**2. Biomass characteristics and general conversion**

is stored in the organic form in the chemical state and supports human beings' daily life since our ancestor apes knew how to use fire to cook. In these millions of years, bioenergy was mostly used in small scale like household cooking. Now, people have realized that efficient exploitation of biomass resource can actually reduce their dependency over fossil fuel. Biomass gasification has been regarded as an effective pathway to utilization of bioresource. It takes biomass as raw materials and employs pyrolysis or thermal cracking under anoxic conditions. This is an energy conversion process including a group of complex chemical reactions that large organic molecules degrade into carbon monoxide, methane and hydrogen and other flammable gases in accordance with chemical bonding theory. Biomass feedstock with the gasification agent is heated inside an integrated gasifier. With temperature increase, biomass goes through dehydration, volatilization and decomposition. Eventually, the produced gases are used for central gas supply and power generation. This technology has already been developed over several decades and progressively achieved commercialization all over the world, especially in Sweden, Germany, Canada, the United States, India and China. In the early stage, downdraft gasifier had been implemented at a large scale in China and India due to its relatively low tar production. Recently, the development of circulating fluidized bed (CFB) gasifier makes it adaptable for both biomass quality and the raw particle size. Besides,

China, as a large agricultural country, produces a large number of crop straw, poultry manure, agricultural by-products and other plant biomass every year. Thus, research and development on key technologies and integrated peripherals of biomass gasification become very necessary. China has already developed various gasifiers, the size of which range from 400 KW to 10 MW. However, compared with fossil fuel, biomass has lower bulk density and energy density, which make it uneconomic for collection and transportation. Therefore, biomass gasification coupled with distributed power generation in small communities with abundant biomass

In recent years in China, the yield of domestic waste has increased every year and exceeds 400 million tonnes per year. Chinese government's 13th five-year plan proposed that the proportion of waste harmless treatment should be no less than 70% by 2020. But waste landfill is still the primary method used to deal with waste in rural areas. Compared with landfill, gasification has advantages of lower environmental impacts and does not consume land resource. When contrasting gasification with incineration, the gasification technology has better quality of gaseous emissions with much lower capital input, which makes gasification more suitable for distributed deployment in rural area. Therefore, there will be a great demand for deployment of waste gasification treatment plants in Chinese rural areas, and more and more people are now focusing on the development of more efficient small-scale gasifiers with capacity under 300 tonne/day. The relevant equipment has also been deployed in Iran, Thailand, Burma and Laos. However, several technical barriers are still there such as effective removal of tar with low cost, environmental influence, accuracy control of gasifier

Therefore, this chapter introduces both technological and logistics challenges of biomass gasification via introducing biomass characters and gasifier technologies. The details of tar minimization and socio-environmental impacts of biomass gasification are also presented as main contents to help understand the primary barriers for the deployment of biomass gasification.

CFB is also easy for scale-up and ash cleaning.

4 Gasification for Low-grade Feedstock

resource would be the way out in future [1].

inner temperature, solidification of fly ash and so on.

#### **2.1. Composition of biomass and its common characteristics**

Biomass includes all the living or recently living organisms, like land plants, grasses, waterbased vegetation and manures [2], and these organisms consist of a number of major elements such as C, H, O, N, P and S. The classification of biomass into different categories is based on their properties. One feasible way is based on the appearances and the growth environment of biomass: woody plants, herbaceous plants/grasses, aquatic plants, manures and wastes [2]. Biomass could also be divided into two types: low moisture content and high moisture content. The low moisture content biomass can be used in thermo-chemical processes (i.e., gasification, combustion and pyrolysis), while the high moisture content plants are more suitable to be used in some wet processing technologies (i.e., fermentation and anaerobic digestion) [3]. Such high moisture contents would consume a large amount of energy for the drying process if employed as resources for thermo-chemical processing.

