Biochemical production of ethanol via enzymatic route

Currently there are three main biochemical pathways to produce bio-ethanol from lignocellulose which are based upon their specific method of lignocellulose hydrolysis.These pathways include hydrolysis by enzymes, dilute acid or concentrated acid. THe biochemical pathway utilising enzymes in the hydrolysis is an interesting one for me as it can have many different variations with regards to the required pretreatment, actual hydrolysis conditions as well as the with regards to the fermentation. Have a look at the schematic below for the route utilising enzymes to accomplish the hydrolysis.

As can be seen from the image above, this biochemical lignocellulose to ethanol process requires that the raw material firstly undergoes a pretreatment, followed by enzymatic hydrolysis, fermentation and distillation of the final product to ethanol. The pretreatment is required for the enzymes to overcome the recalcitrant nature of lignocellulose which prevents enzymes from efficiently hydrolysing lignocellulose. Many different pretreatment approaches can be followed but the main ones currently employed include steam explosion, dilute acid pretreatment and hydrothermal pretreatment to name a few. These all have the common aim of hydrolysing the hemicellulosic fraction of the lignocellulose as well as to disrupt the lignin fraction of lignocellulose which makes the cellulose fraction more susceptible to enzyme hydrolysis. The liquid stream coming from the pretreatment is usually rich in C5 (pentose sugars) while the solid fraction from the pretreatment is rich in C6 carbohydrates.

The C6 sugars are still bound up in the solid material in their carbohydrate form and need to be hydrolysed before these sugars can be fermented. Enzymatic hydrolysis is suited to this and can be performed using enzymes to reduce the long chain polymers into short sugar monomers.

A couple of variations exist as to how the fermentation is performed including, SSF, SHF and SSCoF, all of which have the advantages and disadvantages which change according to the available enzymes and microbes.

Ideally a microbe that can produce the required enzymes while fermenting the hydrolysed sugars would be the most favourable but currently this is not possible. Until this is possible economics will determine whether SHF or SSF is preferred.

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What is Bio-oil?

Bio-oil is another option currently being researched to replace fossil fuels. A number of lignocellulosic biomass sources can be used for bio-oil production including purposely grown crops as well as agricultural residues.

Fast pyrolysis is currently the preferred option to produce Bio-oil, requiring high temperatures between 350 – 600˚C and short residence times of under 2 seconds. The process must also be performed in the absence of oxygen. The product from this process is comprised of mostly oil, around 70%, with the remainder being char and a number of gaseous products.

Bio-oil is a promising bio-fuel resembling light crude oil which will hopefully be one of the renewable fuel options that will replace and alleviate our reliance on fossil fuels.

Bio-oil produced from fast pyrolysis

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Current (2011) production of second generation fuel ethanol

To date most fuel ethanol produced comes from first generation processes and volumes produced in 2009 rose to 76 billion liters for the year. Most fuel ethanol produced from second generation processes involving biomass are currently produced in pilot and demonstration plants and this technology has not been extensively established. There are a number of commercial plants that are planned for the future as well as a number of plants for which construction has begun. So far the following companies that i have come across have established second generation production facilities are:

1. Iogen – based in Canada, this company opened its demonstration facility in 2004 and is capable of handling 30 tons of biomass per day which corresponds to 5000 – 6000L of cellulosic ethanol per day. Their technology is based on a modified steam explosion process followed by enzymatic hydrolysis and fermentation.

2. Inbicon – based in Denmark, this company opened its first pilot plant in 2003 capable of processing 2.4 metric tonnes of biomass per day. In 2005 a new plant capable of handling 24 metric tonnes of biomass per day. Their technology is based on hydrothermal pretreatment of lignocellulosic biomass at around 180-200ºC for 5-15 minutes, followed by enzymatic hydrolysis and fermentation.

3. Weyland Bioethanol – Opened in October 2010, this company has set up a pilot plant in Normway operating a concentrated acid hydrolysis process. The plant has a capacity equivelent to around 200 000 Litres per year of bio-ethanol.

Inbicon Hydrothermal Pretreatment plant

If you have any information regarding other demonstration of commercial second generation bio-ethanol refineries please let me know.

