Boiler Low Load Operation
Package and field erected boilers with single or dual burners are designed to operate at as low as 10% of maximum continous rating (MCR). There are limitations on the boiler and its control for any continous operating at this low of a load.
At a very low load, the resulting flue gas temperatures are very low. There is relatively little heat available to be transferred to the generating bank tubes in the last sections or the last pass of the boiler tube bank. This results in low circulation rates in these tubes which can potentially cause solids to settle out and build up in the mud drum, bottom headers and horizontal floor tube sections. A steady operation for long periods of time at low loads can then result in the accumulation of deposits and scaling in these tubes of the generating bank. The flue gas temperatures at low loads are low enough that boiler tubes will not overheat due to any accumulated deposits. When the boiler's load is increased though, the flue gas temperature increases, resulting in a potential to overheat any scaled tubes resulting in blisters and possible tube failures.
During a prolonged period of operation at a low load, regular short mud drum blowdowns will assist in moving accumulated solids in these areas. A regular increase in boiler load up to 75% MCR for a brief period can suspend any concentrated settled solids in floor tubes and headers. A test of boiler water will then give the total suspended solids to ensure it is maintained within design operating limits.
The flue gas on natural gas fired boilers will contain about 11% water vapour. At prolonged low boiler firing rates this water vapour can condense and accumulate in the economizer's outlet duct and the boiler's stack. Ensuring stack and boiler exit duct drains are regularly opened to remove all accumulated water, will reduce corrosion in the stack base and boiler's exit breeching.
The forced draft fan has a minimum combustion air flow rate of 20%, by code, and at low loads this results in high excess air. The high combustion air flow can cause problems with burner stability. A single burner has a minimum low load level where the flame will remain stable and the burner management can reliably monitor the flame.
The combustion control system measures air flow, gas flow and excess oxygen. The drum level control system measures the boiler's steam drum level and feedwater flow. At a 10:1 turndown or greater, the limits of the flow transmitters and the control valves for the air, fuel and feedwater may be reached. The combustion control system is usually configured to operate the fuel/air controls in manual at loads less than 10% MCR. The practical limit for the turndown of a boiler and its auxiliary systems is in the range of 7:1.
A gas-fired boiler should be able to accommodate load swings of 15% MCR per minute above its 50% MCR with the proper tuning of the combustion control and drum level control. Below a 50% boiler load, a 10% MCR per minute limit on load swings may be a more practical rate. The limiting of the response rate to load swings at the lower loads, will avoid overheating damage to the boiler's tubes.