**2. The treatment of raw digestate**

A digestate from a pilot biogas plant located at the Experimental Station in Bałdy, Poland (N54° 36′ 1.8073″, E20° 36′8.5295″) was used in this research. The following technological parameters of the fermenter were used [11]:

• Feedstock moisture—90%

contains substantial amount of the elements necessary for plant growth. Liquid fraction can also be further treated and recirculated to the fermenter [3]. Solid fraction can also be used for

Nowadays, the digestate in the regulations is considered as waste in Europe. It does not have the status of a biofuel or an alternative fuel and is most commonly used as a soil improver. Such use often requires additional tanks to store the digestate mass to allow it to be used during fertilization periods. This can generate significant capital expenditure on the construction of liquid fraction storage tanks [5]. Another problem that causes the necessity of processing digestate is too small cultivable area where it can be used directly. Agricultural use of digestate is limited by the maximum allowable nitrogen dose of 170 kg N ha−1 y−1 [6]. For this reason, European countries have begun to use the separation of digestate to solid and liquid fraction. These fractions differ in physicochemical properties. As a result, a much smaller area for storage of digestate is required, while liquid fraction is again used to dilute the substrates to the fermentation process to the required 12% of dry matter. Liquid fraction can also be used as fertilizer by acquiring mineral compounds. It is characterized by lower phosphorus content and a higher content of nitrogen and potassium. Solid fraction is used in areas with low phosphorus content, liquid fraction on the other hand—in

For fertilizer purposes, the liquid fraction is used either directly or in the production of mineral fertilizers through further purification. Further purification can be achieved through ultrafiltration in order to remove solid particles, followed by the reduction of high concentrations of nitrogen through stripping or crystallization of struvite [8]. These methods are used extensively in Germany and the Netherlands, and are mainly derived from a pig farm where

For separation, screw separators with slot filter are most commonly used, which have previously been used successfully to separate liquid manure. Those devices feature low energy consumption because of low pressure of pumped digestate and low rotational speed of the screw shaft. Dry matter content obtained is about 30% of the solid fraction, while the liquid fraction remains about 4% of the dry matter. More advanced equipment, such as decanter centrifuges or belt presses, allow more efficient operation, but are rarely used in small biogas

Often solid fraction of digestate is processed in composting process, which reduces its volume, moisture, and improves fertilizer and storage properties [9]. Separation devices are often directly integrated with composting reactor, e.g., container with moving floor and aera-

Another method often used is drying in belt or drum dryers using the heat from CHP units. Dried digestate is used to produce pellets for energy purposes or for use as bedding for ani-

agriculture or as a solid fuel, e.g., in the combustion process [4].

**1.1. Digestate treatment in Europe**

192 Gasification for Low-grade Feedstock

phosphorus saturated areas [7].

ammonia load is very high.

tion system or drum reactor.

mal farm [10].

plants because of high investment costs.


**Tables 1** and **2** show the properties of the raw digestate.

#### **2.1. Mechanical dewatering of digestate**

Raw digestate from biogas plant has been pre-dewatered in screw separator with slot filter with a filter gap of 0.5 mm. Raw digestate contained 94.55% of water. Energy consumption of the mechanical dehydration process was measured using the Schneider ION7650 electrical network meter. In separation process test 0.035 m3 of raw digestate was used. **Figure 1** shows the photograph of the screw separator.

As a result of the separation process, 30 kg of liquid fraction and 5 kg of solid fraction was obtained. Energy consumption during the experiment was 0.02 kWh. The tests were carried out without the pump forcing the digestate pressure in the separator. Digestate was fed to the separator only under its hydraulic pressure. Absence of pressure force does not interfere with separation operation and performance results may be lower than expected. The usage of a forced pump can increase productivity but can also increase the energy consumption of the process.


**Table 1.** Properties of raw digestate.


The amount of heat released during biological transformation has been investigated by many authors. The range of this value is from 17.8 to 24.7 (kJ/g decomposed dry matter of organic), calculated as removed organic matter, it can reach up to 28.0 kJ/g organic dry matter [15].

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To increase porosity and permeability of the feedstock, the digestate was combined with wood chips in 1:1 mass proportion. The addition of woods chips also intended to reduce the water content of pre-dehydrated digestate to create the mixture with the optimal water content for biodrying process—from 50 to 70% [16]. During biodrying process in the reactor the temperature was measured in four points on different heights. Volume flow rate of the air supplied to the reactor was measured by thermoanemometer. The reactor was set on strain gauges to measure the change of feedstock's mass during the process. During biodrying process, energy consumption was measured using the electricity meter. Stream of air was supplied by side channel blower through floor of the reactor. **Figure 2** presents schematic

The biodrying process was carried out using 730 kg of mechanically dehydrated digestate and 730 kg of wood chips to ensure adequate porosity of the mixture. Aeration ratio was of average 0.025 m<sup>3</sup> kg−1 h−1. The process took about 4 weeks. **Figures 3** and **4** show the results

As a result of the biodrying process was the weight loss of 500 kg—68% of the initial weight of digestate and 34% of initial weight of combination of wood chips and digestate. Total electricity consumption for the biodrying process was 17.792 kWh, equivalent to 0.0295 kWh/kg of reduced weight. **Table 3** shows the energy properties of digestate after the biodrying process.

