**3.2. Functional efficiency of the PAH volatilization unit (2nd and 3rd experiment)**

Table 3 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit (PAH-VU) in the 2nd experiment. On average, the PAH content of the processed gasification residues was 58% lower as compared to the unprocessed residues from the 1st experiment. The difference between the sample means (n=3) of the two analysis methods is significant (p < 0.01) according to permu‐ tation version of ANOVA applied. The standard errors of the mean are indicated after each average value.

Figure 3 depicts the results from Table 3 in two box-and-whisker plots.


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

The PAH content of the three samples (one for each temperature level) was analyzed according

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

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

Figure 2 depicts the results from Table 1 in two box-and-whisker plots.

Figure 3 depicts the results from Table 3 in two box-and-whisker plots.

**3.2. Functional efficiency of the PAH volatilization unit (2nd and 3rd experiment)**

Table 3 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit (PAH-VU) in the 2nd experiment. On average, the PAH content of the processed gasification residues was 58% lower as compared to the unprocessed residues from the 1st experiment. The difference between the sample means (n=3) of the two analysis methods is significant (p < 0.01) according to permu‐ tation version of ANOVA applied. The standard errors of the mean are indicated after each

Table 1 summarizes the Σ EPA16 PAH content in the three gasification residue samples in mg/ kg dry mass (DM) as determined by the two different analytical methods. On average, the analysis according to DIN 13877:B resulted in PAH contents more than 4 times higher compared to the analysis according to DIN 13877:A. The standard errors of the mean (SEM) are indicated after the average values. The difference between the sample means (n=3) of the two analysis methods is significant (p < 0.01) according to the applied permutation version of

in an oxygen-free environment.

364 Environmental Risk Assessment of Soil Contamination

3. For this purpose we used the package "ImPerm" [9].

to DIN 13877:B.

**3. Results**

ANOVA.

average value.

**2.5. Statistical analysis**

**Table 1.** Comparison of the two PAH analysis methods using either acetone or toluene for extraction (1st experiment). SEM indicates standard error of the mean.

quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples after application of the analysis method DIN 13877:A (left) and DIN 13877:B (right). **Figure 2.** Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples after application of the analysis method DIN 13877:A (left) and DIN 13877:B (right).

according to DIN 13877:A

Table 2. Comparison of the two PAH analysis methods using either acetone or toluene for extraction (1st experiment). Detailed results.

Table 3 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit (PAH-VU) in the 2nd experiment. On average, the PAH content of the processed gasification residues was 58% lower as compared to the unprocessed residues from the 1st experiment. The difference between the sample means (n=3) of the two analysis methods is significant (p < 0.001) according to permutation version of ANOVA applied. The standard errors of the

**3.2. Functional efficiency of the PAH volatilization unit (2nd and 3rd experiment)** 

mean are indicated after each average value.

Gasification Residues **Extraction with acetone**

Figure 2. Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper

(untreated) Σ EPA16 PAH content Σ EPA16 PAH content

Naphthalene 460 480 420 1,200 570 1,200 2-Methylnapthalene 63 66 61 490 130 330 1-Methylnapthalene 63 67 62 470 100 260 Acenaphtylene 43 47 43 580 120 340 Acenapthene 14 15 14 110 15 54 Flourene 8,5 8,4 8,9 180 38 120 Phenanthrene 12 15 13 690 170 580 Anthracene 1,8 2,4 2 120 32 120 Flouranthen 1,5 1,9 1,7 94 31 140 Pyrene 1,4 1,6 1,5 70 27 110 Benzo(a)anthracene < 0,01 < 0,01 < 0,01 4,5 2,4 12 Chrysene < 0,01 < 0,01 < 0,01 4,8 2,4 18 Benzo(b)flouranthene < 0,01 < 0,01 < 0,01 1,5 0,43 3,4 Benzo(k)flouranthene < 0,01 < 0,01 < 0,01 0,24 0,05 0,64 Benzo(a)pyrene < 0,01 < 0,01 < 0,01 0,65 0,19 1,9 Indeno(1,2,3.cd)pyren < 0,01 < 0,01 < 0,01 0,25 0,11 0,9 Dibenz(a,h)anthracene < 0,01 < 0,01 < 0,01 0,05 < 0,01 0,31 Benzo(g,h,i)perylene < 0,01 < 0,01 < 0,01 0,16 0,16 0,4 **Σ EPA16 PAH 542 571 504 3,056 1,009 2,702** 

