Abstract

Biomass energy would become the most potential renewable energies, for whether wind power or photovoltaic, would be restricted by the nature thus cannot provide stable power, while biomass energy is the only renewable energy that can be used in the form of gas, liquid or solid stage, it could replace the fossil energy, lead a positive influence on the control of the greenhouse gases. Across the globe, the biomass produced through photosynthesis is about 200 Gt, or 99 Gtce per year. If 10% of the biomass is utilized, more than 4 Gt of fuel ethanol and other bioenergy products can be produced, equivalent to 4.13 Gt of petroleum consumed by the world in 2014. Therefore, bioenergy can be a feasible alternative to fossil energy.

Keywords

Climate change ; Bioenergy ; Greenhouse gas

1. Introduction

According to the Greenhouse Gas Bulletin 2014 published by World Meteorological Organization (WMO, 2015 ), concentrations of major greenhouse gases (GHGs) hit another record in 2014, with the average concentration of CO2 , CH4 , and nitrous oxide being 397.7 × 10−9 , 1833 × 10−6 , and 327.1 × 10−9 , constituting 143%, 254% and 121% of the pre-industrial (1750s) levels, respectively.

Derived from more observations, IPCC Fifth Assessment Report (AR5) proves that the world continues to become warmer (IPCC, 2014 ). AR5 analyzes the causal relationship between human activity and global warming, and highlights the urgency of mitigating climate change and minimizing GHG emissions. It also suggests the prerequisite for the world temperature rise to be no more than 2 °C.

After more than two centuries of industrial civilization, humans have to struggle with a “carbon challenge”, that is, the ever increasingly serious concerns about the environment and climate brought about by application of fossil energy, which has fueled significant progress of the society. The future sustainability of China and even the Earth calls for humans to turn from industrial to ecological civilization based on a new energy revolution that uses green and low-carbon energy in place of high-carbon ones (Du, 2013  ;  Du, 2014 ).

He, 2013  ;  He, 2014 predicted that, by 2030, non-fossil energies, most being renewable, will become comparable to coal, oil and gas, and other fossil energies, accounting for 20%–25% or even 30% and boasting an annual supply of more than 1.5 Gtce. By 2050, new and renewable energies will account for 1/3 or even 1/2. In the second half of the 21st century, the energy system will be sustainable as it will be primarily composed of new and renewable energies. The economic and social growth will no longer rely on the limited non-renewable resources on the Earth and CO2 emissions will approach zero. This will be in line with the trend of new energy revolution and the adaptation to climate change around the globe.

When plants grow through photosynthesis with sunlight, solar energy is stored in the form of chemical energy in biomass and can be converted to conventional fuels of solid, liquid, or gaseous state. Because sunlight and its energy are inexhaustible, bioenergy is consequently called renewable. This is the one and only way for carbon to be renewable in nature.

2. Bioenergy and emission reduction

At present, bioenergy primarily includes the highly commercialized fuel ethanol, industrial marsh gas (biogas), and biodiesel, as well as compressed solid fuel, biomass gasification, gasification for electric power generation, and bio-oil produced through thermal cracking. All these types of bioenergy have been used in different economic sectors.

At the present stage, the main products of global biomass are going through a shift from the first bioenergy generation to the second. The first bioenergy generation, or the traditional bioenergy, is using agricultural products and their byproducts as raw materials. It has been industrialized with commonly improved industry chain in many countries, such as the U.S. and Brazil. While the first generation is facing serious problems during its industrial development, it requisitions the agricultural products which were to use for human consuming and food processing, may results problems such as food safety and price rising. Also, the first bioenergy generation may result in the secondary environmental pollution during its production. Regarding the possible limitations, many attentions are paid to the second bioenergy generation, which mainly focuses on the development of lignocelluloses. Nowadays, the second generation is still in the technological innovation stage, most of the enterprises are facing negative profit situations, thus it haven't been proper industrialized. While it is no doubt that the second bioenergy generation is the inevitable trend in the bioenergy development.

In the global energy system, the first biomass generation has become the second largest energy supplier ranking behind the fossil fuel. According to the statistics from Renewable 2013 Global Status Report released from Renewable Energy Policy Network for the 21st Century (REN21, 2013 ), among 2011 global energy consuming, fossil fuel has been the first time, took proportion under 80%, renewable energy has been the first time to supply more than 19% of all the energy, with the 9.3% contribution from traditional biomass energy. In addition, the International Energy Agency (IEA, 2010 ) predicts, to the year 2050, the annual global biomass energy production may reach 1500 × 1018  J.