Digestate after biodrying process was isolated from wood chips by drum sieve and thermally dried in a flow-through tubular dryer. The drying process was controlled by changing the

diagram of the biodrying reactor.

**2.3. Thermal drying of digestate**

**Figure 2.** Schematic diagram of the biodrying reactor.

of the process.

**Table 2.** Contamination content in raw digestate.

**Figure 1.** The photograph of a screw separator with slot filter.

Based on the power and energy measurements and mass of the separated fractions, the energy consumptions of the mechanical separation process in the screw separator with slot filter was determined. Energy consumed for separating 1 kg of solid fraction from raw digestate was 0.004 kWh (14.4 kJ), while the separation of 1 kg of liquid fraction consumed 0.00066 kWh (2.37 kJ). Solid fraction after mechanical separation process contained an average of 76.1% of water.

#### **2.2. Biodrying of digestate**

The pre-dewatered digestate was used as a feedstock in biodrying process. Biodrying technology is typically used in the mechanical and biological treatment of wastes [12]. This technology involves the usage of heat generated by aerobic microorganisms in organic matter decomposition processes [13]. The general stoichiometric equation for the decomposition of the organic matter has the following form [14]:

the organic matter has the following form [14]:

$$\mathbb{C}\_{a}H\_{b}\mathrm{O}\_{c}N\_{d} + 0.5(\mathrm{ny} + 2\mathrm{s} + \mathrm{r} - \mathrm{c})\mathrm{O}\_{2} \xrightarrow{\text{heat}} \mathrm{n}\,\mathbb{C}\_{w}H\_{\mathrm{x}}\mathrm{O}\_{y}N\_{\mathrm{z}} + \mathrm{s}\mathrm{C}\mathrm{O}\_{2} + \mathrm{r}\,\mathrm{H}\_{2}\mathrm{O} + (\mathrm{d} - \mathrm{nz})\mathrm{N}\,\mathrm{H}\_{\mathrm{3}} \qquad \text{(1)}$$

The amount of heat released during biological transformation has been investigated by many authors. The range of this value is from 17.8 to 24.7 (kJ/g decomposed dry matter of organic), calculated as removed organic matter, it can reach up to 28.0 kJ/g organic dry matter [15].

To increase porosity and permeability of the feedstock, the digestate was combined with wood chips in 1:1 mass proportion. The addition of woods chips also intended to reduce the water content of pre-dehydrated digestate to create the mixture with the optimal water content for biodrying process—from 50 to 70% [16]. During biodrying process in the reactor the temperature was measured in four points on different heights. Volume flow rate of the air supplied to the reactor was measured by thermoanemometer. The reactor was set on strain gauges to measure the change of feedstock's mass during the process. During biodrying process, energy consumption was measured using the electricity meter. Stream of air was supplied by side channel blower through floor of the reactor. **Figure 2** presents schematic diagram of the biodrying reactor.

The biodrying process was carried out using 730 kg of mechanically dehydrated digestate and 730 kg of wood chips to ensure adequate porosity of the mixture. Aeration ratio was of average 0.025 m<sup>3</sup> kg−1 h−1. The process took about 4 weeks. **Figures 3** and **4** show the results of the process.

As a result of the biodrying process was the weight loss of 500 kg—68% of the initial weight of digestate and 34% of initial weight of combination of wood chips and digestate. Total electricity consumption for the biodrying process was 17.792 kWh, equivalent to 0.0295 kWh/kg of reduced weight. **Table 3** shows the energy properties of digestate after the biodrying process.

#### **2.3. Thermal drying of digestate**

Based on the power and energy measurements and mass of the separated fractions, the energy consumptions of the mechanical separation process in the screw separator with slot filter was determined. Energy consumed for separating 1 kg of solid fraction from raw digestate was 0.004 kWh (14.4 kJ), while the separation of 1 kg of liquid fraction consumed 0.00066 kWh (2.37 kJ). Solid fraction after mechanical separation process contained an average of 76.1% of water.

