A biomass boiler burns wood pellets, chips, or logs to heat water, which then circulates through radiators or underfloor pipes just like a conventional gas or oil system. The fuel is classed as renewable because the carbon released during burning was absorbed by the trees during growth, giving it a much lower net carbon footprint than fossil fuels. It is not, however, zero-carbon - combustion still produces particulates and CO2, and the supply chain has its own emissions.
The practical requirements are substantial. You need somewhere to store the fuel - a pellet hopper or chip store that stays dry and is accessible for delivery lorries. In a tight urban churchyard, that can be a dealbreaker. The boiler itself is larger than a gas equivalent and needs a proper flue, which on a listed building will require faculty consent and careful design to minimise visual impact. Someone also needs to manage the system: emptying the ash, arranging deliveries, and keeping an eye on the fuel level.
Biomass works best in rural settings where there is no mains gas, space is not an issue, and there may be a local source of wood fuel. Some churches in wooded parishes have even arranged community wood chip supplies, keeping costs low and the carbon miles minimal.
The Church of England does not currently count biomass as a net-zero technology for its 2030 targets, so a biomass installation alone will not satisfy a diocese looking for a decarbonisation plan. If your church is off the gas grid and needs a practical heating solution now, biomass is a credible interim step while the economics of heat pumps and building fabric improvements continue to develop.