Introduction

The environmental pollution, energy shortage, and climate change are the main bottlenecks which currently affect the economic sustainable development. The utilization of renewable biomass resources is a better way to overcome these problems. Abundant and renewable biomass resources, especially non-grain lignocelluloses, can be used to produce liquid fuels and bulk chemicals to partially replace nonrenewable fossil resources such as oil. Currently, research is mainly focused on this issue and lignocellulosic ethanol which is among the representative topics [1, 2].

The lignocellulosic ethanol has been paid more attention since 1970s due to oil crises and adverse ecological effects of fossil fuels. The developed countries such as United States, Brazil, and Europe heavily invested in research and development (R&D) [3]. The process of ethanol production from lignocellulosic biomass is theoretically feasible; however, a few bottlenecks have affected the development of the industry, from the collection, transportation, and storage system of raw materials to the pretreatment technology of raw materials for destroying the anti-degradation barrier [4, 5], from the analysis and reconstruction of microorganisms with complex cellulose-degrading enzyme system to the screening and construction of fermentation strain for efficient conversion of cellulose sugar [6, 7], There are many scientific, technical, and engineering problems which need to be explored and solved properly. Further improving and perfecting the production technology of lignocellulosic ethanol and improving its economic competitiveness have become the key to the success or failure of lignocellulosic ethanol industrialization [8].

Industrial status

In twenty-first century, the climate change and reduction of carbon emission are highly focused and many countries carried out pilot-scale studies at different level which gradually improved the technology of lignocellulosic ethanol production [9]. Globally, the lignocellulosic ethanol production is promoted and many countries including U.S, Germany, Brazil, Italy, and China are among the leading nations. The main industrialization projects are shown in Table 1.

Table 1 Major global lignocellulosic ethanol industrialization projects

The United States is the leading producer and user of lignocellulosic ethanol having a strong investment policy for funding and research. Three large plants for lignocellulosic ethanol have been established in the United States since 2013 [16]. Among them, The DuPont Company’s plant built in 2015 has the largest output; unfortunately, the production cost of lignocellulosic ethanol of the plant was higher than that of grain ethanol, and the project was shut down in November 2017 [10]. Abengoa, another ethanol plant of United States, tried for ethanol production in September 2014, However, due to project cost overrun and the technical problems arising from the amplification of lignocellulosic ethanol, the company declared bankruptcy at the end of 2015 and sold its cellulosic ethanol plant at a low price [15]. Presently, POET-DSM is the only operational plant using corn straw and corncob as raw materials, and it adopts dilute acid steam-explosion pretreatment and pentose/hexose co-fermentation technology. In 2017, the company began to build an in situ producing enzyme system and announced a major breakthrough in the comprehensive utilization of raw materials [17]. Due to the integration of in situ enzyme production technology, this polygeneration biological refining process is considered to be a breakthrough in the industrialization of lignocellulosic ethanol [18].

Brazil is a significant producer of ethanol. The local sugarcane bagasse and sugarcane leaves are used as raw materials for lignocellulosic ethanol production [11]. GranBIo is the largest lignocellulosic ethanol plant and was operated in 2014. According to the latest report [19], Raízen Company put into operation ethanol plant with a capacity of 32,000 tons of ethanol per year in 2018 and claimed to build a second production line. In general, due to the advantages of raw materials, lignocellulosic ethanol industry is developing smoothly in Brazil.

In Germany, Clariant has been in continuous trial production and operation of ethanol in a 1000-ton pilot plant for more than 4 years using straw as raw material [12]. Its cellulosic ethanol integrated production technology has made breakthroughs in chemical free pretreatment, in situ producing enzyme system, pentose/hexose co-fermentation, and other technologies, and has successively signed license agreements with relevant companies, and demonstration plant with a capacity of 50,000 tons of cellulosic ethanol per year will be built in Romania and Slovakia, respectively [20].

In 2013, Italy built its first lignocellulosic ethanol plant, straw and asparagus were used as raw materials, and residual lignin was used for power generation. However, the alcohol-electricity cogeneration unit has not achieved stable operation. In 2017, it was announced to temporarily stop production [13, 18].

