Transport fuels are one of the major contributors to carbon dioxide emissions. The other major contributor is coal based power generation. Several alternatives like solar, wind, hydro and nuclear power are being promoted to bring down the share of coal-based power in overall energy mix. However, this solution is primarily for static power such as electricity. Transportation is still predominantly dependent on liquid fuels which are derived from fossil petroleum sources. Therefore, alternate cleaner liquid fuels are essentially required to partially replace the petroleum transport fuels to bring down their toxic emissions. Biofuels, derived from renewable bioresources, are one such option. These fuels are sustainable as the feedstock, which is of bio origin, grows regularly and is to a large extent carbon neutral as these need carbon dioxide and sunlight to grow via the process of photosynthesis.
First generation biofuels
Biofuels are graded as per generation depending upon their development cycle. First generation biofuels are products made from sugar, starch, vegetable oil or animal fat using conventional technologies—seeds from vegetable oils, both edible or non-edible, which are chemically triglyceride and can be easily converted by the process of trans-esterification to give biodiesel. Similarly, ethanol obtained from fermentation of sugarcane juice or molasses is also a first generation biofuel. Ethanol obtained by hydrolysis and fermentation of corn is also considered as a part of first generation biofuels.
However, since most of the first generation biofuels are derived from agricultural crops, hence they have been criticised for diverting food into fuel. Additionally, the combination of first generation biofuels can provide only a limited amount of ethanol and biodiesel for blending into petrol and diesel respectively. Several estimates have put a combined contribution of first generation biofuels to transport fuels to the extent of 10 to 15 per cent. However, having said so, the technologies for manufacturing of ethanol from grains and molasses, biodiesel from vegetable oils/fats and their use as a component of transport fuel is well established.
USA and Brazil are two major countries which use ethanol to blend with petrol on a very large scale. While USA produces ethanol from corn, Brazil sources it from sugarcane. European countries produce biodiesel from soyabean, palm and animal fat and blend it with diesel. As first-generation biofuels may have limited impact not to mention their competition with food sources, second generation biofuels are being researched world over to overcome common concerns. Moreover, the reduction of overall green house gases by first generation biofuels is lower as these are specially grown as feedstock and there are several inputs like fertilisers, water and pesticides required for their growth.
None of the biofuels are totally carbon neutral as some energy is spent in producing the feed stock and then on the conversion process.
Corn based ethanol reduces emissions only to the extent of 25 per cent as compared to petrol. This lower reduction in emissions is a result of high energy required to produce corn; all agricultural inputs such as fertilisers, water, pesticides are required for corn cultivation and these processes, combined together, give some emissions. Ethanol from sugarcane is a little better in reducing emissions. However, second generation ethanol, from agricultural wastes like corn stover, rice and wheat straw is able to reduce emissions to an extent of 70-85 per cent as compared to petrol. These are agricultural wastes, therefore the energy to grow these is not counted.
The wide range in reduction of emission is thus due to different technologies applied for production of ethanol from these waste materials. The way to derive at emission reduction capacity of a biofuel is to conduct a complete Life Cycle Analysis (Muñoz, et al., 2014; Roy, 2014; Gopal and Kammen, 2009).
Second generation biofuels
It is widely thought that second generation biofuels can supply larger proportion of alternate fuels in a sustainable, affordable and in an environmentally safer manner as very large amount of feedstock is available. These also do not compete directly with food sources.
Second generation biofuels are produced from non food crops which include waste biomass—wheat straw, corn stover, wood or from special energy biomass crops such as switchgrass. All these materials are chemically classified as Lignocellulosic materials. Producing ethanol from lignocellulosic material is a little more difficult than from starchy materials like grains or sugars. This is because the cellulose and hemicellulose which can be converted to ethanol are well protected and locked by lignin in the plants. Freeing of the sugar molecules from lignocellulosic material involves an additional step of pretreatment. This step separates lignin by breaking the bonds between lignin and cellulose and can be done by physical means like steam explosion or by chemical means of acid/base hydrolysis or by enzymes treatment. The second step involved is to de-polymerise cellulose and hemicellulose into non-numeric 6 or 5 member sugars. Thereafter, fermentation by special bacteria leads to fermentation and production of ethanol.
The technological challenges include low energy pre-treatment, selection of proper enzymes for depolymerisation of cellulose and hemicellulose and finally microbes for fermentation of 5 member sugar molecules to ethanol.
In US, Canada and Europe, huge efforts have been put up by research laboratories and companies to perfect this technology. Several American and European Universities have also contributed to the understanding of this technology. In India, there are several groups who are working on developing this technology. Among these the notable is Bioenergy Research Centre of the Department of Biotechnology-Institute of Chemical Technology (DBT-ICT) at Mumbai. This group developed a unique technology in which enzymes are recycled, resulting in lowering of cost and huge reduction in process time. They demonstrated the technology at 4 tonnes/day pilot plant at India Glycol Limited, Kashipur and later scaled it upto 10 tonnes/day. Among the feedstock tested were rice straw, sugarcane bagasse and cotton stalk. Praj Industries Limited at Pune also demonstrated this technology at 12 tonnes/day pilot plant using acid based pretreatment. The DBT-IOC Centre for Advanced Bioenergy Research developed a technology package at 0.25 tonnes/day pilot and also used indigenously generated enzymes to produce ethanol from rice straw and sugarcane bagasse in higher concentrations. Presently the Centre is in the process of setting up a 10 tonnes/day plant in Mathura.
