Energy Efficiency Analysis: Biomass-to-Wheel Efficiency Related with Biofuels Production, Fuel Distribution, and Powertrain Systems

BACKGROUND: Energy efficiency analysis for different biomass-utilization scenarios would help make more informed decisions for developing future biomass-based transportation systems. Diverse biofuels produced from biomass include cellulosic ethanol, butanol, fatty acid ethyl esters, methane, hydrogen, methanol, dimethyether, Fischer-Tropsch diesel, and bioelectricity; the respective powertrain systems include internal combustion engine (ICE) vehicles, hybrid electric vehicles based on gasoline or diesel ICEs, hydrogen fuel cell vehicles, sugar fuel cell vehicles (SFCV), and battery electric vehicles (BEV). METHODOLOGY/PRINCIPAL FINDINGS: We conducted a simple, straightforward, and transparent biomass-to-wheel (BTW) analysis including three separate conversion elements--biomass-to-fuel conversion, fuel transport and distribution, and respective powertrain systems. BTW efficiency is a ratio of the kinetic energy of an automobile's wheels to the chemical energy of delivered biomass just before entering biorefineries. Up to 13 scenarios were analyzed and compared to a base line case--corn ethanol/ICE. This analysis suggests that BEV, whose electricity is generated from stationary fuel cells, and SFCV, based on a hydrogen fuel cell vehicle with an on-board sugar-to-hydrogen bioreformer, would have the highest BTW efficiencies, nearly four times that of ethanol-ICE. SIGNIFICANCE: In the long term, a small fraction of the annual US biomass (e.g., 7.1%, or 700 million tons of biomass) would be sufficient to meet 100% of light-duty passenger vehicle fuel needs (i.e., 150 billion gallons of gasoline/ethanol per year), through up to four-fold enhanced BTW efficiencies by using SFCV or BEV. SFCV would have several advantages over BEV: much higher energy storage densities, faster refilling rates, better safety, and less environmental burdens

Thermodynamic efficiency of biomass gasification and biofuels conversion

Biomass has great potential as a clean renewable feedstock for producing biofuels such as Fischer-Tropsch biodiesel, methanol, and hydrogen. The use of biomass is accompanied by possible ecological drawbacks, however, such as limitation of land or water and competition with food production. For biomass-based systems a key challenge is thus to develop efficient conversion technologies which can also compete with fossil fuels. The development of efficient technologies for biomass gasification and synthesis of biofuels requires a correct use of thermodynamics. Energy systems are traditionally analyzed by energetic analysis based on the first law of thermodynamics. However, this type of analysis shows only the mass and energy flows and does not take into account how the quality of the energy and material streams degrades through the process. In this review, the exergy analysis, which is based on the second law of thermodynamics, is used to analyze the biomass gasification and conversion of biomass to biofuels. The thermodynamic efficiency of biomass gasification is reviewed for air-blown as well as steam-blown gasifiers. Finally, the overall technological chains biomass-to-biofuels are evaluated, including methanol, Fischer-Tropsch hydrocarbons, and hydrogen. The efficiency of biofuels production is compared with that of fossil fuel

Efficiency analysis of hydrogen production methods from biomass

Abstract: Hydrogen is considered as a universal energy carrier for the future, and biomass has the potential to become a sustainable source of hydrogen. This article presents an efficiency analysis of hydrogen production processes from a variety of biomass feedstocks by a thermochemical method – gasification as well as biochemical methods – fermentation and anaerobic digestion. The exergetic efficiency of H2 production by gasification of more dry biomass is comparable to that of the commonly used Steam Methane Reforming. The detailed exergy analysis of H2 production by biomass gasification shows that the largest exergy losses occur in the gasifier

Comparative analysis of large biomass & coal co-utilization units

The co-utilization of coal and biomass in large power units is considered in many countries (e.g. Poland) as fast and effective way of increasing renewable energy share in the fuel mix. Such a method of biomass use is especially suitable for power systems where solid fuels (hard coal, lignite) are dominating. On the other hand, the admixture of usually wet biomass to the main fuel impacts the steam boiler efficiency and durability. Moreover, the production of electricity generates emission not only by the direct combustion, but also during externally-connected processes, like e.g. preparation and transport of both renewable and fossil fuels. Considering abovementioned aspects, the legitimacy of biomass co-utilization in large, basically coal-fired, power units should be carefully analyzed. The main goal of the presented study is therefore the assessment of energy efficiency and CO2 emission due to biomass use as a secondary fuel in large, basically coal-fired power units. Two methods of fossil and renewable fuel coupling have been analyzed: direct biomass and coal co-combustion by mixing them before coal mills in classical pulverized-fuel unit, as well as, biomass gasification followed by co-combustion of syngas with coal in the same steam boiler. Both systems have been modeled mathematically to determine the mass and energy fluxes crossing their boundaries. Models were prepared using Aspen Plus software. The main assessment factor used for comparison of two biomass utilization methods is cumulative CO2 emission calculated per unit of produced electricity

