Читать книгу Biomass Valorization - Группа авторов - Страница 13

The sustainable production of chemicals and products can be achieved from conversion of biomass, an inherently renewable source. Biomass covers a wide range of bio‐based resources from plants or animals. These resources include plant‐based materials, biowastes, and aquatic organisms. Valorizing renewable biomass feedstocks can offer environmental benefits that include reduced emissions, safer feedstocks, better geographic distribution of resources, and achievement of a circular economy [12,15–18].

In a circular economy, resources – such as carbon, nitrogen, and phosphorous compounds – are used with a circular “take–make–reuse/recycle” approach, as opposed to a linear “take–make–dispose” approach [12]. A closed cycle can be achieved with biomass valorization processes by recycling the generated CO₂ through natural photosynthetic processes [19,20]. This process happens particularly with biodegradable plastics. Further, the existence of nonedible and rapidly growing plants parallel to the development of high‐throughput agricultural technologies can lead to a carbon‐neutral cycle in short periods of time, readjusting the increased levels of CO₂ emission given by the fossil industries [21].

In the context of biofuels, biomass has been subdivided in three categories given as follows along with the major evidenced drawbacks:

1 First‐generation biomass: This includes all edible biomasses (e.g. sugarcane, corn, whey, barley, and sugar beet) that are composed of sucrose or starchy carbohydrates, hence relatively simple macromolecules with low recalcitrance. Biological fermentation of said sugar polymers yields bioethanol, one of the most studied drop‐in biofuels with current industrial production [22]. Food‐derived vegetable oils are also considered as first‐generation biomass and they yield biodiesel through transesterification [23]. The main issue of this type of biomass is the clear competition with food resources (which will be continuously more precious, given the increase of world population) as well as the intensive use of water and land for the growth of said crops [24].

2 Second‐generation biomass: Nonfood raw materials, including by‐products and waste materials. Generally, second‐generation biofuels are produced from lignocelluloses (e.g. grasses, soft or hard wood, and forestry residues) or various wastes/by‐products (e.g. agricultural: stover, wheat straw, corn cob, rice husk, and sugarcane bagasse; industrial: glycerol, grains from distilleries, and paper sludge; or urban: household and municipal solid wastes). Given the structural composition of these feedstocks (mixtures of cellulose, hemicellulose, and lignin), pretreatment is usually required for fermentation to biofuels and biochemicals, and the process economics are hindered by the use of multiple steps, leading to lower overall conversions [25–30]. The main technological challenge of these feedstocks is, in fact, the structural complexity that hinders the efficient use of the lignocelluloses as a whole, calling for pretreatments that in turn possess drawbacks depending on the method (vide infra).

3 Third‐generation biomass: This includes nonedible feedstocks that do not require agricultural lands for their cultivation, namely, aquatic biomass, such as algae and other microorganisms (e.g. cyanobacteria). Depending on the strain, these feedstocks may contain mono/polyunsaturated hydrocarbons to produce gasoline‐like fuels via cracking or higher lipid content for biodiesel applications via transesterification. When considering algae, the main issue is correlated with the high water content that hinders transportation or requires significant energy inputs or long times to dry them, whereas microorganisms require specific operating conditions. Furthermore, the economic challenges of these feedstocks limit their industrial application, given the low cultivation volumes and resource efficiency in processing [31–33].

A fourth generation of biomass is also contemplated and exemplified as modified microorganisms considered in the third generation, finally used to harvest solar energy through photosynthetic processes [34,35]. However, these microbial species require improvements of genomics‐based breeding and carry the usual concerns of modified organisms, such as unexpected microbial resistance.

The available volumes of these types of biomass will play a major role in identifying the biggest driver for chemical sustainability. According to a 2018 report from the European Union (EU), the annual production of agricultural biomass (i.e. first generation) was estimated at 956 million tonnes (Mt) of dry matter of which 54% directly used for food consumption and 46% of residues (e.g. leaves and stems) partially used for animal bedding or bioenergy production. In fact, 80% of the agricultural biomass is used as food and feed, showing the limited potential of using first‐generation biomass for chemicals and energy production. As it concerns third‐generation biomass, in particular algae (including macro and micro), only 0.23 Mt of wet matter was estimated, corresponding to a mere 0.027 Mt of dry mass. On the other hand, the total woody biomass (above ground, second generation) was estimated at 18 600 Mt of dry weight [36]. Looking at the quantities of the different biomasses, the high availability of lignocelluloses in Europe makes them the most attractive. The >18 000 Mt of woody resources can make Europe competitive worldwide and support sustainable processes. Particularly, the efficient use of lignocelluloses and residues would improve the long‐term sustainability of the chemical industry, given the volumes and little impact on the food resources, although these feedstocks still rely on forest management constraints. Other waste materials (e.g. food and municipal) are increasing in volumes, given the concomitant increase of world population and improvement of their living conditions. For example, 61 Mt of food waste are produced yearly in the EU alone [37]. However, the major challenges of these products are the variable seasonal composition as well as the implementation of a proper supply chain of these anthropological side streams to biorefineries [38].

Conversion strategies of biomass, however, generally come with low resource efficiency, causing higher production costs and limited competitiveness with the well‐established petroleum market. Thus, for economic advantage, high volumes, ease of production, and limited competition with other markets (e.g. food) are required. In this sense, the use of lignocellulosic biomass may again offer a promising alternative to the fossil‐based industry. From an energetic perspective, lignocelluloses and other waste materials possess lower energy densities compared to nonrenewable resources such as coal, oil, and natural gas. However, biopower possesses negative emissions thanks to the photosynthetic process, whereas fossil fuels cause significant life cycle greenhouse gas emissions [39]. Also, conversion of biomass to key molecules (e.g. ethanol, 2‐methylfuran, and hydrogenated ethers and fatty acids) can offer biofuel diversification with various energy contents for different transport applications, including aviation; these processes rely on the separation of the different biomass components [21,40]. From a chemical point of view, the use of lignocelluloses can offer a wide variety of platform chemicals for the synthesis of not only traditional but also new products to satisfy different areas in the chemical industry (pharmaceuticals, textiles, and materials), which are discussed in the following paragraph. A separation of bio‐components will be required and explained therein.