ANAEROBIC DIGESTION OF LIGNOCELLULOSIC BIOMASS VIA COTREATMENT: A TECHNO-ECONOMIC ANALYSIS

Open Access
Author:
Amador-Diaz, Isamar
Graduate Program:
Agricultural and Biological Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 13, 2019
Committee Members:
  • Tom Richard, Thesis Advisor
  • Daniel Edward Ciolkosz, Committee Member
  • Rachel Brennan, Committee Member
Keywords:
  • techno-economic
  • anaerobic digestion
  • renewable natural gas
  • cotreatment
  • Aspen Plus
  • switchgrass
  • lignocellulosic
Abstract:
This study evaluates an alternative to pretreatment that represents a biomimetic approach to fermenting recalcitrant cellulosic biomass. This approach is modeled after a biological mechanism that has proven over time to efficiently deconstruct lignocellulosic biomass: the ruminant’s digestive system. In the last century, the world has been paying increasing attention to greenhouse gas (GHG) emissions and climate change, with the agricultural and energy sectors as two of the largest emitters. Lignocellulosic biomass from perennial crops, crop residues, winter crops and manures can reduce or reverse agricultural GHG emissions relative to conventional summer annual crops like maize and soybean. Renewable Natural Gas (RNG) produced by anaerobic digestion (AD) of lignocellulosic biomass can be a sustainable alternative to fossil natural gas to reach renewable energy policy goals. Conventional AD of lignocellulose is usually not cost competitive relative to fossil fuels, largely due to the long-residence times and hence large digester volumes required to convert recalcitrant cellulosic feedstocks. Recent research with pure cultures suggests that mimicking rumination by milling intermittently during fermentation can improve lignocellulose digestibility and has the potential to lower cost by increasing yield and/or by reducing retention time. Our study is motivated by the possibility that this biomimetic strategy, termed cotreatment, can similarly improve AD. Techno-economic assessment of the process is still needed to assess scale-up viability and potential economic implications of cotreatment assisted AD for renewable natural gas production. The following research intends to assess scale-up viability of cotreatment assisted switchgrass fed anaerobic digestion. Two phases are carried out: technical process modeling and economic analysis. Sensitivity analysis was carried out to study potential impacts of carbohydrate solubilization (due to cotreatment) and scale on minimum fuel selling price ($ GGE-1). For the process modeling phase, an Aspen Plus model was developed to determine the mass and energy flows of each process area. Mass flow results show a potential increase of 27% more biomethane production with cotreatment in comparison to no cotreatment for a fixed ten day residence time. These results served as input parameters for the second phase of economic analysis. The cost for cotreatment aided, mixed culture, biomass-fed anaerobic digestion systems for biomethane production at a scale of 2000 Mg day-1 is $3.37 GGE-1 compared to $4.23 GGE-1 for no cotreatment (2014 USD). Thus, cotreatment decreases the MFSP by $0.86 at that scale. The carbohydrate solubilization sensitivity analysis estimates a 5 cent reduction in MFSP per 1% increase in solubilization. At a 94.4% carbohydrate solubilization factor and 2000 dry Mg day-1 scale, cotreatment aided switchgrass fed AD becomes cost competitive relative to CNG. However, when adding a RIN incentive of $3.36 GGE-1 of CNG to a market fossil CNG price of $2.09 GGE-1, switchgrass fed AD becomes economically feasible at a scales greater than 230 dry Mg day-1 without cotreatment, and at scales greater than 104 dry Mg day-1 with cotreatment. These results indicate that under current prices and reasonable conversion assumptions cotreatment could be a favorable option for the production of biomethane from biomass sources. These findings can aid future planning of large-scale anaerobic digesters to reach government renewable energy policy targets, reduce greenhouse gas emissions, and provide a sustainable, cost-efficient bioenergy resource.