Biomass Gasification

The conversion of solid and liquid organic compounds into the gaseous and solid states is the primary objective of the gasification process. The gaseous form of the product is generally termed as “syngas,” while the solid form is “char,” which consists of an inert and unconverted fraction of organic material present in biomass. The biomass gasification process is partial oxidation of biomass at high temperature, so the process is also called a thermochemical conversion process. The product gas has high calorific value and can generate power. This process helps in proper utilization of biomass available for all-around production of energy. It has high potential to be used similarly to fossil fuels and can control the continuous increase in the price of oil and fossil fuel products. This section mainly deals with the chemistry of gasification, gasification media, the equivalence ratio (ER), temperature, the pressure inside the gasifier, and the steam-to-biomass ratio (Molino, Chianese, and Musmarra 2016).

Gasification Chemistry

Generally, the chemical reaction that takes place during gasification is endothermic. The main steps of the gasification process are as follows (Molino, Chianese, and Musmarra 2016):

  • • Oxidation (exothermic process)
  • • Drying (endothermic process)
  • • Pyrolysis (endothermic process)
  • • Reduction (endothermic process)

Oxidation steps: The oxidation of biomass is carried out in the partial presence of oxygen in order to maintain the stoichiometric ratio for oxidizing only part of the fuel. The main reactions that take place during this process are the follow'ing:

(1) Char combustion

(2) Partial oxidation

(3) Hydrogen combustion

These steps are mainly responsible for supplying the energy required for the entire process, and the product of combustion is a mixture of CO, C02, and H20; in this mixture, nitrogen from the air is used instead of pure oxygen.

Drying steps: In drying steps, the moisture present in feedstocks is mostly evaporated. The amount of heat needed for drying is proportional to the presence of moisture in the feedstock.

Pyrolysis steps: In these steps, the thermochemical decomposition of biomass takes place, and solids are converted into char and volatiles. The main reaction which shows the entire pyrolysis phenomenon is the following:

Reduction steps: In reduction steps, all the products of the above steps are involved; the gas mixture and char react w'ith each other, which gives the final product: syngas. The reactions involved in these steps are the following:

(1) Boudouard reaction

(2) Char reforming reaction

(3) Water-gas shift reaction

(4) Methanation reaction

The Boudouard reaction and the char reforming reaction are endothermic, while the water-gas shift reaction and the methanation reaction are exothermic in nature; the overall reduction reaction is endothermic. The temperature at which the reduction reaction is carried out plays a significant role in determining the composition of syngas and their characteristics (Molino, Chianese, and Musmarra 2016).

Gasifying Medium

Gasifying agents react with solid carbon and higher hydrocarbon to reduce them into lower-molecular-weight compounds like CO, H2, and CH4. The main gasifying agents used for gasification are the following:

  • (1) Steam
  • (2) Oxygen
  • (3) Air

Steam and oxygen are the important gasifying media, which lead to the production of a mixture of syngas with a heating value ranging between 10 and 15 MJ/ Nm and thus are considered to be desirable properties for the synthesis processes (Ramalingam, Rajendiran, and Subramiyan 2020).

Steam helps in the steam-methane reforming reaction, so it is proved to be better for the adjustment of the CO/H2 ratio in syngas. Also, steam carries heat as it enters the gasifier as a saturated state. If steam is the only gasifying medium, then it needs an external heat supply for maintaining the reactor temperature. Oxygen is used as a gasifying medium as it oxidizes and reacts with carbon-containing compounds via the exothermic route and supplies heat for the gasification process. Thus, with the addition of oxygen, the gasifier becomes autothermal. Air is also a good oxidizing agent, as it is cheap and abundantly available in the atmosphere, but the problem with the use of air is the presence of nitrogen, which reduces the heating value of the syngas (Ramalingam, Rajendiran, and Subramiyan 2020).

Equivalence Ratio

The equivalence ratio is a parameter that indicates quantitatively whether a fuel oxidizer mixture is lean, rich, or stoichiometric. The equivalence ratio is defined by the following equation (Jangsawang, Laohalidanond, and Kerdsuwan 2015)

Three different cases that come into the picture are below.

  • (1) ER >1, fuel rich mixture
  • (2) ER <1, lean fuel mixture
  • (3) ER = 1, stoichiometric mixture

The main motive behind the gasification process is to produce synthesis gas by varying the equivalence ratio and gasifying agent temperature. For optimization of the gasification, process ER is critical. Two different cases of chemical equilibrium in gasification processes are identified. The first case is with an excess of carbon present in the gasification process, while the second is with an excess of a gasifying agent with all carbon gasified. In conclusion, the optimized values of the ER are given in the table below (Jangsawang, Laohalidanond, and Kerdsuwan 2015).

