PATHWAY OF ENERGY TRANSITION

Climate change due to human-caused emissions is responsible for global warming. Due to global warming, extreme and abnormal weather conditions like – untimely rainfalls, floods, heatwaves, snowfall, and droughts are taking place. Before the industrial revolution, the CO₂ level was around 280 ppm which increased to 421 ppm in 2022. Emissions from burning fossil fuels for power generation are one of the major factors responsible for global warming.

Energy transition is a global initiative to phase out fossil fuels to renewable and clean energy sources by 2050. Energy transition is a shift in technologies that are needed to replace one source of energy with another.

For energy transition, the following promising technologies/methods are the future pathways.

1. Modification of operating practices in existing thermal power plants.
2. Substitution of fossil fuel with biomass by using the following ways;
   •  Direct combustion (Non-torrefied and torrefied pellets)
   •  Thermochemical conversion (gasification and pyrolysis)
   •  Biological conversion (bioethanol).

3. Increasing renewable energy in the power mix.
4. Renewable energy with storage.
5. Green hydrogen
6. Green ammonia
7. Waste heat recovery.
8. Carbon capture

1. MODIFICATION IN EXISTING THERMAL POWER PLANT OPERATION PRACTICES

The addition of renewable energy to the power mix will create challenges in the operation of power plants. To maximise the use of renewable energy, following modified operating practices will be required.

1. Demand side management.
2. Two-shift operation.
3. Frequent start stops.
4. Minimum start-up time.
5. Higher load ramp rate.
6. Lower minimum technical load.
7. Managing consequences of cycling.
8. Performance improvement.

During low-load or high-load demands, renewable energy availability or during fluctuations in renewable energy sources, the load on the thermal power plant is to be adjusted. So, thermal power plants will need to be operated at higher load ramp rates (2%-3%/Min.), lower minimum technical load and frequent unit start-ups with minimum start-up time. The minimum technical load (target TML is 40%) is the possible minimum operating load on a thermal power plant without affecting the operating parameters and performance. As coal-based power plants are not designed for prolonged operation at minimum load, reducing to a minimum sustainable load will create many challenges concerning reliability, efficiency, and equipment health.

Two-shift operation is another option available to maximise RE Power. Mostly older and small-size units can be put into operation from 5 PM to 10 AM and shut down during the solar generation period and kept in hot standby. Reserved shutdown is the temporary closure of a thermal power plant until the demand for electricity rises. Higher efficiency of power plants reduces fuel consumption in thermal power plants.

    2. SUBSTITUTION OF FOSSIL FUEL WITH BIOMASS

The use of biomass, which is a carbon-neutral and renewable source of fuel, is a solution to reduce fossil fuel consumption.  Large amounts of agro waste and corp residue are available in India. The use of this biomass is the best energy transition solution.

Various technologies are available to use biomass for power generation. Following are some options.

1. Direct combustion (burning) in the boiler (non-torrefied and torrefied pellet).
2. Thermochemical conversion (pyrolysis and gasification)
3. Biological or enzymatic conversion (bioethanol)

A. Direct combustion (burning) in the boiler

Biomass cofiring with coal is one option to reduce coal consumption. But it has some challenges. The alkali metals in biomass like- potassium (K) and sodium (Na) along with chlorine, are the main elements in biomass ash causing deposition/slagging of pressure parts. Also due to the low energy density of biomass, it is a challenge to fire raw biomass in a boiler. Densification of biomass is a solution to this problem. A pellet is a sized and densified material made from raw biomass.

Non-torrefied and torrefied biomass pellets are suitable to cofire along with coal in a boiler.

A biomass pellet is a densified, high-energy density material made from raw biomass and corp waste. Biomass pellet is made from raw biomass in the following process.

Crop waste——– Drying——–Crushing ——— Compression ———Pellet

Torrefaction improves the physical properties, chemical composition, energy and storage properties of biomass. During torrefaction, biomass is heated slowly in an inert or oxygen-deficit environment to a maximum temperature of 300°C. When biomass is heated, the moisture and various low-calorific volatiles, contained in the biomass are driven out. During this process, mainly the hemicellulose in the biomass decomposes. The torrefaction process creates a solid uniform product with lower moisture and higher energy content than the raw biomass. The property of torrefied biomass is close to that of coal. The torrefied pellet is very suitable to cofire in the boiler.

