**3. The gasification of digestate**

#### **3.1. Configuration of gasification system**

Main features of the research gasification reactor:


**Figure 6** shows schematic diagram of research gasification reactor. The reactor construction was mounted on strain gauges to real-time measurement of the reactor mass. This allows to determine the conversion speed of the batch material. The temperature measurement is carried out in four gasification zones and additionally in the outlet of syngas by thermocouples. Gasification agent is fed into the reactor by a side channel compressor, volumetric flow rate is measured by a rotameter. Syngas composition and calorific value are measured using the industrial GAS 3100R Syngas Analyzer.

**Figure 6.** Schematic diagram of gasification reactor.

Reactor construction can be divided into:


**Figure 7** shows 3D model of the gasification reactor with support frame. Biomass for the reactor is provided by screw feeder to the hopper and then through two knife gate valve to the interior of the reactor chamber. Such construction of the biomass feed system ensures the tightness of the installation.

**Figure 7.** 3D model of gasification reactor.

**Figure 6.** Schematic diagram of gasification reactor.

Digestate after thermal drying process has been pelletized to increase of bulk density. Energy

**Figure 6** shows schematic diagram of research gasification reactor. The reactor construction was mounted on strain gauges to real-time measurement of the reactor mass. This allows to determine the conversion speed of the batch material. The temperature measurement is carried out in four gasification zones and additionally in the outlet of syngas by thermocouples. Gasification agent is fed into the reactor by a side channel compressor, volumetric flow rate is measured by a rotameter. Syngas composition and calorific value are measured using the

consumption for pelletization process was 0.085 kWh.

**3. The gasification of digestate**

198 Gasification for Low-grade Feedstock

**3.1. Configuration of gasification system**

Main features of the research gasification reactor:

• Reactor without "throat" in oxidation zone

• Fixed bed reactor

• Thermal power about 200 kW

• Gasification agent—atmospheric air

industrial GAS 3100R Syngas Analyzer.

• Reactor was designed and constructed as a downdraft

**Figure 8.** Air supply system to gasification reactor.

The gasification agent supply system consists of four nozzles arranged at the periphery of the oxidation zone (**Figure 8**). Atmospheric air as a gasification agent is forced into the oxidation zone of the reactor through a piping system using a side channel compressor. In order to achieve greater uniformity of the aeration, the nozzles were made at an angle of 15° to produce a vortex of air inside the reactor chamber.

**3.3. The gasification process**

**Figure 9.** Ash removal unit from gasification reactor.

average 26.63 kg/h.

The main purpose of the experiments was to evaluate the gasification potential of the fuel produced from digestate and to analyze the obtained syngas. Gasification process was started using a wood pellet. After stabilizing the reactor work, supplying fuel from the digestate was

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has been setup to maintain fuel mass of about 30–31 kg. Syngas from gasification process was

The results of the gasification process of the fuel produced from digestate include composition of syngas, calorific value (**Figure 12**), and the temperature in the oxidation zone of the process (**Figure 13**). In addition, the mass of the reactor measured by strain gauges during process is presented. From the chart, it is possible to read the conversion rate of biomass to syngas. For comparison purposes, the **Table 5** also shows composition of syngas from wood pellets.

During digestate gasification process a high fluctuation of temperature was observed in the oxidation zone of the reactor. Fluctuations reached ±50°C (**Figure 13**). The average temperature in the oxidation zone was 940°C. **Figure 14** shows fuel mass change in reactor chamber during gasification process. The thermal conversion speed of the digestate to syngas was

Due to the low melting temperature of ash from digestate, during gasification process, ash slagging caused some problems. Because of this, temperature fluctuations in the oxidation zone probably occurred. During longer work of reactor, conglomerate prevented the

/h. The dispenser feeder

started. Process was carried out with air volumetric flow rate of 30 m<sup>3</sup>

burned in atmospheric conditions immediately after leaving the reactor.

The ash removal unit from reactor consists of a rotary grate integrated with an ash scraper and a screw conveyor (**Figure 9**). The ash removal rate mainly depends on rotational speed of the grate. The holes of grate were made in the form of cones increasing their diameter toward the bottom. Such a construction of the holes results in a lower risk of collimation and enables free removal of ash from the reactor space.

