**2. Gasification of individual biomass feedstocks**

The first study begins with the first set of experimental studies using a 10 kW energy output down-draft gasification system (**Figure 3**). The type and characteristics of biomass can significantly affect the performance of gasification process and consequently result in different reaction temperatures, synthesis gas high heating value and tar content [10–12].

As shown in **Figure 3**, the complete system for syngas production from the gasification unit is composed of six major parts: (1) feeding; (2) main reactor; (3) filtering system, (4) burner assembly, (5) calorimetric units, and (6) purification system.

Feeding consists of a cylindrical painted steel drum bolted to a double layer drying bucket for drying of raw material. The main purpose of the painted steel drum is to increase the biomass storage capacity of the gasifier which multiplies by 6 the continuous operational time in comparison of using only the storage capacity of the main reactor. The drying bucket is used Experimental Observation on Downdraft Gasification for Different Biomass Feedstocks http://dx.doi.org/10.5772/intechopen.77119 83

**Figure 3.** A downdraft gasification unit for producing advanced biofuels (McGill University, Canada).

gasifying agent [9]. The gas composition evolved from biomass gasification strongly depends

The production of renewable energy from biomass using a gasification system is an environmentally friendly method that helps reduce dependence on fossil fuels. Biomass gasification offers advantages over the direct burning of biomass in a boiler. The sustainability of biomass utilization will greatly increase the overall sustainability of biomass management. In this chapter, the technical aspects of sustainable biomass management, with specific focus on recycling and energy recovery via gasification technology, are investigated. The Chapter is consisting of four interconnected studies to examine the basics towards advancements in the

The first study begins with the first set of experimental studies using a 10 kW energy output down-draft gasification system (**Figure 3**). The type and characteristics of biomass can significantly affect the performance of gasification process and consequently result in different

As shown in **Figure 3**, the complete system for syngas production from the gasification unit is composed of six major parts: (1) feeding; (2) main reactor; (3) filtering system, (4) burner

Feeding consists of a cylindrical painted steel drum bolted to a double layer drying bucket for drying of raw material. The main purpose of the painted steel drum is to increase the biomass storage capacity of the gasifier which multiplies by 6 the continuous operational time in comparison of using only the storage capacity of the main reactor. The drying bucket is used

reaction temperatures, synthesis gas high heating value and tar content [10–12].

on the gasification process, the gasifying agent, and the feedstock composition [9].

**2. Gasification of individual biomass feedstocks**

**Figure 2.** Biomass conversion pathways for producing bioenergy.

assembly, (5) calorimetric units, and (6) purification system.

gasification process.

82 Gasification for Low-grade Feedstock

to reduce the moisture content of the biomass that will enter the main reactor. The drying bucket is a double layer container with the biomass feedstock in the middle and hot syngas coming from the cyclone in a separate compartment surrounding the feedstock. From feeding point of view, the capacity of the employed mini-scale gasification system ranges averagely between 2.45 and 3.75 kg hr−1 of biomass. However, this range may change depending on the conditions of the reactor as well as the biomass feedstock.

The main reactor, which is a cylinder-shaped vessel, receives the gasifying medium (i.e. air, oxygen, steam or carbon dioxide) using internal pipes rolled around the reactor. These pipes are used to preheat the air prior the injection at the top of the reduction bell allowing a more stable gasification. Thermocouples are installed at different heights into the reactor to monitor the gasification process. An ignition port provides an access to introduce a flame directly at the top of the reduction bell to start the gasification process. The pressure into the reactor is maintained at atmospheric pressure by an ejector venturi located prior the swirl burner. This negative pressure siphons the syngas produced at the bottom of the reduction bell into the air particulate cyclone. There is a reticular grate under the reduction bell that allows the ash to be separated from the unprocessed feedstock. The ash accumulates under the grate and can be removed from the reactor by the ash trap. The compartment comprised between the reduction bell and the grate is filled with wood charcoal. A bed of pyrolysis materials facilitates the ignition of the system. The compartment between the lid and the reduction bed is filled with the biomass that will be gasified. A manometer is connected to the reactor frame to measure the vacuum pressure inside the reactor. A schematic view from cross section of the reactor along with the different zones during the gasification is shown in **Figure 4**.

