Introduction

Biomass power generation is the largest and most mature modern biomass energy utilization technology. China is rich in biomass resources,

mainly including agricultural waste, forestry waste, livestock manure, urban domestic waste, organic wastewater and waste residue. The total

amount of biomass resources that can be used as energy every year is equivalent to about 460 million tons of standard coal. In 2019, the

installed capacity of global biomass power generation increased from 131 million kilowatts in 2018 to about 139 million kilowatts, an increase

of about 6%. The annual power generation increased from 546 billion kWh in 2018 to 591 billion kWh in 2019, an increase of about 9%,

mainly in the EU and Asia, especially China. China’s 13th Five Year Plan for Biomass Energy Development proposes that by 2020, the total

installed capacity of biomass power generation should reach 15 million kilowatts, and the annual power generation should reach 90 billion

kilowatt hours. By the end of 2019, China’s installed capacity of bio power generation had increased from 17.8 million kilowatts in 2018 to

22.54 million kilowatts, with the annual power generation exceeding 111 billion kilowatt hours, exceeding the goals of the 13th Five Year Plan.

In recent years, the focus of China’s biomass power generation capacity growth is to use agricultural and forestry wastes and urban solid wastes

in the cogeneration system to provide power and heat for urban areas.

Latest research progress of biomass power generation technology

Biomass power generation originated in the 1970s. After the world energy crisis broke out, Denmark and other western countries began to

use biomass energy such as straw for power generation. Since the 1990s, biomass power generation technology has been vigorously developed

and applied in Europe and the United States. Among them, Denmark has made the most remarkable achievements in the development of

biomass power generation. Since the first straw bio combustion power plant was built and put into operation in 1988, Denmark has created

more than 100 biomass power plants so far, becoming a benchmark for the development of biomass power generation in the world. In addition,

Southeast Asian countries have also made some progress in the direct combustion of biomass using rice husk, bagasse and other raw materials.

China’s biomass power generation started in the 1990s. After entering the 21st century, with the introduction of national policies to support the

development of biomass power generation, the number and energy share of biomass power plants are increasing year by year. In the context of

climate change and CO2 emission reduction requirements, biomass power generation can effectively reduce CO2 and other pollutants emissions,

and even achieve zero CO2 emissions, so it has become an important part of researchers’ research in recent years.

According to the working principle, biomass power generation technology can be divided into three categories: direct combustion power generation

technology, gasification power generation technology and coupling combustion power generation technology.

In principle, biomass direct combustion power generation is very similar to coal-fired boiler thermal power generation, that is, the biomass fuel

(agricultural waste, forestry waste, urban domestic waste, etc.) is sent into a steam boiler suitable for biomass combustion, and the chemical

energy in the biomass fuel is converted into internal energy of high-temperature and high-pressure steam by using the high-temperature combustion

process, and is converted into mechanical energy through the steam power cycle, Finally, the mechanical energy is transformed into electrical

energy through the generator.

Biomass gasification for power generation involves the following steps: (1) biomass gasification, pyrolysis and gasification of biomass after crushing,

drying and other pre-treatment under high temperature environment to produce gases containing combustible components such as CO, CH₄ and

H ₂; (2) Gas purification: combustible gas generated during gasification is introduced into the purification system to remove impurities such as ash,

coke and tar, so as to meet the inlet requirements of downstream power generation equipment; (3) Gas combustion is used for power generation.

Purified combustible gas is introduced into gas turbine or internal combustion engine for combustion and power generation, or it can be introduced

into boiler for combustion, and the generated high-temperature and high-pressure steam is used to drive steam turbine for power generation.

Due to dispersed biomass resources, low energy density and difficult collection and transportation, direct combustion of biomass for power generation

has a high dependence on the sustainability and economy of fuel supply, resulting in high cost of biomass power generation. Biomass coupled power

generation is a power generation method that uses biomass fuel to replace some other fuels (usually coal) for co combustion. It improves the flexibility

of biomass fuel and reduces the coal consumption, realizing the CO₂ emission reduction of coal-fired thermal power units. At present, biomass coupled

power generation technologies mainly include: direct mixed combustion coupled power generation technology, indirect combustion coupled power

generation technology and steam coupled power generation technology.

