Wood is made of cellulose, hemicellulose, and lignin.
Cellulose is made of glucose, a 6 carbon sugar easily fermentable by yeast to produce ethanol.
Hemicellulose in hardwood is made of 5 and 6 carbon sugars. Soft wood hemicellulose is made of 6 carbon sugars only.
Lignin is an unholy mess of aromatics (benzene rings) and short chains. When fungi break it down, it often produces poisons. This way lignin protects each precious fibre of cellulose from microorganisms. Research is looking at ways to suppress the lignin producing process. This leads to greater biofuel production.
Sources of lignocellulose are: Trees, Switchgrass, giant miscanthus, Wheat straw, bagasse from sugar cane, various food processing wastes.
To make ethanol, the matrix of lignin and hemicellulose must be broken up in a process known as pre-treatment. Then the accessible cellulose and hemicellulose is hydrrolized, or broken down into sugars with cellulase and hemicellulase enzymes. Then the sugars are fermented to ethanol.
On wood chips, cellulase enzyme will only convert 20% of the cellulose to sugar. Pretreatment allows access to the cellulose and hemicellulose.
Pre-treatment - Getting past the Lignin
Just one look at lignin and you can see why bugs can’t get past this wall of nasties.
The challenge was to break it down without destroying the cellulose and leaving the lignin in a usable form.
Pre-treatments to make cellulose available to cellulase enzymes include:
Acid hydrolysis was first used in Germany during WW2.
A good collection of pretreatment options
Poplar bred to suppress ligin is red. It produces much more ethanol.
Protic ionic liquids
A 2013 research project looks interesting.
An acid such as acetic, is combined with an amine to make a "protic ionic liquid" which dissolves lignin from the lignocellulose. The liquid can be distilled off for reuse.
There is quite a lot of work in this field:
Steam explosion pre-treatment
With hardwoods, high pressure steam will break down the 5 carbon hemicellulose to produce acetic acid. At high temperature water becomes acidic. The combination of both produces acid hydrolysis of the lignin. This breaks down the lignin without damaging the cellulose. It does not work with softwoods because it has 6 carbon hemicellulose which does not break down to acetic acid.
Enzyme digestion improves from 15-20% to 90% of the cellulose converted to sugar. The hemicellulose produces a low yield of sugar.
Improvements can be made by adding SO2, SO3, or CO2 to the steam.
Liquid hot water pre-treatment
Hot water at pH 5-7 has an advantage in converting 80% of the hemicellulose to sugar.
Supercritical water hydrolysis
Supercritical hydrolysis is a method of converting biomass into cellulosic sugar by employing water under supercritical conditions. The water, acting as a solvent, extracts C6 sugar (glucose) from cellulose plant matter. Lignin remains as a solid particle. Users of the process, including Renmatix, claim advantages in reaction time and reactor size compared to alternative biological and chemical processes, including acid hydrolysis and enzymatic hydrolysis. Source
Dilute sulfuric acid
90% of the hemicellulose has been recovered as sugar.
Concentrated sulfuric acid
Concentrated Phosphoric acid with ethanol can dissolve bamboo and give a high yield of sugars from hemicellulose and good access to the cellulose for cellulase enzyme.
Lime, NaOH, Ammonium hydroxide, etc give good yields and remove lignin.
This process uses super critical CO2 under pressure at 35-80oC to penetrate the cells and make the cellulose available for conversion to sugar. The claim is that as it is carried out at 35 the sugars are not destroyed and more is available than with other processes. Ref...
Ammonia fibre explosion
White rot fungi attacks the lignin. However the process is slow and consumes the sugars.
Cellulose to sugar
Once the cellulose is exposed, cellulase enzymes produced by fungi can break the cellulose down to sugars ready for fermentation.
Enzymes are sourced from microorganisms collected from termites, leaf cutter ants, cow's rumen, etc. One fungus,Trichoderma Viridae was collected in PNG during WW2. The army research laboratory had been asked to find out why the soldiers cotton uniforms were rotting so quickly. The fungus produces cellulase enzyme that breaks down the cellulose into sugar. However it does not break down the lignin.
