The key to a successful cellulosic ethanol project

December 27, 2021 |

By Mallikarjun Navalgund, President, Advanced Biofuels and Chemicals, Praj Industries

Special to The Digest

The need

From the World Energy Outlook Report, 2021 it is clear that the pledges made so far in the Nationally Determined Contributions (NDCs) are grossly inadequate to stay under the 2 °C, let alone the 1.5 °C target. Humankind, more so the ones living in the island countries and coastal cities know the dire need to meet the net-zero target. Immediate actions are needed to ensure the climate impacts do not become dangerous and unpredictable.

Wading through the deluge of debates, articles, podcasts during the recently concluded COP26, it was striking to note that the current efforts at mitigation are merely reducing the rate of GHG emissions. Most of the removal of carbon from the atmosphere to achieve pre-industrial levels will take place in the next couple of centuries. So, every technology available today and those coming from a green climate technology revolution underway need to be exploited to mitigate climate change.

Some hard-to-abate sectors (e.g., cement and steel) and transport have proven to be particularly recalcitrant. To quote “To get on track for the emission cuts and carbon removal required by 2030 the world needs to increase the share of low-emission fuels twelve times faster”.

Solution at our fingertips

Ethanol is a currently available solution and is being deployed to mitigate emissions from the transportation sector via the legacy energy infrastructure that has existed for over a century. The first generation(1G)/conventional ethanol industry is well established all over the world and more than 110 billion liters (~29 BGPY/88MTA) of ethanol is being produced every year. This eliminates around 70 million metric tonnes of CO2 emissions. What if we could amplify the emissions reductions many times over by producing ethanol using a feedstock that is far more abundant and is currently of low value.

Cellulose is the most abundant molecule around us, of which quite a large part is a residue or waste. Advanced ethanol produced from this cellulosic feedstock is a low-carbon fuel. To top it off, many supporting policy initiatives have been put in place by legislators in the US and EU amongst others. With this backdrop, there is a strong rationale for the industry to ramp up the production capacity of advanced ethanol.

However, a few technology demonstration projects have not delivered the expected outcomes.   Hence, the maturity of the underlying technology to process lignocellulosic feedstock to bioethanol has been perceived as being one of the hurdles in not achieving its potential.

The pre-treatment process step is the major differentiating step between the conventional and advanced ethanol production technologies. (To get deeper insights into the pre-treatment process, Click here.)

The perceived risk of cellulosic ethanol technology can be overcome, and the benefits of this already available solution can be reaped using Praj’s enfinity technology.

The following are reportedly the technology challenges, encountered in the first few cellulosic ethanol technology demonstration plants:

  • In-efficient pretreatment system
  • Hurdles in bioprocessing
  • Difficulty in synchronized operations

1. Successful pre-treatment is key for improved economics

The pre-treatment step has presented some challenges to the first few technology demonstration plants like reduced efficiency, the presence of extraneous components like soil and debris, and clogging of the reactor.

A. Low Process efficiency: Process efficiency is determined by the extraction of the highest amount of product ethanol from the feedstock. As the feedstock cost is the most important element impacting project economics, higher efficiency is paramount to project economics.

The pre-treatment step ‘prepares’ the feedstock for hydrolysis. This ‘preparation’ takes place in two steps:

  • Deconstructing the biomass i.e., separation/ loosening of lignin from cellulose and hemicellulose. This makes the cellulose available for enzymatic hydrolysis.
  • Opening of the carbohydrates makes them porous for more efficient hydrolysis. This improved efficiency not only converts more of the carbohydrates to sugars but at the same time achieves it with the lowest quantity of enzymes.

B. Compromised plant availability: One of the reasons for the plant availability being compromised is the extraneous material that invariably accompanies the biomass feedstock which is also accompanied by soil/gravel. Equipment is prone to get damaged due to the abrasive effect of sand/soil.

Handling fibrous and contaminated feedstock like cassava (1G Feedstock) has provided Praj valuable experience to tackle challenges associated with its processing.

Secondly, pre-treatment reactor scaling and deposition has been a major stumbling block for the first few projects. Praj has utilized multiple strategies in the design and operations of the pre-treatment system based on insights gained from its varied experience. These strategies have been validated and ensure the pre-treatment system stays clog-free.

2. Bioprocessing backed by experience

Challenges that have been reportedly encountered in some of the technology demonstration projects are similar to those encountered in 1G ethanol technology. Having resolved these challenges, Praj is placed in an advantageous position to ensure a successful cellulosic biomass processing technology deployment.

Cellulosic feedstock, difficult feedstock

Physical complexity

Many of the challenges posed by cellulosic feedstock are similar to those posed by cassava, (a tuberous starch-rich feedstock) due to similar physical characteristics. The experience garnered from 10 licensees using cassava as a feedstock for bioethanol production provides Praj with the right credentials to resolve them. The similarities between the feedstock are broadly

  • Cassava is fibrous like a cellulosic material and exhibits similar flow characteristics.
  • Being a tuber, it is typically delivered to the factory along with a significant amount of extraneous matter like soil sand, etc.

Chemical complexity

In terms of the sheer number of constituents that are present, no other feedstock comes even close to molasses. Many of these constituents are inimical to biological processes like fermentation. The number and toxicity of the constituents in the substrate after the pre-treatment step in the cellulosic ethanol technology is similar to molasses.

Praj is using the numerous lessons learned from tackling challenges posed by the physical and chemical complexity of feedstock while processing first-generation feedstock to make the cellulosic ethanol technology robust. So, leveraging the vast bio-processing experience can help minimize risks and improve the maturity of the technology.

3. End-to-end integration of all the process units viz. pre-treatment, enzymatic hydrolysis, fermentation to product separation steps like distillation and ethanol dehydration, and at times evaporation of the stillage. Here are a couple of examples to highlight the importance of ‘integration’.

A. Energy and water integration: To achieve the lowest GHG footprint possible it is essential that the technology uses the lowest net energy. There are many opportunities to cascade energy used in one unit operation again in other unit operations. This obviously results in lower overall net energy use. The same concept is used to reduce net water use by recycling. All in all, ‘good physical integration’ is necessary to ensure a low GHG footprint.

B. Process integration: Removal of the constituents causing harm during bioprocessing would obviously be capital and operating costs additive. Strategies like directed evolution will ensure robust and economic performance.


Three Fortune 500 energy companies have already shown their confidence by signing licenses for Praj’s enfinity technology for their commercial-scale projects. The first of the projects is scheduled for completion by the middle of 2022 with the other two projects succeeding with a gap of 6 months each.

In addition to enfinity, Praj’s offers Celluniti technology in collaboration with Sekab, Sweden. Celluniti harnesses soft-wood from forestry residues to produce cellulosic sugars, biofuels & chemicals. Both Cellutniti & enfinity technology are also useful for the production of sustainable aviation fuels (SAFs) & marine biofuels paving the way for greening these sectors too.

While mankind needs many more technologies from the green climate technology revolution that is underway, the advanced ethanol solution is one that is ready and available to start moving the needle in the journey towards net zero.


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