Biomass is derived from solar energy via photosynthesis. Under a good illumination condition, carbon dioxide in the atmosphere can be converted into organic materials or, in another way, the solar energy is stored as chemical energy, which existed as chemical bonds in the organisms [4]. The said chemical energy is released when these bonds are broken either via thermo-chemical or wet processing. This is an ongoing energy transfer from the sun and hence the sustainability of biomass resource could be ensured. As we have known, the total energy captured annually in biomass is more than that of the annual energy consumption globally [5]. On the other hand, biomass is clean as it is carbon neutral. On the view of carbon network, the net emission of carbon dioxide into the environment during the harvesting of energy from biomass is zero. The final products of conversion of biomass (CO2 and H2 O) are originally absorbed into the plants from the atmosphere during photosynthesis. The conversion of biomass also has less harmful releases such as NOx and SOx compared with fossil fuels [6].

However, the characters of biomass also create many barriers during its actual application. On the aspect of species diversity, biomass usually does not behave as steady as fossil fuels, which causes a lot of difficulty during project planning stage including gasifier type, plant size and the way of energy output. On the other hand, the varieties of biomass resource also lead to different heating values and moisture contents. Compared with other energy carriers, biomass has much lower heating values. Taking wood and wheat straw as examples, their lower heating values are only 18.6 and 17.3 MJ/kg, respectively, while the lower heating value of coal is as high as 23–28 MJ/kg [2, 7]. The reason for this disparity is that the oxygen content of biomass carbohydrates is very high while the combustible elements such as C and H are low. In addition, the intrinsic moisture content in biomass is also very high, which requires more energy for drying before further processes take place [3]. Hence, use of biomass requires the complexity in material handling, pre-treatment and the design of processing facilities [3]. For the purpose of transportation and collection, biomass is unlike any other renewable resources (solar, wind, hydropower) where it is able to be stored directly and transported somewhere else. However, biomass is highly dispersed in regional distribution and the low volumetric of biomass makes it a bit more difficult for the collection and transportation. Therefore, smallscale gasification unit operated in small communities with abundant biomass resource or domestic waste would be the way out in future.

#### **2.2. General conversion technologies of biomass except gasification**

For the utilization purpose, the conversion technologies of biomass could be classified in three categories: mechanical extraction; thermo-chemical conversion; and biological conversion, as illustrated in **Figure 1** [3, 8]. Among them, direct combustion, gasification and pyrolysis are considered as the thermo-chemical processes; fermentation and anaerobic digestion are regarded as biological conversion.

(up to 35 wt% yield), while high temperatures (700–1100° C) and short reaction time favor the production of gases (up to 80 wt% yield) [11]. Bio-oil production is normally favored at 500° C,

Biomass Gasification: An Overview of Technological Barriers and Socio-Environmental Impact

http://dx.doi.org/10.5772/intechopen.74191

7

Fermentation is a bio-chemical process which is used for the production of about 80% of the world's ethanol [13]. The main process of fermentation involves using microorganisms to convert sugars into ethanol under a warm and wet environment. The sugar is typically obtained from the mechanical handling (crushing and mixing with water) of sugar-rich crops, such as sugar cane and sugar beet. However, the high cost of sugar-rich crops has diminished its proportion of utilization in fermentation. The starch-based biomass is also commonly used for ethanol production. However, it requires an extra step to convert starch

Anaerobic digestion involves using anaerobic microorganisms to convert biomass into bio-

anaerobic environment, the organic material in biomass is decomposed into usable-sized molecules, such as sugar, as the first step. The sugar molecules is then converted into organic

Gasification process converts biomass, a low-energy density material, into a gaseous product

tial oxidation process and it is commonly operated at 800–900° C for biomass gasification [2]. In some cases, steam is also used as the gasification agents. The gaseous products from the gasifier can be utilized in gas engines or gas turbines for the generation of electricity. In terms of economics, it has also been proven that the performance of a biomass gasification plant with a combined cycle gas turbine (CCGT) is comparable to that of a conventional coal power

The gasifier, as the principle component of a gasification plant, actually provides a space for biomass and gasification agent being mixed to a certain extent, in some cases with catalysts or additives [14]. The different selection of gasifiers is actually responsible for keeping steady the production of syngas regarding the variations of biomass. Literature shows that gasifiers could be categorized into three main types: fixed bed gasifiers, fluidized gasifiers and the

feasible technology and is widely applied in the rural areas of China.

