**4. Conclusions**

Briquettes made of waste paper produced in Ouagadougou were combusted in the present work. The temperature variation, the heavy metal in the flue gas and the produced ash and NPK content in the produced ash were studied in the present work. The combustion of the briquette made of corrugated cardboard achieved the highest mean temperature of 950 ◦C and lowest duration of 10.5 min during the established phase of the combustion. The lowest mean temperature of the established phase was

achieved for the combustion of briquettes of type 2 due to their low HHV. The highest combustion time was obtained for the combustion of briquettes made of the mixture of all paper waste due to flame retardant which may be present in the mixture of waste paper and cardboard. The combustion produced considerable residual ash which proportion varied from 11% to 16% of the raw briquettes mass. The analysis of the ash and the flue gas shown that large proportions of the heavy metal were transferred predominantly in the flue gas than in the ash. In addition, the manufacturing process and the usage of paper had increased some heavy metal content (Pb, Hg and Mn) comparatively to the parent wood. The heavy metal contents in the ash were under the limit content fixed by the standard in the context of soils amendment. Thus, the ash obtained after the briquettes combustion can be used to amend soils. For this purpose, the ash may be mixed with other element in order to improve fertilizer content since the ash fraction of NPK are lower than the minimum required by the standard.

**Author Contributions:** H.G.I. realized the experiments and wrote the paper. S.K.O. and A.Y. conceived and supervised the study. O.S. contributed in the data analysis. The paper presentation and formulation was improved by T.D., J.K. had made a critical review which improved the scientific quality of the paper.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors want to acknowledge Dissa Alfa Omar and Sanon Pierre for their assistance with the present work.

**Conflicts of Interest:** The authors declare no conflict of interest.
