At present,with the continuous consumption of fossil resources such as coal,oil,and natural gas around the world,the development and utilization of biomass energy has attracted more and more attention.Biomass is all organic substances formed by photosynthesis of green plants that are directly or indirectly used.It includes plants,animals,and microorganisms other than fossil fuels,as well as their excretions and metabolites.Biomass energy refers to a kind of energy stored in organisms in the form of chemical energy from solar energy.Plants photosynthesize and form energy directly or indirectly.The carrier is biomass.Biomass energy has a wide range of uses,such as ethanol fuel that can be used in automobiles made from corn,wheat and other plants as raw materials.

With the continuous research on biomass energy and petroleum substitute products at home and abroad and the development and application of related technologies,the conditions for large-scale industrial production are now in place.Relevant experts and scholars believe that biomass energy sources such as polyacid,bioethylene,ethanol fuel and other products that are alternatives to petroleum products have the hope of becoming China's future petroleum alternatives and new energy sources.Not only will they not pose a threat to China's food security,they will also promote China's food production.At the same time,it will greatly promote the safe,stable and rapid development of China's energy industry.

1The characteristics of biomass energy

Biomass is a complex material,mainly composed of cellulose,hemicellulose and lignin,as well as a small amount of tannic acid,fatty acids,resins and inorganic salts.This renewable raw material has great potential and can be used for power generation and the production of high value-added chemicals.As a new type of renewable energy,biomass energy has many advantages compared with other fossil energy sources.：

1)Renewability.Because biomass energy can be regenerated through photosynthesis of plants,it is a renewable energy source compared with fossil energy sources such as coal,petroleum,and natural gas,and it is rich in resources,which can ensure the permanent use of energy.

2)Low pollution.The content of sulfur,nitrogen and other non-metals in biomass is low,and the sulfur-containing compounds and nitrogen oxides produced by combustion are less.Since the amount of carbon dioxide absorbed by biomass during growth is the same as the amount of carbon dioxide emitted,the net carbon dioxide emissions to the atmosphere are almost zero when used as fuel,which can effectively reduce carbon dioxide emissions and the generation of acid rain.

3)The total amount is very rich.Biomass energy is the fourth largest carbon-containing energy source in the world after coal,oil and natural gas.Biomass energy plants(referred to as“biomass resources”)are widely distributed.With the continuous development of agroforestry,there will be more and more biomass resources.Biomass resources can grow rapidly through large-scale planting,which can ensure their yield.

4)Security.The use of biomass energy is quite safe,and there will be no major safety accidents such as explosions and leaks.

5)Wide applicability.It has applications in many fields of the national economy such as biogas,solid fuel compression molding,pyrolysis and gasification to produce gas,power generation,fuel alcohol production,and biodiesel.

As far as the current international situation is concerned,there is an urgent need to adopt unconventional and sustainable energy sources to meet the growing demand for liquid fuels.The use of biomass for the production of biodiesel,biofuel and ethanol is approaching commercialization.The development of biomass energy will play an active role in ensuring energy supply,reducing dependence on the oil market,and stabilizing economic development.It is an effective new way to solve future energy problems.Many experts pointed out that the 21st century will be an era of great development of biomass energy.

2 Biomass energy utilization technology

The biggest advantage of biomass is that it is the only carbon-containing renewable resource.Liquid and gaseous fuels can be prepared by chemical conversion method,biochemical conversion method and photochemical conversion method,involving pyrolysis,gasification,liquefaction,molding and direct combustion technologies.Clean fuels that are easy to store,transport,and use can be obtained,and their physical forms are liquids,solids,and gases.Figure 1 lists various biomass energy utilization technologies.

2.1 Direct biomass combustion technology

2.1.1 Direct biomass combustion fluidized bed technology

Direct biomass combustion technology is mainly divided into stove combustion,boiler combustion and dense forming technology.The technologies involved are direct biomass combustion fluidized bed technology and direct biomass combustion layered combustion technology.