From the view of global bioenergy industry distribution, it is concentrated in some developed countries and areas with less energy but abundant biomass materials. From the point of production scale, the total biomass ethanol production of the U.S. and Brazil has accounted for 70% of global output. From the bioenergy consuming proportion, Finland and Sweden stand the top. In the total energy consumption of Finland, more than 12% are provided from biomass energy (Zhang and Zhang, 2014 ).

In 2014, the world witnessed a 7.4% increase in bioenergy production and 6.0% increase in ethanol output for a consecutive second year, driven by the Central and South America and Asia–Pacific. In 2014, global biodiesel output was increased by 10.3% (BP, 2015 ).

Bioenergy is commonly recognized as green as its low CO2 and SO2 emissions. CO2 is the dominant GHG, and SO2 is the main reason for acid rain. Recently many researchers (Dwivedi et al., 2015 ; Dunn et al., 2013  ;  Slade et al., 2009 ) have issues on bioenergy. The main focus is environmental issues such as CO2 emissions.

The growth of biomass need to absorb CO2 from the air, so theoretically, biomass energy has less CO2 net emissions than fossil fuel, while these researches (Searchinger et al., 2008  ;  Fargione et al., 2008 ) only considered a certain stage of biomass energy, for example, the grow, abstract and consuming process. The precisely analysis should take the whole life circle into account, and should consider the results of land use change in CO2 emissions.

Currently the source of biomass is mainly agricultural crops. Farms would damage the forest and grassland when plant the crops, thus results in the carbon from forest and grassland to enter the atmosphere. If take this part into consideration, CO2 net emissions from the conversion of crops to bioenergy would not be less than fossil fuel. Therefore, to precisely calculate the CO2 net emissions of the bioenergy, it is needed to monitor and observe the whole carbon cycle caused by the biomass energy development and consumption. The CO2 net emissions would be different for different produce methods. Disafforesting or destroying the local vegetations to grow crops and produce bioenergy, its CO2 net emissions would be larger than the fossil fuel; utilizing local biomass and wastes from agriculture and forest, its CO2 emissions would be reduced; reclaiming land and grow proper plants to supply bioenergy, the CO2 net emissions would be even lesser (Hu et al., 2012 ).

Sun et al. (2014) have shown that E85 containing alcoholic fuels converted from biomass emits much less CO2 equivalent than conventional gasoline does. GHG emissions measured in CO2 equivalent in the pathway of biochemical transformation of cellulose are about 0.2–0.7 time (e.g. 20%–70%) of conventional gasoline, 0.6–0.9 time (e.g. 60%–90%) in the thermal chemical pathway, and 0.8–1 time (e.g. 80%–100%) in dry corn processing. Regarding the ester fuels converted from fat and oil biomass, the reduction of GHG emissions by biodiesel is animal fats > gutter oils, palm oil > soybean oil, and coconut oil > colza oil. Biodiesel made from animal oils and gutter oils does best in reducing GHG emissions by 70%–90%; while that from plants contributes to a reduction by 10%–90%. Among the hydrocarbon fuels converted from biomass, the renewable colza-oil-based jet fuel produced through oil hydrogenation reduces GHG emissions by 13%–55%. F-T synthetic oil reduces emissions better than those from oil hydrogenation. BTL (Biomass-to-liquid) generally reduces GHG emissions by more than 80%, and pyrolysis gasoline and diesel reduce the emissions by 58%–70%.

Comparison on the reduction of GHG emissions by four major types of bioenergy using life cycle assessment (LCA) method (Table 1 ) shows that both the net energy ratio (NER) and the net reduction of GHG emissions (GGENR) of the same type of biomass is related to the raw materials.

