling using ASPEN plus. Table 1 provides a detailed literature review on ASPEN plus modelling studies for co-gasification of different biomass and plastic waste. Various biomass such as Rice Husk (RH), sawdust, wood chips, wheat straw, olive pomace, hazelnut shells, Barley and vine pruning’s have been reported to be co-gasified with different waste plastics such as Polyethylene (PE), High Density Polyethylene (HDPE), Polypropylene (PP), Polyethylene Terephthalate (PET), Polystyrene (PS), Plastic Solid Waste (PSW), and Sachet Water Plastic Waste (SWPW). Except for one study that utilized CO₂ and steam as gasifying agents, most of the reported thermodynamic studies have used air, O₂ or steam both to produce syngas with varying H₂/CO ratios. The major driving force of our study is to examine the thermodynamic viability and potential benefits of integrating “co-gasification of waste biomass and plastic” with “Carbon Capture and Utilization (CCU)” in terms of syngas generation, H₂/CO ratio, CO₂ utilization, and net plant efficiency. Co-gasification coupled with CCU process are still not developed sufficiently for commercialization, necessitating additional research to make ground-breaking discoveries. The current work is primarily focused on thermodynamic investigation of an integrated process for co-gasification of Rice Husk (RH) and High-Density Polyethylene (HDPE) for syngas production with CCU. Steady state plant wide models are developed using ASPEN Plus, for the co-gasification of RH and HDPE (at varying composition ranging from 0-100% RH) with CCU for syngas production with H₂/CO of 2. The integrated scheme is made self-sustainable by burning a fraction of syngas produced to meet the process heat and steam demand along with the incorporation of Heat Recovery and Steam Generation (HRSG) and power generation sections. Further, sequential parametric sensitivity analysis is performed to determine the optimal process parameters and investigate the impact of feed composition (i.e., wt.% of plastic in total feed), steam to carbon ratio, and CO₂ to carbon ratio on the overall performance of the integrated scheme (Table 1).

Table 1: Modelling studies on plastic waste and biomass conversion.

Materials and Methods

Process description

Figure 1 provides the plant-wide schematic for the conversion of waste plastic to syngas. In this work, a feed mixture consisting of RH and HDPE is investigated at mass ratios ranging from 0 to100% of RH in the feed mixture. The proximate and ultimate analysis of RH and HDPE is provided in Table 2. Initially, the waste plastics are cleansed to remove the dirt and sand with an equal mass ratio of water, then crushed to 1-3 mm particle size. Plastic waste is then completely dried to remove all the moisture content, at 140°C. Similarly, RH was crushed to 1-3 mm size without washing. The crushed waste materials were then co-fed to the gasifier, where they were initially paralyzed at 500°C into their volatile constituents and char. The char obtained is then completely gasified at 850°C using steam to produce syngas. The lower hydrocarbons/volatile matters obtained during gasification and pyrolysis are then reformed using CO₂ and steam at 850°C to obtain syngas with H₂/CO molar ratio of 2. The outlet gas (or reformer gas) from the gasifier is then cooled to 150°C in Heat Recovery and Steam Generation (HRSG) section to produce High Pressure (HP) steam at 80 atm, which is expanded in a Steam Turbine (ST) to generate power. The exhaust from ST at 1 atm is sent to the gasification and reforming sections to meet the process steam demand. The reformer gas at 150°C from the HRSG section is then used for drying/preheating the feed mixture to 140°C. The reformer gas then enters the gas cleanup section where it undergoes cleaning in three stages for the removal of moisture in condenser, sulfur impurities via Selexol process and finally CO₂ capture. The energy required during the Selexol process and CO₂ capture unit were reported to be 5000 kJ/kg of SO_x and 3950 kJ/kg of CO₂, respectively. A fraction of the CO₂ captured was utilized during the gasification/reforming. The clean syngas obtained with H₂/CO of 2 was then compressed to 5 atm and stored. A fraction of the compressed syngas was burnt in the combustor using compressed air at 5 atm. The products of combustion were then expanded in the gas turbine to generate power. The hot exhaust from the gas turbine was further used to jacket the gasifier to supply the required heat to the process (Figure 2 and Table 2).

Figure 2: Process description for waste plastic conversion to syngas.

Table 2: Proximate and ultimate analysis of different waste plastics.