Sample 1 Sample 2 Sample 3 Sample 1 Sample 2 Sample 3 mg/kg DM mg/kg DM

**Extraction with toluene**  according to DIN 13877:B


**Table 2.** Comparison of the two PAH analysis methods using either acetone or toluene for extraction (1st experiment). Detailed results.


**Table 3.** PAH reduction in PAH volatilization unit (2nd experiment). SEM indicates standard error of the mean.

mg/kg DM mg/kg DM

Production batch with treatment in the PAH-VU from 2nd experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Production batch without treatment from 1st experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Table 3. PAH reduction in PAH volatilization unit (2nd experiment). SEM indicates standard error of the mean.

**Average 2,255 ± 516 (SEM) 943 ± 143 (SEM)** 

Gasification Residues

Figure 3 depicts the results from Table 3 in two box-and-whisker plots:

Gasification Residues

366 Environmental Risk Assessment of Soil Contamination

Detailed results.

Gasification Residues

**Extraction with acetone** according to DIN 13877:A

(untreated) Σ EPA16 PAH content Σ EPA16 PAH content

Naphthalene 460 480 420 1,200 570 1,200 2-Methylnapthalene 63 66 61 490 130 330 1-Methylnapthalene 63 67 62 470 100 260 Acenaphtylene 43 47 43 580 120 340 Acenapthene 14 15 14 110 15 54 Flourene 8,5 8,4 8,9 180 38 120 Phenanthrene 12 15 13 690 170 580 Anthracene 1,8 2,4 2 120 32 120 Flouranthen 1,5 1,9 1,7 94 31 140 Pyrene 1,4 1,6 1,5 70 27 110 Benzo(a)anthracene < 0,01 < 0,01 < 0,01 4,5 2,4 12 Chrysene < 0,01 < 0,01 < 0,01 4,8 2,4 18 Benzo(b)flouranthene < 0,01 < 0,01 < 0,01 1,5 0,43 3,4 Benzo(k)flouranthene < 0,01 < 0,01 < 0,01 0,24 0,05 0,64 Benzo(a)pyrene < 0,01 < 0,01 < 0,01 0,65 0,19 1,9 Indeno(1,2,3.cd)pyren < 0,01 < 0,01 < 0,01 0,25 0,11 0,9 Dibenz(a,h)anthracene < 0,01 < 0,01 < 0,01 0,05 < 0,01 0,31 Benzo(g,h,i)perylene < 0,01 < 0,01 < 0,01 0,16 0,16 0,4

**Σ EPA16 PAH 542 571 504 3,056 1,009 2,702**

mg/kg DM mg/kg DM

**Table 2.** Comparison of the two PAH analysis methods using either acetone or toluene for extraction (1st experiment).

Production batch without treatment from 1st experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

sample 1 3,056 1,291 sample 2 1,009 731 sample 3 2,702 806 **Average 2,255 ± 516 (SEM) 943 ± 143 (SEM)**

**Table 3.** PAH reduction in PAH volatilization unit (2nd experiment). SEM indicates standard error of the mean.

Sample 1 Sample 2 Sample 3 Sample 1 Sample 2 Sample 3 mg/kg DM mg/kg DM

**Extraction with toluene** according to DIN 13877:B

Production batch with treatment in the PAH-VU from 2nd experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Figure 3. Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples from the 1st experiment (left) and from the 2nd experiment (right). **Figure 3.** Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples from the 1st experiment (left) and from the 2nd experiment (right).

