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Mass Balance Modeling and Life Cycle Assessment of Microalgae-derived Biodiesel Production.

机译：微藻衍生生物柴油生产的质量平衡建模和生命周期评估。

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摘要

Biofuels produced from agricultural starch, sugar and oil crops such as corn, sugarcane, and palm, or first-generation biofuels, are produced at commercial scales worldwide. Though most biofuels are produced with the intent to reduce greenhouse gas (GHG) emissions and fossil fuel dependency, these first-generation biofuels have increasingly been shown to be problematic; achieving little to no reduction in GHG emissions compared to their fossil fuel counterparts, competing with food and feed crops, and causing direct and indirect land use change. Second generation biofuel feedstocks, such as microalgae, are hoped to reduce or eliminate the drawbacks of first-generation feedstocks.;This dissertation investigates the environmental impacts of biodiesel production from microalgae, with the main focus on primary energy requirements and life cycle GHG emissions. The dissertation includes a critical review of existing studies; a mass balance model of a simulated microalgae biodiesel production system; a detailed life cycle assessment (LCA) of the production system with a variety of technology options for each step of the production process; and a scenario analysis with alternative utilization scenarios for the primary co-product from the system, lipid-extracted algal biomass residual.;In addition to assessing and informing technology choices and strategies for environmentally preferable pathways among current algal biodiesel technologies, this research also addresses an important methodological issue in LCA, co-product allocation, and proposes some possible solutions to reduce the uncertainty caused by this issue.;Results of the critical review show that significant variation exists among existing LCA studies of algal biodiesel production, which arises from inconsistency in both parameter assumptions and methodological choices. Even after a meta-analysis was conducted, which corrected for some differences in scope and key assumptions, the reviewed studies show a large range in life cycle primary energy and GHG emissions; 0.2 to 8.6 MJ per MJ of algal biodiesel, and -30 to 320 g of CO2e per MJ of algal biodiesel. This range is so large that very little can be concluded regarding the potential for algal biodiesel to meet the goals of second-generation biofuels, and provides the motivation for development an independent and original model for algal biodiesel production.;A mass balance model for an integrated algal oil and biogas system was developed to understand nutrient, water and carbon flows and identify recycling opportunities. The model showed that recycling growth media and recovering nutrients from residual algal biomass through anaerobic digestion can reduce the total demand for nitrogen (N) and phosphorus (P) by 66% and 35%, respectively. Freshwater and carbon dioxide requirements can also be reduced significantly under these conditions.;The mass balance model provided the basis for developing a LCA model capable of incorporating multiple technology options and identifying preferable pathways. The LCA found the best performing scenario consists of normal nitrogen cultivation conditions (as opposed to nitrogen deficient conditions which can increase algal lipid content, but decrease overall productivity), a combination of bioflocculation and dissolved air flotation for harvesting algal cells from cultivation media, centrifugation for dewatering of separated algae, oil extraction from wet biomass using hexane