Possible routes for decarbonisation

Poll results and the discussions: A recent poll conducted in Linkedin and the results discussed as follows:

1.According to the poll recently conducted 73% of people said, “decarbonization” means to reduce Carbon emission. How to reduce CO2 emission when every time we switch our lights on or start our car engine CO2 is automatically emitted? It is possible only when the electricity we use (lights or Electric car) have zero or substantially reduced carbon footprint. Each individual house can have roof top solar panel with storage battery just for their consumption so that they can achieve zero carbon footprint. Alternatively small house holds (hundreds to thousands) can collectively install fully automated micro grids for their power generation and distribution network using solar and wind with battery storage and not to export or import from the centralized grid meant for large power generators for industrial applications. They can also have their own gas network (mixture of 80% natural gas + 20% renewable Hydrogen) for individual CHP applications. The centralized grid should have a zero emission or substantially reduced Carbon emission highlighted in the following paragraphs.

2. Zero percent people said Carbon should be substituted entirely by Hydrogen. The top 10 GHG emitting countries can use either EV or Fuel cell vehicles or a combination of these two for transport applications provided the electricity supply have a zero or substantially reduced Carbon footprint. For power and heating/cooling requirements individual houses can install their own CHP units using gas network (a mixture of 80% natural gas + 20% renewable hydrogen). Fuel cell cars can use renewable Hydrogen generated using PV solar/ wind turbine.

3. 13% of the people voted for adding Hydrogen to carbon. A distributed power system using syngas (a mixture of CO and Hydrogen) as a fuel to generate electricity and district heating and cooling using waste heat can be installed. The resulting CO2 emission along with water vapor can be captured and recycled in the form of syngas using PEM or SOFC electrolyzers.

4. 13% of the people voted for Carbon to disappear. I guess they prefer Carbon capture and use or storage (CCUS) or Carbon capture and sequester deep underground. This technology is yet to be proven commercially on large scale especially by power plants using coal. But “making carbon disappear” is impossible because it violates the fundamental law of physics (matter can neither be created or destroyed). It can be stored temporarily deep underground, but I question the technical feasibility and economic viability of such a scheme. Coal has been used for power generation due to its cheap availability and cheap cost of power generation despite a low electrical efficiency at 32%. But CO2 content in the flue gas is only around 11% and recovery of CO2, compression, long distance transportation and sequestration may substantially increase the cost of CO2 disposal making electricity very expensive. It will be simply unviable.

Top 10 GHG (greenhouse gases) emitters in the world

(Source: World resources institute)

The top three GHG emitters- China, EU and USA contribute 41.5% of the total global emissions while the bottom 100 countries account for only 3.6%. Collectively the top 10 emitters account for over two third of the global GHG emissions according to WRI.

Summary of Life cycle GHG emission intensity (Source: World nuclear association report) 

Technology	Mean	Low	High
tones CO2e/GWh
Lignite	1,054	790	1,372
Coal	888	756	1,310
Oil	733	547	935
Natural Gas	499	362	891
Solar PV	85	13	731
Biomass	45	10	101
Nuclear	29	2	130
Hydroelectric	26	2	237
Wind	26	6	124

About 84% % of the world’s energy in the year 2020 was met only by fossil fuels according to Forbes based on BP’s annual review.  Therefore, CO2 emission reduction should be targeted mainly by power generation and transportation industries two major users of fossil fuels.

Various methods of using fossil fuels for power generation and their CO2 emissions are shown below assuming Oxy combustion and gasification are used.

Fuel                  Process                      Reaction               CO2 emission by wt. percentage 

1. Coal           combustion              C + O2 => CO2           100% 

• Coal            Gasification        2C + H2O + O2 => CO +H2 +CO2.       97.30% 

• Natural gas  Combustion        CH4 + 2O2 => CO2 + 2H2O              52 % 

• Diesel   Combustion     C13H28 + 20 O2 => 13 CO2 + 14 H2O.     69.4 % 

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Note: 

THE OXIDANTS USED IN ALL THE ABOVE PROCESSES ARE PURE OXYGEN AND NOT AIR

(Air oxidation will show low CO2 emission by weight percentage due to large portion of Oxides of Nitrogen, Nitrogen and excess oxygen present in the flue gas)

1.By simply closing all coal operations and switching over to natural gas for power generation the CO2 emission can be reduced by 48% compared to coal and by 17.4 % compared to Diesel.  It is critical top 10 emitters of GHG emission should close all their coal fired power plants by 2022 or impose Carbon tax at the rate of $250/Mt to force such closures. CCS or CCUS can be allowed by coal fired power plants provided such technologies are commercially proven and verifiable. Otherwise, Carbon penalty should apply retrospectively.