Temperature range (К)

ER

600-900

3.0

1000-1500

2.0

1600-2500

1.5

Gasifier Temperature

The equivalence ratio is the main factor that may affect the gasifier temperature. With an increase in the equivalence ratio, the combustion process is enhanced, and thus the gasifier temperature increases in the combustion or oxidation zone. The increase in gasifier temperature helps in cracking of tar, which improves the endothermic char gasification, which results in the production of producer gas with a higher heating value. Mostly, the oxidation zone has the highest temperature as compared to other zones, and the temperature is always changing during gasification in all zones. This happens because of the self-regulating property of the gasifier (Guo et al. 2014).

Gasifier Design

Gasifiers are classified into two main categories, namely fixed bed gasifiers and fluidized bed gasifiers. Their designs are mainly based on certain parameters like gasification chemistry, gasifying media, equivalence ratio, steam-to-biomass ratio, gasifier temperature, and pressure. These parameters are also responsible for product yields. Further, fixed bed gasifiers are broadly classified into three main categories, namely (Prins, Ptasinski, and Janssen 2007)

  • 1. Updraft gasifiers,
  • 2. Downdraft gasifiers,
  • 3. Cross draft gasifiers.

And fluidized bed gasifiers are classified into two main categories, namely

  • 1. Circulating fluidized bed gasifiers,
  • 2. Bubbling fluidized bed gasifiers.

Fixed Bed Gasifiers

The main parts of the fixed bed gasification system consist of a gasifier or reactor having a cleaning and gas cooling system. The gas and gasifying media may move in an upward or downward direction through the bed of solid fuel particles in the fixed bed gasifier. Its construction is simple, consisting of cylindrical space for fuel and a gasifying agent having a feeding unit, ash collection unit, and gas exit unit. The material used for the construction of a fixed bed gasifier system is concrete, steel, and/or firebricks. As gasification occurs in the fixed bed gasifier, the fuel bed moves slowly in a downward direction (Prins, Ptasinski, and Janssen 2007). The fixed bed gasifier generally operates with high conversion of carbon, high residence time, low gas velocity, and low ash flow as mentioned in Golden, Reed and Das (1988) and Carlos (2005). The major problem occurring in fixed bed gasifiers is with the removal of tar; however, current development in thermal and catalytic conversion of tar gives huge options. The fixed bed gasifier is more suitable for small-scale applications of heat and power. The main equipment used for the gas cleaning and cooling system is a cyclone separator, a dry filter, and wet scrubbers as mentioned in Ghosh, Sagar, and Kishore (2006).

Updraft Gasifier

The updraft gasifier, also called a counter-current gasifier, is a type of fixed bed gasifier which can use solid biomass having moisture content up to 60%, low volatile matter, and high ash content up to 25%. Some of the advantages of the updraft gasifier are as follows (Ayyadurai, Schoenmakers, and Hernandez 2017).

  • 1. Good thermal efficiency
  • 2. Suitable for the high value of moisture in feedstocks
  • 3. Low pressure drops across the reactor
  • 4. Lower slag formation.

Direct firing, in which gases produced are directly utilized in a furnace or boiler, is used in the updraft gasifier. In the updraft gasifier, biomass is fed from the top and the gasifying medium is fed from the bottom of the gasifier. In this updraft gasifier product, gas comes out from the top and ash releases from the bottom. In designing the updraft gasifier, more attention is focused on the amount of tar in the final product (Ayyadurai, Schoenmakers, and Hernandez 2017).

The biomass fed into the reactor is slowly burned with air. The upper layer of biomass is cracked first, followed by the next layer, and both layers are converted into char. The directional movement of tar is from bottom to top. In the pyrolysis zone, the temperature is around 600-800°C, where most of the tar was thermally braked, which results in tar-free product gas. The liquid fraction reaches a temperature of around 500°C, which shows that gas production was dominant because the liquid fraction is thermally cracked above 500°C. The main benefit which comes into the picture is minimization of tar content, which leads to improving the composition of the producer gas. Tar cracking is observed between 700 and 1250°C (Ciferno and Marano 2002).

The gasifier design focuses on the process and hardware. The factors under process include the following parameters.

  • 1. Gasifier types,
  • 2. Producer gas yield,
  • 3. Operating conditions,
  • 4. Reactor size.