In 2010, a genetically engineered yeast strain has been developed that produces its own cellulose-digesting enzymes. Assuming this technology can be scaled to industrial levels, it would eliminate one or more steps of cellulolysis, reducing both the time required and costs of production.
The process is claimed to be ten times more effective at saving CO2 than making ethanol from corn. This is a puzzling claim as half of the energy in ethanol from corn is used to distil the ethanol out of the water, and the cellulosic ethanol process still has to pass through fermentation and distillation. The energy balance looks good if wood chips are burnt to distil the ethanol, and then not included in the energy cost.
One company, Iogen, is now working with Shell and others producing ethanol in their pilot plant. They are using wheat straw and sugar cane bagasse.
(Declaration: I was head of research at this company in 1979. It was then Iotech. - John Davis)
A competing company, Stake technologies, used a lower temperature steam with longer resident times. It has sold the technology to SunOpta. It then sold the process to Massachusetts-based Celunol Corp. They merged with Diversa to become Verenium Biofuels Corporation. They seem to be most interesed in enzymes.
Bypassing pre-treament via gasification
Ethanol from Synthesis gas (CO + H2)
One way to avoid the difficulties of pretreatment is to partially burn the biomass (or fossil fuel) to produce synthesis gas, a mixture of carbon monoxide and hydrogen.
The anaerobic bacterium Clostridium ljungdahlii, discovered in commercial chicken waste, can produce ethanol from syn. gas.
The process can produce 100 gal US / ton of dry biomass. (380 KG/1000 KG = 38% by mass)
Coskata is also following this path, however they say their organism was found in anaerobic mud. Technology review. The organism is grown on fine porous tubes with syn. gas flowing through the tubes and seeping into the growing film of organism. More
Market Assessment of Biomass Gasification and Combustion Technology for small- and Medium-Scale Application - National Renewable Energy Laboratory (US)
Enerkem in Canada converts municlipal waste to syn gas, then to methanol, then to ethanol.
Syn gas can produce oils via the Fischer-Tropsch synthesis.
6CO + 3H2O ---> C2H5OH + 4CO2
6H2 + 2CO2 ---> C2H5OH
In 2012, cellulosic ethanol production cost $0.94 per litre, compared to the $0.67 per litre cost of corn-based ethanol.
By 2016 it is expected that the price will be similar to ethanol from corn.
Greenhouse cost of ethanol
The greatest problem with ethanol is that it takes energy to make it. The question is how much profit is there from each method, and is it worth it?
It is very difficult to get a consistent view on this.
Depending on what you include, the results vary. Do you include the goods the workers consume? Do you give the bye-products any share of the energy cost?
More: Efficiency of biofuels
The figures are puzzling and I don't rally believe them. It takes about 50% of the energy in ethanol to distil it, so if this is included, the profit can never exceed 2 X.
Using the lignin
As lignin makes up one third of wood, it is important to make use of it.
It is the second most common bio polymer after cellulose.
A thermoplastic is made from lignin by German firm Tecnaro. It can be reinforced with cellulose fibres and when discarded burned the same as wood.
Can be used to make phenol formaldehyde resins, but it depends on the price for phenol from oil.
Some lignin can be used to make expanded polyurethane foam. Again it depends on price.
The Toyota MOB concept electric car car. The body panels are made from liquid wood in two mobius strips.
D. Sandor and R. Wallace, National Renewable Energy Laboratory
Techno-economic comparison of process technologies for biochemical ethanol production from corn stover
Research at USDA and UC–Berkeley involves patching a gene from corn called corngrass into switchgrass, to create a grass that is incapable of aging.
The new switchgrass stays in an early stage of life in which it never goes dormant, and it never produces seeds or flowers.
Without the need to expend energy on flowers and seeds, the grass keeps up to 250 percent more starch in its stem than other varieties, yielding more sugar for fermentation into biofuels.
The leaves are much softer than those in unmodified switchgrass, and they contain a different kind of lignin.
Ref: Clean Technica