the development, detailed technical aspects, and potential failure scenarios of the gasification process using woody biomass. The wood pellets are cylindrical in shape and are characterized according to American Society for Testing and Materials (ASTM) for finding out the

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The pressure fluctuations between the top and bottom pressure sensors cause material jamming such as bridging. Bridging of the feedstock results in a stopping of the downward flow of the pellet biomass inside the main reactor. A schematic of bridging scenario is depicted in **Figure 5a**. As can be seen, the bridging happens in the above-side zones of the reactor where only drying and pyrolysis take place. Bridging starts at early stage of gasification when the raw biomass is not yet influenced by the heat from the combustion zone (**Figure 5b**). The materials are however impacted by the gaseous products generated from the pyrolysis zone (**Figure 5c**). The pyrolysis does not take place completely due to the feedstock bridging. Therefore, the blocked materials do not allow an appropriate flow of heat towards drying and pyrolysis zones. As a result, the level of biomass does not change across the reactor in different times while the materials are slightly impacted by incomplete pyrolysis products and only transforms to darker color after a while. It should be noted that there is no chemical

The gasification temperature controls the equilibrium of the chemical reactions [15, 16]. The results for five different types of feedstocks including switchgrass (representative of energy crops), woody materials (representative of agro-forestry), chicken manure (representative of animal waste) and fiber and cardboard (representative of municipal solid waste) are presented in **Figure 6** [17]. The gas compositions are recorded at five different instant temperatures of 600, 700, 800, 900 and 1000°C during the operation and at an air-biomass ratio of 0.3. All the feedstock reached instantaneous temperature over 1000°C while the average bed temperature measured is different for each feedstock. It is of worth mentioning that depending on the type of the feedstock and the conditions of the reactor in the time of operation, the producer gas flow into the swirl burner rate was recorded averagely in the range of 9.6–11.4 Nm<sup>3</sup> hr−1.

The desired element in the syngas composition is producing hydrogen (first preference) and then carbon monoxide (second preference). The production of hydrogen is so appropriate for using in the secondary combustion chamber downstream to the gasification unit for conditioning the syngas and producing ultimate end-products. This, however, never happens in practice to have only these two elements in the syngas. There have been numerous studies working on the composition of syngas [17–21] at which there are always four components hydrogen, carbon monoxide, carbon dioxide and methane reported. Majority

**Properties (%) Value Properties (%) Value** Moisture content 5.2 Carbon 42.64 Ash content 1.2 Hydrogen 8.5 Fixed carbon content 21.1 Oxygen 42.40 Volatile matter 72.45 Nitrogen 0.06

**Table 1.** Proximate and ultimate analysis of pelletized woody biomass.

proximate and ultimate analysis (**Table 1**).

transformation happening under this scenario.

The filtering system consists of an air particulate cyclone and a charcoal filtering system. The filtration process assists to remove particulates of larger diameters than 5–25 μm. The efficiency of the particulate removal varies between 50 and 90% depending on the conditions of the gaseous entered into the cyclone.

The burner assembly consists of an ejector venturi and a swirl burner. Compressed air is provided to the ejector venturi by a compressor capable of delivering an air flow between 10.2 and 13.6 m<sup>3</sup> hr−1 at a pressure of 750 kPa as specified by the manufacturer's recommendations.

The calorimetric unit consists of an insulated drum filled with water in which a spiral chimney is installed. The swirl burner is connected at the bottom of the drum to the chimney. The flue gas produced by syngas-air combustion travels inside the spiral chimney and transfers its heat to the water surrounding the chimney. The calorific value of the syngas is calculated based on the calculation of the rejected heat from the flue gas and the change in temperature of the water [13].

Low energy density of biomass is a major restriction for using biomass which typically ranges from 60 to 400 kg m−3 for different feedstocks [10]. There are also two operational parameters bed temperature and pressure across the reactor which affects the process of gasification as well as ultimate heating values and the syngas composition. The pressure gradient, which is monitored using pressure sensors positioned at different levels across the reactor, is a function of system configuration, geometry, feedstock porosity, permeability and physical properties of the feedstock [14].

