Measurement and Testing
43
Some of these include carbon fi bers, oleo-gels, polymer blends, and phenols which have applications in industries like pharmaceutical, cosmetics, and fuels [26]. Economic analyses have been conducted on the impact of lignin valorization on bioethanol product economics. In one model of a specifi c industrial process, researchers analyzed the economic impact of two potential co-products of cellulosic ethanol production: Compressed natural gas (CNG) and lignin plastic composite material (LPCM) both of which have various applications apart from bioethanol. In this model, it was found that by including CNG and LPCM as co-products, the capital costs could be distributed among these products, resulting in a feedstock cost decrease of 19.4% [27]. Instead of discarding the byproducts of second- generation bioethanol production, valorization could help make the industry more competitive and economically viable.
Third-Generation Ethanol Figure 3: Changes in 2022 8-hour ozone design values for nonattainment areas between the RFS2 scenario and the RFS1 scenario [8].
per bushel have been predicted to increase due to the rise of ethanol production [3]. Second-generation ethanol is considered an improvement to fi rst-generation ethanol. Also known as cellulosic ethanol, second-generation ethanol is sourced from biowaste [17] rather than crop sugars or starches. This type of ethanol allows for biofuel production without affecting food sources. In the US, instead of corn grain, corn stover could be used for cellulosic ethanol production [18]. Corn stover includes the leaves and stalk of a corn, which is then used as the basis for second-generation ethanol. Depending on the method and feedstock used, cellulosic ethanol could result in a larger carbon reduction than fi rst-generation bioethanol [19]. Although the EPA mandated at least 16 billion gallons of cellulosic biofuel, there is currently no commercial cellulosic bioethanol production in the US due to technological and economic challenges [17, 3]. Such challenges include fi nding effi cient pretreatment methods as well as production methods that can make cellulosic ethanol more economically viable.
Cellulosic Ethanol Production Methods
The process in which cellulosic ethanol is made differs signifi cantly from fi rst-generation ethanol. Instead of fermenting plant starches directly into ethanol, hydrolysis is the primary method. Brazil, the second largest producer of ethanol worldwide, uses sugarcane as its primary feedstock [20]. Although most of its production is fi rst-generation, the country is home to the only plant in the world that commercially produces cellulosic ethanol [1]. Making cellulosic ethanol is a more complicated process than conventional fi rst-generation ethanol because the cellulose must fi rst be broken down into sugars before starting fermentation. These free sugar molecules, through hydrolysis, then can be fermented for ethanol production [21]. One vital step, however, is the pretreatment of the feedstock. Known as fi rst-stage hydrolysis, it aims to break down the lignin and hemicellulose that surround the feedstock cellulose [22]. One of the most common pretreatments, acid hydrolysis, is performed by soaking the feedstock in a dilute acid solution and heating the mixture [21,22]. Enzymatic hydrolysis, a second pretreatment also known as second-stage hydrolysis, introduces a cellulose-degrading enzyme that converts the complex carbohydrate into sugars for fermentation [21]. Cellulosic bioethanol requires complex methods, which is partially why many countries are reluctant to incorporate it into their commercial biofuel production.
Recently, there have been efforts to make cellulosic ethanol production more favorable by potentially mitigating the amount of pre-processing requirements needed for the feedstock. A potential idea that is being researched is the incorporation of biological microorganisms into pre-treatments. Microorganisms play an important role in the fermentation process in common cellulosic production systems, but they are rarely used in the saccharifi cation stage. Consolidated bioprocessing (CBP) is an approach that aims to merge both the saccharifi cation stage with
the fermentation stage by using a single organism to convert the material to sugars as well as ferment it to ethanol. By combining these processes, the costs of breaking down the cellulose could be reduced by avoiding the usage of expensive commercial enzymes [23]. CBP organisms have also been the target of genetic modifi cation in order to increase ethanol yield. Some of the organisms researched for engineering include various yeasts, bacteria, and fungi and it has been predicted that a consortium of microbials could make the process even more robust [24].
The International Energy Agency (IEA) Bioenergy Organization has created a classifi cation that aims to quantifi ably measure the readiness of a system for commercial deployment, the Technology Readiness Level (TRL) which uses values 1 to 9 [25]. When measuring both the feedstock and the concept of specifi c biofuel refi neries, it was found that while conventional (fi rst- generation) biorefi neries have a readiness of 9, lignocellulosic biorefi neries only have a TRL of 6-8. One way researchers are aiming to make cellulosic bioethanol production more economically viable is by fi nding ways to utilize the lignin component of cellulosic material that would have otherwise been discarded as residue, also known as valorization. Due to the specifi c structure of lignin and certain features it possesses such as high reactive groups and hydrophobicity, it has the potential to be made into many types of chemicals and materials.
Cellulosic ethanol does not come without its drawbacks. Despite not being a food source, cultivation still produces harmful emissions and other environmental detriments. Harvesting excessive stover from corn, for example, can harm the soil, lead to increased levels of erosion and water pollution [18]. In addition, cellulosic ethanol can be costly and requires advanced technologies in order to facilitate the complicated process. A newer feedstock for ethanol is being researched and the results are promising: third-generation bioethanol. Third- generation bioethanol uses algae as its feedstock- a remarkable feat. Algae is a promising alternative due to its high lipid and carbohydrate content, low land usage, and low levels of lignin and hemicellulose [28]. The low levels of lignin and hemicellulose make this option more attractive than second-generation ethanol since the main obstacle for cellulosic ethanol production is that advanced technology needed for substance breakdown.
Third-generation bioethanol is still being researched and there are still no commercial producers of it. Currently, the main drawback is the large amount of energy needed to cultivate it. For this reason, the net energy return is lower than what is needed to compete with fossil fuels [29]. Different species of algae as well as cultivation and fermentation methods are being researched so that algal bioethanol could become more economically attractive. Various algae species have different carbohydrate contents, and ones with higher carbohydrate percentages may have higher conversion into biofuel [30]. There are two main types of algae: microalgae and macroalgae, which include seaweed. Researchers propose that seaweed has the most potential due to its low lignin composition. Because of this, the greatest potential for investment in third-generation ethanol is in eastern Asian countries, where 98.9% of the world’s macroalgae cultivation occurs a there [28]
The algal-ethanol process is similar to cellulosic production in the sense that both have to undergo pretreatment, fermentation, and purifi cation. Discovering appropriate algae species for cultivation is important, but research is also being conducted on different pretreatment and hydrolysis methods [31]. One study was done on a fungal pretreatment method for the algae
K.alvarezii and
G.amansii which are potentially suitable for bioethanol
Figure 4: Global seaweed distributions across the different regions of the world [28].
WWW.PETRO-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88