Non-Torrefied and torrefied pellets

B. Thermochemical conversion to produce solid, gaseous, and liquid fuels.

• Gasification
• Pyrolysis

Gasification

Gasification is a process that converts biomass to gas at high temperatures (>700°C), without combustion, with a controlled amount of air/oxygen/steam. The product gas, known as synthesis gas or syngas is a mixture of carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), methane (CH₄), water (H₂O), and nitrogen (N₂). The calorific value of this gas depends on the gasification agent used.  It is low if produced using air and is more if produced with oxygen and/or steam. Syngas can be used in a boiler to generate steam for power generation.

Biomass Gasification process

Biomass gasification is also a hydrogen production route other than the electrolysis method. Green hydrogen can be produced using renewable energy. Hydrogen can be produced from the syngas. Carbon monoxide reacts with water to form carbon dioxide and hydrogen via a water-gas shift reaction. Adsorbers or special membranes can separate the hydrogen from this gas stream.

CO + H₂O → CO₂ + H₂ (+ small amount of heat).

Methanol can also be produced from the biogas gasification process. The syngas (a mixture of carbon monoxide and hydrogen) are conditioned to remove impurities such as tars and methane and to adjust the hydrogen-to-carbon monoxide ratio to 2:1. The syngas are then reacted over a catalyst at elevated temperatures and pressures to form methanol(CH₃OH).

Pyrolysis

Pyrolysis is a thermochemical technology available to convert biomass to a liquid product that can be refined to get biofuels. Fast pyrolysis of biomass produces three major components: a solid charcoal-like material (biochar), a liquid tar-like oil (bio-oil), and non-condensable gases (syngas). This process yields roughly 60% bio-oil, 20% biochar and 20% syngas.

During pyrolysis, biomass is heated rapidly at high temperatures (500°C–700°C) in an oxygen-free environment. The heat breaks down biomass into pyrolysis vapour and gas. Most of these combustible gases can be condensed into a combustible liquid, called pyrolysis oil or bio-oil. Non-condensed gases like CO₂, CO, and H₂ can be used to provide the heat for the process. In a fast pyrolysis process, the yield of bio-oil is optimum when the pyrolysis temperature is around 500°C and the heating rate is high (1000°C/s).

Pyrolysis can be performed at a relatively small scale and in remote locations and reduce transport and handling costs. 

C. Biological or Enzymatic conversion (bioethanol)

Making alcohol from natural and agricultural products with high starch or sugar contents is an old-known process. Sugar is converted into ethanol by fermentation and distillation process. Ethanol (CH₃CH₂OH) or ethyl alcohol is a clear colourless liquid.

Plant materials (grain, stems and leaves) are composed mainly of sugars, so almost any plant can be used to produce ethanol. Ethanol is used as a blending agent with Petrol.

Generally, the bioethanol production process from biomass consists of four main steps pre-treatment, hydrolysis, fermentation distillation.

Cellulosic biomass, such as crop residue, consists of a complex mixture of carbohydrate polymers like- cellulose, hemicellulose, and lignin. The pre-treatment process reduces the feedstock size, breaks down the hemicellulose into sugars, and opens the structure of the cellulose component. In enzymatic hydrolysis, cellulose chains are broken into glucose molecules by cellulose enzymes, in a process like what occurs in the stomach of a cow to convert grass or fodder cellulose into sugar. Fermentation is a biological process where yeast converts sugars into alcohol and carbon dioxide as by-products. Distillation is done to separate the ethanol by boiling and condensing.

3. INCREASING RENEWABLE ENERGY IN THE POWER MIX

Renewable energy, often referred to as clean energy, comes from nature that is replenished. Most renewable energy sources are sustainable sources and available plentily around us, like- sunlight, wind, ocean tide, geothermal etc.