A variety of biomass feedstock is deployed for the gasification process. In the first step, pelletized woody biomass is selected as the primary biomass of interest to run in the reactor. Forests are a major source of wealth for Canadians, providing a wide range of economic, social and environmental benefits. Therefore, choosing woody biomass to run the gasifier for baseline tests matches with the available natural resources in Canada. This section elaborates

**Figure 4.** (a) Cross section view of the main reactor of the downdraft gasifier, (b) different zones across the gasifier [10].

the development, detailed technical aspects, and potential failure scenarios of the gasification process using woody biomass. The wood pellets are cylindrical in shape and are characterized according to American Society for Testing and Materials (ASTM) for finding out the proximate and ultimate analysis (**Table 1**).

vacuum pressure inside the reactor. A schematic view from cross section of the reactor along

The filtering system consists of an air particulate cyclone and a charcoal filtering system. The filtration process assists to remove particulates of larger diameters than 5–25 μm. The efficiency of the particulate removal varies between 50 and 90% depending on the conditions of

The burner assembly consists of an ejector venturi and a swirl burner. Compressed air is provided to the ejector venturi by a compressor capable of delivering an air flow between 10.2 and 13.6 m<sup>3</sup> hr−1 at a pressure of 750 kPa as specified by the manufacturer's recommendations. The calorimetric unit consists of an insulated drum filled with water in which a spiral chimney is installed. The swirl burner is connected at the bottom of the drum to the chimney. The flue gas produced by syngas-air combustion travels inside the spiral chimney and transfers its heat to the water surrounding the chimney. The calorific value of the syngas is calculated based on the calculation of the rejected heat from the flue gas and the change in temperature of the water [13]. Low energy density of biomass is a major restriction for using biomass which typically ranges from 60 to 400 kg m−3 for different feedstocks [10]. There are also two operational parameters bed temperature and pressure across the reactor which affects the process of gasification as well as ultimate heating values and the syngas composition. The pressure gradient, which is monitored using pressure sensors positioned at different levels across the reactor, is a function of system configuration, geometry, feedstock porosity, permeability and physical proper-

A variety of biomass feedstock is deployed for the gasification process. In the first step, pelletized woody biomass is selected as the primary biomass of interest to run in the reactor. Forests are a major source of wealth for Canadians, providing a wide range of economic, social and environmental benefits. Therefore, choosing woody biomass to run the gasifier for baseline tests matches with the available natural resources in Canada. This section elaborates

**Figure 4.** (a) Cross section view of the main reactor of the downdraft gasifier, (b) different zones across the gasifier [10].

with the different zones during the gasification is shown in **Figure 4**.

the gaseous entered into the cyclone.

84 Gasification for Low-grade Feedstock

ties of the feedstock [14].

The pressure fluctuations between the top and bottom pressure sensors cause material jamming such as bridging. Bridging of the feedstock results in a stopping of the downward flow of the pellet biomass inside the main reactor. A schematic of bridging scenario is depicted in **Figure 5a**. As can be seen, the bridging happens in the above-side zones of the reactor where only drying and pyrolysis take place. Bridging starts at early stage of gasification when the raw biomass is not yet influenced by the heat from the combustion zone (**Figure 5b**). The materials are however impacted by the gaseous products generated from the pyrolysis zone (**Figure 5c**). The pyrolysis does not take place completely due to the feedstock bridging. Therefore, the blocked materials do not allow an appropriate flow of heat towards drying and pyrolysis zones. As a result, the level of biomass does not change across the reactor in different times while the materials are slightly impacted by incomplete pyrolysis products and only transforms to darker color after a while. It should be noted that there is no chemical transformation happening under this scenario.

The gasification temperature controls the equilibrium of the chemical reactions [15, 16]. The results for five different types of feedstocks including switchgrass (representative of energy crops), woody materials (representative of agro-forestry), chicken manure (representative of animal waste) and fiber and cardboard (representative of municipal solid waste) are presented in **Figure 6** [17]. The gas compositions are recorded at five different instant temperatures of 600, 700, 800, 900 and 1000°C during the operation and at an air-biomass ratio of 0.3. All the feedstock reached instantaneous temperature over 1000°C while the average bed temperature measured is different for each feedstock. It is of worth mentioning that depending on the type of the feedstock and the conditions of the reactor in the time of operation, the producer gas flow into the swirl burner rate was recorded averagely in the range of 9.6–11.4 Nm<sup>3</sup> hr−1.