The industrialization of lignocellulosic ethanol in China began in 2012, when Longlive, a Chinese company built the largest lignocellulosic ethanol plant and was the only approved cellulosic ethanol producer of its time in China. Unfortunately, due to the lack of advantages in production cost, the unit is currently in shutdown state [14]. Henan Tianguan Group built gas-electric cogeneration unit in 2013 with a capacity of 30,000 tons of ethanol per year, but yet not operated due to raw material supply, cost, and financial crises [15]. Jinan Shengquan Group produced cellulosic ethanol production unit with a capacity of 20000 tons of ethanol per year in 2012, but it is also in a state of closure [15].

The completion and trial production of these industrial plants show that the cellulosic ethanol production technology has been basically mature, and more investment is needed to improve the technology. However, in the second half of 2014, the international oil price and ethanol price fell sharply, which made the production cost of cellulosic ethanol difficult to accept, and the industrialization process also fell to a low point [12, 14]. From the follow-up operation of the above companies, the impact of cost continues to this day. The production process of cellulosic ethanol is complex and the investment is large (RMB 100–200 million/10000 tons of ethanol production capacity [12]). The low product price makes the return of investment hopeless. It can be seen that the only way out for the industrialization of cellulosic ethanol (second-generation ethanol) is to reduce the comprehensive production cost, make it reach the level of starchy raw material ethanol (first-generation ethanol), and realize the supplement and substitution of first-generation ethanol.

Technical difficulties and solutions

Technical difficulties

As far as cellulosic ethanol production technology is concerned, there are three core technologies in addition to collection of raw materials and difficulties in purchasing and storage [21]. First, environmental friendly, low cost, and low energy consumption raw material pretreatment technology; it can effectively improve the enzymatic hydrolysis performance of enzymes with cellulose as substrate and minimize the generation of harmful substances that inhibit enzymatic hydrolysis, fermentation or cause environmental pollution [22, 23]. Second, the technology is cellulose-degrading enzyme system with high performance and low cost [24, 25]. Third, the cultivated strain should be tolerant to pretreatment inhibitors, capable of utilizing non-fermentable sugars (xylose, arabinose, and various cello-oligosaccharides) to produce ethanol with high concentration [26,27,28].

An in-depth research has been carried out around these key core technologies by scientists and biological engineers, there are still no effective solutions to some problems due to biodegradation complexity of lignocellulose biomass at current stage, and the economy of the whole production process is still unable to compete with grain ethanol [29, 30]. However, the objective needs of the sustainable development of human society are still promoting the relevant researchers to carry out in-depth research and make continuous progress around these core technologies [31,32,33].

Solution

Solution of raw material pretreatment

Lignocellulose widely exists in nature, and different raw materials and plant tissues have differences in composition and structure [34]. As shown in Table 2, lignocellulose consists mainly of cellulose, hemicellulose, and lignin. However, the structure of natural lignocellulose is very complex [35, 36]. The hemicellulose is connected with cellulose by hydrogen bond and the side chain is connected with lignin by ferulic acid or aldehyde acid. The hemicellulose and lignin wrap cellulose to form a polymer that is difficult to degrade. Moreover, cellulose itself is highly crystalline and difficult to be hydrolyzed by enzymes. The pretreatment of lignocellulosic biomass is necessary which increases the conversion rate of cellulose as well as ethanol yield [37]. The pretreatment cost accounts for 20% of the cost of overall fuel ethanol production process. Therefore, an efficient pretreatment process is the key to the industrialization of lignocellulosic ethanol [38, 39].

Table 2 Chemical composition of common lignocelluloses

Depending on the compositional and structural differences of raw materials, a series of pretreatment methods has been developed by scientists [46,47,48], such as steam-explosion pretreatment technology, dilute acid pretreatment technology, sulfite pretreatment technology, organic solvent pretreatment technology, etc. The pretreatment technologies that have been industrialized are shown in Table 3. A pretreatment capable of industrialization should be fulfilled the following conditions: it is simple, feasible, cheap in terms of equipment; it has less environmental pollution, high saccharification efficiency of raw materials, less carbohydrate loss, and less by-products; it has good compatibility with fermentation process [49,50,51,52].

Table 3 Pretreatment technology of lignocellulose industrialization

While the pretreatment technology destroys the structure of lignocellulose and improves the enzymatic hydrolysis performance of raw materials, severe reaction conditions lead to a series of side reactions and produce a variety of complex compounds, mainly including furans, organic acids, and lignin derivatives [66,67,68,69]. All these substances have a complex shape and strongly inhibit the saccharification and fermentation processes, and these inhibitors must be removed, that is, the pretreated materials must be detoxified [46, 62, 70]. The frequently-used detoxification methods are shown in Table 4.