1. Biomass direct combustion power generation technology

Based on the current biomass direct fired generator sets, according to the furnace types used more in engineering practice, they can be mainly divided

into layered combustion technology and fluidized combustion technology [2].

Layered combustion means that the fuel is delivered to the fixed or mobile grate, and the air is introduced from the bottom of the grate to conduct

combustion reaction through the fuel layer. The representative layered combustion technology is the introduction of water-cooled vibrating grate

technology developed by BWE Company in Denmark, and the first biomass power plant in China – Shanxian Power Plant in Shandong Province was

built in 2006. Due to the low ash content and high combustion temperature of biomass fuel, grate plates are easily damaged due to overheating and

poor cooling. The most important feature of water-cooled vibrating grate is its special structure and cooling mode, which solves the problem of grate

overheating. With the introduction and promotion of Danish water-cooled vibrating grate technology, many domestic enterprises have introduced

biomass grate combustion technology with independent intellectual property rights through learning and digestion, which has been put into large-scale

operation. Representative manufacturers include Shanghai Sifang Boiler Factory, Wuxi Huaguang Boiler Co., Ltd., etc.

As a combustion technology characterized by fluidization of solid particles, fluidized bed combustion technology has many advantages over bed

combustion technology in burning biomass. First of all, there are a lot of inert bed materials in the fluidized bed, which has high heat capacity and

strong adaptability to biomass fuel with high water content; Secondly, the efficient heat and mass transfer of the gas-solid mixture in the fluidized

bed enables the biomass fuel to be heated up quickly after entering the furnace. At the same time, the bed material with high heat capacity can

maintain the furnace temperature, ensure the combustion stability when burning low calorific value biomass fuel, and also have certain advantages

in unit load adjustment.With the support of the national science and technology support plan, Tsinghua University has developed the “Biomass

Circulating Fluidized Bed Boiler Technology with High Steam Parameters”, and has successfully developed the world’s largest 125 MW ultra-high

pressure once reheat biomass circulating fluidized bed boiler with this technology, and the first 130 t/h high-temperature and high-pressure

circulating fluidized bed boiler burning pure corn straw.

Due to the generally high alkali metal and chlorine content of biomass, especially agricultural wastes, there are problems such as ash, slagging

and corrosion in the high-temperature heating area during the combustion process. The steam parameters of biomass boilers at home and abroad

are mostly medium temperature and medium pressure, and the power generation efficiency is not high. The economy of biomass layer direct fired

power generation restricts its healthy development.

2. Biomass gasification power generation technology

Biomass gasification power generation uses special gasification reactors to convert biomass wastes, including wood, straw, straw, bagasse, etc.,

into combustible gas. The generated combustible gas is sent to gas turbines or internal combustion engines for power generation after dust

removal and coke removal and other purification processes [3]. At present, the commonly used gasification reactors can be divided into fixed bed

gasifiers, fluidized bed gasifiers and entrained flow gasifiers. In the fixed bed gasifier, the material bed is relatively stable, and the drying, pyrolysis,

oxidation, reduction and other reactions will be completed in sequence, and finally converted into synthetic gas. According to the difference of flow

direction between gasifier and synthetic gas, fixed bed gasifiers mainly have three types: upward suction (counter flow), downward suction (forward

flow) and horizontal suction gasifiers. The fluidized bed gasifier is composed of a gasification chamber and an air distributor. The gasifying agent is

uniformly fed into the gasifier through the air distributor. According to the different gas-solid flow characteristics, it can be divided into bubbling

fluidized bed gasifier and circulating fluidized bed gasifier. The gasification agent (oxygen, steam, etc.) in the entrained flow bed entrains biomass

particles and is sprayed into the furnace through a nozzle. Fine fuel particles are dispersed and suspended in high-speed gas flow. Under high

temperature, fine fuel particles react rapidly after contacting with oxygen, releasing a lot of heat. Solid particles are instantly pyrolyzed and gasified

to generate synthetic gas and slag. For the updraft fixed bed gasifier, the tar content in the synthesis gas is high. The downdraft fixed bed gasifier

has simple structure, convenient feeding and good operability.