The desired element in the syngas composition is producing hydrogen (first preference) and then carbon monoxide (second preference). The production of hydrogen is so appropriate for using in the secondary combustion chamber downstream to the gasification unit for conditioning the syngas and producing ultimate end-products. This, however, never happens in practice to have only these two elements in the syngas. There have been numerous studies working on the composition of syngas [17–21] at which there are always four components hydrogen, carbon monoxide, carbon dioxide and methane reported. Majority


**Table 1.** Proximate and ultimate analysis of pelletized woody biomass.

**Figure 5.** (a) Schematic of bridging formation across the reactor [10], (b) bridging after 30 minutes from beginning of the gasification process, (c) bridging after 90 minutes from beginning of the gasification process.

of studies have shown only up to 50% of the syngas composition filled with hydrogen and carbon monoxide, and the rest is contaminated with the other elements. As can be seen in **Figure 6**, energy crops (switchgrass) along with municipal solid waste (fiber and cardboard) showed promising performance in hydrogen and carbon monoxide. It should be also noted that the ratio of hydrogen to carbon monoxide is an important factor for condition the syngas. Madadian et al. reported that a higher percentage of hydrogen to carbon monoxide does not necessarily indicated a rich syngas [17]. This could be the main reason that the gas needs a secondary conditioning after production. Furthermore, the syngas heating values measured using the calorimetric unit fell in the range 16.84–9.24 MJ kg−1 which belongs to switchgrass and chicken manure, respectively. The heating values of the other biomass were recorded at 15.7, 14.45, 14.19 and 13.94 MJ kg−1 for hardwood, cardboard, softwood and fiber. The values may be found to some extent lower from what is reported in literature which is mainly attributed to the calorimetric unit errors which was developed by the author.

For the abovementioned biomass feedstocks in this section, the ratio of hydrogen to carbon monoxide is presented in the range of 0.15–1.5 under different temperatures. As shown in **Figure 6**, there is a proportional relationship between the value of hydrogen to carbon monoxide and the temperature profile within the reactor. In higher temperature, the

amount of carbon monoxide drops which can be related to the reduced environment at higher elevation within the reactor. The step-by-step procedure under which this observed

**Figure 6.** The recorded variations in syngas composition with respect to temperature profile during gasification of

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(a) switchgrass, (b) chicken manure, (c) soft wood, (d) hardwood, (e) fiber, and (f) cardboard [17].

phenomenon took place is explain below:

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of studies have shown only up to 50% of the syngas composition filled with hydrogen and carbon monoxide, and the rest is contaminated with the other elements. As can be seen in **Figure 6**, energy crops (switchgrass) along with municipal solid waste (fiber and cardboard) showed promising performance in hydrogen and carbon monoxide. It should be also noted that the ratio of hydrogen to carbon monoxide is an important factor for condition the syngas. Madadian et al. reported that a higher percentage of hydrogen to carbon monoxide does not necessarily indicated a rich syngas [17]. This could be the main reason that the gas needs a secondary conditioning after production. Furthermore, the syngas heating values measured using the calorimetric unit fell in the range 16.84–9.24 MJ kg−1 which belongs to switchgrass and chicken manure, respectively. The heating values of the other biomass were recorded at 15.7, 14.45, 14.19 and 13.94 MJ kg−1 for hardwood, cardboard, softwood and fiber. The values may be found to some extent lower from what is reported in literature which is mainly attributed to the calorimetric unit errors which was

**Figure 5.** (a) Schematic of bridging formation across the reactor [10], (b) bridging after 30 minutes from beginning of the

gasification process, (c) bridging after 90 minutes from beginning of the gasification process.

For the abovementioned biomass feedstocks in this section, the ratio of hydrogen to carbon monoxide is presented in the range of 0.15–1.5 under different temperatures. As shown in **Figure 6**, there is a proportional relationship between the value of hydrogen to carbon monoxide and the temperature profile within the reactor. In higher temperature, the

developed by the author.

86 Gasification for Low-grade Feedstock

**Figure 6.** The recorded variations in syngas composition with respect to temperature profile during gasification of (a) switchgrass, (b) chicken manure, (c) soft wood, (d) hardwood, (e) fiber, and (f) cardboard [17].

amount of carbon monoxide drops which can be related to the reduced environment at higher elevation within the reactor. The step-by-step procedure under which this observed phenomenon took place is explain below:

