An internally fired boiler has which of the following characteristics?

An internally fired boiler is characterized by having its combustion chamber located inside the boiler itself. This design allows for better heat transfer efficiency as the heat generated by the combustion gases can directly interact with the water or steam inside the boiler.

When grates, which are used to hold fuel during combustion, are enclosed within the boiler shell, they facilitate this internal combustion process. This configuration also typically leads to a more compact design and can help with effective control of the combustion process and heat generation.

In contrast, the other characteristics presented in the options do not align with the design and functionality of an internally fired boiler. For example, having the combustion chamber outside the boiler describes an externally fired boiler. Operating without water would not classify it as a boiler at all, as the primary function of a boiler involves heating water to generate steam. Additionally, the presence of moving components, which could include mechanisms for fuel feeding, ash removal, or other operational features, contradicts the typical design of an internally fired boiler and does not consider the various mechanisms typically involved in boiler operation.