Table 1. Properties of different types of bioenergy.
Category Raw material Calorific value Net energy ratio (NER) Fossil energy Emission factor (kg CO2 (TJ)−1 ) Net reduction of GHG emissions (GGENR) (kg CO2 (TJ)−1 )
Ethanol (MJ L−1 ) Corn 21.26 1.25 Vehicle gasoline 69,300 10,660
Sugarcane 22.32 9.30 61,418
Maize straw 21.10 4.39 52,603
Biodiesel (MJ L−1 ) Soybean 32.93 1.93 Diesel 74,100 36,121
Microalgae 35.40 1.34 19,399
Biogas (MJ m−3 ) Animal wastes, crop straw 22.36 Natural gas 56,100 56,100
Biomass direct combustion (MJ kg−1 ) Crop straw 20 Anthracite coal 98,300 98,300

Note: Data are sourced from 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2006 ).

NER much varies depending on the raw material, which involves considerably different production and pretreatment to which different amount of exogenous energy has to be input. Among the fuel ethanol and biodiesel made from different raw materials, if the NER is higher, the GGENR will be higher, too. Because the application of a specific type of bioenergy always reduces the same amount of GHG emissions that would otherwise be released by fossil energy, higher NER means less emissions released from the production of the bioenergy, thereby resulting in higher GGENR. Fuel ethanol made from maize straw, thanks to its less energy input in the production from raw materials, has a GGENR (52,603 kg CO2 (TJ)−1 ) that is five times as high as that of corn ethanol (10,600 kg CO2 (TJ)−1 ); and the sugar-based ethanol produced in Brazil, where both the climate and geography are special, has a NER as high as 9.3, resulting in a GGENR of 61,418 kg CO2 (TJ)−1 , higher than the above two types of ethanol and about 6 times as high as that of corn ethanol (Li et al., 2014 ).

Biogas is a rare type of carbon neutral fuel. Theoretically, it releases about 30% less CO2 emissions than gasoline does (Table 2 ), and its net carbon emissions are even negative in certain circumstances. Borjesson and Mattiasson (2008) compared the CO2 equivalent released by different types of energy consumed to produce 1 MJ energy to drive vehicles. The values are 80 g by gasoline, 75 g by diesel, 30 g by biodiesel, and 23 g by bioethanol. If CH4 and biogas made from animal manure is used to fuel vehicles, the emissions of CO2 equivalent would be −62 g as a result of the carbon flux of the biomass throughout the lifecycle as well as the CH4 (its GHG equivalent is 25 times as many as that of CO2 ) that would be released if the manure is not immediately processed. This is why all EU countries strategically promote the application of biogas on vehicles to reduce carbon emissions, especially that from transportation (Cheng et al., 2013a  ;  Cheng et al., 2013b ).

Table 2. Calorific value and effect on CO2 emissions of several fuels.
Type Hydrogen content (%) Lower calorific value CO2 emissions to output 1 kW h work (g CO2 -eq) Theoretical reduction of CO2 emissions from gasoline (%)
(MJ kg−1 ) (kW h kg−1 )
Biomethane 25.0 50.0 198.0 29.2
Propane 18.2 45.6 12.67 236.8 15.3
Butane 17.2 45.3 12.58 241.2 13.7
Diesel 13.5 42.7 11.86 267.5 4.3
Gasoline 13.5 42.4 11.77 279.5 0.0

Note: Data are sourced from bioCH4 vehicle trial.

Renewable Fuel Standard Program (RFS2) of the U.S. requires that producers of second generation bioenergy, especially energy from wood fiber, may not release GHG emissions in their production by more than 50%–60% of the standard set forth for all energy producers in the country, and must measure the emissions throughout product lifecycle. In order to ensure energy supply, energy conservation, and emission reductions, EU has developed energy strategies for different timeframes. By 2020, GHG emissions will be reduced by 20% compared to the 1990 level, renewable energy will account for 20% of the total energy consumption, energy efficiency should increase by 20%, and renewable fuel will account for 10% of energy for road transportation.

In spite of all the queries and issues, a common view of the bioenergy development has been reached in most of the bioenergy countries, that is, the positive effects of bioenergy to replace traditional fossil fuel, optimize the energy structure, and reduce GHG emissions are indispensable. The future bioenergy policy should take an overall consideration to cope with climate change, secure energy safety, protect ecological environment, protect agriculture and secure crops safety, and maintain economic stability, etc, through building a sustainable bioenergy system to rationalize the bioenergy industry development, reduce the negative effects on society, economy and environment.