Many foreign companies such as the United States Idaho Energy Products Company,the United States B&W,the United States CE Company,etc.Use fluidized bed technology to develop fluidized bed power generation boilers to treat biomass has a considerable scale and certain operating experience.Sweden uses branches,leaves,etc.as fuel for large fluidized bed boilers,and the thermal efficiency of the boiler has reached 80%.Denmark burns hay and coal in accordance with a 6:4 mass ratio using a high-magnification circulating fluidized bed boiler,with a thermal power of 80MW and a boiler output of 100t/H.Liu Hao,Lin Zhijie,etc.Based on the physical and chemical properties of rice husk,and taking into account its combustion characteristics,the traditional fluidized bed combustion boiler has been improved;using a unique combustion and air distribution method,a boiler with good fluidization performance,stable combustion,and not easy to coking has been developed.The advantages of boiler.

Chen Guanyi and others cooperated with Wuxi Boiler Factory to design and develop a pneumatic conveying device that not only has a large conveying capacity,but also safe conveying.It is used for 35t/h rice husk-burning fluidized bed boiler.The unique design of the conveying device not only avoids the interruption of feeding,but also reduces the wear of buried pipes in the bed and the loss of heating area;at the same time,it also expands the scope of fuel application of the boiler.The future development of biomass boilers depends on the fossil fuel market and decision-making on the biomass market.

2.1.2 Direct combustion layer combustion technology

2.1.2.1 Agroforestry waste development and utilization technology

Biomass layer combustion technology has a wide range of applications in the development and utilization of agroforestry waste.The Benson-type boiler funded by Denmark's ELSAM company can not only make straw,sawdust and other materials fully burn on the grate,but also its furnace and pipes are also equipped with fiber filters to reduce the corrosion and wear of equipment by harmful substances in the flue gas.After practical operation,it has been proved that the transformed biomass boiler is stable in operation and has good social and economic benefits.Based on the combustion mechanism of bagasse,Zhai Xuemin has developed a bagasse boiler with a closed furnace structure arranged separately from the radiation heating surface,which is beneficial to the timely ignition and stable combustion of bagasse.Since bagasse is representative in biomass fuels,this furnace type has certain versatility for biomass fuels such as bark and rice husk.

He Yuheng has developed and designed a layer-fired boiler with a novel structure that can burn waste materials such as sawdust,wood powder and bark.The boiler can ensure the full combustion of sawdust and wood powder;it can prevent wood powder from deflagration;the boiler is a negative pressure combustion to ensure that the wood powder does not breathe fire outside the furnace when burning,which provides valuable experience for the development and design of boilers that burn sawdust,wood powder and other forestry waste.

2.1.2.2 Municipal solid waste incineration technology

With the development of urban construction and the progress of society,the output of municipal solid waste is increasing year by year.Therefore,it is very necessary to develop a new type of waste incineration treatment technology.Since the last century,in addition to Shanghai Pudong Yuqiao,many cities in our country such as Beijing,Guangzhou,Xiamen,etc.have been building thousand-ton waste incineration plants.Waste incineration can not only reduce environmental pollution,but also save a lot of land resources.Therefore,this technology has become a mainstream technology for the treatment of domestic waste in large cities.However,waste incineration technology still needs to be continuously improved and improved in terms of exhaust gas treatment,secondary pollution and incinerator combustion efficiency in accordance with our country's basic national conditions.Due to the many shortcomings of incineration technology,continuous research and exploration at home and abroad have developed a process that uses anaerobic digestion technology to treat urban domestic waste.Compared with traditional incineration technology,this process has great advantages in terms of treatment efficiency,treatment cost,and resource recycling.At present,this process has been widely used in urban waste treatment.

2.1.3 Dense forming technology

After drying,agroforestry waste with a certain particle size such as sawdust,rice husk,branches,straw,etc.can be continuously squeezed into rod-shaped,granular,bulk and other solid forming fuels under a certain pressure.The processing technology is called biomass dense forming technology.Using the special adhesive effect of lignin,or adding certain additives or binders to bond them together into a forming fuel,the biomass raw materials are extruded and formed,the volume is reduced,the density will be significantly larger,and the moisture content will decrease,which is convenient for storage and transportation.The technology is widely used in high-efficiency combustion furnaces,biomass gasifiers and small boilers.

