Boilers: Automating operations, innovating for a more sustainable future

Subramaniam C.R., Group Head, Process Engineering at Thermax Babcock & Wilcox Energy Solutions Limited, explains how sophisticated automation and control technologies have transformed boiler operations.

What is the role of the boilers in power generation, and how do they convert steam into electricity?
Fossil fuel and biomass-based thermal power plants constitute approximately 60 percent of total power generation in India. The steam generated from the boiler is fed to the turbine, which rotates the turbine blades. Thus, the thermal energy in the steam is converted to mechanical energy in the turbine, which eventually drives the electrical generator.

What are the differences between gas and steam boilers, and what are the advantages and disadvantages of each?
Gas turbines generally work on the Brayton cycle, using mostly natural gas as fuel. The internal combustion of natural gas generates pressurised flue gases, which expand and flow through a series of stationary and rotating turbine blades. The gas turbine shaft is connected to a generator, which eventually converts it to electrical energy.

Gas turbines demand high fuel gas purity to avoid any potential damage to turbine internals and are viable when fuel gas costs are cheaper. Thus, in India, gas-based power generation still accounts for less than 7 percent of total power generation. Steam boilers work on the Rankine cycle, where the fossil fuel is burned in a combustion chamber and the hot flue gases heat water into high-pressure steam, which drives the steam turbine.

Steam boilers can accept a wide range of fuels, ranging from coal, lignite, biomass, charcoal, and petcoke, and efficiently combust and convert them into steam. However, these boilers need pollution control equipment to control particulate matter, SOx, NOx, and mercury. There are steam boilers employed as a waste heat recovery module downstream of HRSGs and downstream of steelmaking processes, cement kilns, SRUs, etc. These boilers convert the waste heat available from the process flue gas into useful steam and are thereby considered “green boilers” (due to no burning of fossil fuels).

What are the key factors to consider when selecting a boiler for backup power, and how do you determine the appropriate size and capacity?
The key factor in selecting a boiler is to assess the minimum electrical load that needs to be supported during a power outage. This includes systematic identification of the process requirements, the minimum number of pieces of equipment that need to be operated, and their power ratings during the power outage. Once the study is completed, the required boiler capacity can be determined. Generally, these backup boilers work on steam-driven fans and pump to drive the boiler operation using steam from the plant network. Thus, it is also important to assess the availability of steam for those steam-driven turbo equipment.

The other key factor is the speed of minimum power restoration. Thus, the backup boiler shall be of the type to ramp up quickly to provide the required backup power. Hence, the backup boilers are kept on standby and ramped up during power outages, and these boilers generally fire NG or LDO to cater for the same.

How do boilers operate, and what are the different types of fuel that can be used to power them?
Steam boilers take deaerated water, convert it to steam in the evaporator section, and then superheat it to the required temperature. This follows the principles of the Rankine cycle. The energy for steam generation is received through fuel combustion ions. The combustion technologies could be stoker, fluidised bed, circulating fluidised bed, pulverised coal-fired, etc. The typical fuels are Indian coal, imported coal, lignite, agricultural waste, and process waste like char and petcoke. Certain boilers use steel plant gases like BFG, COG,

and RFG as fuel, and the same is fired through specially designed burners.

What are the main maintenance considerations for boilers, and how can you ensure they operate safely and efficiently?
Generally, Oil and gas-fired Boilers need less maintenance due to the absence of ash. Solid fuel-fired Boilers need minor refractory makeovers, cleaning of the convection area, fuel feeder tuning, etc. during their annual shutdown. Boiler safety is ensured by advanced safety controls through alarms, interlocks, and trips configured for a given combustion technology and upgrading the set points continuously based on operation experience. The development of advanced automation and control systems has revolutionised boiler operations. Computerised control systems, special sensors, and actuators enable precise monitoring and regulation of various parameters such as temperature, pressure, fuel flow, and air-to-fuel ratio. These safety controls also monitor abnormalities in operation and will automatically shut down the boiler during adverse conditions with the help of advanced logic.

Also with IoT Boilers are now smartly connected and monitored remotely. Boilers can now self-correct their operation based on neural network-based machine learning programmes and can identify potential failures well in advance to take preventive action. Thermax’ “Edge Live” is focused on providing such an innovative solution to plant owners to provide insights for efficient operation and cues to prevent any unplanned shutdowns.

Regarding the efficiency of boilers, oil, and gas boilers need burner tuning for proper combustion, and for solid fuel-fired boilers, efficiency is ensured by adjusting the combustion equipment, such as the speed of the travelling grate, the optimum fluidising velocity of fluidised bed boilers, the tuning of excess air, maintaining the right fuel particle size distribution, and ensuring the cleanliness of heat transfer areas through the right amount of soot blowing.

What are the environmental impacts of boilers, and what steps can be taken to minimise their carbon footprint?
Any fossil fuel-fired boiler will add additional CO2 to the environment. Hence, the focus is now shifting to renewable fuel-based steam generation with good emission controls. There is a sea change in the boiler industry due to soaring fossil fuel prices and supply uncertainties. Most boiler designs now include a minimum percent of the firing capability of biomass fuels along with coal. In addition to this, the main efficiency improvement measures taken in the efficient burning of fuels will reduce fuel consumption and the parasitic power consumption of boilers. On an overall power plant level, the selection of an efficient turbine cycle will help reduce the energy required per unit of power generation.

How have technological advancements impacted the design and operation of boilers over time?
The boiler industry has navigated a transition from manned operation to automatic operation in the past 30 years. Technological advancements in terms of the introduction of special alloy steels to address high-temperature turbine cycles and welding procedures ensure safe operation and reliability. Specialised burners were developed by different OEMs and burner manufacturers to fire difficult fuels, making it possible to utilise them as fuels for steam generation and saving precious fossil fuels. The next major advancement has happened in emission control technologies such as FGD, SNCR, SCR, and PAC, which have enabled end users to use many waste fuels such as MSW, RDF, NRSW, and Petcoke. With increased focus on decarbonisation across various industries and the declaration of the carbon footprint of their products, they shifted to renewable-based steam and power. Thus, integrating fossil fuel boilers with renewable-based steam generation such as biomass boilers, solar panels, and wind turbines has gained focus recently.

What are the future trends and challenges in the development and use of these technologies, particularly in the context of sustainable energy and climate change?
The major trend in the boiler industry is to shift to low-carbon or renewable fuels such as biomass and green hydrogen. The challenge lies in developing an efficient combustion system and deploying the right safety mechanisms. The next untapped potential is to identify industrial waste heat sources and deploy waste heat recovery systems using advanced heat exchange technologies, which require advanced materials and the manufacturing of suitable metallurgy to withstand corrosion and erosion of those waste gas streams. There is a growing emphasis on the life cycle analysis of boilers, which makes boiler OEMs focus on reducing carbon footprint start raw material selection (use of recycled steel), optimising the manufacturing process, developing efficient combustion systems, reducing boiler parasitic power consumption, stack emission reduction, and finally end-of-life disposal.

In summary, the future trend is focused on renewable-based steam generation, waste heat recovery, and smartconnected boilers for efficient operation. This requires innovation at various levels, industry-academia collaborations, and supportive government policies and regulations to make the development sustainable.