According to Ciferno and Marano (2002) hardware consists of the following mechanical components:

  • 1. Grate,
  • 2. Reactor body,
  • 3. Insulation,
  • 4. Other components that are specific to reactors.

During the design of the gasifier, the specification of the plant plays a significant role; specification factors include fuel, gasification agent, and product gases. Generally, fuel specification consists of ultimate and proximate analysis. The specification for gasifying agents includes the selection of steam, oxygen, or air and their proportions (Sansaniwal, Rosen, and Tyagi 2017).

The parameters that drastically affect the design of updraft gasifiers are the following:

  • 1. Gasifying agent: if air is used as a gasifying agent, the product gas lower heating value is in the range of 45 MJ/m3 (Roy, Datta, and Chakraborty 2013). High moisture and oxygen content in biomass results in lower heating value.
  • 2. The equivalence ratio is the major parameter in conversion efficiency.
  • 3. Capital cost; capital cost is least for air, followed by steam and pure oxygen.

Table 9.1 lists geometric, operating, and performance output parameters for the designed process.

According to Basu (2010) gasifier design generally begins with mass and energy balance. The mass and energy balance is common to all types of gasifiers. It involves the calculation of producer gas flow rate and biomass feed rate. The power output

TABLE 9.1

Performance Parameters for Optimum Output

Geometric parameter

Operating parameter

Performance parameters

Reactor configuration

Preheat temperature of the air

Carbon conversion efficiency

Height

Reactor temperature

Cold-gas efficiency

Cross-sectional area

Amount and approximate quantity of gasifying medium

required for gasification is a primary input parameter denoted by Q in megawatts (MW). The volume flow rate of the producer gas, Vg (Nm3/s), can be found by the following relationship.

The net heating value can be calculated by using the composition of the gas. The equation for finding the biomass feed rate Mf is as follows:

The equation that is used to relate LHVand HHVis given by the following relationship.

Where, H = hydrogen mass fraction in the fuel, M = moisture mass fraction, and HHV is in kJ/kg on a dry moisture ash-free basis. The relationship between HHV on a moisture ash-free basis and that on a dry basis is given by the following equation (Basu 2010).

Where subscript d refers to dry, ASH=ash fraction in fuel on a raw fuel basis, and M= moisture fraction. HHVd is generally taken as 1821 MJ/kg. Finding HHVd from the elemental composition of biomass is done by the following equation:

Where С, H, S, N, O, and ASH represent the fractions of carbon, hydrogen, sulfur, nitrogen, oxygen, and ash, respectively, in the fuel on a dry basis.

The stoichiometric air requirement of fuel for complete combustion is related as follows:

Where Ma = amount of air required for unit mass of biomass, M„, = theoretical amount of air for the unit mass of biomass, and ER = equivalence ratio. For the gasification of biomass, 0.25 ER is the initial guess.

The Calculation for Reactor Diameter (D)

The size of the reactor, especially the cross-section of a cylinder, is calculated in terms of the diameter in which the fuel is being burned. This is given in terms of fuel consumed per unit time to the specific gasification rate of the fuel ranging from 100 to 250 kg/m2-h. The reactor diameter is calculated using the following relationship:

Where FCR = fuel consumption rate, and SGR=specific gasification rate given by the following:

The Calculation for Height (H)

The distance between the top and bottom ends of the reactor is the height of the reactor. The height will determine how long the gasifier will operate with a single load of fuel. Generally, it is the function of the time required to operate the gasifier (T), the specific gasifier rate, and the density of the fuel. The following relationship can be used to calculate the height of the gasifier:

For an operating time of 2.5 h, the density of the fuel is assumed to be 300 kg/m3. Time is the total time required to completely convert fuel into gases inside the reactor. This is the combination of time for igniting the fuel, generating the gas, and completely burning the fuel inside the reactor. It is the function of the density of the fuel, reactor volume, and fuel consumption rate. The relationship used for calculation is given by equation 9.20 (also see Table 9.2):

Where symbols have the usual meaning.

TABLE 9.2

Dimensions of Updraft Gasifier

D

0.6 m

H

1.0 m

T

2.45 hour

FCR

30 kg/hr

AFR

0.347 mVs

The Calculation for Airflow Rate (AFR)

The rate of flow of air required to gasify the fuel is called the AFR. It helps in determining the size of blower required for a reactor in gasification of fuel. This can be determined using FCR, the stoichiometric air of fuel (SA), the density of air (p), and ER (for wood 0.3 to 0.5). The formula used for calculation is given by the following relationship (Basu 2010):

Downdraft Gasifier

On the basis of utility, downdraft gasifiers are more prominent in small-scale applications. Their capacities range between Ю kW and l MW. Their construction and operation are simple and they contain less tar in producer gas. However, some of their disadvantages are below:

  • 1. Grate blocking
  • 2. Feedstock should have low bulk density
  • 3. Only suitable for feedstock with lower moisture content.