> Σ EPA16 PAH content (DIN 13877:B) Sample 1 Sample 2 Sample 3 mg/kg DM

Gasification Residues (processed)



**Table 4.** PAH reduction in PAH volatilization unit (2nd experiment). Detailed results.

Table 5 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit in the 3rd experiment. On average, the PAH content of the processed gasification residues was 36% lower as compared to the unprocessed residues from the first experiment. The difference between the sample means (n=3) of the two analysis methods is not significant (p < 0.05) according to permutation version of ANOVA applied. The standard errors of the mean are indicated after each average value.


**Table 5.** PAH reduction in PAH volatilization unit (3rd experiment). SEM indicates standard error of the mean.

mg/kg DM mg/kg DM

Production batch with treatment in the PAH-VU from 3rd experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Benzo(g,h,i)perylene 0.04 0.07 0.05 **Σ EPA16 PAH 1,291 731 806**

Table 5 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit in the 3rd experiment. On average, the PAH content of the processed gasification residues was 36% lower as compared to the unprocessed residues from the first experiment. The difference between the sample means (n=3) of the two analysis methods is not significant (p < 0.05) according to permutation version of ANOVA applied. The standard errors of the mean

> Production batch without treatment from 1st experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

**Average 2,255 ± 516 (SEM) 1,434 ± 113 (SEM)** 

Figure 4 depicts the results from Table 5 in two box-and-whisker plots: Table 5. PAH reduction in PAH volatilization unit (3rd experiment). SEM indicates standard error of the mean.

Figure 4 depicts the results from Table 5 in two box-and-whisker plots:

Table 4. PAH reduction in PAH volatilization unit (2nd experiment). Detailed results.

are indicated after each average value.

Gasification Residues

Gasification Residues (processed)

368 Environmental Risk Assessment of Soil Contamination

**Table 4.** PAH reduction in PAH volatilization unit (2nd experiment). Detailed results.

Production batch without treatment from 1st experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

sample 1 3,056 1,713 sample 2 1,009 1,292 sample 3 2,702 1,298

**Average 2,255 ± 516 (SEM) 1,434 ± 113 (SEM)**

**Table 5.** PAH reduction in PAH volatilization unit (3rd experiment). SEM indicates standard error of the mean.

value.

Gasification Residues

Flouranthen 26 32 33 Pyrene 19 26 18 Benzo(a)anthracene 1.5 2 1.2 Chrysene 2.4 3.4 1.9 Benzo(b)flouranthene 0.27 0.54 0.23 Benzo(k)flouranthene 0.05 0.04 0.05 Benzo(a)pyrene 0.06 0.1 0.06 Indeno(1,2,3.cd)pyren 0.18 <0.01 0.03 Dibenz(a,h)anthracene <0.01 <0.01 <0.01 Benzo(g,h,i)perylene 0.04 0.07 0.05

**Σ EPA16 PAH 1,291 731 806**

Table 5 summarizes the Σ EPA16 PAH content in the gasification residue samples of the production batch after the treatment in the PAH volatilization unit in the 3rd experiment. On average, the PAH content of the processed gasification residues was 36% lower as compared to the unprocessed residues from the first experiment. The difference between the sample means (n=3) of the two analysis methods is not significant (p < 0.05) according to permutation version of ANOVA applied. The standard errors of the mean are indicated after each average

mg/kg DM mg/kg DM

Production batch with treatment in the PAH-VU from 3rd experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Σ EPA16 PAH content (DIN 13877:B) Sample 1 Sample 2 Sample 3 mg/kg DM

> Figure 4. Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples from the 1st experiment (left) and from the 3rd experiment (right). **Figure 4.** Boxplots indicating the median (central black bar), the minimum and maximum (lower and upper whisker) and the lower and upper quartile (lower end upper end of the box) of the Σ EPA16 PAH contents in mg/kg DM in three gasification residue samples from the 1st experiment (left) and from the 3rd experiment (right).

