Bioenergy

 

 

Introduction & Motivation Plant Biomass is Part of the Solution Biomass Resources & Issues Unique Role of Biomass Biomass Composition Biomass to Ethanol Conversion Process Opportunities & Challenges Summary

 

Figure: conceptual Biomass to fuel conversion plant

 

Introduction & Motivation

In the current scenario, energy crisis is one of the biggest problems facing the world. One of the solutions to this complex problem is the study of alternative sustainable energy sources that can compete against the petroleum based fuels. Biofuels are being extensively promoted for their potential to contribute to energy security, stable energy prices, and climate change mitigation. The United States government has recently subsidized biofuels and over 99% of biofuels produced in the United States at this time is ethanol that is distilled from corn as an additive for gasoline. Brazil and the United States account for nearly 90 percent of all ethanol production.

 

Currently, ethanol is produced by fermentation of sugar and starch in various plants (corn, soybean) with Saccharomyces species that break down the sugars to ethanol. This practice of turning food that would be consumed by humans or livestock into fuel has driven up food prices around the globe, making ethanol a less viable alternative to petroleum-based fuels. However, cellulosic ethanol, which is made by breaking down the structural parts of the plant that cannot be used for food, appears as a solution.

Furthermore, increasing environmental concerns coupled with record-high oil prices have created a demand for cleaner and more efficient liquid fuels. In 2007 the U.S. Department of Energy adopted a mandate that transportation fuel include 21 billion gallons of advanced biofuels by 2022 and 2 billion gallons as soon as 2012. Known as the Energy Independence and Security Act, the bill would eventually require 80% of the biofuel to be derived from ligno-cellulosic biomass. Along with the ligno-cellulosic biomass, algae biofuels are also generating considerable interest around the world. They may also represent a sustainable pathway for helping to meet the U.S. biofuel production targets set by the Energy Independence and Security Act of 2007.

 

Biofuel can be derived from biomass in various ways and used for many applications. Traditionally, biomass is burned and used for heat or in a Rankine steam cycle. New technologies are using biomass in biochemical / thermochemical conversion processes to produce useful transportation fuel. 

 

Plant Biomass is Part of the Solution

The search for sustainable energy sources has led us exploring many areas such as wind, solar, geo-thermal etc. Amongst these, plant biomass came across as a part of solution for the sustainable energy future. The plant biomass has many advantages that makes it a vital part in search for the sustainable resources. Some of the key points are highlighted below.

 

Energy Security

Reduce dependence on imported oil

Biomass is the only renewable that directly reduces dependence on petroleum for liquid transportation fuels

 

Economic Prosperity

Reinvigorate rural economies by creating a new bioindustry

Envision creation of a large biorefining industry producing a range of renewable fuels and chemicals from a variety of biomass feedstocks

 

Environmental Health

Carbon neutral processes with lower net GHG emissions

Cleaner, more efficient processes

Biomass to Hydrogen and Distributed Energy

 

 

Biomass Resources & Issues

Biomass are available in very wide range in nature and each resource have its own unique characteristic for conversion to fuel. The bio-fuels may be derived from a woody resources in trees to a micro-algae that are found in a totally different environment and have to undergo a totally different approach for conversion to fuels. A list of potential bio-fuel resources and its conversion issues are tabulated below.

 

Resources Wood Residues Sawdust Wood waste Pulp miss wastes Agricultural Residues Corn stover Rice hulls Sugarcane bagasse Animal Wastes Energy Crops Switchgrass Hybrid poplar Willow Micro-algae

Issues Quality Composition Ease of conversion Cost Production Collection and Transportation Quantity available Sustainability Land Air Water

 

Unique Role of Biomass

While the growing need for sustainable electric power can be met by other renewables, biomass is our only renewable source of carbon-based fuels and chemicals. Bio-fuels have a great potential to replace the transportation fuel. Hence, it is very important to learn about the role that the bio-fuels may play and its various components that may be converted to fuels. Biomass biochemistry is probably the best place to start while exploring the potential of certain bio-fuel resource.

 

Biochemistry

Carbohydrates (CxH2xOx)

Polymers of sugars

(monomer) Glucose, Fructose, Pentose, Hexose, etc…

(dimer) Lactose, Sucrose

(polymer) Starch

(polymer) Cellulose

Fatty acids

Proteins

Lignin

 

Lignin has a structure similar to the valuable aromatics that the petroleum industry routinely processes. This constituent has a high energy content—the highest of all forms of biomass. A key characteristic is that it is resistant to biochemical conversion. In all current biorefinery schemes, the lignin fraction is converted through high-temperature processes.

 

Hemicellulose is the easiest constituent to convert to sugars. It is very readily hydrolyzed to release a complex mixture of C5 and C6 sugars. The difficulty is in the fermentation of C5 sugars. Today’s fermentation organisms are slow and inefficient in converting C5 sugars to ethanol.

 

Cellulose is the most abundant form of carbon in the biosphere. It is a polymer of glucose, just like starch. However, unlike starch, the manner in which the glucose units are connected makes cellulose resistant to hydrolysis. This characteristic is what makes wood so durable. This also is the basis for much of the research going into the conversion of lignocellulosic biomass. NREL and other organizations continue to work to develop low-cost technology to convert cellulose to glucose (a simple C6 sugar). The research in this area involves chemical as well as enzymatic processes.

