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  • Writer's pictureArnab Sinha

Enhancing the Viability of Coal Gasification: The Advantages of Novonanmek’s Hybrid Sorbent technology over Amine Scrubbing for CO2 Capture





Introduction


The viability of coal gasification hinges on two critical pillars: the energy efficiency of the process (energy generated from syngas – energy required to gasify coal) and the removal of CO2 from syngas, both essential for its sustainability. Currently, coal gasification is not highly efficient, with an estimated energy requirement of 12,308 MJ to gasify one tonne of coal, producing syngas with an energy content of 18,750 MJ. This results in a net gain of 6,442 MJ considering 100% efficiency of energy generation from Syngas. Even with an optimistic energy scenario of around 50%, the net gain becomes negative. The net gain becomes – 2993 MJ/tonne of coal gasified.


When combined with amine scrubbing for CO2 capture, which requires around 4000-5000 MJ of energy per tonne of CO2 captured in the process of its regeneration, the net energy gain could reduce significantly or even become further negative (Net gain of – 6993), rendering the process almost non-viable.


In contrast, Novonanmek’s hybrid sorbent process (Closed Loop Hybrid Sorbent Carbon Capture Technology) using NaOH and CaO involves no heating and requires minimal energy for CO2 capture, making it a more suitable solution for coal gasification. This process efficiently captures CO2 from syngas without the high energy penalties associated with amine scrubbing, thus enhancing the overall sustainability and feasibility of coal gasification as an energy conversion technology.


Energy Required to Gasify 1 Tonne of Coal


Key Components in Energy Requirement Calculation:


  • Heat Required to Raise Temperature: Energy needed to heat the coal and gasification agents (e.g., steam, oxygen) to the operating temperature of the gasifier.

  • Endothermic Reaction Energy: Energy consumed by the endothermic reactions within the gasifier.

  • System Efficiency: Overall efficiency of the gasification process, including heat losses and other inefficiencies


Typical Energy Requirements


Heat to Raise Temperature:


  • The operating temperature of a gasifier typically ranges from 800°C to 1500°C.

  • Specific heat capacity of coal is approximately 1 kJ/kg·°C.

  • For a rough estimate, assuming an average heating requirement and coal starting at ambient temperature (~25°C):


Qheat = m. c. DT

Qheat = 1000 Kg x 1KJ/kg 0C x (1000 0C – 25 0C)

Qheat = 975,000 KJ

Qheat = 975 MJ

 

Endothermic Reaction Energy:


The primary endothermic reaction in coal gasification is the reaction of carbon with steam:

C + H2O → CO + H2

This reaction requires approximately 131 kJ/mol of carbon.

 

For one tonne of coal (~70% carbon by mass), the amount of carbon is:

Carbon mass = 1000kg×0.70=700kg

 

Moles of carbon: Moles of carbon=700 kg/12 g/mol=58,333 mol

Energy required for the reaction:

 

Qreaction = 58,333mol × 131 kJ/mol

Qreaction = 7,641,423kJ

Qreaction = 7,641MJ

 

Total Energy Requirement and Efficiency:

Summing the energy required for heating and the endothermic reactions:

Qtotal = Qreaction + Qheat

Qtotal​=975MJ+7,641MJ  

Qtotal​=8,616MJ

 

Considering the efficiency of the gasification process (assuming a typical efficiency of 70%):

Effective Energy Requirement = Qtotal/Efficiency

Effective Energy Requirement = 8616 MJ/0.7

Effective Energy Requirement = 12,308MJ

 

Energy that can be produced from Syngas that is produced from 1 tonne Coal


Syngas typically contains hydrogen (H2) and carbon monoxide (CO) as its main components, along with smaller amounts of carbon dioxide (CO2) and methane (CH4). The energy content of syngas can vary based on its composition, but for estimation purposes, we can use the lower heating value (LHV) of syngas, which is typically in the range of 10 to 15 MJ/Nm3 (mega joules per normal cubic meter).


Calculation


Syngas Production:

The gasification process converts coal into syngas.

Let us assume a typical yield of syngas from coal gasification is around 1,500 Nm3 per tonne of coal.


Energy Content of Syngas:

The gasification process converts coal into syngas.

 

Typical Syngas Composition (volume percent):

H2: 30-40%

CO: 20-30%

CO2: 5-15%

CH4: 0-5%

N2 and other gases: remainder

 

Let us assume a typical yield of syngas from coal gasification is around 1,500 Nm3 per tonne of coal. Assuming an average lower heating value (LHV) of 12.5 MJ/Nm3 for coal-based syngas, we can calculate the total energy content of the produced syngas.


Energy Content of Syngas = Syngas Production × LHV of Syngas

Energy Content of Syngas=1,500 Nm3 × 12.5 MJ/Nm3

Energy Content of Syngas=18,750 MJ


Theoretically, from one tonne of coal gasified, approximately 18,750 MJ of energy can be generated from the produced syngas, assuming an average lower heating value (LHV) of 12.5 MJ/Nm3 for the syngas. This energy can be utilized for various purposes, such as power generation, heat production, or conversion to fuels and chemicals.