Table 4 Comparison of different detoxification methods

The above detoxification methods have some shortcomings, such as increased water consumption, more wastewater discharge, and incomplete removal of inhibitors [66]. For the most widely used water washing method, although it can remove soluble acetic acid and furfural substances, about 20% of cellulose is lost during solid–liquid separation, which directly reduces the yield of ethanol [29, 53]. Therefore, it is necessary to optimize the pretreatment process to reduce the generation of inhibitor. Additionally, application of microorganisms which is able to tolerate higher inhibitor concentration is a useful way to avoid loss from detoxification process [57, 58, 61, 71].

Commercial enzymes solutions

Cellulase, a composite class of enzymes that is produced by Penicillium oxalicum, Trichoderma Richter, and other microorganisms, has the ability to degrade cellulose under the synergistic action of various enzymes [72]. In general, a cellulase complex includes exo-β-1, 4-glucanase (EC 3.2.1.91), endo-β-1, 4-glucanase (EC 3.2.1.4) and cellobiase (EC 3.2.1.21). The proportion of three components in cellulase preparation products from different sources is different, and the final enzyme activity is also different [73]. Different kinds of enzymes and different enzyme components have synergistic effects during hydrolysis, that is, the action efficiency of the combined enzyme system is significantly higher than the sum of the degradation efficiency of each single component, From the experience of enzyme preparation, more and more scientists and engineers have come to the same conclusion: the detection activity of cellulase can only be used as a reference, and the actual application effect should be judged according to the application experiment but not enzyme activity [70].

The complexity of enzymatic hydrolysis of lignocellulose is also reflected in: the substrate specificity and catalytic characteristics of enzymes obtained from different strains are different even if they have the same EC number. To improve the application of cellulase, it is also necessary to be able to describe, analyze, predict and control the characteristics and proportion of various components, and study and analyze the interaction between different enzyme molecules and the dynamic synergy of different enzyme molecules. Its purpose is to provide a basis for the artificial construction of an efficient cellulase complex [46, 74].

The use cost of cellulase accounts for about 35% of the cost of lignocellulosic ethanol processing, which is the highest link except the raw material [75]. Currently, the commercialized cellulases used for the lignocellulosic ethanol include Novozymes’CellicCTec [76], DuPont’s Accellerase series [77], Royal DSM’s Filtrase NL, MethaPlus S/L100, Cytolase CL [78], and cellulase produced by Shandong University [79]. The components of commercial cellulases used presently are listed in Table 5.

Table 5 Industrialized cellulase

Novozymes has developed enzyme preparation for the production of cellulosic ethanol since 2000 and launched CellicCTec in 2009, which is the first standardized composite cellulase. Then, Novozymes launched CellicCTec2 in 2010 and CellicCTec3 in 2012. CellicCTec3 is an advanced compound enzyme of cellulase and hemicellulase. By optimizing the pretreatment and hydrolysis process, the pretreated lignocellulose can be transformed into fermentable sugar. Compared with CellicCTec2, the conversion rate is increased by 50%, and its adaptability to temperature and pH is also enhanced [73, 76].

Accellerase®1000, the first commercial enzyme for cellulose degradation was launched by Genencor (now DuPont) in 2007; then, Accellerase®1500 was launched in February 2009, and it contains endo-cellulase, exo-cellulase and high content of β- Glucosidase. Subsequently, better enzymes, such as Accellerase®DUET and Accellerase®TRIO, have been put on the market one after another [13, 72]. The Accellerase®TRIO is extracted from Trichoderma Richter and its dosage is two times less than Accellerase®DUET and nearly three times less than Accellerase®1500. Accellerase®TRIO has been successfully applied to different kinds of lignocellulose materials, such as corn straw, wheat straw, corncob, bagasse, etc., and is compatible with a series of pretreatment technologies. The conversion rate of five-carbon sugar and six-carbon sugar reaches more than 80% [77].

Royal DSM gave full play to the advantages of the industrial chain and launched a full set of solutions for lignocellulosic ethanol, including raw material pretreatment, enzyme preparation, and fermentation strains. The company promoted in situ producing enzyme system, and the corresponding enzyme preparation was less sold separately [73, 78].