According to the differences in process characteristics,the biomass dense forming process is divided into three types:cold-pressed dense forming,hot-pressed dense forming and carbonized dense forming.Each process has its own characteristics and application scope.In recent years,scientific research units at home and abroad have conducted research on biomass molding theory,biomass utilization devices,and molding fuel combustion technology,and have made breakthroughs.They have developed and produced various biomass dense molding machines with different functions and application ranges.In addition,there are single-head and multi-head screw extrusion rod dense forming machines,which have been put into actual production in small batches,and have achieved certain social and economic dual benefits.Due to the influence of a variety of factors,there are also problems of one kind or another in the molding machine,molding raw materials and supporting equipment,and it cannot be expected to be fully applied in the short term.In the future,we will focus on the practicality of equipment,reducing energy consumption,reducing wear and tear,the applicability of raw materials,and serialization,so as to provide the necessary technical reserves for large-scale development and better use of biomass energy.

2.2 Biochemical transformation technology

2.2.1 Anaerobic digestion to produce biogas

Biogas fermentation is a(micro)biological process.Crop stalks,human and animal manure,and organic matter contained in wastewater discharged from industry and agriculture can all be used as raw materials for biogas fermentation.Under suitable conditions and anaerobic environments,organic matter is finally converted into biogas by microorganisms.The process is mainly divided into three stages:liquefaction,acid production and methane production.A schematic diagram of the basic process is shown in Figure 2.

2.2.2 Preparation of ethanol or methanol by enzyme technology

Various green plants(such as corn cob,fruits,sugar beets,sweet sorghum,straw,straw,wood chips,grasses and many cellulose-rich raw materials)can be used as raw materials for extracting ethanol.Ethanol is also known as alcohol,and people usually refer to ethanol used as fuel as“green petroleum.”There are many methods for producing ethanol,including:①Direct fermentation using sugary raw materials;②Indirect fermentation using carbohydrates or starch;and乙醇Acid hydrolysis or enzymatic hydrolysis of cellulose raw materials such as wood to produce ethanol.

2.3 Thermochemical conversion technology

Thermochemical conversion technology refers to the use of chemical means to convert biomass into high-grade,easy-to-store,easy-to-transport,high-energy-density and commercially valuable solid,liquid and gaseous fuels,as well as energy products such as thermal energy and electrical energy,under the conditions of heating and hypoxia.Fuel material technology J.Thermochemical conversion is an effective method of producing biofuel from biomass,which mainly includes baking,liquefaction,pyrolysis and gasification technologies.Through these conversion technologies,solid,liquid and gaseous biofuel produced from biomass are used for heating and power generation.Liquid bio-oils can be further converted into chemicals,while syngas can be synthesized into liquid fuels.

Pyrolysis is one of the better processes for treating solid waste.The temperature is generally 300-600℃.There are three methods:slow pyrolysis,fast pyrolysis and flash pyrolysis.The process can be divided into four stages:material drying,hemicellulose pyrolysis,cellulose and lignin pyrolysis.In the process of biomass pyrolysis,heat is transmitted layer by layer from the outside to the inside.The first is the surface of the particles,and then it is transmitted from the surface to the inside of the particles.The heated part of the particles quickly cleaves into charcoal and volatile components,and the cracked product will continue to cleave under the action of temperature.Reaction.Most of the biomass pyrolysis processes in practical applications are atmospheric pressure or near atmospheric pressure reactions,and the products obtained by pyrolysis are mainly composed of bio-oil,gas and solid carbon.

Biomass gasification is also a kind of thermochemical conversion of biomass.The basic principle is to heat the raw material in the case of incomplete combustion to cleave the higher molecular weight compound into a lower molecular weight mixture such as H2,CO,small molecule hydrocarbons and CO2.The process.Air or a mixture of oxygen,water vapor,water vapor and oxygen is usually used as a gasifier.The product of gasification is syngas,which is further converted into liquid fuels such as methanol and ethanol through Fischer-tropsch synthesis or biosynthesis,and can also be directly used as fuel for gas motors.

2.4 Solid waste treatment technology

For the treatment of solid waste,there are three traditional methods:landfill,incineration,and biochemical treatment.Landfill and incineration methods cause a waste of resources and energy.At present,more in-depth research has been done on the reuse of biomass energy in waste at home and abroad,mainly using feed,fertilizer,biogas,fuel and other methods for scientific and effective use of biomass.Such as processing it into feed and making full use of its nutrients;composting technology is used to treat garden waste,organic biological waste,organic residual sludge and agricultural waste;biogas technology is used to treat all kinds of organic waste in agriculture,industry and human life.While generating biogas,in order to avoid secondary pollution,the biogas residue and biogas liquid must be reused.The biogas residue and biogas liquid can be directly used as fertilizer or separated into commercial fertilizers by solid-liquid separation.The compression molding technology of biomass can compress the original loose,finely broken,and amorphous biomass raw materials into rod-shaped,granular,and block-shaped fuels under certain conditions.