2 All gas fired power plants can use either natural gas or Syngas (H2 +CO mix) using Oxy combustion to generate power and achieve an electrical efficiency of at least 65% by bottom cycling with sCO2 power cycle using waste heat or 85% using CHP application. Synthetic natural gas (SNG) can substitute natural gas (fossil origin) by using DIC dissolved inorganic in the form of CO2 recovered from seawater and renewable Hydrogen so that SNG will be Carbon negative. Alternatively, CO2 recovered directly from air can be used to synthesize SNG using renewable hydrogen. Carbon pricing will encourage such Carbon negative fuels.  Fuels synthesized from captured CO2 from natural gas fired power plants and hydrogen should be treated as “Carbon neutral’ till 2022 and it should attract carbon tax beyond 2022.

3.Oxy combustion closed super critical CO2 power cycle using natural gas is to be encouraged by enabling pipeline CO2 to be recycled in the form of renewable synthetic methane gas (RSMG) using renewable Hydrogen thus achieving zero emission. It should be confined to individual location and RSMG should not be allowed to be exported but recycled within the premises.

4.CO2 emissions by transport can be reduced by 17.8% by substituting diesel vehicles with CNG by countries other than the top 10 emitters. Top emitting countries can use Fuel cell using renewable Hydrogen banning IC engine using fossil fuels or allow Electric vehicles with Fuel Cell extenders.

5. Deployment of largescale renewables such as solar and wind as well as biomass technologies substituting coal fired power plants will be the key. However renewable energy is only intermittent and will require large scale battery for energy storage. Even battery production emits 150-200 kgs of CO2 per kwh based on the energy consumption @97-181 kwh per kwh battery production (Nearly 200 times more CO2 emission than coal fired power plants). Therefore, utility scale batteries should be justified. Therefore, Bioenergy can play a major role in countries like Australia, African countries, Indonesia, India and Brazil in decarbonization especially biocrude can be converted into renewable synthetic fuels as Carbon neutral fuels.

6. Renewable energy such as solar and wind can be stored in the form of syngas by electrolysis of CO2 emissions from Oxy combustion of natural gas or by gasification of coal as shown above. Low temperature electrolysis using PEM or high temperature electrolysis using SOFC (solid oxide fuel cell) can convert CO2 into syngas. Both the processes have been already demonstrated. Syngas can be stored under pressure, and it can be used as a fuel for a continuous production electricity using Oxy combustion such as sCO2 Brayton cycle and recycling CO2 in the form of Syngas.

CO2 + H2O => H2 + CO (by electrolysis using PEM or SOFC)

7.Using Oxy combustion of natural gas in closed super critical CO2 Bryton power cycle and recycling CO2 internally in the form of RSMG using renewable Hydrogen, ZERO EMISSION base load power can be achieved. The advantage of this system it requires natural gas only for the start-up and it can generate RSMG internally using renewable Hydrogen. It can generate baseload power with zero emissions. And the electrical efficiency of such as system can be up to 65%. It runs completely using only renewable energy sources such as solar and wind. Water electrolysis using PEM or Alkaline Electrolyzer have been commercially proven.

It does not require any energy storage at all. The power can be directly exported to the centralized grid as well as imported from the grid for hydrogen generation. 

By adopting CRT (Carbon recycling technology) outlined above it is possible to achieve zero emissions by power plants and supply power to all industries including transport industries. 

By the introduction of Electric vehicles and Fuel cell vehicles replacing petrol/diesel vehicles the electricity demand will sharply increase in some countries which will proportionately increase GHG emissions. CRT can eliminate GHG emissions as shown above.

The best option is to generate base load electricity with zero GHG emissions using CRT using sCO2 power cycle and recycling CO2 in the form of RSMG and converting waste heat into electricity by bottom cycling using sCO2 power cycle thus increasing the electrical efficiency to more than 70-75%. Advanced bioenergy to convert biomass directly into biomethane can play a major role in decarbonization. It will require massive plantation of high CO2 absorbing short life plant varieties all over the world but unlikely to happen.

Implementation of the above technologies will require massive amount of water especially for renewable hydrogen and for biomass production and gasification and the major source will be the sea. Advancement in seawater desalination such as high recovery, low energy consumption, better concentrate management by recovering value added chemicals and minerals and substituting solar salt by high purity brine directly from seawater desalination will be required, achieving zero liquid discharge in SWRO plants will be critical to eliminate global warming by highly concentrated effluent discharge. All SWRO plants should use only renewable energy sources sch as solar and wind or Hydro.

The above suggestions are purely based on the author’s assessment based on his personal experience in the industry for the past 40 years.