Cu Zn Mn Fe Salinity S Cl mg/kg D.M. mg/kg D.M. mg/kg D.M. mg/kg D.M. g/dm<sup>3</sup> % D.M. mg/dm<sup>3</sup> 8.24 94.55 68.97 7.16 1830.0 2.38 6.61

The pre-dewatered digestate was used as a feedstock in biodrying process. Biodrying technology is typically used in the mechanical and biological treatment of wastes [12]. This technology involves the usage of heat generated by aerobic microorganisms in organic matter decomposition processes [13]. The general stoichiometric equation for the decomposition of

*heat*→*<sup>n</sup> Cw Hx Oy Nz* <sup>+</sup> *sC <sup>O</sup>*<sup>2</sup> <sup>+</sup> *<sup>r</sup> <sup>H</sup>*<sup>2</sup> *<sup>O</sup>* <sup>+</sup> (*<sup>d</sup>* <sup>−</sup> *nz*)*<sup>N</sup> <sup>H</sup>*<sup>3</sup> (1)

**2.2. Biodrying of digestate**

**Parameter**

194 Gasification for Low-grade Feedstock

the organic matter has the following form [14]: *Ca Hb Oc Nd* <sup>+</sup> 0.5(*ny* <sup>+</sup> <sup>2</sup>*<sup>s</sup>* <sup>+</sup> *<sup>r</sup>* <sup>−</sup> *<sup>c</sup>*) *<sup>O</sup>*<sup>2</sup> ⎯

**Figure 1.** The photograph of a screw separator with slot filter.

**Table 2.** Contamination content in raw digestate.

Digestate after biodrying process was isolated from wood chips by drum sieve and thermally dried in a flow-through tubular dryer. The drying process was controlled by changing the

**Figure 2.** Schematic diagram of the biodrying reactor.

**Figure 3.** Temperature change graph during biodrying process.

The tubular dryer was designed to allow steam and condensate drain-off in the initial part of the dryer. This design enables to transfer part of the heat energy of steam and condensed water to the dryer part before the electric heater, increasing the energy efficiency of the dry-

**Water content Ash content Heat of combustion Calorific value**

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43.67 18.07 17.82 8.97

After biodrying % % D.M. MJ/kg D.M. MJ/kg

Energy consumption test was carried out using 36.65 kg of digestate after biodrying process. The material feed rate was 2 m/min, temperature of drying process was about 150°C, and the ambient temperature was 12°C. The result of the drying process was mass reduction of 22.72 kg. The weight loss was 13.92 kg, and the water content of dried product was 9.15%. The drying process consumed 10.28 kWh, and the unitary electricity consumption for evaporation of 1 kg of water was 0.73 kWh/kg. The energy required to produce 1 kg of digestate with water content of 9.15% was about 0.136 kWh. **Table 4** shows the energy properties of digestate after

After thermal drying % % D.M. MJ/kg D.M.

**Water content Ash content Heat of combustion**

9.15 18.11 17.516

ing process.

**Figure 5.** Schematic diagram of tubular dryer.

**Type of digestate Parameter**

**Table 4.** Energy properties of digestate after thermal drying process.

**Type of digestate Parameter**

**Table 3.** Energy properties of digestate after biodrying process.

thermal drying.

**Figure 4.** Mass change graph during the process.

feed rate of the dried material and the power of the electric heater. Energy consumption of the digestate thermal drying process was measured using the Schneider ION7650 electrical network meter. **Figure 5** shows a schematic of a tubular dryer.


**Table 3.** Energy properties of digestate after biodrying process.

**Figure 5.** Schematic diagram of tubular dryer.

The tubular dryer was designed to allow steam and condensate drain-off in the initial part of the dryer. This design enables to transfer part of the heat energy of steam and condensed water to the dryer part before the electric heater, increasing the energy efficiency of the drying process.

Energy consumption test was carried out using 36.65 kg of digestate after biodrying process. The material feed rate was 2 m/min, temperature of drying process was about 150°C, and the ambient temperature was 12°C. The result of the drying process was mass reduction of 22.72 kg. The weight loss was 13.92 kg, and the water content of dried product was 9.15%. The drying process consumed 10.28 kWh, and the unitary electricity consumption for evaporation of 1 kg of water was 0.73 kWh/kg. The energy required to produce 1 kg of digestate with water content of 9.15% was about 0.136 kWh. **Table 4** shows the energy properties of digestate after thermal drying.


**Table 4.** Energy properties of digestate after thermal drying process.

feed rate of the dried material and the power of the electric heater. Energy consumption of the digestate thermal drying process was measured using the Schneider ION7650 electrical

network meter. **Figure 5** shows a schematic of a tubular dryer.

**Figure 4.** Mass change graph during the process.

**Figure 3.** Temperature change graph during biodrying process.

196 Gasification for Low-grade Feedstock

Digestate after thermal drying process has been pelletized to increase of bulk density. Energy consumption for pelletization process was 0.085 kWh.

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

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• Gasification agent supply system

• Biomass feeding unit

• Ash removal unit • Reactor chamber

tightness of the installation.

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