> > Σ EPA16 PAH content (DIN 13877:B)

Gasification Residues (processed)



**Table 6.** PAH reduction in PAH volatilization unit (3rd experiment). Detailed results.

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

Table 7 summarizes the Σ EPA16 PAH content in the gasification residue sample of the production batch after the redesign of the PAH volatilization unit (PAH-VU) in the 4th experiment. The PAH content of the processed gasification residues was 82% lower as compared to the average PAH content in the unprocessed residues from the 1st experiment.


**Table 7.** PAH reduction in PAH volatilization (4th experiment). SEM indicates standard error of the mean.

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

Table 9 summarizes the Σ EPA16 PAH content in the gasification residue samples treated in the laboratory scale experiment in an oxygen-free environment. Following a temperature treatment of at least 650 °C, the PAH contents in the gasification residues decreased drastically.


**Table 8.** PAH reduction in PAH volatilization (4th experiment). Detailed results.


**Table 9.** PAH content in gasification residues from the laboratory scale experiment (5th experiment)

#### **4. Discussion**

Gasification Residues (processed)

370 Environmental Risk Assessment of Soil Contamination

**Table 6.** PAH reduction in PAH volatilization unit (3rd experiment). Detailed results.

Production batch without treatment from 1st experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

sample 2 1,009 sample 3 2,702

**Average 2,255 ± 516 (SEM)**

sample 1 3,056 396

**Table 7.** PAH reduction in PAH volatilization (4th experiment). SEM indicates standard error of the mean.

Table 9 summarizes the Σ EPA16 PAH content in the gasification residue samples treated in the laboratory scale experiment in an oxygen-free environment. Following a temperature treatment of at least 650 °C, the PAH contents in the gasification residues decreased drastically.

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

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

Gasification Residues

Σ EPA16 PAH content (DIN 13877:B) Sample 1 Sample 2 Sample 3 mg/kg DM

> Production batch with treatment in the PAH-VU from 4th experiment

Σ EPA16 PAH content (DIN 13877:B, extraction with toluene)

Benzo(b)flouranthene <0.01 <0.01 <0.01 Benzo(k)flouranthene <0.01 <0.01 <0.01 Benzo(a)pyrene <0.01 <0.01 <0.01 Indeno(1,2,3.cd)pyren <0.01 <0.01 <0.01 Dibenz(a,h)anthracene <0.01 <0.01 <0.01 Benzo(g,h,i)perylene <0.01 <0.01 <0.01

**Σ EPA16 PAH 1,713 1,292 1,298**

Table 7 summarizes the Σ EPA16 PAH content in the gasification residue sample of the production batch after the redesign of the PAH volatilization unit (PAH-VU) in the 4th experiment. The PAH content of the processed gasification residues was 82% lower as compared to the average PAH content in the unprocessed residues from the 1st experiment.

mg/kg DM mg/kg DM

The comparison of the PAH analysis methods described in section 2.1 confirms the recom‐ mendation of Hilber et al. 2012 [7]: Soxhlet extraction with toluene resulted in a much higher extraction of PAH from the gasification residues as compared to the cold extraction with


**Table 10.** PAH content in gasification residues from the laboratory scale experiment (5th experiment). Detailed results.

acetone (see Table 1). Thus, analysis method DIN 13877:B was applied for the PAH analysis in the subsequent experiments 2-5.

[10] observed a positive correlation between the pyrolysis temperature (between 200 °C and 500 °C in the experiment carried out) applied for the production of biochars from lake sediments and the sorption of Phenanthrene to these biochars. As gasification temperatures are higher [1] than the pyrolysis temperatures applied in this experiment, we explain our analysis results by a strong sorption of PAH to the carbonaceous matrix of the gasification residues.

