What is Biomass? Biomass is biological material derived from living, or recently living organisms. It most often refers to plants or plant-based materials which are specifically called lignocellulosic biomass. Wood remains the largest biomass energy source to date. Wood-based Biomass comes in various forms. The two main types are woodchip and pellets. Harp Renewables Biomass energy systems can be supplied to use either woodchips or pellets. This provides Harp Renewables with the flexibility to present their clients with the best biomass system for their site. To date, we have used the Twin Heat Biomass Boiler in the majority of our site projects.