Penicillium oxalicum cellulase developed by Professor Yinbo Qu’s team of Shandong University has been realized industrialized production in 2012, and its performance has also been close to the indicators of multinational enzyme preparation companies [18, 80]. It represents China’s current technical level in this field. The second-generation cellulase products were introduced to the market in 2016, its fermentation activity is higher than that of the first-generation enzyme preparation, and the dosage is 50% of that of the first-generation enzyme preparation, so the use cost of enzyme preparation is lower.

Solutions for fermentation strains

After pretreatment and enzymatic hydrolysis of lignocellulose, the hydrolysate mainly containing glucose and xylose is produced and is utilized by microorganisms for ethanol production through fermentation [81, 82]. After enzymatic hydrolysis of hemicelluloses, the hydrolysate contains a large amount of xylose, and its content reaches 30–40% of the total sugar. However, traditionally Saccharomyces cerevisiae which is used for ethanol production is an efficient glucose consumer but unable to utilize xylose. Therefore, scientists try to introduce the xylose metabolic pathway into Saccharomyces cerevisiae through genetic engineering, so that Saccharomyces cerevisiae has the ability to ferment xylose to improve ethanol yield, but there is still no strain that is efficient and perfect [83,84,85,86], and the efficient strain should be able to tolerate high concentration inhibitors, make full use of the difficult fermentable sugar in the enzymatic hydrolysate, and have high ethanol yield.

In December 2013, Royal DSM and Inbicon, a subsidiary of Danish DUNG Company, announced that they had verified the pentose/hexose co-fermentation with straw as raw material on industrial scale, the combined fermentation technology can increase the ethanol yield per ton of straw by 40%, which greatly reduces the production cost of lignocellulosic ethanol. The special yeast independently developed by Royal DSM has been used in the fermentation for 2 month trial. It is a milestone in the industrialization of cellulose fermentation technology for Royal DSM [87].

In September 2018, the first project on Sunliquid® cellulosic ethanol technology developed by Clariant Company was started construction in Podari, Romania. Sunliquid® technology provides fully integrated process design based on mature technology. The main technical characteristics of its innovation include the joint operation of raw materials, specific processes and enzymes, pentose/hexose co-fermentation technology, etc. [88, 89]. In the fermentation step, a strain of yeast can be reused for 20 times, so its cost can be ignored. Clariant Company has announced a license agreement for Sunliquid® technology with Anhui Guozhen group of China and EtaBio Company of Bulgaria in January and August 2020 [90, 91].

Economic analysis

Production costs

Lignocellulosic ethanol has a long industrial chain, and it involves many links such as grain planting, harvesting, storage and transportation, production, and sales. There are many solutions for each link, so the economy of each production scheme is different [92]. With different raw materials, the equipment, raw, and subsidiary material consumption and detoxification methods of the pretreatment process are also different, and the utilization of glucose, xylose, and arabinose for alcoholic fermentation by yeast is also different [93, 94]. However, it is very important for lignocellulosic ethanol industry to make a rigorous and rational economic evaluation under the scale of industrialization.

Since 1999, the National Renewable Energy Laboratory (NREL) has published the design scheme of cellulose ethanol plant based on strict process simulation calculation and continuously updated it. The updated process in the autumn of 2011 is considered to be closer to the current actual situation [95]. The process includes the following nine sections: preliminary treatment of raw material, pretreatment and detoxification, in situ enzyme production, saccharification and fermentation, product purification, wastewater treatment, lignin residue combustion and power generation, product storage and utilities, etc. Based on the above process simulation, the economic indicators such as ethanol output, energy consumption, and wastewater output are shown in Table 6. In this case, the conversion rate of cellulose to glucose is 85%, the conversion rate of glucose to ethanol is 90%, the conversion rate of xylose to ethanol is 40%, and the conversion rate of arabinose to ethanol is 0.

Table 6 Basic economic indicators and minimum selling price of basic case [8, 46, 95,96,97]

The NREL technical report is an open document of the U.S. Department of energy, which is open to the public. To a great extent, it reflects the expectation level of the U.S. official on the technology of producing liquid fuel ethanol from renewable resources, rather than the actual technical level [98]. In addition, it is worth noting that the above minimum price of ethanol only includes the production cost, not the enterprise operation cost. According to the experience of corn ethanol, the operation cost of lignocellulosic ethanol is inferred as shown in Table 7.