), which is a mixture of CO, H2

as the main gaseous products) by means of decomposition. Under the

, CH4

and CO2

[10]. Gasification is a par-

gas. This process has been proven as a commercially

with very short retention time (<1 s) [12].

into sugar by enzymatic reactions.

acids and further decomposed to CH4

**3. Technologies of biomass gasification**

*2.2.4. Anaerobic digestion*

(LHV at 4–11 MJ/N/m3

plant [7], if not better.

**3.1. Types of gasifiers**

entrained flow gasifiers [15].

and CO2

gas (CH4

*2.2.3. Fermentation*

#### *2.2.1. Direct combustion*

The direct combustion of biomass is widely applied in small-scale cooking and domestic heating by converting chemical energy stored in biomass into heat [9]. In modern industrial technology, combustion is also employed in large-scale applications to produce mechanical power and electricity with the aid of boilers, steam turbines and turbo-generators. The temperature range of biomass combustion is within 800–1000 ° C. Materials with the moisture content higher than 50 wt% are not suitable for combustion processes [3]. The net efficiency of electricity generation from biomass combustion varies between 20 and 40% [8]. The efficiency could be improved either by scaling up the system to over 100 MWe or co-firing with coal (<10 wt% by weight) [10].

#### *2.2.2. Pyrolysis*

Pyrolysis is a thermo-chemical process, in which biomass decomposes into fuel gas, bio-oil and solid char in the absence of oxygen. The selectivity leading to different types of products could be controlled by manipulating the operating conditions (temperature and residence time). Low temperatures (<500° C) and long residence time favor the production of solid char

**Figure 1.** The main processes for the biomass conversion technologies [3].

(up to 35 wt% yield), while high temperatures (700–1100° C) and short reaction time favor the production of gases (up to 80 wt% yield) [11]. Bio-oil production is normally favored at 500° C, with very short retention time (<1 s) [12].

#### *2.2.3. Fermentation*

**2.2. General conversion technologies of biomass except gasification**

regarded as biological conversion.

*2.2.1. Direct combustion*

6 Gasification for Low-grade Feedstock

(<10 wt% by weight) [10].

*2.2.2. Pyrolysis*

For the utilization purpose, the conversion technologies of biomass could be classified in three categories: mechanical extraction; thermo-chemical conversion; and biological conversion, as illustrated in **Figure 1** [3, 8]. Among them, direct combustion, gasification and pyrolysis are considered as the thermo-chemical processes; fermentation and anaerobic digestion are

The direct combustion of biomass is widely applied in small-scale cooking and domestic heating by converting chemical energy stored in biomass into heat [9]. In modern industrial technology, combustion is also employed in large-scale applications to produce mechanical power and electricity with the aid of boilers, steam turbines and turbo-generators. The temperature range of biomass combustion is within 800–1000 ° C. Materials with the moisture content higher than 50 wt% are not suitable for combustion processes [3]. The net efficiency of electricity generation from biomass combustion varies between 20 and 40% [8]. The efficiency could be improved either by scaling up the system to over 100 MWe or co-firing with coal

Pyrolysis is a thermo-chemical process, in which biomass decomposes into fuel gas, bio-oil and solid char in the absence of oxygen. The selectivity leading to different types of products could be controlled by manipulating the operating conditions (temperature and residence time). Low temperatures (<500° C) and long residence time favor the production of solid char

**Figure 1.** The main processes for the biomass conversion technologies [3].

Fermentation is a bio-chemical process which is used for the production of about 80% of the world's ethanol [13]. The main process of fermentation involves using microorganisms to convert sugars into ethanol under a warm and wet environment. The sugar is typically obtained from the mechanical handling (crushing and mixing with water) of sugar-rich crops, such as sugar cane and sugar beet. However, the high cost of sugar-rich crops has diminished its proportion of utilization in fermentation. The starch-based biomass is also commonly used for ethanol production. However, it requires an extra step to convert starch into sugar by enzymatic reactions.

#### *2.2.4. Anaerobic digestion*

Anaerobic digestion involves using anaerobic microorganisms to convert biomass into biogas (CH4 and CO2 as the main gaseous products) by means of decomposition. Under the anaerobic environment, the organic material in biomass is decomposed into usable-sized molecules, such as sugar, as the first step. The sugar molecules is then converted into organic acids and further decomposed to CH4 gas. This process has been proven as a commercially feasible technology and is widely applied in the rural areas of China.