solvent, transesterification of algal oil to biodiesel, and anaerobic digestion of biomass residual with the liquid digestate returning to cultivation ponds. This pathway results in a life cycle energy requirement and GHG emissions of 1.08 MJ and 73 g CO 2-equivalent per MJ of biodiesel, with cultivation and oil extraction dominating energy use and emissions. This result suggests that current technologies can neither achieve a positive net energy return for algal biodiesel, nor achieve substantial reductions in CO2e emissions compared to petroleum diesel.;A comparison between different scenarios for using the major co-product from algae biodiesel production, lipid-extracted algal biomass residual, suggests that utilizing the co-product within the production system for nutrient and energy recovery is preferable than utilizing it outside as animal feed from a life cycle perspective.;A number of possible ways to allocate the environmental burdens between co-products were tested. Among them, system expansion and economic allocation return favorable results compared value-based allocation methods; however, there are still unsolved issues when applying system expansion, for example, current practices do not consider future market values in the context of a consequential LCA.;This dissertation shows that the near-term performance of biodiesel derived from microalgae does not achieve the significant reductions in fossil energy dependence and GHG emissions hoped for from second-generation feedstocks. Furthermore, there is substantial uncertainty in technology performance and other key modeling parameters that could influence these findings. However, some promising, but still uncertain technologies, such as hydrothermal gasification, have the potential to achieve greater reduction in life cycle GHG emissions and energy consumption.
机译：由农业淀粉，糖和油料作物（例如玉米，甘蔗和棕榈）生产的生物燃料或第一代生物燃料，在世界范围内都以商业规模生产。尽管大多数生物燃料的生产都是为了减少温室气体（GHG）的排放和减少对化石燃料的依赖，但这些第一代生物燃料已越来越有问题。与化石燃料相比，温室气体的排放量几乎没有减少，甚至与粮食和饲料作物竞争，并导致直接和间接的土地利用变化。希望第二代生物燃料原料，例如微藻类，能够减少或消除第一代原料的弊端。本论文研究了微藻类生产生物柴油对环境的影响，主要集中在一次能源需求和生命周期温室气体排放上。论文包括对现有研究的评论。模拟微藻生物柴油生产系统的质量平衡模型；生产系统的详细生命周期评估（LCA），在生产过程的每个步骤均具有多种技术选择；以及对系统主要副产品，脂质提取的藻类生物质残留物进行替代利用情景的情景分析。除了评估和告知当前藻类生物柴油技术中对环境更有利的途径的技术选择和策略的信息外，本研究还针对LCA中一个重要的方法学问题，即副产品分配，并提出了一些解决方案，以减少由该问题引起的不确定性。严格审查的结果表明，现有LCA研究中藻类生物柴油生产之间存在显着差异，这是由不一致引起的在参数假设和方法选择上。即使进行了荟萃分析，并纠正了范围和主要假设的某些差异，所审查的研究仍显示生命周期中一次能源和温室气体排放的范围很大；每MJ藻类生物柴油0.2至8.6 MJ，每MJ藻类生物柴油-30至320 g CO2e。这个范围如此之大，以至于几乎无法得出藻类生物柴油满足第二代生物燃料目标的潜力，并为开发一个独立且原始的藻类生物柴油生产模型提供了动力。开发了藻类油和沼气综合系统，以了解养分，水和碳的流量并确定回收机会。该模型显示，通过厌氧消化回收生长培养基和从藻类残余生物中回收养分，可使氮（N）和磷（P）的总需求分别减少66％和35％。在这些条件下，淡水和二氧化碳的需求量也可以大大降低。质量平衡模型为开发LCA模型提供了基础，该模型能够结合多种技术选择并确定可取的途径。 LCA发现，表现最佳的情况包括正常的氮培养条件（与氮缺乏条件相反，氮缺乏条件会增加藻类脂质含量，但会降低总生产率），生物絮凝和溶气浮选相结合以从培养基中收获藻类细胞，离心分离用于分离藻类的脱水，使用己烷溶剂从湿生物质中提取油，将藻类油酯交换为生物柴油以及将生物质残留物进行厌氧消化，然后将液体消化物返回培养池。该途径导致生命周期的能源需求和每MJ生物柴油1.08 MJ和73 g CO 2当量的温室气体排放，其中耕种和采油占能源使用和排放的主导。该结果表明，与石油柴油相比，目前的技术既无法为藻类生物柴油实现正的净能量回报，也无法实现二氧化碳排放量的大幅减少。使用藻类生物柴油生产的主要副产品脂质-脂肪的不同方案之间的比较从生命周期的角度来看，提取的藻类生物质残留物表明，在生产系统内利用副产品进行营养和能量回收比在室外将其作为动物饲料更可取。；在副产品之间分配环境负担的多种可能方式产品经过测试。其中，与基于价值的分配方法相比，系统扩展和经济分配取得了良好的效果。但是，例如，在应用系统扩展时仍然存在未解决的问题因此，目前的做法在相应的LCA的背景下并未考虑未来的市场价值。本论文表明，源自微藻类的生物柴油的近期性能并未实现对化石能源依赖性和温室气体排放的大幅降低，而第二次一代原料。此外，技术性能和其他可能影响这些发现的关键建模参数存在很大的不确定性。但是，一些有前途但仍不确定的技术（例如水热气化）有潜力实现生命周期温室气体排放量和能源消耗的更大减少。

著录项

  • 作者

    Yuan, Juhong.;

  • 作者单位

    University of California, Davis.;

  • 授予单位 University of California, Davis.;
  • 学科 Natural resource management.;Environmental management.;Transportation.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 157 p.
  • 总页数 157
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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