A Glance at Drop-in Biofuels

drop-in-biofuelsBiofuel commercialization has proved to be costly and lingering than expected due to its high production cost and modification to flexibility in engines. Drop-in fuels are alternatives to existing liquid fuels without any significant modification in engines and infrastructures. According to IEA, “Drop-in biofuels are liquid bio-hydrocarbons that are functionally equivalent to petroleum fuels and are fully compatible with existing petroleum infrastructure”.

What are Drop-in Biofuels

Drop-in biofuels are can be produced from oilseeds via trans-esterification, lignocellulosic biomass via thermochemical process, sugars and alcohol via biochemical conversion or by hybrids of the above methods. Drop-in fuels encompass high hydrogen to carbon ratio with no/low sulfur and oxygen content, low water solubility and high carbon bond saturation. In short drop-in fuel is a modified fuel with close functional resemblance to fossil fuel.

Existing biofuels – bioethanol and biodiesel – have wide variation from fossil fuels in their blend wall properties – high oxygen content, hydrophilicity, energy density and mainly compatibility in existing engines and infrastructures. Oxygenated groups in biofuel have a domino effect such as reduction in the energy density, production of impurities which are highly undesirable to transportation components, instability during storage etc.

Major advantages of drop-in fuels over existing fuels are as follows:

  • Reduced sulphur oxide emissions by ultra low sulphur content.
  • Reduced ignition delay by high cetane value
  • Reduced hydrocarbons and nitrogen oxides emissions
  • Low aromatic content
  • Low olefin content, presence of olefin compounds undergo auto-oxidation leading to surface depositions.
  • High saturates, therefore leaving minimum residues
  • Low particulate emissions
  • No oxygenates therefore has high stability.

Potential Biomass Feedstock

Drop-in biofuels can be produced from various biomass sources- lipids (vegetable oils, animal fats, greases, and algae) and lignocellulosic material (such as crop residues, woody biomass, and dedicated energy crops). The prominent technologies for biomass conversion to drop-in fuel are the thermochemical and the biochemical process.

The major factor playing role in selection of biomass for thermochemical methods is the energy content or heating value of the material, which is correlated with ash content. Wood, wood chips accounts for less than 1% ash content, which is favorable thermal processing than biochemical process, whereas straws, husks, and majority of the other biomass have ash content ranging up to 25% of dry mass.

Free sugar generating plants such as sugarcane and sweet sorghum, are desirable feedstock for Acetone-Butanol-Ethanol fermentation and have been widely implemented. Presently there is a focus to exploit lignocellulosic residues, rich in hydrocarbon, for fuel production. However, this biomass requires harsh pretreatment to remove lignin and to transform holocellulose (cellulose & hemicelluloses) into fermentable products.

The lignocellulose transformation technology must be circumspectly chosen by its life cycle assessment, as it resists any changes in their structural integrity owing to its complexity. Lignocellulosic biomass, when deoxygenated, has better flexibility to turn to drop-in fuels. This is because, in its native state of the feedstock, each oxygen atom consumes two hydrogen atoms during combustion which in turn reduces effective H: C ratio. Biomass feedstock is characterized with oxygen up to 40%, and higher the oxygen content higher it has to be deoxygenated.

Thermochemical Route

Thermochemical methods adopted for biomass are pyrolysis and gasification, on thermolysis of biomass produce intermediate gas (syngas) and liquid (bio crude) serving as precursors for drop-in fuel. Biomass when exposed to temperature of 500oC-600oC in absence of oxygen (pyrolysis) produce bio-oil, which constitutes a considerable percentage of oxygen. After down streaming by hydroprocessing (hydrotreating and hydrocracking) the rich hydrocarbon tar (bio-oil) can be converted to an efficient precursor for drop-in fuel.

At a higher temperature, above 700, under controlled oxygen, biomass can be converted to liquid fuel via gas phase by the process, gasification. Syngas produced is converted to liquid fuel by Fischer-Tropsch with the help of ‘water gas shift’ for hydroprocessing. Hydroprocessing after the thermochemical method is however costly and complex process in case of pyrolysis and inefficient biomass to fuel yield with gasification process.

Biochemical Pathway

The advanced biocatalytic processes can divert the conventional sugar-ethanol pathway and convert sugars to fatty acids. Modified microbial strain with engineered cellular machineries, can reroute the pathway to free fatty acid that can be transformed into butanol or drop-in fuel with necessary processing.

Schematic for the preparation of jet fuel from biomass

Schematic for the preparation of jet fuel from biomass

Biological processing requires operation under the stressful conditions on the organisms to reroute the pathways, in additional to lowering NADPH (hydrogen) consumption. Other value added products like carboxylic acid, polyols, and alcohol in the same biological routes with lower operational requirements have higher market demands and commercial success. Therefore little attention is given by chemical manufacturers to the biological pathways for drop-in fuel production.

The mechanisms of utilization of lignocellulosic biomass to fuel by biological pathway rely heavily on the availability of monomeric C5 and C6 sugars during fermentation. Ethanol is perhaps the best-known and commercially successful alcohol from ABE fermentation. However, butanol has various significant advantages over ethanol- in the perception of energy content, feasibility to existing infrastructures, zero blend wall, safety and clean aspects. Although butanol is a closer drop-in replacement, existing biofuel ethanol, is a major commercial competitor. Low yield from fermentation due to the toxicity of butanol and complexity in down streaming are the vital reasons that hamper successful large scale butanol production.

