(561a) Pulp and Paper Sludge Valorization: Techno- Economic and Life Cycle Assessment for Enhanced Methane Production

The pulp and paper industry (PPI) generates a significant volume of wastewater that undergoes physical and biological treatment [1,2], resulting in a considerable quantity of pulp and paper sludge (PPS) [3], which is one of the major waste streams of PPI [4]. As global energy demand escalates and concerns over energy security and climate change intensify, anaerobic digestion (AD), which transforms organic materials into methane and carbon dioxide without oxygen [5], has emerged as a versatile technology for renewable energy production. This study undertakes a life-cycle assessment (LCA) and techno-economic analysis (TEA) to explore the valorization of PPS. The focus is on utilizing AD of PPS to generate combined heat and electricity.

The LCA aims to estimate the life cycle environmental impacts of electricity and heat co-generated in a combined heat and power (CHP) unit using biogas produced from AD of PPS. This assessment assumes the plant is in the United States and employs the OpenLCA software. Concurrently, the TEA employs a process simulation of PPS AD built utilizing Aspen Plus. Economic parameters such as net present value (NPV), cash flow rate of return (CFRR), and payback period are estimated and compared across three scenarios.

Scenario A, the Base Case, replicates the conventional operational conditions of AD, serving as a benchmark for assessing the economic and environmental viability and methane production efficiency of a standard AD that utilizes PPS as the feedstock. Scenario B, PPS is pretreated via alkaline pretreatment before AD. Alkaline pretreatment is a promising method to enhance enzymatic hydrolysis for various lignocellulosic materials [6,7]. Studies indicate that alkaline pretreatment effectively disrupts the floc structure of PPS, leading to improved biodegradation efficiency [6]. Research has shown a significant increase in soluble chemical oxygen demand (SCOD) removal efficiency, ranging from 83% to 93%, compared to 70% for untreated sludge [6]. Alkali pretreatment, typically involving hydroxides like NaOH or KOH [8], could be substituted with the by-product of the kraft pulping process, Green Liquor Dregs (GLD), offering a cost-effective alternative [9]. Scenario C explores the co-digestion of food waste (FW) with PPS, leveraging FW's nitrogen-rich composition [10]. Previous studies have identified nitrogen deficiency as a challenge in PPS AD [11,12]. The optimal carbon-to-nitrogen ratio for AD feedstock typically falls between 20-30% [13]. Pulp and paper sludge, with its high carbon-to-nitrogen ratio, accelerates nitrogen consumption by methanogens, hindering microbial population growth and prolonging digestion. Conversely, a low ratio increases ammonia release, inhibiting the AD process.

In this study, for the sake of simplicity in comparison, the management of 500 tons of sludge is chosen as the functional unit. All emissions, materials, wastewater treatment, energy consumption, and product generation are standardized based on this functional unit.

For the AD-CHP process of sludge valorization, as depicted in Fig. 1, the system boundary encompasses biogas production in the AD unit, electricity, heat generation in the CHP unit, and the storage and disposal of digestate. While some generated heat and electricity are utilized within the AD plant, excess electricity is exported to the national grid, and excess heat is utilized either onsite or offsite of the pulp and paper plant. This study does not consider sludge collection and transportation in the comparison and assumes that sludge is generated on-site. This assumption is consistent across all scenarios; thus, it does not affect the comparison.

The life cycle inventory (LCI) involves compiling and quantifying all inputs and outputs that pass through the system boundaries, including input mass/energy and output mass/work done, as depicted in Figure 2. The LCI analysis aggregates emissions and extractions related to the consumption of raw materials, water, energy, wastewater, and transportation.

The impact assessment phase follows inventory analysis in the LCA process. This phase evaluates potential environmental impacts based on inventory flow results. Life Cycle Impact Assessment (LCIA) aims to translate LCI results into environmental impacts, such as effects on natural resources, the environment, and human health [14]. In the final step, results from the inventory and impact phases are interpreted to meet the study objectives. This involves checking for completeness, sensitivity, and consistency and deriving conclusions, limitations, and recommendations.

We developed an Aspen Plus simulation that integrates the AD stoichiometric model (Buswell), CHP production, and recirculation system (Figure 3). This study focuses on utilizing gas turbine technology for CHP due to its reliability, low emissions, abundant high-grade heat, and lack of cooling requirement [15,16]. The gas turbine engine system, based on the Brayton cycle, is depicted in Figure 3. Biogas from the digester is compressed, then combusted, and the resulting high-temperature gases expand through a turbine, generating power. Exhaust gases are directed to a Heat Recovery Steam Generating Unit (HRSG) for further energy recovery. The simulation calculates the rate of biogas generation and biogas composition. Furthermore, the simulation results, including heat and power consumption required for plant operation and power generated in the gas turbine, are used to precisely determine the appropriate CHP unit sizing, and estimate energy consumption within the AD-CHP system.

Under typical operational conditions (Scenario A), before entering the anaerobic process, the PPS is heated to 35 °C (mesophilic temperature). Within the digester, the feedstock undergoes agitation with a recirculated stream of biogas in the absence of oxygen, producing 10,444 m³ of methane per day. The daily consumption of 500 tonnes of feedstock with 12% total solids (TS) and a 30% conversion rate equates to a methane yield of 179.46 ml CH₄/g volatile solids (VS). The biogas is conveyed from the top of the AD to the CHP unit. The CHP unit generates 1.65 GW of electricity and 1.56 GW of heat, totaling 3.21 GW of energy output. The CHP plant operates with a total efficiency of 83%, comprising an electricity generation efficiency of 43% and a heat generation efficiency of 40%. Of the generated heat, 24% (366 kW) is allocated for heating the sludge and compensating for heat loss from the digester walls. Approximately 174 kW of the electricity generated is dedicated to compressing the recirculated biogas stream (RECIR1) to match the pressure at the bottom of the digester (3.5 atm), with the surplus electricity being exported to the grid.

This talk will discuss the detailed results of the TEA and LCA for all three scenarios. The study findings highlight the potential for commercially producing biogas from PPS, presenting promising solutions to enhance sustainability and resource efficiency.

References

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