The Indian government has adopted a national biofuel policy which mandates upto 20 per cent addition of biofuels in the transport fuels, mainly petrol and diesel. It has also recognised that the existing source of ethanol—sugarcane molasses will not be able to meet more that 10 per cent of blending in petrol. The maximum blending of ethanol in petrol achieved in 2015-16 was approximately 4 per cent (Ministry of Petroleum & Natural Gas, 2016). Therefore, the government has decided to support the production of second generation ethanol from non food, non fodder agricultural wastes.
India has planned to set up six to eight very large lignocellulosic ethanol plants (400 tonnes of biomass/day) in the country in next three years. Few of these plants are being set up by oil marketing companies—Indian Oil Corporation, Hindustan Petroleum Corporation and Bharat Petroleum Corporation, using indigenously developed technology. It may be mentioned here that as the technology is still under developed, the cellulosic ethanol produced will be about 10-30 per cent higher in cost as compared to ethanol from sugarcane molasses. However, economies of scale will result with further development and this price gap will be reduced.
A noteworthy fact about lignocellulosic ethanol is its very high impact on green house gas emissions. This is due to the fact that the feedstock is a byproduct of the main food crop. Depending upon the technology configuration, lignocellulosic ethanol has shown 65-80 per cent reduction in GHG emissions as compared to petrol.
Thermo-chemical methods to produce biofuels from biomass
Conversion of vegetable oil by the hydro treating process to produce green diesel is also classified as second generation biofuel. This process straight away gives green diesel which is much more stable than bio-diesel obtained from the trans-esterification process. This process is likely to replace all current trans-esterification processes and will be the process of choice when large availability of vegetable oil is made possible. Indian Oil Corporation’s Research and Development wing has undertaken pilot plant studies and is currently working to further scale up this process.
Though the process for production of biodiesel is well established using any edible or non edible vegetable oils, but availability of feedstock is very limited. Unlike Europe and USA which have surplus soybean and rapeseed oil, India is the largest importer of vegetable oils. Earlier attempts were made to grow non edible oil seeds like Jatropha and Karanjia in marginal unirrigated land without much care. However, the very low seed yield put a question mark on the use of these oils on a large scale.
Biomass to liquids and biomass to synthesis gas
Biomass to liquids (BTL) and biomass to synthesis gas—followed by Fischer-Tropsch, known as FT to diesel are also second generation biofuel processes. Pyrolysis of biomass gives synthesis gas which can then be converted by the FT process to diesel. Shell technologies have developed a novel process where biomass can be directly converted to fungible fuels such as diesel.
Third generation biofuels
Algal materials are being researched as source of lipids which can be easily converted to biodiesel. The interest in algae is due to the fact that algal production per unit area is 10 times more than biodiesel production from seeds. Another advantage of algal growth is the sequestration of carbon dioxide which is required for its growth. There are several US and European companies who are offering technologies for algal biofuels. The Department of Biotechnology (DBT), Government of India has an algal task force for screening of high yielding algal strains. Some groups are working on production of methane by fermentation of algal material. DBT Centre of Bioenergy Research at the Institute of Chemical Technology (ICT), Mumbai; International Centre for Genetic Engineering and Biotechnology (ICGEB), Delhi and Indian Oil Corporation (IOC, Research and Development), Faridabad are amongst several institutes in India working on development of algal technology.
Biohydrogen is considered as the third generation biofuels technology. Dark fermentation of sugar containing materials, aided by microbes or by algae, produces hydrogen by biosplitting of water. This area is actively being researched for production of hydrogen from biowaste and lignocellulosic material. The US National Renewable Energy Laboratory and Universities of Birmingham, Michigan and Cambridge have large groups working on this technology. In India, Indian Oil Corporation’s Research and Development wing, Indian Institute of Technology, Kharagpur, Indian Institute of Chemical Technology, Hyderabad and a group from The Energy and Research Institute (TERI) are active in this area.
Biofuels are required to be blended in fossil based transport fuels to lower the environmental emission impacts and also for sustainability. First generation biofuels like ethanol and biodiesel are already commercial but the quantities available are limited. Furthermore, as a national policy, India needs to produce biofuels from feedstock which do not compete with food or fodder.
Second generation biofuels can make a definite contribution to the transport fuel pool and this area is currently under sharp focus of research laboratories and industry. Globally several large scale plants have been set up mainly in USA, Brazil and Europe. India has also made significant advances and has developed indigenous technology for production of biomass based biofuels. However, there still remains a large requirement for research and development inputs to make these processes cost effective.