Renewable energy is generally defined as energy that comes from resources which are continually replenished on a human timescale such as sunlight, wind, tides etc. The biomass (wood chips and shavings) forming the source of most of the energy used in the kraft pulping process can be a renewable energy source, provided that the forests were managed in a fully sustainable manner.
Through the process of photosynthesis plants capture the sun's energy. When the plants are burnt, they release the sun's energy that they contain. In this way, biomass functions as a sort of natural battery for storing solar energy and the kraft process is an efficient way to reuse this energy.
(Based on a Wikipedia definition)