Increased share of renewable energy can reduce dependency on fossil fuel power generation. Electricity generation techniques from renewable sources like- solar PV, wind turbines, tidal and wave turbines, geothermal and hydropower can provide green energy in our power mix.

4. RENEWABLE ENERGY WITH STORAGE

Due to the growing share of Renewable power, the importance of energy storage is increasing. Energy storage can help with fluctuations in solar and wind power. It can respond rapidly to large fluctuations and demand.

Following are the prospective energy storage solutions.

• Battery energy storage systems (BESS)

Lithium-ion batteries and flow batteries are the two technologies most preferred for Energy storage. Lithium-ion batteries (LIBs) are the most popular battery storage option today. Other options like lead-acid, sodium, and nickel-based batteries are also available.

The 400MW/1,600MWh Moss Landing Energy Storage Facility, California, US is the world’s largest lithium-ion battery storage system in the world. Battery systems consist of energy storage containers of battery modules placed in vast areas.

LIBs are having higher safety risk as they are flammable, toxic and explosive. Also, increasing the use of LIBs will require a huge amount of Lithium. Lithium mining has a very negative impact on the environment. Disposal of battery waste is also a challenge. 

A flow battery, using newer technology is the future of energy storage. In redox-flow batteries, two electrolyte solutions known as catholyte, and anolyte are forced to flow in a membrane that permits protons to pass through while electron transfer occurs. Dalian Flow Battery Energy Storage Peak- Shaving Power Station, China uses a vanadium flow battery of 100 MW/400 MWh capacity.

• Thermal storage

Thermal energy storage is a technology in which a fluid, such as water or molten salt (sodium nitrate NaNO₃ and potassium nitrate KNO₃) is used to store heat. Thermal energy storage technology is installed along with concentrated solar power plants (CSP). Solar heat collected during the daytime is stored in hot molten salt tanks. Later, the hot molten salt is used to generate steam in a steam generator to drive turbines for making electricity on demand. The cooled molten salt is again heated and the cycle continues.

• Pumped storage hydropower

A pumped storage hydropower plant is an electricity storage arrangement in which surplus electricity is stored for later use. It has two water reservoirs at different elevations. Power is generated when water passes down from the Upper reservoirs (discharge), through a turbine. During the surplus period, water from the lower reservoir is pumped back into the upper reservoir (recharge).

5. GREEN HYDROGEN

Hydrogen is emerging as one of the leading options for storing energy from renewables. Renewable hydrogen or green hydrogen is obtained by electrolysis of water using green power.

Green hydrogen is obtained by separating hydrogen from oxygen through the electrolysis of water by using renewable energy. To produce 1 kg of hydrogen, about 9 litres of water and about 50 kWh of electricity are required. GCV of hydrogen is around 35850 kcal/kg.

6. GREEN AMMONIA

Ammonia can be used as fuel to produce electricity. Green ammonia is produced by using renewable energy sources. It is produced by using hydrogen from water electrolysis and nitrogen separated from the air. These are then fed into the Haber process (also known as Haber-Bosch). In the Haber process, hydrogen and nitrogen are reacted together at high temperatures and pressures to produce ammonia, NH₃.

7. WASTE HEAT RECOVERY

Large quantity of hot flue gases is generated from boilers, kilns, ovens and furnaces. If some of this waste heat can be recovered, much fossil fuel could be saved. So due importance to waste heat recovery is to be given.

8. CARBON CAPTURE

Carbon dioxide (CO₂) is naturally captured from the atmosphere through biological, chemical, and physical processes. Biological carbon sequestration is the natural ability of ecosystems to store carbon– such as forests and oceans. It removes CO₂ from the atmosphere by plants and microorganisms and stores it in vegetative biomass and soils. The process can be accelerated through changes in land use and agricultural practices, such as converting cropland into land for non-crop fast-growing plants.

Artificial/geological carbon capture involves the capture of carbon dioxide (CO₂) from the gas stream. The captured CO₂ gas is then compressed to liquefy. This liquid carbon is then transported to a storage site, generally through a pipeline and stored deep underground, so it will not enter the atmosphere.

Author

A R Mallick

(Author -Practical Boiler Operation

Engineering and Power Plant)