How do modern biomass boilers work?

In the case of wood chip and wood pellet boilers, the fuel is fed automatically into the boiler from the fuel store using an auger screw system. The fuel then burns in the combustion chamber, where a regulated flow of oxygen ensures a clean and efficient combustion process. The resulting hot gases then heat water in a heat exchanger which feeds the hot water storage tank, or for smaller stoves a back boiler, and ultimately the heating circuit (radiators in most cases). You can read more about the specific design of combustion systems on most boiler installe and manufacturer websites; it makes interesting reading for the technically minded!
Some controls even allow the user to switch on the boiler remotely via text messaging!
Selecting the right boiler
There are several factors that will influence the type of boiler suitable for a  articular project. Below is a list of the most important points for consideration.
Space  If there is limited space on site then storage of wood fuel could be a restriction. Wood chips will occupy up three times more room than wood pellets for the same weight of wood.
Biomass boilers also tend to be larger than conventional fossil fuel boilers, you will need to have a large enough space to house the unit.
Size of the building  Typically the larger the building is, the greater requirement for space heating and therefore the larger the boiler needs to be. Larger systems will consume more fuel and therefore tend to be automated systems with minimal manual intervention. Biomass boilers with automatic auger feed mechanisms and fuel stores are generally too large for domestic installations. However, domestic systems allow the user to fill a hopper attached to the boiler.
Access  For most systems a fuel delivery vehicle will need to access your site. Fuel can be delivered in a variety ways, however for bulk chips and pellet orders direct access to the fuel store is critical. For small domestic deliveries it is important to have a dry place to stack logs or store bags of pellets. Projects in urban areas will need to be mindful of the number of fuel deliveries (large vehicle movements) necessary throughout the year as this may have planning implications.
Fuel supply You may have your own fuel supply, in which case using logs or chips would dictate the boiler type. Generally if space and access are not a problem larger projects would consider wood chip; however if space is at a premium or the area is sensitive to a greater number of fuel deliveries then pellets are the preferred option.