3. Coproduction of bioenergy

To build an economic-friendly factory, it is necessary to have real profit products. Most of the biofuel enterprises are in the exploring stages. They need more economic materials, more efficient production mode. From time to time, refine biofuel has always cost more than traditional fossil fuel such as oil or gas, and it is more difficult. Take ethanol, the main force of biofuel, as an example, it is originally abstracted from carbohydrate, starch or oil abundant plants, while these plants could be used as human or livestock food. In the U.S., around 40% of the corn is used to produce ethanol. To fix this, many biofuel companies start to seek other materials. For example, Shells American branch is cooperating with Brazilian company Cosan to form a new joint venture Raizen, choose to utilize sugarcane juice to abstract ethanol; POET-DSM and Dupont have built factories in Idaho, Abengoa in Kansas, they apply waste corn cob, corn leaf and corn husk to produce cellulosic ethanol; besides that, Abengoa also developed a method to acquire plastic from recovering plastic bottles from beverage companies, also, they are working on city trash, they plan to build factory in California, convert city trash to fuel.

Despite the fact that biofuel companies are working vigorously to extend the raw materials, the widespread popularity of biofuel is more expensive and slower than expected. It is confirmed that abstract biomass from cellulosic materials such as factory waste, wood dust and city trash is more complicated and expensive than expected. Therefore, the U.S. government offers millions of dollars of subsidy and tax preference, many companies start to expand the business ways, develop peripheral products and other chemical products to make up for the loss. For example, Dupont has reached a deal with famous American consumer goods company P&G, and Dupont will provide part chemical materials for P&G in use of soap production (Li, 2014 ).

3.1. Coproduction process of cellulosic ethanol, CH4 , electricity and organic fertilizer

Ethanol is not only a substitute to gasoline, but also an excellent additive and a high-octane component of gasoline. With an oxygen content as high as ∼34.8%, ethanol features even combustion and high thermal efficiency, leading to the improvement of power, economy, and emission of engines. It can be used as a gasoline oxygenate in place of methyl tert-butyl ether (MTBE), which is the most popularly used one at present but has been banned as an additive in the U.S. because of its irreversible pollution to groundwater. Moreover, it works as a high-octane component that meets the needs for high-octane-number gasoline.

According to the Auto/Oil Air Quality Improvement Research Program Report (Burns et al., 1995 ), Californias new formula gasoline containing 6% ethanol reduces hydrocarbon emissions by 10%–27%, CO by 21%–28% and poisonous gases by 9%–32%, compared to conventional gasoline (EI, 2015 ). According to Oak Ridge National Laboratory (ORNL) Report (Storey et al., 2010 ), the application of E10 ethanol gasoline reduces NMHC (non-methane hydrocarbons) by 20% and CO by 14% in automobile exhaust; and the emissions of primary particulates (PM) are reduced by 6.0%–6.6% with E10, and by 29.4%–41.8% with E20 ethanol gasoline. PM2.5 emissions can be reduced by 36%–64.6% by adding 10% ethanol to gasoline (ED, 2014 ).

The Environmental Protection Agency (Mulawa et al., 1997 ) shows that aromatic compounds in gasoline are the most important contributors to fine particulate matter (PM2.5); and using ethanol in place of aromatic compounds to increase the octane number of gasoline can effectively reduce the emissions of particulate matter (PM) in automobile exhaust. The oxygen content in ethanol gasoline is significantly higher than that of ordinary gasoline, which is very favorable for the reduction of PM. Therefore, in the case of ethanol and aromatic compounds can increase the octane number of gasoline, the use of ethanol instead of aromatic compounds as a gasoline additive, can effectively reduce PM emissions. Using fuel ethanol to replace and modify gasoline has been proven, and put into application in developed and developing countries. Bioenergy, which primarily consists of fuel ethanol, has been incorporated in the national energy strategies of China, the U.S., and Brazil, and become one of the important elements to change international strategy. In 2014, the U.S. produced 43 Mt of fuel ethanol, and E10 (gasoline to which fuel ethanol is added by 10%) accounts for more than 99% of the market share.

The massive application of generation-1 and generation-1.5 ethanol results in higher grain price and even grain crisis, especially in countries and regions where grain supply is not sufficient enough. Not only food crops such as wheat and corn, but also non-food crops such as cassava, sugarcane, and sweet sorghum. If they were extensively planted, the local plant diversity, soil, environment, and water resource would be affected.