Table 7 Operation cost calculation [95, 99]

Analysis of main indicators

It can be seen from the Table 6 that the cost of corn straw raw material is 2427 RMB/T ethanol, accounting for 37% of the production cost, and it includes preliminary treatment of raw material sections such as collection, transportation, crushing and dust removal, etc. The cost of in situ enzyme production is 1786 RMB/T ethanol, which accounts for 27% of the production cost. The cost of pretreatment and detoxification accounts for 13% of the production cost; the cost of wastewater treatment accounts for 9% of the production cost. Solid-waste incineration and power generation generates part of the power surplus, and this makes this section offset the production cost of 82 RMB/T ethanol. Therefore, the technical indicators of enzymatic hydrolysis yield and ethanol yield have a significant impact on the economy of lignocellulosic ethanol [100,101,102].

The price of straw raw materials depends on the cost of straw collection, storage and transportation. To reduce the cost of straw, we need to start with straw harvesting, purchase, bundling, storage and transportation [92, 103]. The development of lignocellulosic ethanol should be based on local conditions. The production plant should be laid out and constructed in areas rich in straw resources. The comprehensive utilization of resources should be considered for the new equipment, and the construction mode of alcohol/electricity cogeneration can be adopted to improve the economy of the process [104].

The cost of cellulase has always been a focus in the process of lignocellulosic ethanol industrialization. In the above economic analysis of NREL, the cellulase is in situ produced, i.e., it is in the same production system with biological refinery and piped to the saccharification tank. The purification, storage and transportation process of cellulase can be omitted, so the cost can be reduced by about 1/3 [18, 46, 79].

At present, the cost of fuel ethanol production from lignocellulose is much higher than that of fuel ethanol production from corn. The enterprises will suffer serious losses without financial subsidies. Therefore, the development of lignocellulosic fuel ethanol industry cannot be separated from the strong support of fiscal and tax policies [100, 105].

Industrialization trend

For a technology suitable for industrial production, the feasibility of the technical route and reaching the standard of economic benefits are two necessary conditions. Scientists and entrepreneurs are trying to make lignocellulosic ethanol industry have economic advantage, and their attempts mainly include the following aspects: the construction of raw material collection and storage system, on-site production of the enzymes, utilizing of components of materials with high value, mature equipment and overall industrialization solutions, etc. [106,107,108,109].

The composition of plant biomass such as straw is very complex. It is difficult to be competitive economically using only some of its components to produce cheap liquid fuel without converting other components into relatively high-value products [110, 111]. The traditional lignocellulosic ethanol production only aims at the biotransformation of one component, which not only wastes raw materials seriously, but also brings environmental pollution problems. We should break the concept of single production of ethanol from biomass raw materials with complex components and make full use of the three main components of cellulose, hemicellulose and lignin in raw materials through advanced biorefinery technology, and then turn them into different products. Finally, the goal of making full use of raw materials, maximizing product value, and maximizing land-use efficiency will be realized, and the economic feasibility of the whole process will be improved [112,113,114].

Conclusion and prospect

For the production of lignocellulosic ethanol, several industrial scale plants have been installed successfully, although the production efficiency is low. However, large-scale industrial production has taken an important and critical step. It is expected that green energy in the form of lignocellulosic ethanol may be the leading contributor of bioenergy in near future. The developed countries have raised and highlighted the dominant position of renewable energy in energy supply chain and formulated appropriate policies such as “Comprehensive Energy Strategy” and “Energy Technology Roadmap to 2050”, However, in fact, there is still a long way to go to realize the industrialization of lignocellulosic ethanol production.

The technical difficulties of lignocellulosic ethanol industry that still need continuous research and practice include: from the perspective of raw materials, reveal their physicochemical intrinsic characteristics, and deeply analyze the restrictive factors such as inhibition effect, mass transfer efficiency, and changes of hydrodynamic properties, so as to break through the high-efficiency and low-cost raw material pretreatment technology; construct in situ enzyme production system and develop enzymatic hydrolysis process with low cost and high hydrolysis performance; recombinant engineered strains that can make full use of various sugars in hydrolysate. Enterprises producing grain ethanol (first-generation ethanol) can also combine and build the first-generation and second-generation ethanol plants, share some industrial infrastructure, and apply alcohol-electricity cogeneration to improve quality and efficiency. A set of mature production technology must comprehensively consider all the above engineering and technical problems, we integrate the solution into a closely connected and complete process package and use mature industrial equipment as much as possible, and then, we will jointly promote the healthy development of lignocellulosic ethanol industry.