**Abstract**

The Nordic countries have a long tradition of utilizing agro-industrial sidestreams for heat and power production and recovery of chemicals. A typical example is black liquor from pulp mills. Here, the woody biomass undergoes a digestion process where the fibers are separated to produce pulp for paper production. The liquid by-product from the digester, black liquor, contains wood lignin and the spent cooking chemicals. Through the chemical recovery cycle, the black liquor is utilized for heat and power production and recovery of cooking chemicals. Worldwide, there are several challenging biomass sidestreams that can be utilized in a similar fashion as with black liquor. Some examples of these are vinasse from the integrated sugar-ethanol production process; straw and manure from agriculture sources; forest residues; by-products from the food industry; etc. This book chapter will review the availability of these types of feedstocks and discuss their applicability and challenges to be used for energy and chemicals. Pyrolysis, gasification, and combustion are the potential thermal conversion options considered for the utilization of these types of challenging biomass feedstocks.

**Keywords:** biomass feedstock, ash, impurities, thermal conversion, renewable energy, circular economy