2.5 Biomass liquefaction technology

The liquefaction technology of biomass energy refers to the technology of converting biomass into liquid fuel through hydrolysis,pyrolysis or catalysis.Through the chemical processing of biological substances,liquid fuels such as fuel ethanol,methanol,bio-oil,etc.are prepared;under certain conditions,biological fermentation or acid hydrolysis technology can be used to convert biomass into ethanol for use in automobiles or other industries.The use of biomass liquefaction technology can not only improve the utilization efficiency of biomass,but also expand its application range.The preparation of liquid fuels through the liquefaction of biomass will be a cutting-edge technology with potential for development.

2.6 Supercritical conversion technology of biomass

Supercritical fluid(SCF)is a fluid with good fluidity,transferability,diffusion and solubility that is above the critical temperature and critical pressure and has physical properties between a gas and a liquid.It has the dual properties and advantages of gas and liquid.Near its critical point,small changes in pressure and temperature will cause large changes in physical properties such as fluid density,solubility,and dielectric constant.

Supercritical water has the characteristics of dissolving most organic matter and gases,high density,and low viscosity.The heat utilization efficiency of biomass gasification and pyrolysis in supercritical water is higher than conventional ones,so that biomass supercritical water vaporization technology has received widespread attention and has great potential for development.Due to the high reaction temperature and reaction pressure required for supercritical water vaporization,the requirements for equipment and materials are high,and the research on supercritical water vaporization of biomass has been restricted.Research in this area in my country started late.With people's continuous understanding and research on the unique physical and chemical properties of supercritical water,it has been found that supercritical fluids can be used for the conversion and efficient utilization of biomass resources in the pretreatment,pyrolysis,liquefaction and preparation of biodiesel of biomass.Such as supercritical pyrolysis of biomass to produce hydrogen,supercritical preparation of biodiesel,supercritical liquefaction of biomass and supercritical pretreatment of biomass.The supercritical conversion technology of biomass has great application potential,especially in the field of hydrogen production by gasification of supercritical water and esterification of supercritical methanol to biodiesel.However,due to the demanding reaction conditions of supercritical fluids,high energy consumption and equipment requirements and large losses,resulting in high production costs,most of the current applications of supercritical fluids in biomass energy conversion are in the research stage,and there is still huge room for development in the research and improvement of technologies and systems.

3 Conclusion and outlook

As the only renewable carbon-containing resource that can replace fossil energy,biomass energy is becoming more and more important in the development of new energy sources.Due to the non-renewable nature of fossil fuels such as coal,petroleum,and natural gas and the damage to the environment during use,biomass energy will become one of the main modern energy sources,and biomass conversion and utilization technology will also become the key to this transformation.At present,the applications related to biomass conversion and utilization include biomass gasification for power generation,gasification for hydrogen production,thermal cracking for hydrogen production,fermentation for the production of fuel ethanol,thermal cracking for bio-oil,curing molding for solid fuel,compost fermentation for fertilizer,anaerobic digestion for the production of biogas,catalytic cracking of biomass for the production of biofuel,etc.,have a wide range of applications in many fields.

The world's annual biomass production is about 146 billion t,and the amount of agroforestry waste and livestock manure resources alone can reach 1 billion t per year.According to the United Nations Conference on Environment and Development(UNCED),the conversion and utilization of biomass energy will account for about half of global energy consumption by 2050.Biomass energy is expected to contribute half of the EU's renewable energy goals.By 2020,bioenergy is expected to remain the main contributor to renewable energy at the EU level.Therefore,the sustainable development of biomass is a key issue.The use of biomass conversion technology can reuse biomass resources,which plays an important role in saving resources,protecting and improving the ecological environment,promoting the harmonious development of the regional economy,and alleviating the human energy crisis.Due to the different types of biomass,the suitable conversion and utilization technologies are also different.Due to the inability to completely unify the practicality and economy,most of these technologies are difficult to popularize,but with the continuous deepening of research at home and abroad,more new technologies and integrated technologies for biomass conversion and utilization will continue to emerge.