Process modelling and simulation

Self-sustained steady state plant-wide ASPEN Plus models were developed for co-gasification of RH with HDPE to produce syngas coupled with CCU. The models were developed and optimized individually for 11 different compositions of RH and HDPE feed mixture ranging from 0-100% RH. The ASPEN Plus model flow sheet for the integrated process is shown in Figure 2. A stream class called MCINCPSD is defined before the simulation begins, as the overall process contains RH and HDPE, which are categorized as non-conventional streams with particle size distribution, while H₂, CO, CH₄, CO₂, NO₂, NO, O₂, H₂O, S, SO₂, and SO₃ are regarded as conventional streams, and C is a pure solid representing char. The enthalpy and density of the feed mixture in this case were evaluated using the HCOALGEN and DCOALIGT methods. The Peng Robinson-Boston Mathias (PR-BM) method was used to simulate the properties of all components. No tar formation is considered during thermochemical conversion due to high gasification temperature and use of oxidizing agents (steam and CO₂) in excess [9,10]. 100 kg/h of feed mixture was taken as the basis for the comparison of the 11 different cases of feed composition. The crushed feed of RH and HDPE were initially pyrolyzed using separate RYield reactors (B5 and B7) to break them down into its constituent elements based on their respective ultimate analysis as reported in Table 2. The constituent’s elements of RH and HDPE obtained from the RYield reactors are mixed and co-fed to the RGibbs reactor (B9) which acts as a gasifier. The char is gasified using steam and products are obtained from the RGibbs reactor based on the minimization of Gibbs free energy. Further the product gas from B9 unit enters another RGibbs reactor which acts as the reformer where the unconverted hydrocarbons/volatile matters such as CH₄ is converted using CO₂ and steam to syngas with H2/CO ratio of 2. The syngas obtained is cooled in HeatX (B9) unit while simultaneously producing HP steam. The partially cooled syngas is further used as jacket (B13) for pre-heating/drying the plastic waste. The cooled syngas is then purified by moisture removal in Flash (B14), sulfur removal in Separator (B15), CO₂ removal in Separator (B16) units. The energy required for sulfur and CO₂ separation were assumed to be 5000 kJ/kg of SO_x and 3950 kJ/kg of CO₂, respectively. The cleaned syngas was then compressed and a fraction of it was burnt to meet the process energy needs [11,12]. Table 3 gives detailed specifications of the ASPEN Plus modules that were used during the modelling. Multiple design specs and calculator blocks are used as feed backward and feed forward controllers to manage the process parameters heat, steam, CO₂ and temperature demands. The details of the design specs and calculator blocks used is provided in Tables 4 and 5, respectively (Figure 3 and Tables 3-5).

Figure 3: Plant wide model for waste plastic conversion to syngas.

Table 3: Details of ASPEN Plus modules used.

Table 4: Design Specifications (DS) considered in the proposed schemes model development.

Table 5: Calculator Blocks (CB) considered in the proposed schemes model development.

Results and Discussion

Sequential parametric sensitivity analysis is used to analyze and obtain the optimal process parameters for each of the 11 cases of different feed mixture ranging from 0 to 100% HDPE. Figure 4 shows the effect of varying the Steam to Carbon (S/C) ratio from 0.8 to 2, on carbon conversion within the gasifier for the case of 50% of RH and 50% of HDPE at 850°C. It was found that as the S/C ratio rises, the amount of unconverted carbon in the gasifier reduces which becomes zero at a S/C ratio of 1.2 [13-15].

Figure 4: Effect of S/C mass ratio on carbon conversion within the gasifier.

Figure 5 shows the effect of S/C ratio on the overall syngas production, net amount of syngas consumed to meet the process heat demand and net plant efficiency. It was observed that as the S/C ratio increased from 0.8 to 1.4 the net plant efficiency increased from 50.57% to a maximum of 61.08%. Further increase in S/C ratio from 1.4 to 2 tends to gradually decrease the net plant efficiency from 61.08% to 52.07% due to the increase in consumption/burning of produced syngas from 50.76% to 56.87%, respectively. Therefore, S/C of 1.4 within the gasifier is chosen as the optimal ratio for all the cases [16].

Figure 5: Effect of S/C on total syngas produced/burnt (kg/ h) and net plant efficiency (%).