[11] recently compared recovery rates after reflux extraction with toluene and a 1:1 acetone/ cyclohexane mixture of the three surrogate PAH Acenaphtene-d10, Phenanthrene-d10 and Chrysene-d10 which had been previously added to a pyrolysis char made form orchard pruning. The experiment resulted in higher recovery rates with the solvent toluene as com‐ pared to the solvent mixture acetone/cyclohexane for the high molecular weight PAH Phe‐ nanthrene-d10 (68% compared to 41%) and Chrysene-d10 (58% compared to 7%). The recovery rate with the solvent toluene was though lower as compared to the acetone/cyclohexane mixture for the low molecular weight PAH Acenaphtene-d10 (68% compared to 80%).

The differences in the recovery rates stated above are yet small if compared to our analysis results (see Table 1). Reflux extractions are usually carried out at temperatures close to the boiling point of the solvent applied. We thus presume that the strong difference in our analysis results after applying the analysis methods DIN 13877:A (low temperature extraction) and 13877:B (high temperature extraction) can be mainly explained by the temperature difference between the two extraction methods. We recommend to directly compare Soxhlet extractions with the solvents toluene and a 1:1 acetone/cyclohexane mixture for the determination of the Σ EPA16 PAH content of gasification residues for future research.

Based on the results described above, the effectiveness of the legal provisions of the German Federal Soil Protection Act (Bundesbodenschutzgesetz [BBodSchG]) [12] and the German Federal Soil Protection and Contaminated Sites Ordinance (Bundesbodenschutzverordnung [BBodSchV]) [13] for the protection of agricultural soils against PAH pollution in Germany was analyzed. Special attention was paid to the suitability of the PAH analysis methods specified in these regulations. The German Federal Soil Protection and Contaminated Sites Ordinance defines precautionary values for the PAH content of soils. These precautionary values are set at 10 mg/kg DM for the Σ EPA16 PAH content in the finely granulated part of soils with humus contents larger than 8% and at 3 mg/kg DM for the Σ EPA16 PAH content of soils with humus contents of less or equal 8%. The German Federal Soil Protection and Contaminated Sites Ordinance specifies a number of analysis methods which may be used to determine the PAH content of soils in accordance with this ordinance. Among them are the analysis methods DIN 13877:A (cold extraction with acetone) and DIN 13877:B (Soxhlet extraction with toluene). However, our analysis results in section 3.1 have clearly shown that DIN 13877:A is not suitable to determine the PAH content in gasification residues. Thus, the regulations of the German Federal Soil Protection Act and the German Federal Soil Protection and Contaminated Sites Ordinance currently cannot prevent the application of gasification residues with high PAH contents to agricultural soils, if the cold extraction with acetone is applied for their characterization. It is recommended to close this legislative loophole. Apart from that, it should be noted that there is no boundary value for PAH loads added to soil (however for the PAH content in the soil) in the German soil legislation.

While thermal processes are already used to remove PAH from contaminated soils [6], this principle was applied for the PAH removal from wood gasification residues for the first time to our knowledge. The results displayed in Table 3, Table 5 and Table 7 indicate a reduction of the Σ EPA16 PAH content of the gasification residues by 36% to 82% after the treatment in the PAH volatilization unit. Still, the residual PAH contents in the gasification residues are too high to allow for an agricultural use.

acetone (see Table 1). Thus, analysis method DIN 13877:B was applied for the PAH analysis

**Table 10.** PAH content in gasification residues from the laboratory scale experiment (5th experiment). Detailed results.