In order to achieve a high level of tar conversion, reactor structure was extended in relation to the diameter to increase the gas flow time through the hot zone. In the research reactor the internal diameter D = 300 mm, while the length of the gasification chamber L = 1200 mm.

#### **3.2. Energy consumption for substrate preparation to gasification process**

The substrate of the gasification process was pelletized digestate, after mechanical separation process, biodrying process, and thermal drying process. **Figure 10** shows energy consumption for each stage of preparation process to prepare 1 kg solid fuel from digestate. **Figure 11** shows picture of pelletized digestate. The total energy consumption for production 1 kg of fuel was 0.2545 kWh/kg. The largest part (about 53.43%) of energy was spent on thermal drying process in tubular dryer. Dehydration of digestate in mechanical separation process and in biodrying process was characterized by high energy efficiency of the processes.

**Figure 9.** Ash removal unit from gasification reactor.

#### **3.3. The gasification process**

The gasification agent supply system consists of four nozzles arranged at the periphery of the oxidation zone (**Figure 8**). Atmospheric air as a gasification agent is forced into the oxidation zone of the reactor through a piping system using a side channel compressor. In order to achieve greater uniformity of the aeration, the nozzles were made at an angle of 15° to pro-

The ash removal unit from reactor consists of a rotary grate integrated with an ash scraper and a screw conveyor (**Figure 9**). The ash removal rate mainly depends on rotational speed of the grate. The holes of grate were made in the form of cones increasing their diameter toward the bottom. Such a construction of the holes results in a lower risk of collimation and enables

In order to achieve a high level of tar conversion, reactor structure was extended in relation to the diameter to increase the gas flow time through the hot zone. In the research reactor the internal diameter D = 300 mm, while the length of the gasification chamber L = 1200 mm.

The substrate of the gasification process was pelletized digestate, after mechanical separation process, biodrying process, and thermal drying process. **Figure 10** shows energy consumption for each stage of preparation process to prepare 1 kg solid fuel from digestate. **Figure 11** shows picture of pelletized digestate. The total energy consumption for production 1 kg of fuel was 0.2545 kWh/kg. The largest part (about 53.43%) of energy was spent on thermal drying process in tubular dryer. Dehydration of digestate in mechanical separation process and

**3.2. Energy consumption for substrate preparation to gasification process**

in biodrying process was characterized by high energy efficiency of the processes.

duce a vortex of air inside the reactor chamber.

**Figure 8.** Air supply system to gasification reactor.

200 Gasification for Low-grade Feedstock

free removal of ash from the reactor space.

The main purpose of the experiments was to evaluate the gasification potential of the fuel produced from digestate and to analyze the obtained syngas. Gasification process was started using a wood pellet. After stabilizing the reactor work, supplying fuel from the digestate was started. Process was carried out with air volumetric flow rate of 30 m<sup>3</sup> /h. The dispenser feeder has been setup to maintain fuel mass of about 30–31 kg. Syngas from gasification process was burned in atmospheric conditions immediately after leaving the reactor.

The results of the gasification process of the fuel produced from digestate include composition of syngas, calorific value (**Figure 12**), and the temperature in the oxidation zone of the process (**Figure 13**). In addition, the mass of the reactor measured by strain gauges during process is presented. From the chart, it is possible to read the conversion rate of biomass to syngas. For comparison purposes, the **Table 5** also shows composition of syngas from wood pellets.

During digestate gasification process a high fluctuation of temperature was observed in the oxidation zone of the reactor. Fluctuations reached ±50°C (**Figure 13**). The average temperature in the oxidation zone was 940°C. **Figure 14** shows fuel mass change in reactor chamber during gasification process. The thermal conversion speed of the digestate to syngas was average 26.63 kg/h.

Due to the low melting temperature of ash from digestate, during gasification process, ash slagging caused some problems. Because of this, temperature fluctuations in the oxidation zone probably occurred. During longer work of reactor, conglomerate prevented the

**Figure 10.** Scheme of digestate processing and energy consumption of each processing stages.

gasification reactor from working properly. A long-term operation of digestate gasification process would be possible by modifying the ash removal system for example by eliminating the solid grate. Some technological solutions of gasification reactors include a system of injecting pure oxygen below the oxidation zone. Molten ash is removed from reactor by

**Figure 13.** Temperature in oxidation zone during gasification process.

**Figure 12.** Syngas composition and its calorific value from the digestation gasification process.

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**Figure 11.** Photo of pelletized digestate—fuel for gasification reactor.