Under high temperature, the tar generated can be fully cracked into combustible gas, but the outlet temperature of the gasifier is high. The fluidized

bed gasifier has the advantages of fast gasification reaction, uniform gas-solid contact in the furnace and constant reaction temperature, but its

equipment structure is complex, the ash content in the synthesis gas is high, and the downstream purification system is highly required. The

entrained flow gasifier has high requirements for material pretreatment and must be crushed into fine particles to ensure that the materials can

react completely within a short residence time.

When the scale of biomass gasification power generation is small, the economy is good, the cost is low, and it is suitable for remote and scattered

rural areas,which is of great significance to supplement China’s energy supply. The main problem to be solved is the tar produced by biomass

gasification. When the gas tar produced in the gasification process is cooled, it will form liquid tar, which will block the pipeline and affect the

normal operation of power generation equipment.

3. Biomass coupled power generation technology

The fuel cost of pure incineration of agricultural and forestry wastes for power generation is the biggest problem restricting the biomass power

generation industry. The biomass direct fired power generation unit has small capacity, low parameters and low economy, which also limits the

utilization of biomass.Biomass coupled multi source fuel combustion is a way to reduce the cost. At present, the most effective way to reduce

fuel costs is biomass and coal-fired power generation. In 2016, the country issued the Guiding Opinions on Promoting Coal fired and Biomass

Coupled Power Generation, which greatly promoted the research and promotion of biomass coupled power generation technology. In recent

years, the efficiency of biomass power generation has been significantly improved through the transformation of existing coal-fired power plants,

the use of coal coupled biomass power generation, and the technical advantages of large coal-fired power generation units in high efficiency

and low pollution. The technical route can be divided into three categories:

(1) direct combustion coupling after crushing/pulverizing, including three types of co combustion of the same mill with the same burner, different

mills with the same burner, and different mills with different burners; (2) Indirect combustion coupling after gasification, biomass generates

combustible gas through gasification process and then enters the furnace for combustion; (3) Steam coupling after combustion of special biomass

boiler.Direct combustion coupling is a utilization mode that can be implemented on a large scale, with high cost performance and short investment

cycle. When the coupling ratio is not high, the fuel processing, storage, deposition, flow uniformity and its impact on boiler safety and economy

caused by burning biomass have been technically solved or controlled. The indirect combustion coupling technology treats biomass and coal

separately, which is highly adaptable to the types of biomass, consumes less biomass per unit power generation, and saves fuel. It can solve the

problems of alkali metal corrosion and boiler coking in the direct combustion process of biomass to a certain extent, but the project has poor

scalability and is not suitable for large-scale boilers. In foreign countries,the direct combustion coupling mode is mainly used. As the indirect

combustion mode is more reliable, the indirect combustion coupling power generation based on circulating fluidized bed gasification is currently

the leading technology for the application of biomass coupling power generation in China. In 2018,Datang Changshan Power Plant, the country’s

first 660MW supercritical coal-fired power generation unit coupled with 20MW biomass power generation demonstration project, achieved a

complete success. The project adopts the independently developed biomass circulating fluidized bed gasification coupled power generation

process, which consumes about 100000 tons of biomass straw every year, achieves 110 million kilowatt hours of biomass power generation,

saves about 40000 tons of standard coal, and reduces about 140000 tons of CO₂.