**Σ EPA16 PAH 1,000 1.3 0.28**

Naphthalene 590 0.77 0.18 Acenaphtylene 0.3 0.19 < 0.01 Acenapthene 1.3 < 0.01 < 0.01 Flourene 0.2 < 0.01 < 0.01 Phenanthrene 230 0.23 < 0.01 Anthracene 43 0.04 < 0.01 Flouranthen 57 0.02 < 0.01 Pyrene 57 0.03 < 0.01 Benzo(a)anthracene 6.6 < 0.01 < 0.01 Chrysene 13 < 0.01 < 0.01 Benzo(b)flouranthene 3,6 < 0.01 < 0.01 Benzo(k)flouranthene 1,3 < 0.01 < 0.01 Benzo(a)pyrene 0.95 < 0.01 < 0.01 Indeno(1,2,3.cd)pyren 2.5 < 0.01 < 0.01 Dibenz(a,h)anthracene 0.1 < 0.01 < 0.01 Benzo(g,h,i)perylene 0.56 < 0.01 < 0.01

[10] observed a positive correlation between the pyrolysis temperature (between 200 °C and 500 °C in the experiment carried out) applied for the production of biochars from lake sediments and the sorption of Phenanthrene to these biochars. As gasification temperatures are higher [1] than the pyrolysis temperatures applied in this experiment, we explain our analysis results by a strong sorption of PAH to the carbonaceous matrix of the gasification

[11] recently compared recovery rates after reflux extraction with toluene and a 1:1 acetone/ cyclohexane mixture of the three surrogate PAH Acenaphtene-d10, Phenanthrene-d10 and Chrysene-d10 which had been previously added to a pyrolysis char made form orchard pruning. The experiment resulted in higher recovery rates with the solvent toluene as com‐ pared to the solvent mixture acetone/cyclohexane for the high molecular weight PAH Phe‐

in the subsequent experiments 2-5.

Gasification Residues (processed)

372 Environmental Risk Assessment of Soil Contamination

Σ EPA16 PAH content (DIN 13877:B) 550 °C 650 °C 700 °C mg/kg DM

residues.

The residual PAH content in the gasification residues was higher in the third experiment as compared to the second experiment. This result might be explained by a difference in the PAH content of the untreated gasification residues (although these were produced under the same production conditions) or by a change in the airflow conditions within the PAH volatilization unit caused by modifications of the sealing air stream and the heating chamber air stream volumes. As already mentioned, gasification residues which had already passed through the hot heating chamber got in contact with the volatilized PAH contained in the sealing air which was blown into the lower conveyor screw. We assume that this is one of the reasons for the high residual PAH content of the gasification residues in the 2nd and 3rd experiment.

This hypothesis is supported by the comparably low PAH content of 396 mg/kg DM which was reached after the redesign of the PAH volatilization unit which prevented the recirculation of volatized PAH. Independent from the avoided recirculation effect, the increase in the air supply to the heating chamber in the 4th experiment has with high probability promoted the oxidation (combustion) of gasification residues in the heating chamber. The resulting temper‐ ature increase might have supported a more complete volatilization of the PAH from the gasification residues as compared to the 2nd and 3rd experiment.

The 5th experiment proves that thermal volatilization processes are capable in reducing the PAH content of gasification chars to levels which are acceptable for agricultural applications. It can be derived from the data presented in Table 9, that minimum process temperatures of about 650 °C are necessary for an effective removal of PAH from gasification residues.

Based on the laboratory scale experiment, the following additional technical design modifi‐ cations are suggested to further improve the functional efficiency of the PAH volatilization unit:


Since the use of biochars in agriculture promises beneficial effects for soil amelioration and climate change mitigation (see [15] and [16]), it is recommended to continue the technical development to obtain gasifier residues with low PAH contents which might subsequently be used as soil amendment. The joint composting of gasification residues and organic feedstock sources could help to further reduce any remaining PAH in the gasification residues via biodegradation [14] and to prevent nutrient losses in the composting process. In case further experiments would affirm the viability of the preparation of gasification residues with very low PAH contents which also meet all other applicable environmental standards for soil amendments, a disposal problem could be turned into the valuable resource supply option for the agricultural sector.