Cellulosic ethanol, which is produced from agricultural crop straw, sawdust, energy grass and other cellulosic materials, can mitigate energy security, reduce pollution resulting from straw combustion, ensure grain security, increase farmers' income and promote the adjustment of industrial structure. This is why it is said to be the mark of and hope for the sustainable development of human society.

Cellulosic ethanol has been categorized into three types of mainstream processes: production of ethanol through C5 and C6 fermentation; production of ethanol through C6 fermentation, together with the coproduction of furfural, xylitol, butanol and other chemicals from C5; and coproduction of ethanol, CH4 , electricity, and organic fertilizer.

What the first type of process confronts lies in unstable engineering bacteria used for total sugar, slow metabolism of C5 sugar, and extremely low efficiency in energy conversion of straw. With the second type of process, the ethanol output is limited by the market capacity of co-produced chemicals so that it is hard to meet energy demand.

Tianguan Group has implemented classified utilization of cellulose, hemicellulose, and lignin in maize straw and wheat straw by utilizing the Groups advantages, adhering to the national and local conditions, and performing integrated innovation. Two bio-refining modes have been created: one being the primary route of ethanol and CH4 production, and the other being the generation of electricity and the production of bio-compressed natural gas (BCNG). This has preliminarily established a complete system for the industrial production of cellulosic ethanol. In addition to utilizing all components of wood fiber, the process also features that all its products, such as ethanol, CH4 and electricity, are renewable energy that has tremendous downstream market and complies with the ultimate objective of sustainable development.

Tianguan Groups cellulosic ethanol process begins with the pretreatment of straw materials in high-temperature stream, followed by enzymatic saccharification of solids, yeast vaccination in liquid glucose, conversion of fermentation sugar into ethanol, and distillation, condensation, and dehydration of fermentation marsh, before dry biomass residue is supplied to fuel boilers that produce steam to the plant. The acetic acid, furfural, oligosaccharide, and phenols generated in the pretreatment of straw materials via steam blasting are collected and, along with production sewage, used for the production of CH4 , which is used to generate electricity for production. Biogas slurry and biogas residues are used to produce nuisance less organic fertilizer to improve agricultural production. A technical system for the efficient utilization of straw has been created, whereby ethanol is produced from cellulose, CH4 is produced from hemicellulose, electricity is generated using lignin as solid fuel, and organic fertilizer is returned to farms. This realizes a coproduction system of ethanol, CH4 , electricity, and organic fertilizer (Fig. 1 ). The technology features a mode of raw material collection suitable to Chinas circumstance. In fact, the mode is practicable, efficient, and satisfactory to the requirements of industrialization. The production process is advanced and unique in the coproduction of ethanol, gas, electricity and fertilizer, and the production process without using fossil energy.


Coproduction process of cellulosic ethanol, gas, electricity and fertilizer.


Fig. 1.

Coproduction process of cellulosic ethanol, gas, electricity and fertilizer.

On average, 7.0 t of absolutely-dry straw material coproduces 1 t of fuel ethanol, 3.6 t of fermentation residue with the calorific value of 4000 kcal kg−1 (mainly lignin), and 800 Nm3 biogas that are made into 480 Nm3 BCNG for automobiles. The fermentation residue fuels biomass boilers to produce steam, which generates electricity to be used by the production process. Moreover, 1010 kW h of the electricity can be supplied to the power grid. 4.7 t of straw is consumed to produce energy of 1 t ethanol equivalent. The ratio for energy input and output is up to 1:1.93 (Table 3 ).

Table 3. Energy consumption indices of cellulosic ethanol coproduction.
Item Index of the project World-leading index
Cellulase preparation 0.55 t 0.53 t cellulose
Energy consumption (t ethanol equivalent) Fresh water 13.7 t 12.4
Electricity −1010 kW ha 800
Steam 0 4.0

a. 1010 kW h of the electricity generated in the production of each ton of ethanol can be output to the grid in addition to meeting the need of production process; Data are sourced from Special Plan for Industrialization of Cellulosic Ethanol (Draft for Comment) issued by the National Development and Reform Commission of China and industrial reports.