Thermodynamic evaluation of biomass-to-biofuels production systems

Biomass is a renewable feedstock for producing modern energy carriers. However, the usage of biomass is accompanied by possible drawbacks, mainly due to limitation of land and water, and competition with food production. In this paper, the analysis concerns so-called second generation biofuels, like Fischer-Tropsch fuels or Substitute Natural Gas which are produced either from wood or from waste biomass. For these biofuels the most promising conversion case is the one which involves production of syngas from biomass gasification, followed by synthesis of biofuels. The thermodynamic efficiency of biofuels production is analyzed and compared using both the direct exergy analysis and the thermo-ecological cost. This analysis leads to the detection of exergy losses in various elements which forms the starting point to the improvement of conversion efficiency. The efficiency of biomass conversion to biofuels is also evaluated for the whole production chain, including biomass cultivation, transportation and conversion. The global effects of natural resources management are investigated using the thermo-ecological cost. The energy carriers' utilities such as electricity and heat are externally generated either from fossil fuels or from renewable biomass. In the former case the production of biofuels not always can be considered as a renewable energy source whereas in the latter case the production of biofuels leads always to the reduction of depletion of non-renewable resources. (C) 2013 Elsevier Ltd. All rights reserved

Exergy analysis of thermochemical ethanol production via biomass gasification and catalytic synthesis

In this paper an exergy analysis of thermochemical ethanol production from biomass is presented. This process combines a steam-blown indirect biomass gasification of woody feedstock, with a subsequent conversion of produced syngas into ethanol. The production process involves several process sections, including biomass drying and gasification, syngas cleaning, reforming, conditioning, and compression, ethanol synthesis, separation of synthesis products, and heat recovery. The process is simulated with a computer model using the flow-sheeting software Aspen Plus. The exergy analysis is performed for various ethanol catalysts, including Rh-based and MoS2-based (target) catalysts as well as for various gasification temperatures. The exergetic efficiency is 43.5% for Rh-based and 44.4% for MoS2-based (target) catalyst, when ethanol is considered as the only exergetic output. In case when by-products of ethanol synthesis are considered as the additional output the exergetic efficiency for Rh-based catalyst increases to 58.9% and 65.8% for MoS2-based (target) catalyst. The largest exergy losses occur in biomass gasifier and ethanol synthesis reactor. The exergetic efficiency for both ethanol catalysts increases with decreasing gasification temperature

Torrefaction of wood. Part 1: Weight loss kinetics

Torrefaction is a thermal treatment step in the relatively low temperature range of 225–300 °C, which aims to produce a fuel with increased energy density by decomposing the reactive hemicellulose fraction. The weight loss kinetics for torrefaction of willow, a deciduous wood type, was studied by isothermal thermogravimetry. A two-step reaction in series model was found to give an accurate description. For the two steps, activation energies of 76.0 and 151.7 kJ/mol, respectively, and pre-exponential factors of 2.48 × 104 and 1.10 × 1010 kg kg-1 s-1, respectively, were found. The first reaction step has a high solid yield (70–88 wt%, decreasing with temperature), whereas less mass is conserved in the second step (41 wt%). The fast initial step may be representative for hemicellulose decomposition, whereas the slower subsequent reaction represents cellulose decomposition and secondary charring of hemicellulose fragments. The kinetic model is applied to give recommendations for industrial torrefaction process conditions, notably operating temperature, residence time and particle siz

Exergy analysis of biomass-to-synthetic natural gas (SNG) process via indirect gasification of various biomass feedstock

This paper presents an exergy analysis of SNG production via indirect gasification of various biomass feedstock, including virgin (woody) biomass as well as waste biomass (municipal solid waste and sludge). In indirect gasification heat needed for endothermic gasification reactions is produced by burning char in a separate combustion section of the gasifier and subsequently the heat is transferred to the gasification section. The advantages of indirect gasification are no syngas dilution with nitrogen and no external heat source required. The production process involves several process units, including biomass gasification, syngas cooler, cleaning and compression, methanation reactors and SNG conditioning. The process is simulated with a computer model using the flow-sheeting program Aspen Plus. The exergy analysis is performed for various operating conditions such as gasifier pressure, methanation pressure and temperature. The largest internal exergy losses occur in the gasifier followed by methanation and SNG conditioning. It is shown that exergetic efficiency of biomass-to-SNG process for woody biomass is higher than that for waste biomass. The exergetic efficiency for all biomass feedstock increases with gasification pressure, whereas the effects of methanation pressure and temperature are opposite for treated wood and waste biomas

Utilisation of reactor heat in methanol synthesis to reduce compressor duty : application of power cycle principles and simulation tools

The chemical conversion in a methanol reactor is restricted by equilibrium, therefore the synthesis loop is operated at high pressure and unconverted gas is recycled. Such a synthesis loop consumes large amounts of compression work. In this paper a new flow sheet for methanol synthesis is presented. In this flow sheet the high recycle and operating pressure of the reactor is exploited to produce power. A turbine expander and compressor pair is placed in the recycle stream and utilises the reactor heat at the maximum possible temperature in a process gas power cycle. In conventional systems the reaction heat is often transferred to generate steam to drive steam turbines, but the heat is reduced in quality due to the temperature-driving forces in the heat exchange equipment. Simulation models of the new flow sheet and a conventional flow sheet are created to compare the systems based on energy consumed per kg methanol produced. In the conventional flow sheet the reaction heat is used to generate steam for use in steam turbines. In the new flow sheet a portion of the reaction heat is still transferred to a steam cycle to limit the temperature in the reactor. The remaining heat is used to drive the process gas cycle. The simulation results showed that the new flow sheet consumed overall 24% less energy than the conventional flow sheet