When converting the energy produced from syngas (18,750 MJ) to kilowatt-hours (kWh), it is essential to consider the efficiency factor of the system that converts the syngas energy into usable electricity. This efficiency factor accounts for losses during energy conversion processes, such as in power generation.


Typically, the efficiency of power generation systems varies depending on the technology used. For example, conventional steam turbine power plants have an efficiency ranging from about 30% to 40%, while combined cycle gas turbine (CCGT) plants can achieve efficiencies of up to 60%.


In an optimistic scenario, let us assume an efficiency of 50% for power generation from syngas. This means that only 50% of the energy contained in the syngas can be effectively converted into electricity, while the rest is lost as waste heat.


Calculation with Efficiency Factor

Energy =Energy from Syngas (MJ) × Efficiency Factor

Energy =18,750MJ × 0.50

Energy =9375 MJ

Energy (KWh) = 2,604 KWh


Therefore, when accounting for the efficiency factor of 50%, approximately 2,604 kilowatt-hours of electricity can be generated from the produced syngas from one tonne of coal gasified. This adjustment reflects the reality that not all the energy from the syngas can be converted into usable electricity due to inefficiencies in the conversion process.

It is clear from the above that the net-gain is negative.


Net Gain = 9375 MJ - 12,308MJ

Net Gain = - 2933 MJ


CO2 Produced Per Tonne of Coal Gasified


The amount of CO2 produced per tonne of coal gasified can vary depending on the type of coal, the gasification process, and the specific conditions under which the gasification occurs. However, we can estimate the CO2 production based on typical compositions and reactions involved in coal gasification.

 

Typical Coal Composition: Coal is primarily composed of carbon (C), along with hydrogen (H), oxygen (O), sulphur (S), and other elements. For simplicity, let's assume a typical bituminous coal composition as follows:


Carbon (C): 60-80%

Hydrogen (H): 5-6%

Oxygen (O): 5-20%

Sulphur (S), Nitrogen (N), Ash: remainder


Gasification Reactions: During gasification, coal reacts with steam and oxygen to produce syngas (a mixture of CO, H2, CO2, and other gases). The main reactions include:

C+H2O→CO+H2

C+(½)O2→CO

CO+H2O→CO2+H2

C+O2→CO2


Carbon Content and Conversion: Assume that one tonne of coal contains 70% carbon (typical for bituminous coal), which equates to 700 kg of carbon


Calculation

Complete Combustion to CO2: If all the carbon in the coal were to be completely converted to CO2, the reaction would be:

C+O2→CO2

The molar mass of carbon (C) is 12 g/mol, and the molar mass of CO2 is 44 g/mol. Therefore, each mole of carbon produces one mole of CO2.


CO2 Production

Mass of CO2 = (44/12) * Mass of Carbon

For 700 Kg of Carbon (Assuming coal contains 70% Carbon)

Mass of CO2 = (44/12) * 700 Kg

Mass of CO2 = 2567 Kg = 2.57 Tonnes.


With 100% efficiency of conversion (Theoretically):


The energy generated with Syngas produced by gasifying a tonne of coal = 18,750 MJ

Energy required to gasify a tonne of coal = 12, 308 MJ

Net gain = 18,750 MJ – 12,308 MJ 

Net gain = 6442MJ



Considering Amine Based CO2 Capture


If Amine based scrubbing is used to capture CO2 from the gasification process, 4 - 5GJ of energy is consumed per tonne of CO2 captured only in the regeneration of the amine sorbent – also called the reboiler duty. Gasification of 1 tonne coal releases around 2.57 tonne of CO2. Therefore, considering an optimistic scenario, 4 GJ or 4000 MJ of energy is consumed per tonne of CO2 captured.

 

Energy consumed in capturing 2.57 tonne of CO2 with amine-based technology, considering an optimistic scenario = 10,280 MJ

This completely nullifies the net gain, making the net gain negative even when the efficiency of power generation system used is 100%.


But the efficiency of power generation systems is never 100% but range between 30%-50%.

Novonanmek’s Closed Loop Hybrid Sorbent Carbon Capture Technology involves no heating and the energy requirement is minimal, thereby making it a very suitable choice for carbon capture in coal gasification projects.


Conclusion


The viability of coal gasification as a sustainable energy conversion technology relies heavily on achieving high energy efficiency and effective CO2 removal from syngas. While amine scrubbing has been a widely used method for CO2 capture, its significant energy penalties associated with sorbent regeneration make the process less efficient and economically unfeasible. In contrast, the hybrid sorbent process using NaOH and CaO presents a more promising solution, requiring minimal energy for CO2 capture and avoiding the high energy costs of amine scrubbing. By improving energy efficiency and reducing the energy demands of CO2 capture, the hybrid sorbent method enhances the overall feasibility of coal gasification, making it a more attractive and sustainable option for meeting future energy needs while addressing environmental concerns.


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