3.2. Comprehensive utilization of straw for the production of CH4 , electricity, heat, and fertilizer

In recent 20 years, biogas has become an industry with a considerable scale in EU countries. At the beginning of the 21st century, EU sets a strategic objective that renewable energy would account for 20% in the primary energy consumption and one half of the 20% would be contributed by bioenergy, especially by CH4 , by 2020. Sweden and Finland tried to use CH4 on automobiles as early as 1960s and 1970s. Since the beginning of the 21st century, the production of CH4 from domestic materials has been stimulated by the mandatory legislations of increasing the ratio of green electricity and minimizing CO2 emissions, and the limited resources for the production of renewable energy, especially bioethanol and biodiesel, in Sweden, Germany, and Austria. These efforts began to give results in 2006. By far, biogas has become a large industry in EU countries, with the annual biogas output of approx. 20 × 109  m3 (bio CH4 equivalent). By 2010, 9000 large-to medium-scaled CH4 plants had been established in these countries, and most of them were new plants that have been established after 2000.

Regarding raw materials, 32.7% of the biogas is produced from landfill waste, and 12.0% from activated sludge in urban sewage. The largest portion (55.3%) comes from agriculture, including energy crops and other organic wastes. Germany is a representative in the production and utilization of CH4 from straw. Its rapid growth of CH4 industry in recent years is ultimately driven by the Renewable Energy Act (Erneverbare Energien Gesetz, EEG), which was enacted in 2000 and revised for two times thereafter. The pivot of EEG is preferential price of electricity supplied to the grid. Most of typical CH4 plants are built on farms in Germany. The foremost material is shredded whole-maize-plants supplied from nearby farms (ensilaged for year-round application). Other materials mixed for joint fermentation include stable dung, butchery wastes, kitchen wastes, and domestic sewage and sludge. The general capacity of power generation and heat supply is 0.5–1.0 MW. The energy crop used as raw materials for CH4 works is transported from sites that are not more than 5 km away typically and not more than 10 km at the maximum. Because of EEGs extremely preferential subsidy to the electricity supplied to the grid, CH4 was once mainly used for heat and power cogeneration. Later, it was found that the heat and electricity produced are rarely practicable while the policy bans the subsidized electricity price in absence of simultaneous heat supply. Therefore, CH4 began to be refined for grid supply or automobile applications. In 2009, Germany had 90,000 biogas-fueled automobiles. In 2011, the country had 83 CH4 refining plants with an annual biogas output of 460 million m3 . By the end of 2012, the CH4 refining plants with the refining capacity of more than 700 m3  h−1 for each had increased to 110 in the country. According to German governments objective that its annual biogas output would be 6 billion m3 by 2020, 12,000 new CH4 refining plants have to be built in the next 8 years. In a longer term, Germany expects that biogas will account for 10 billion m3 , or 11.5%, of its total gas consumption of 87 billion m3 by 2030 (Cheng et al., 2013a  ;  Cheng et al., 2013b ).

Tests and research on the production of CH4 from raw materials with high cellulose content through anaerobic fermentation began as early as 1980s. Initially, the fermentation process was a traditional one whereby all the four anaerobic reaction steps, namely, hydrolysis, acidification, acetoxylation, and methanogenesis, were completed in a single anaerobic system. In such a process, each ton of cellulosic material could produce about 20 m3 of CH4 . By 1990s, the two-phase anaerobic systems began to be used in CH4 works. The process avoided the effect of lowered pH resulting from acidification on the methanogenesis microorganism system. The anaerobic fermentation system had much increased resistance against acid impact and became more stable. Moreover, raw materials could be hydrolyzed and acidized more specifically. In addition, the control theory was improved. With the improvement in the exclusive hydrolysis microorganism system and enzyme preparation technologies, more than 300 m3 of gas could be produced from each unit of raw material.

The traditional biogas technology had limited adaptability to raw materials. Constrained by these materials, the facilities had limited scale and low conversion rate of the materials. Moreover, they were not flexible enough to become distributed energy supplies. In addition, the downstream application of their products was not sufficiently developed. This made it impossible to fully realize the technical advantages of bioenergy.