Improvement in the design of downdraft gasifiers can be based on feeding system modification, air supply, producer gas recirculating system, and discharge system. The downdraft or co-current gasifier operates from top to bottom from the drying zone followed by pyrolysis, oxidation, and reduction zones. The heat generated in the oxidation zone helps in drying or reducing the moisture content of biomass. Also, the excess heat helps in the pyrolysis and reduction process. During volatile pyrolysis, the matter is released. The producer gas obtained during the reduction process comes out through the gas outlet (Susastriawan, Saptoadi, and Purnomo 2017).

The parameters playing a crucial part in the design of downdraft gasifier, resulting in high performance in terms of the quality of the producer gas and gasification efficiency, are mentioned below:

  • 1. Biomass characteristics such as composition, moisture content, and size
  • 2. Air-fuel ratio
  • 3. Gasification temperature
  • 4. Feeding rate.

The equation for gasification efficiency estimation is given by the following relationship.

where HVpg represents the heating value of producer gas (kJ/Nm3) and HVb represents the heating value of biomass feedstocks (kJ/kg).

The heating value of producer gas can be evaluated using the following relationship.

Where x,, ,v2, and x3 are the percentages of CO, H2, and CH4 respectively in producer gas. These are measured using a gas chromatograph. The heating values of CO, H2, and C#4 are 12.71, 12.78, and 39.76 MJ/m respectively (Susastriawan, Saptoadi, and Purnomo 2017).

Cross Draft Gasifier

The higher exit temperature and poor carbon dioxide reduction are the major disadvantage of the cross draft gasifier. The ash pin, fire, and reduction zones are separated in cross draft gasifiers, unlike downdraft and updraft gasifiers. The design characteristic of the cross draft gasifier is to use wood, charcoal, and coke as a biomass feedstock for gas generation. It uses charcoal gasification with a high temperature of 1500°C in the oxidation zone, which can lead to material problems. The gas composition obtained from the cross draft gasifier is of low hydrogen and methane content, with higher carbon monoxide content (Golden, Reed, and Das 1988). The disadvantages of cross draft gasifiers are minimal tar cracking and very high temperatures in the oxidation zone.

Fluidized Bed Gasifier

The fluidized bed gasifier technique is a recent development offering excellent mixing and a high reaction rate for solid-to-gas conversion. Isothermal operation is an advantage of the fluidized bed gasifier. The operating temperature of the gasifier is around 800-850°C. To keep operating temperature in a state of suspension, air is blown through a bed of solid particles at a particular velocity. After reaching the optimum temperature, the biomass feedstock is fed into the gasifier (Siedlecki, de Jong, and Verkooijen 2011).

9.3.2.1 Circulating Fluid Bed Gasifier

To entrain large amounts of solids, the fluidizing velocity in the circulating fluid bed is high enough to convert the extensively developed wood waste conversion in pulp and paper mills for firing lime and cement kilns and steam generation for electricity (Raskin, Palonen, and Nieminen 2001).

9.3.2.2 Twin Fluid Bed Gasifier

To obtain a high heating value of gas, twin fluid bed gasifier is employed. It consists of two defined zones. In the upper zone, drying, low-temperature carbonization, and cracking of gases will take place. But in the lower zone, gasification of charcoal takes place. The temperatures of the gasifier’s upper and lower zones are around 460 and 520°C, respectively. The pressure is around 30 mbar. To encourage carbon-steam reactions and hydrogen enrichment, steam is added to the reactions. The quality of gas is very clean with high energy content (Butt et al. 1991).

9.3.1.3 Entrained Bed Gasifier

In entrained flow gasifiers, a finely reduced feedstock is required, but no inert material is present. The entrained bed gasifiers operate at a very high temperature of 1200°C, depending on gasification medium employed, and hence the producer gas obtained from gasifier has a low concentration of tar and condensable gases. However, due to the operation of higher temperatures, material selection and ash melting are major problems in this gasifier. A conversion efficiency of 100% is reported for this gasifier, but not with the biomass feedstock (Bridgwater 1995).

 
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