**2. Materials and methods**

#### **2.1. Comparison of PAH analysis methods for gasification residues (1st experiment)**

Hilber et al. 2012 [7] demonstrated that the selection of an appropriate solvent is crucial to determine the PAH content in biochars. They recommend a Soxhlet-extraction with toluene for the PAH analysis of biochars. To complement and to reassess this work, the PAH content (sum of the 16 PAH defined by the US Environmental Protection Agency EPA) of three samples of the same production batch of gasification residues was determined using two different extraction methods (cold extraction with acetone according to DIN 13877:A and Soxhlet extraction with toluene for 5 hours according to DIN 13877:B). The gasification residues had been produced from poplar wood chips in a commercially operated fixed-bed Joos-Spanner biomass gasifier (30 kW electrical capacity) of the German manufacturer Spanner Re² GmbH. The PAH analyses of all experiments described on the following pages were carried out by the commercial laboratory Görtler Analytical Services GmbH in Vaterstetten, Germany.

### **2.2. Testing the functional efficiency of the PAH volatilization unit (2nd and 3rd experiment)**

In a next step, the functional efficiency of a PAH volatilization unit developed by a cooperation of the RWTH Aachen and the German manufacturer Spanner Re² GmbH was assessed. Gasification residues produced under identical process conditions as described in the first experiment were treated in the PAH volatilization unit as illustrated in Figure 1:

This experimental setting was slightly modified for a 3rd experiment: The sealing air stream was reduced. In addition to that, air from the external source (17) was intermittently (within the standstill periods of the conveyor screws) blown into the gasification residue stream within the heating chamber to supply oxygen to the hot gasification residues. In this technical design version of the process, about 4% of the flue gas from the PAH volatilization unit was recircu‐ lated back into the air stream which supplied both the combustion chamber and the sealing air stream.

The PAH content of three samples of the processed batches of gasification residues was analyzed according to DIN 13877:B in both experiments.

#### **2.3. Redesign of the PAH volatilization unit (4th experiment)**

In a redesigned version of the PAH volatilization unit, the manufacturer prevented the recirculation of PAH into the PAH volatilization unit by blowing clean air from an external

Thermal Removal of Polycyclic Aromatic Hydrocarbons from Gasification Biochars http://dx.doi.org/10.5772/57269 363

residues in line with the soil protection regulations in Germany. As a precondition for this evaluation, the suitability of the analysis methods DIN 13877:A and DIN 13877:B for the determination of the PAH content in wood gasification residues had to be checked. Based on the results of [7], our working hypothesis in this regard was that analysis method DIN 13877:B would be more suitable for the analysis task as compared to analysis method DIN 13877:A.

**2.1. Comparison of PAH analysis methods for gasification residues (1st experiment)**

commercial laboratory Görtler Analytical Services GmbH in Vaterstetten, Germany.

experiment were treated in the PAH volatilization unit as illustrated in Figure 1:

analyzed according to DIN 13877:B in both experiments.

**2.3. Redesign of the PAH volatilization unit (4th experiment)**

**2.2. Testing the functional efficiency of the PAH volatilization unit (2nd and 3rd experiment)**

In a next step, the functional efficiency of a PAH volatilization unit developed by a cooperation of the RWTH Aachen and the German manufacturer Spanner Re² GmbH was assessed. Gasification residues produced under identical process conditions as described in the first

This experimental setting was slightly modified for a 3rd experiment: The sealing air stream was reduced. In addition to that, air from the external source (17) was intermittently (within the standstill periods of the conveyor screws) blown into the gasification residue stream within the heating chamber to supply oxygen to the hot gasification residues. In this technical design version of the process, about 4% of the flue gas from the PAH volatilization unit was recircu‐ lated back into the air stream which supplied both the combustion chamber and the sealing

The PAH content of three samples of the processed batches of gasification residues was

In a redesigned version of the PAH volatilization unit, the manufacturer prevented the recirculation of PAH into the PAH volatilization unit by blowing clean air from an external

Hilber et al. 2012 [7] demonstrated that the selection of an appropriate solvent is crucial to determine the PAH content in biochars. They recommend a Soxhlet-extraction with toluene for the PAH analysis of biochars. To complement and to reassess this work, the PAH content (sum of the 16 PAH defined by the US Environmental Protection Agency EPA) of three samples of the same production batch of gasification residues was determined using two different extraction methods (cold extraction with acetone according to DIN 13877:A and Soxhlet extraction with toluene for 5 hours according to DIN 13877:B). The gasification residues had been produced from poplar wood chips in a commercially operated fixed-bed Joos-Spanner biomass gasifier (30 kW electrical capacity) of the German manufacturer Spanner Re² GmbH. The PAH analyses of all experiments described on the following pages were carried out by the

**2. Materials and methods**

362 Environmental Risk Assessment of Soil Contamination

air stream.