Analysis and prospect of the development trend of biomass power generation technology

With the improvement of China’s carbon emission reduction system and carbon emission trading market, as well as the continuous implementation

of the policy of supporting coal-fired coupled biomass power generation, biomass coupled coal-fired power generation technology is ushering in good

development opportunities. The harmless treatment of agricultural and forestry wastes and urban domestic waste has always been the core of the

urban and rural environmental problems that local governments need to solve urgently. Now the planning right of biomass power generation projects

has been delegated to local governments. Local governments can bind agricultural and forestry biomass and urban domestic waste together in project

planning to promote waste integrated power generation projects.

In addition to combustion technology, the key to the continuous development of biomass power generation industry is the independent development,

maturity and improvement of supporting auxiliary systems, such as biomass fuel collection, crushing, screening and feeding systems. At the same time,

developing advanced biomass fuel pretreatment technology and improving the adaptability of single equipment to multiple biomass fuels are the basis

for realizing low-cost large-scale application of biomass power generation technology in the future.

1. Coal fired unit biomass direct coupling combustion power generation

The capacity of biomass direct fired power generation units is generally small (≤ 50MW), and the corresponding boiler steam parameters are also low,

generally high pressure parameters or lower. Therefore, the power generation efficiency of pure burning biomass power generation projects is generally

not higher than 30%. The biomass direct coupling combustion technology transformation based on 300MW subcritical units or 600MW and above

supercritical or ultra supercritical units can improve the biomass power generation efficiency to 40% or even higher. In addition, the continuous operation

of biomass direct fired power generation project units depends entirely on the supply of biomass fuel, while the operation of biomass coupled coal-fired

power generation units does not depend on the supply of biomass. This mixed combustion mode makes the biomass collection market of power generation

enterprises have stronger bargaining power. The biomass coupled power generation technology can also use the existing boilers, steam turbines and

auxiliary systems of coal-fired power plants. Only the new biomass fuel processing system is needed to make some changes to the boiler combustion

system, so the initial investment is lower. The above measures will greatly improve the profitability of biomass power generation enterprises and reduce

their dependence on national subsidies. In terms of pollutant emission, the environmental protection standards implemented by biomass direct fired

power generation projects are relatively loose, and the emission limits of smoke, SO2 and NOx are respectively 20, 50 and 200 mg/Nm3. Biomass coupled

power generation relies on the original coal-fired thermal power units and implements ultra-low emission standards. The emission limits of soot, SO2

and NOx are respectively 10, 35 and 50mg/Nm3. Compared with the biomass direct fired power generation of the same scale, the emissions of smoke, SO2

and NOx are reduced by 50%, 30% and 75% respectively, with significant social and environmental benefits.

The technical route for large-scale coal-fired boilers to carry out the transformation of biomass direct coupled power generation can currently be summarized

as biomass particles – biomass mills – pipeline distribution system – pulverized coal pipeline. Although the current biomass direct coupled combustion

technology has the disadvantage of difficult measurement, the direct coupled power generation technology will become the main development direction

of biomass power generation after solving this problem, It can realize the coupling combustion of biomass in any proportion in large coal-fired units, and

has the characteristics of maturity, reliability and safety. This technology has been widely used internationally, with biomass power generation technology

of 15%, 40% or even 100% coupling proportion. The work can be carried out in subcritical units and gradually expanded to achieve the goal of CO2 deep

emission reduction of ultra supercritical parameters+biomass coupled combustion+district heating.

2. Biomass fuel pretreatment and supporting auxiliary system

Biomass fuel is characterized by high water content, high oxygen content, low energy density and low calorific value, which limits its use as a fuel and

adversely affects its efficient thermochemical conversion. First of all, the raw materials contain more water, which will delay the pyrolysis reaction,

destroy the stability of the pyrolysis products, reduce the stability of the boiler equipment, and increase the system energy consumption. Therefore,

it is necessary to pretreat biomass fuel before thermochemical application.