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Process Routes for 2nd Generation Ethanol fuel

Two different routes can be taken to produce second generation bio-ethanol. Both are able to produce the same final product but require completely different capital equipment and have different advantages and disadvantages.

The first route is the thermochemical route in which the biomass is gasified in a gasification process to produce synthetic gas comprising of hydrogen and carbon monoxide. The syn gas is then either bubbled through a specially designed fermenter in which genetically engineered organisms convert the syngas to ethanol. Otherwise the syngas is fed into a reactor containing a catalyst responsible for converting the gas into fuel ethanol.

The second route is via the biochemical route in which the biomass is first pretreated to expose and open up the lignocellulosic matrix to enzymatic attack. Following pretreatment, enzymes hydrolysis available carbohydrates into sugar monomers which are then fermented to ethanal. The final product from ethanol is then distilled to seperate out the ethanol which will be utilised as fuel ethanol. A number of pretreatment strategies as well as seperate and combined enzymatic hydrolysis strategies are available for this process route.



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Effect of Steam Explosion on Biomass

Before enzymes can efficiently hydrolyse the sugar components of biomass, biomass needs to be pre-treated to expose these sugars to enzymatic attack. Why is this necessary? Well the highly complex lignocellulosic matrix consisting of Cellulose, hemicellulose and lignin forms a barrier against enzymes which prevents degradation of the biomass into its individual components.

Steam explosion is one man-made way in overcoming this barrier, or in other words overcome the recalcitrant propery of biomass that is observed in nature. By subjecting biomass to high pressure steam for a certain period of time and then explosively depressurizing the biomass through release of steam/biomass one can break apart the lignocellulosic structure effectively yielding a highly digestible substrate.

Compare the following pictures of raw un-pretreated triticale straw with that that has been steam exploded at a steam temperature of around 200 degrees and a residence time of 5 – 10 minutes. One can visually see how the biomass has been fragmented and destroyed by this pretreatment technology.

Raw straw for steam explosion

Raw unpretreated triticale straw

Pretreated biomass

Steam Exploded triticale straw

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Mandatory Blending of Biofuels with Petrol and Diesel

Last week the Department of Energy (South Africa) published draft regulations with regards to blending of Bioethanol with petrol/gasoline and biodiesel with diesel. This should hopefully provide security for investors as well as stimulate the local biofuels industry.

The draft deals with three issues:

1. Conditions for mandatory blending

2. Prohibition of certain actions

3. Records to be kept by licensees

With regards to mandatory blending, the draft stipulates that the minimum concentration allowed is 2% v/v bioethanol in petrol and 5% v/v biodiesel in diesel. This poses a couple of questions, the main one being, from where will South Africa source this fuel until we can produce this locally. My hope though is that this mandatory blending will encourage our local biofuels industries to quickly step up to the challenge while at the same time creating a large number of jobs (if the biofuel blending target was increased to 10%, it is estimated that 125 000 jobs could be created directly).

Check out the draft at http://www.info.gov.za/view/DownloadFileAction?id=150835

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Process Development for lignocellulosic ethanol

There are currently a number of process paths that could be used for the production of lignocellulosic ethanol from biomass via the biochemical route.  Generally the production route starts off with preparation of your raw material through milling, communition or chipping of your material to reduce the particle size. The type of preparation depends on the raw material being considered as well as the type of pretreatment that is used to open up the structure.

Following this the prepared raw material undergoes pretreatment in which the aim is to open up structure of the raw material for enzymatic attack. Different pretreatments will results in different products. For example if one performs a pretreatment such as dilute acid hydrolysis, generally the hemicellulosic fraction will be hydrolysed leaving the cellulosic portion intact. This will result in a pretreatment liquor that will containing the hydrolysed hemicellulosic sugars and a solid fraction which contains lignin, cellulose and any remaining hemicellulose that was not hydrolysed during the pretreatment.

Once the pretreatment has finished, the next processing step must be chosen. Currently there are two routes that I know of that could be followed, either seperate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF). Both of these have their own advantages and disadvantages. The result of these two process choices are similar though, resulting in the hydrolysis of the remaining cellulose fraction to monomeric sugars and the fermentation of hydrolysed sugars to ethanol. Finally the product from SSF or SHF can be distilled to produce a pure ethanol which could be used as biofuel.

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