**Figure 1.** PAH volatilization unit (Source: Patent Specification) The gasification residues entered the unit on the left side (1) and were being transported by a motor- (4) driven long conveyor screw (5) in an upper tube (24) through a heat exchanger (23) until the heating chamber (16) on the far right side of the unit. From this point, they were trans‐ ported back through the heat exchanger by a motor- (11) driven second long conveyor screw (12) in a second tube (22) on the bottom side of the unit until the exit (6) on the bottom left site of the unit. Both conveyor screws were operated according to the following time schedule: 15 seconds operation, 30 seconds stop, 15 seconds operation and so forth... Within the heating chamber, the gasification residues got in contact with the outer side of a hot (surface temperature: about 600 °C) tube (26) transporting hot wood gas produced in the wood gasifier (not shown in the figure). It is technically possible to supply air from an external source (16) to the gasification residue stream inside the heating chamber. However, this option was not used for the 2nd experiment. A hot external air stream used for the air supply of the wood gasifier entered (13) the heat exchanger at a temperature of about 300 °C and left (14) the heat exchanger at a temperature of about 375 °C, thereby effectively cooling the wood gasification residues on their way through the heat exchanger. Sealing air from the same external source as the heating chamber air stream was blown into the left side (6) of the bottom tube of the PAH volatilization unit in counter flow principle and left the unit on the top left side (1) of the unit. This counter current air flow was introduced to prevent volatized PAH from leaving the PAH volatilization unit through the exit for the processed gasification residues. In the 2nd experiment, about 5 % of the air leaving the unit was recirculated back into the sealing air stream.

source in the sealing air nozzle and by discharging the airstream with the volatized PAH in the gas engine where the wood gas of the gasifier was burned. Further modifications of the redesigned PAH volatilization unit included larger conveyor screw diameters and conveyor screw tube diameters. Gasification residues from a 45 kWel wood gasifier fed by a mixture of chips from different sort of woods were treated in this modified PAH volatilization unit in a 4th experiment. The sealing air stream and the heating chamber air stream volumes were increased as compared to the 2nd and 3rd experiment. The PAH content of one sample of the processed batch of gasification residues was analyzed according to DIN 13877:B.

#### **2.4. PAH volatilization in a laboratory scale experiment (5th experiment)**

To evaluate the general capability of thermal processes to volatize and subsequently remove PAH from gasification residues, the following experiment was carried out:

20 gram samples of gasification residues from a 45 kWel wood gasifier fed by a mixture of chips from pine trees and spruce were filled in an open steel container (20mm width x 20mm depth x 100mm height). This container was placed in an electrical box furnace (Nabertherm, Model LH 30/14) which had been heated to specified temperature levels (550° C, 650° C and 700° C in three consecutive trials). During the experiments, the box furnace was purged by inert gas of type Argon 4.6. After 30 minutes, the container was removed from the furnace and imme‐ diately cooled down in a water quench. During the cooling process, the container was purged from above with Argon 4.6. Due to that, the complete heating and cooling procedure took place in an oxygen-free environment.

The PAH content of the three samples (one for each temperature level) was analyzed according to DIN 13877:B.

### **2.5. Statistical analysis**

All statistical tests were conducted with the open source software R 3.0.1 (R CORE TEAM 2012). Due to the low number of samples per treatment (n=3), particular care and attention was paid to the statistical requirements and assumptions. In this respect, our treatments didn't show neither a normal distribution nor homogenous variances among groups so that the Student's t-test couldn't be applied. Therefore, we applied a permutation version of ANOVA according to [8] for the statistical analysis of the obtained results from the experiments 1, 2 and 3. For this purpose we used the package "ImPerm" [9].