Biomass densification processing technology can reduce the increase in transportation and storage costs caused by the low energy density of biomass

fuel. Compared with drying technology, baking biomass fuel in an inert atmosphere and at a certain temperature can release water and some volatile

matter in biomass, improve the fuel characteristics of biomass, reduce O/C and O/H. The baked biomass shows hydrophobicity and is easier to be

crushed into fine particles. The energy density is increased, which is conducive to improving the conversion and utilization efficiency of biomass.

Crushing is an important pretreatment process for biomass energy conversion and utilization. For biomass briquette, the reduction of particle size can

increase the specific surface area and the adhesion between particles during compression. If the particle size is too large, it will affect the heating rate

of the fuel and even the release of volatile matter, thereby affecting the quality of gasification products. In the future, it can be considered to build a

biomass fuel pretreatment plant in or near the power plant to bake and crush biomass materials. The national “13th Five Year Plan” also clearly points

out that the biomass solid particle fuel technology will be upgraded, and the annual utilization of biomass briquette fuel will be 30 million tons.

Therefore, it is of far-reaching significance to vigorously and deeply study the biomass fuel pretreatment technology.

Compared with conventional thermal power units, the main difference of biomass power generation lies in the biomass fuel delivery system and related

combustion technologies. At present, the main combustion equipment of biomass power generation in China, such as boiler body, has achieved localization,

but there are still some problems in the transportation system of biomass. Agricultural waste generally has a very soft texture, and the consumption in

the power generation process is relatively large. The power plant must prepare the charging system according to the specific fuel consumption. There

are many kinds of fuels available, and the mixed use of multiple fuels will lead to uneven fuel and even blockage in the feeding system, and the fuel

working condition inside the boiler is prone to violent fluctuations. We can make full use of the advantages of fluidized bed combustion technology in

fuel adaptability, and first develop and improve the screening and feeding system based on the fluidized bed boiler.

4、 Suggestions on independent innovation and development of biomass power generation technology

Unlike other renewable energy sources, the development of biomass power generation technology will only affect the economic benefits, not the

society. At the same time, biomass power generation also requires harmless and reduced treatment of agricultural and forestry wastes and household

garbage. Its environmental and social benefits are far greater than its energy benefits. Although the benefits brought by the development of biomass

power generation technology are worth affirming, some key technical problems in biomass power generation production activities cannot be effectively

addressed due to factors such as the imperfect measurement methods and standards of biomass coupled power generation, the weak state financial

subsidies, and the relatively lack of development of new technologies, which are the reasons for limiting the development of biomass power generation

technology, Therefore, reasonable measures should be taken to promote it.

(1) Although technology introduction and independent development are both the main directions for the development of domestic biomass power

generation industry, we should clearly realize that if we want to have a final way out, we must strive to take the road of independent development,

and then constantly improve domestic technologies. At this stage, it is mainly to develop and improve biomass power generation technology, and

some technologies with better economy can be used commercially; With the gradual improvement and maturity of biomass as the main energy and

biomass power generation technology, biomass will have the conditions to compete with fossil fuels.

(2) The social management cost can be reduced by reducing the number of partial pure burning agricultural waste power generation units and the

number of power generation companies, while strengthening the monitoring management of biomass power generation projects. In terms of fuel

purchase, ensure sufficient and high-quality supply of raw materials, and lay a foundation for stable and efficient operation of the power plant.

(3) Further improve the preferential tax policies for biomass power generation, improve the system efficiency by relying on cogeneration

transformation, encourage and support the construction of county multi-source waste clean heating demonstration projects, and limit the value

of biomass projects that only generate electricity but not heat.

(4) BECCS (Biomass energy combined with carbon capture and storage technology) has proposed a model that combines biomass energy utilization

and carbon dioxide capture and storage, with dual advantages of negative carbon emissions and carbon neutral energy. BECCS is a long-term

emission reduction technology. At present, China has less research in this field. As a large country of resource consumption and carbon emissions,

China should include BECCS in the strategic framework to deal with climate change and increase its technical reserves in this area.