In its development of cellulosic ethanol, Tianguan Group successfully developed large-scaled household CH4 works suitable to the construction of new rural communities and towns. The so-called distributed green energy project simultaneously supplies CH4 , electricity, and heat. It can be used to build up demonstrative rural communities of new energy and may be rolled out in new rural communities and towns around the nation. Being a typical distributed system of clean energy supply, the project makes full use of agricultural wastes and supplies renewable clean energy to new villages and towns. The highly adaptable project is easy to roll out. Its scale may be determined based on the population of residents in a town or village, and energy may be converted depending on changes to the type of residents' energy consumption. In fact, CH4 may be used for domestic purpose, purified and refined into compressed natural gas (CNG), or used to generate electricity. Marsh residue is the material of efficient organic biofertilizer. Biogas slurry may be used to irrigate farm (Fig. 2 ).


Comprehensive utilization for the production of CH4, electricity, heat and ...


Fig. 2.

Comprehensive utilization for the production of CH4 , electricity, heat and fertilizer (CHP: Cooling-heating-power; BCNG: bio-compressed natural gas).

Tianguan Groups production technology of CH4 from straw accepts a wide variety of raw materials, such as dry straw, fresh straw, other green fiber plants, cattle and sheep manure, and kitchen waste. Boasting the state-of-the-art pretreatment technology of biomass straw, exclusive hydrolase and exclusive microorganism system, two-phase fermentation technology and energy-saving efficient stirring technology, it can produce 360 m3 of CH4 out of each ton of straw, increase by 15%–30% than the conventional technology. The facilities may be flexibly sized to suit habitations of different population. Proven technology of CH4 application enables combined solutions to meet different energy needs. A properly sized facility can supply the fuel gas and electricity required for the living of 2000–30,000 residents.

3.3. CO2 -based fully degradable plastic

CO2 is not only the foremost GHG, but also a type of inexhaustible and cheap material for chemical industry. Synthesizing polymers from CO2 as part of industrial exhaust enables us to less rely on petroleum resources, and make use of this useless gas as a valuable resource. The synthesized polymers, being fully bio-degradable, are typical materials with environmental friendliness. Therefore, this type of synthesis is one of the most focused fields of development in polymer technology.

CO2 is the foremost byproduct in the fermentation process of ethanol production. Together with each ton of ethanol product, 0.965 t of CO2 can be obtained and the CO2 concentration in the ethanol fermentation gas can be higher than 99%. At a 300 × 103  t ethanol plant, 200,000 t of CO2 can be recycled each year. Such recycling, if possible, not only reduces GHG emissions, but also increases the profitability of companies.

Working with Sun Yat-sen University, Tianguan Group has developed the synthesis of a CO2 -based full-degradable plastic, polypropylene carbonate (PPC), from the CO2 byproduct of fuel ethanol production. The synthesis facility has been put into operation and satisfied the design requirements, outputting products of stable quality. Moreover, the sales have reached a certain scale. PPC is a fully bio-degradable plastic that completely degrades in natural environment. It can be used to make disposable package materials, tableware, fresh-keeping materials, disposable medical materials, mulching film, etc. Compared with other products, its value of application includes:

  • Carbon capture. As a part of industrial exhaust, is fixed and turned to a type of usable material. This mitigates CO2 emissions and greenhouse effect and recycles CO2 as a resource (Tianguans PPC contains 43 wt% CO2 ).
  • Bio-based material. With raw materials sourced from plant fermentation industry, the plastic is a bio-based material (PPC contains 25% bio-based carbon).
  • Energy saving. It can substitute for conventional petroleum-based plastics for many purposes, thereby reducing CO2 emissions and saving petroleum resources (2–3 t of petroleum can be saved by using 1 t of PPC).
  • Environmental friendliness. The application product of this bio-degradable material may be disposed of through composting, whereby it ultimately decomposes into CO2 and water without damaging the environment.

3.4. China liquid biofuel development roadmap

Tsinghua University (TU, 2014 ) analyzed the efficiency and cost of potential large scale produced and promoted liquid biofuels. According to the report, most of the second generation bioenergy technology (including cellulosic ethanol, F-T diesel (aviation kerosene, gasoline), and APR gasoline (aviation kerosene)) could reduce its cost significantly during 2020–2030, to around CN¥8000 (toe)−1 , achieve the economic competitive to a large degree.