Table 6 compares the optimal results from the 11 different cases with various feed ratios. The comparison was based on the cogasification of 100 kg/h of feed mixture to produce syngas with H₂/CO ratio of 2 while supplying process heat by burning a portion of the produced syngas [17].

Table 6 shows that when RH concentration rises, less steam and CO₂ are needed for gasification and reforming since there is less carbon in the feed mixture. Further, the concentration of O₂ in the feed mixture also increases as a result of the increase in RH weight percent, which tends to partially oxidize and reform the hydrocarbons and char, thereby reducing the demand for steam and CO₂ as oxidizing agents. Further it can also be observed from the Table 6 and Figure 6 that with the increase in HDPE weight% in the feed mixture (or decrease in weight% of RH) the amount of syngas produced is increased due to the higher carbon content in feed which is in agreement with the results reported by Chai, et al. [3,18]. Air compression requires the majority of the process power, followed by syngas compression and CO₂ separation. Results also show that the power needed for CO₂ compression rises with an increase in RH weight percent because more CO₂ is produced during the gasification stage due to an increase in intrinsic O₂ content in the feed. Almost the same amount of power was required for crushing RH and HDPE in each case. The amount of water pumped for washing HDPE and HP steam generation was found to be decreasing with an increase in RH weight percent. The energy required for sulphur removal was also found to be decreasing with the increase in RH weight percent due to the lower sulphur content of RH compared to HDPE [11]. Further, it was observed that increase in the weight% of RH in the feed mixture resulted in decrease in the amount of syngas production and amount of syngas burnt to meet the process heat demand. However, it was also observed that a rise in RH weight percent in feed was inversely proportional to the fraction produced syngas that was burnt. Approximately 50% of the produced syngas was burnt in case of pure HDPE, compared to 60% in case of pure RH [12]. Table 6 also shows that more than 90% of the captured CO₂ was recycled back for co-gasification, demonstrating the efficacy of the proposed process to utilize CO₂. However, as the HDPE content in the feed mixture drops, the CO₂ utilization is found to decline, and it falls to just 11% in the case of pure RH. Finally, the net power generation and net plant efficiency tend to decrease with the increase in RH content in the feed mixture. The overall plant efficiency was found to be highest at 68% in case of pure HDPE which decreased to almost half i.e., 35% in case of pure RH, due to the high ash content of the RH in comparison to no ash content in case of HDPE [19,20].

Table 6: Performance comparison for syngas production.

Figure 6: Effect of feed composition on net plant efficiency and syngas composition.

Conclusion

Thermodynamic models for the co-gasification of Rice Husk (RH) and High-Density Polyethylene (HDPE) with a Carbon Capture and Utilization (CCU) system was developed using ASPEN Plus for the generation of syngas with H₂/CO ratio of 2. The integrated system was made self-sustainable by adding heat recovery and steam generation and power generation section to meet the process steam and power requirements. The heat demand of the gasifier and reformer is met by burning a fraction of the produced syngas. Sequential parametric sensitivity analysis performed for the case of 50% RH and HDPE showed that complete carbon conversion to syngas within the gasifier can be attained at Steam to Carbon (S/C) ratio of 1.2. However, the net plant efficiency was found to be maximizing at S/C ratio of 1.4. Further the comparative analysis of the 11 different co-gasification cases revealed that the increase in weight percent of RH in feed mixture, resulted in lower carbon content, but higher O₂ and ash content during gasification which resulted in

• Decrease in process steam and CO₂ demand.
• Decrease in amount of syngas produced.
• Increase in fraction of produced syngas that was burnt to meet process heat demand.
• Decrease in utilization of the captured CO₂.
• Decrease in net power generation and net plant efficiency.

Due to the high ash concentration of the RH in comparison to the absence of ash content in the case of HDPE, the total plant efficiency was found to be at its best in the case of pure HDPE at 68% and decreased to practically half (35%), in the case of pure RH.

Acknowledgments

The authors are grateful to the Director, CSIR-Indian Institute of Chemical Technology, for providing a platform and facilities for carrying out the research. The authors also would like to acknowledge the financial support of Council of Scientific and Industrial Research, India RA Fellowship (Award No. 31/0014(13248)/2022-EMR-I).

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