 

Biomass Composition

Based on the biochemistry, biomass composition had led us to understand the various products that may be developed from a specific biomass. A woody biomass that has a very rich concentration of carbon in form of cellulose (carbohydrate) may be converted to ethanol, biogas or may be directly combusted in heat engines. On the other hand, algae bio-fuel that  has a very high concentration of  lipids may be converted to bio-diesel. The methods of biomass to fuel conversion is shown in schematic diagram below.

 

Woody Material

Process based on enzymatic hydrolysis of cellulose to glucose with cellulase enzyme systems.  While past designs have been SSF, an HHF design was implemented to take advantage of the potential cost savings that could come from higher temperature hydrolysis.  Enzyme cost assumed at $0.11/gal.

 

Bioenergy Products

• Ethanol (From carbohydrates)
• Biodiesel (From lipids)
• Biogas
• Electricity via heat engine (From lignin)

 

Bioenergy "Platforms"

The schematic diagram below shows a possible efficient biomass to fuel conversion process where a variety of products are produced from single biomass species that increases the overall utilization of biomass. 

 

Big Drivers in the US in the area of the bio-fuel are listed below:

 

Corn Grain

• Largest volume grain and source of ethanol in U.S.
• Potential to produce 13-20 billion gallons/year

Cellulosic  Biomass

• Over 1 billion tons/year of lignocellulosic biomass
• Non-food supply
• Technology needed to make the 60 billion gallon ethanol goal by 2030
• 15%-25% lignin
• 23%-32% hemicellulose
• 38%-50% cellulose

Algae Biomass 

• Largest source of lipid for the conversion to bio-diesel. Has the highest yield of bio-diesel per hectare. Algae cost more per unit mass yet can yield over 30 times more energy per unit area than other, second-generation biofuel crops
• 10% -80% lipid content per dry mass depending upon the species.

 

Biomass to Ethanol Conversion Process

There are various ways in which biomass can be converted to ethanol. Amongst them, starch process and stover process is most widely accepted. The flow diagram for these process is shown below.

 

Starch Process

Feedstock Collection and Delivery --> Pre-Processing --> Grain Mashing --> Sugar Fermentation --> Ethanol and Solids Recovery

 

Stover Process

Feedstock Collection and Delivery --> Pre-processing --> Thermochemical Pretreatent --> Conditioning --> Cellulose Hydrolysis --> Sugar Fermentation --> Ethanol and Solids Recovery

 

From Grass to Gas

Step 1: Thermochemical treatment The raw plant feedstock is treated with chemicals — often dilute sulfuric acid — to break down cell walls and make the cellulose accessible.

Step 2: Enzymes A mix of cellulase enzymes is then added to convert the cellulose and hemicellulose molecules into the simple sugars glucose and xylose.

Step 3: Fermentation Yeast or bacteria are added, converting the sugar into a mixture of ethanol and water, what refineries call "the beer."

Step 4: Distillation The ethanol is refined and purified, producing a fuel that could one day end up in your gas tank.

Read More http://www.wired.com/print/science/planetearth/magazine/15-10/ff_plant_formula#ixzz151HQwckh

 

 

Opportunities & Challenges

One of the challenges for bio-mass to fuel conversion technologies is energy efficiency. With respect to utilization of biomass for bio-fuel production, a lot of attention has to been given on the energy balance for the process. A summary of the opportunities and challenges for the bio-fuel production is listed below:

 

• Lower operating cost
• Operating cost potentially 20%-40% lower processing cellulosics
• Diversifying feedstock options hedges against rising grain or utilities prices
• Higher capital cost
• $2.50-$4.00/annual gallon for cellulosics vs. $1.00-$1.50 for grain
• Potential for novel higher value of products and co-products
• Opportunity: New process streams provide opportunities.
• Example: Advances in the pre-treatment for the ligno-cellulosic ethanol production, development of cheaper and more effective enzymes for hydrolysis, plant genetic engineering for higher yield etc. may prove valuable.

 

Summary

Bio-fuels are definitely a part of the solution to the current energy crisis. There are various ways in which biomass to energy conversion process may be designed.

 

• The plant that uses biomass in its solid form directly in a boiler that is operating a Rankine Cycle steam plant to generate electricity.
• The plant that gasifies the biomass and uses the biogas in the existing natural gas system, where the existing system consists of both natural gas‐fired power plants and natural gas pipeline for residential/commercial heat needs.
• The plant that separates the biomass into two components: starch and the rest. The starch component is converted into sugar and fermented to create ethanol.
• The plant that separates the biomass into four components: starch, cellulose/hemicellulose, lignin, and oils. The starch is converted into sugar which is fermented to create ethanol. The cellulose/hemicellulose is also converted into sugar and fermented to create ethanol. The lignin is combusted to operate steam turbine. The oil is converted into biodiesel.

 

The choice of the various options depends on the economic viability of the given option in which the consideration of bio-fuel cost, transportation (both from source to end uses), capital cost for construction and its end use possibilities (demand fulfillment) has taken into account. Further, the option of using a biomass to fuel conversion to other sources of energy has to analyzed and depends from case to case.