**Figure 12.** Syngas composition and its calorific value from the digestation gasification process.

**Figure 13.** Temperature in oxidation zone during gasification process.

**Figure 11.** Photo of pelletized digestate—fuel for gasification reactor.

**Figure 10.** Scheme of digestate processing and energy consumption of each processing stages.

202 Gasification for Low-grade Feedstock

gasification reactor from working properly. A long-term operation of digestate gasification process would be possible by modifying the ash removal system for example by eliminating the solid grate. Some technological solutions of gasification reactors include a system of injecting pure oxygen below the oxidation zone. Molten ash is removed from reactor by


The result of the research of the gasification of dried digestate was gaseous fuel that does not differ from the quality of fuels obtained from the thermal treatment of other types of biomass.

Small-Scale Energy Use of Agricultural Biogas Plant Wastes by Gasification

be used for combustion in the engine or turbine systems; however, they require adequate

\* and Janusz Piechocki<sup>1</sup>

1 Faculty of Technical Sciences, Department of Electric and Power Engineering, University

2 Faculty of Technical Sciences, Department of Systems Engineering, University of Warmia

[1] Börjesson P, Berglund M. Environmental systems analysis of biogas systems—Part I: Fuel-cycle emissions. Biomass & Bioenergy. 2006;**30**(5):469-485. DOI: 10.1016/j.biombioe.

[2] Börjesson P, Berglund M. Environmental systems analysis of biogas systems—Part II: The environmental impact of replacing various reference systems. Biomass & Bioenergy.

[3] Al Seadi T, Drosg B, Fuchs W, Rutz D, Janssen R. Biogas digestate quality and utilization. In: Wellinger A, Murphy J, Baxter D, editors. The Biogas Handbook. 1st ed. Woodhead

[4] Kratzeisen M, Starcevic N, Martinov M, Maurer C, Muller J. Applicability of biogas digestate as solid fuel. Fuel. 2010;**89**(9):2544-2548. DOI: https://doi.org/10.1016/j.

[5] Al Seadi T, Lukehurst C. Quality Management of Digestate from Biogas Plants Used as Fertiliser [Internet]. 2012. Available from: https://www.iea-biogas.net/files/daten-redaktion/download/publi-task37/digestate\_quality\_web\_new.pdf [Accessed: September 07,

[6] EU. Directive 91/676/EC of the European parlament and the council of 12 December 1991. Concerning the protection of waters against pollution caused by nitrates from agricultural sources. In: E. Comission (Ed.) Official Journal L. 1991;**375**:pp. 0001-0008

Publishing; 2013. pp. 267-301. DOI: https://doi.org/10.1533/9780857097415.2.267

. This type of fuels can

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http://dx.doi.org/10.5772/intechopen.71700

The calorific value of obtained syngas was approximately 5 MJ/Nm<sup>3</sup>

, Mariusz Siudak<sup>2</sup>

\*Address all correspondence to: siudak.mariusz@gmail.com

of Warmia and Mazury in Olsztyn, Olsztyn, Poland

and Mazury in Olsztyn, Olsztyn, Poland

conditioning in advance.

**Author details**

Dariusz Wiśniewski<sup>1</sup>

**References**

2005.11.014

2007;**31**:326-344

fuel.2010.02.008

2017]

**Table 5.** Syngas composition and its calorific value—for comparison, table shows also syngas composition from wood pellets.

**Figure 14.** Slice of recorded mass change during gasification process.

a separate channel. This solution results in an increase of energy consumption for gasification process. To solve this problem, authors are currently working on the use of a double screw conveyor on the entire lower surface of the gasification reactor.

#### **4. Summary**

It is possible to produce second-generation fuels from dried digestate. The residues from thermal treatment of digestate can be used in the production of mineral fertilizers. Difficulties may occur during the gasification in downdraft reactors with fixed bed. High ash content, which in the case of biomass of agricultural origin features a low melting temperature, can cause problems with slagging.

The result of the research of the gasification of dried digestate was gaseous fuel that does not differ from the quality of fuels obtained from the thermal treatment of other types of biomass. The calorific value of obtained syngas was approximately 5 MJ/Nm<sup>3</sup> . This type of fuels can be used for combustion in the engine or turbine systems; however, they require adequate conditioning in advance.