Challenges to Overcome

Zero oxygen and sulphur content mark major challenges for production of drop-in fuels from conventional biomass. This demands high hydrogen input on the conventional biomass, with H: C ratio below 0.5, like sugar, starch, cellulose, lignocellulose to meet the effective hydrogen to carbon ratio of 2 as in drop-in fuel. This characterizes most of the existing biomass feedstock as a low-quality input for drop-in fuels. However oleochemicals like fats, oils, and lipids have closer H: C ratio to diesel, gasoline and drop-in fuels, thus easier to conversion. Oleochemical feedstock has been commercially successful, but to prolong in the platform will be a major challenge. Lipid feedstock is generally availed from crop-based vegetable oil, which is used in food sectors. Therefore availability, food security concerns, and economics are the major constraints to sustaining the raw material. Consequently switching to lignocellulosic biomass feedstock for drop-in holds on.

Conclusions

Despite the hurdles on biomass characteristics and process technology for drop-in fuel, it is a vital requirement to switch to better replacement fuel for fossil fuel, considering environmental and economic benefits. Understanding its concepts and features, drop-in fuel, can solve existing greenhouse emission debate on current biofuels. Through crucial ambiguities existing on future of alternative fuels, drop-in fuel has a substantial potential to repute itself as an efficient sustainable eco-friendly fuel in the naear future.

References

  • Neal K Van Alfen: ENCYCLOPEDIA OF AGRICULTURE AND FOOD SYSTEMS, Elsevier, Academic Press.
  • Pablo Domínguez de María John: INDUSTRIAL BIORENEWABLES:A Practical Viewpoint: Wiley & Sons.
  • Ram Sarup Singh, Ashok Pandey, Edgard Gnansounou: BIOFUELS- PRODUCTION AND FUTURE PERSPECTIVES, CRC Press.
  • Satinder Kaur Brar, Saurabh Jyoti Sarma, Kannan Pakshirajan : PLATFORM CHEMICAL BIOREFINERY-FUTURE GREEN CHEMISTRY, Elsevier.
  • Sergios Karatzos, James D. McMillan, Jack N. Saddle: Summary of IEA BIOENERGY TASK 39 REPORT-THE POTENTIAL AND CHALLENGES OF DROP-IN BIOFUELS, IEA Bioenergy.
  • Vijai Kumar Gupta, Monika Schmoll, Minna Maki, Maria Tuohy, Marcio Antonio Mazutti: APPLICATIONS OF MICROBIAL ENGINEERING, CRC Press.

Trends in Waste-to-Energy Industry

The increasing clamor for energy and satisfying it with a combination of conventional and renewable resources is a big challenge. Accompanying energy problems in almost all parts of the world, another problem that is assuming critical proportions is that of urban waste accumulation. The quantity of waste produced all over the world amounted to more than 12 billion tonnes in 2006, with estimates of up to 13 billion tonnes in 2011. The rapid increase in population coupled with changing lifestyle and consumption patterns is expected to result in an exponential increase in waste generation of up to 18 billion tonnes by year 2020. Ironically, most of the wastes are disposed of in open fields, along highways or burnt wantonly.

Size of the Industry

Around 130 million tonnes of municipal solid waste (MSW) are combusted annually in over 600 waste-to-energy (WTE) facilities globally that produce electricity and steam for district heating and recovered metals for recycling. The global market for biological and thermochemical waste-to-energy technologies is expected to reach USD 7.4 billion in 2013 and grow to USD 29.2 billion by 2022. Incineration, with energy recovery, is the most common waste-to-energy method employed worldwide. Since 1995, the global WTE industry increased by more than 16 million tonnes of MSW. Over the last five years, waste incineration in Europe has generated between an average of 4% to 8% of their countries’ electricity and between an average of 10% to 15% of the continent’s domestic heat.

Advanced thermal technologies, like pyrolysis, and anaerobic digestion systems are beginning to make deep inroads in the waste-to-energy sector and are expected to increase their respective market shares on account of global interest in integrated waste management framework in urban areas. Scarcity of waste disposal sites coupled with growing waste volumes and solid waste management challenges are generating high degree of interest in energy-from-waste systems among policy-makers, urban planners, entrepreneurs, utility companies etc.

Regional Trends

Currently, the European nations are recognized as global leaders of waste-to-energy movement. They are followed behind by the Asia Pacific region and North America respectively. In 2007 there are more than 600 WTE plants in 35 different countries, including large countries such as China and small ones such as Bermuda. Some of the newest plants are located in Asia. China is witnessing a surge in waste-to-energy installations and has plans to establish 125 new waste-to-energy plants during the twelfth five-year plan ending 2015.

Incineration is the most common waste-to-energy method used worldwide.

The United States processes 14 percent of its trash in WTE plants. Denmark, on the other hand, processes more than any other country – 54 percent of its waste materials. As at the end of 2008, Europe had more than 475 WTE plants across its regions – more than any other continent in the world – that processes an average of 59 million tonnes of waste per annum. In the same year, the European WTE industry as a whole had generated revenues of approximately US$4.5bn.

Legislative shifts by European governments have seen considerable progress made in the region’s WTE industry as well as in the implementation of advanced technology and innovative recycling solutions. The most important piece of WTE legislation pertaining to the region has been the European Union’s Landfill Directive, which was officially implemented in 2001 which has resulted in the planning and commissioning of an increasing number of WTE plants over the past five years.