Many factors would influence the liquid biofuel production cost, including materials price, conversion efficiency and production scale (Table 4 ). For the 1.5 generation liquid biofuel, raw materials cost take a large ratio compared to the overall cost. Besides the material cost, artificial cost and land cost in the planting process, bioenergy would also pay costs in the harvest, store and transport part, and because of the competitive opportunity cost (for example, cassava could also be used to make starch) deeper the gap between the materials acquisition cost and real cost, thus raise the proportion of materials cost in the overall cost for the 1.5 generation liquid biofuel. To reduce the cost, possible ways could be build large-scale planting base, promote good quality varieties, increase yield production, and improve mechanization level of plant, harvest, store and transport, along with improve the conversion technology. In general, the overall cost of the 1.5 generation in 2020 is likely to be uncertain, around CN¥6000–12,000 t−1 , while in some biomass abundant areas, with the application of improved raw materials and conversion technology, the cost is likely to reach CN¥6000–8000 t−1 , qualify the preliminary competitive potential compared with fossil fuels.

Table 4. Levelized cost structure of liquid bio fuels.
Liquid biofuel Feedstock cost Investment cost Operating and management cost
Current 2050 Current 2050 Current 2050
1.5G fuel ethanol 74%–86% 59%–83% 3%–6% 4%–7% 11%–20% 14%–35%
1.5G biodiesel 79%–89% 63%–86% 2%–4% 3%–4% 9%–18% 12%–33%
2G Cellulosic ethanol 31%–37% 37%–43% 15%–16% 15%–18% 48%–52% 39%–48%
2G F-T liquid fuel 35%–36% 37%–40% 30%–31% 24%–29% 34% 30%–39%
2G APR liquid fuel 27%–38% 46%–48% 23%–27% 10%–14% 37%–47% 38%–44%
3G Algae biodiesel 94%–96% 76%–81% 1% 2% 3%–5% 17%–23%

Compared with 1.5 generation, the 2nd generation cost more in the operating and management (O&M) cost and investment, material cost take a low proportion. Different 2nd generation technology has different cost structure. Cellulosic ethanol has a main problem of low sugar concentration after hydrolysis pretreatment, thus could not ferment xylose in the same time, result in low efficiency, so its operation and management cost, especially the changes cost is relatively higher, enzyme costs account for more than 80% of the overall costs. The main problem of the F-T synthesis biofuel is to match and integrate the biomass gasification technology of proper biomass materials with the corresponding liquid fuel synthesis process, investment cost take a large place in the overall costs. The uncertain cost of the 2nd generation liquid biofuel would reduce as the expanding of production scale and increasing of the conversion efficiency, the key point to reduce the cost is the outbreak of enzyme technology and scale expanding. Energy saving protocols and comprehensive utilization of byproducts would also reduce the cost in a certain extent, for example, the microalgae based biofuel, it is critical of the high value comprehensive utilization (TU, 2014 ).

4. Conclusions

The United Nations Climate Change Conference in Paris has reached a historical agreement in reducing CO2 emissions, in which biomass energy would play a key role. Biomass energy would become the most potential renewable energies, for whether wind power or photovoltaic, would be restricted by the nature thus cannot provide stable power, while biomass energy is the only renewable energy that can be used in the form of gas, liquid or solid stage, it could replace the fossil energy, lead a positive influence on the control of GHGs.

Although questions and debates exist, biomass energy has positive impacts in replacing traditional fossil energy, optimizing energy structure, and reducing GHGs. The future biomass energy police framework should take comprehensive considerations to deal with factors such as climate change, secure energy safety, protect the ecological environment, secure crops safety, and maintain economic stability, cut down its negative influences on society, economy and environments.

Currently the global oil price is at low position, and biomass energy industry development is facing both technical and economic pressures, Tianguan Group demonstrate with their industrial experiences that, only by applying all-round methods of bioraffinerien and comprehensive utilization can the new industry has breakthrough in the economic index, and compete with the traditional fossil energy.

The biomass produced through photosynthesis is about 200 Gt, or 99 Gtce per year. In 2014, the world consumed coal, oil and gas of about 13 Gtoe. If 10%, or 20 Gt, of the biomass is utilized, more than 4 Gt of fuel ethanol and other bioenergy products can be produced, equivalent to the 4.13 Gt of petroleum consumed by the world in 2014. Therefore, bioenergy can be a feasible alternative to fossil energy.

Acknowledgments

This research gained supports from the National Key Technology Support Program (2012BAC18B03 , 2014BAC33B01 ), and the National 863 Program (2009AA034901 ).

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