Introduction:
Steam boilers are widely used in various industries for generating steam, which is used for heating, power generation, and other industrial processes. Coal is one of the primary fuels used in steam boilers due to its abundance and relatively low cost. Different varieties of coal have different properties, which determine their suitability for use in steam boilers.

Explanation:
1. Non-coking bituminous coal:
Non-coking bituminous coal is a type of coal that has relatively high carbon content and low moisture content. It is commonly used in steam boilers due to its favorable properties such as high calorific value, low ash content, and good combustion characteristics. The high carbon content ensures efficient combustion, while the low ash content reduces the formation of ash deposits in the boiler. This variety of coal is widely available and economically viable for use in steam boilers.

2. Brown coal:
Brown coal, also known as lignite, is a low-grade coal with high moisture content and low carbon content. It has lower calorific value compared to other varieties of coal, making it less suitable for use in steam boilers. The high moisture content of brown coal leads to inefficient combustion and increased emissions. Therefore, it is not commonly used in steam boilers.

3. Coking bituminous coal:
Coking bituminous coal, also known as metallurgical coal, is primarily used in the production of coke, which is a key component in the steelmaking process. It has high carbon content and low ash content, similar to non-coking bituminous coal. However, coking bituminous coal undergoes a process called coking, where it is heated in the absence of air to drive off volatile components and produce coke. Due to its specialized use in the steel industry, coking bituminous coal is not commonly used in steam boilers.

4. Peat:
Peat is an early stage of coal formation and has relatively low carbon content and high moisture content. It is not suitable for use in steam boilers due to its low calorific value and poor combustion characteristics.

Conclusion:
Among the given options, non-coking bituminous coal is the most suitable variety of coal for use in steam boilers. Its high carbon content, low ash content, and good combustion characteristics make it an ideal fuel for efficient and reliable steam generation. Other varieties such as brown coal, coking bituminous coal, and peat have properties that make them less favorable for use in steam boilers.