Techno-economics of CCU pathways starting with carbon-rich streams

Abstract

Emissions of greenhouse gases (GHG) are the main cause of climate change, with as effect a rise in global temperature. The culprit in these emissions is carbon dioxide (CO2). Carbon capture and utilization (CCU)Emissions of greenhouse gases (GHG) are the main cause of climate change, with as effect a rise in global temperature. The culprit in these emissions is carbon dioxide (CO2). Carbon capture and utilization (CCU) can mitigate a part of these emissions, by converting carbon species to useful products. The typical route for CCU involves: carbon capture, compression, transportation and utilization. This thesis takes on a slightly different approach. Instead of utilization at a centralized location, the utilization of carbon species will be realized on location. This is beneficial, since no transportation of CO2 is required. Three different high partial pressure carbon sources will be addressed: tail-gas (TG) from steam methane reforming, blast furnace gas (BFG) from the steel industry and biogas from the bacterial degeneration of biomass. Each of these gas streams are composed of up to 50-60 mol % CO2 and or CO. By capturing the carbon species in these gases or removing some of the inert components, different ratios of CO2, CO, H2 and inerts can be obtained. Electrolysis technologies will be used to upgrade the captured gases, with the addition of hydrogen or by adjusting the CO/CO2 ratio. The electrolysis technologies considered are PEM and SOEC. The advantage of electrolysis is the potential for integration with renewable energy sources such as, wind, solar and hydro power. Depending on the electrolysis technology and carbon source, syngas with CO/CO2 ratios varying between 0-4 can be obtained. Five different cases were defined with CO/CO2 ratios equal to 0, 0.25, 1, 3 and 4. The upgraded syngas can then be used in the conversion towards several liquid fuels, such as methanol and Fischer-Tropsch (FT) liquids. Both methanol and FT liquids require a stoichiometric ratio of at least 2. For these two product routes a techno-economic analysis was performed. As a first production route, methanol synthesis was discussed. The feasibility for each of the five cases was assessed, by comparing the cost price of the methanol with the respective market price. A reference methanol plant from literature was implemented in Aspen Plus to obtain the energy and mass balances for each case. The mass balances showed the amounts of hydrogen and carbon species required to produce one ton of methanol. The total cost price of the methanol consists of the operational expenses (OPEX) and capital expenses (CAPEX). The OPEX reflects the costs required to run the process. This includes the prices for the feedstock and labour costs for example. The CAPEX consists of the capital investments. Based on the CO/CO2, Case 3 with CO/CO2 = 4, should be the cheapest case as less hydrogen has to be added to produce one ton of methanol. However, Case 4 with a CO/CO2 = 0.25 is the cheapest option. Case 4 originated from the TG stream, which already had a high fraction of H2 present in the TG. Therefore, Case 4 requires less addition of hydrogen. Capture costs were also lower in Case 4, compared to the other cases. At a market price for methanol of €400 per ton, a maximum electricity price of €30 per MWh was allowable. FT liquids were overall found to be an unfeasible product, when considering renewable production of hydrogen and CO from CO2 electrolysis. In FT liquids, waxes (C20+) are a requirement in the product fraction as these can be sold at the highest price. When considering the production of gasoline and diesel, the renewable FT route proved to be unfeasible. In general it was found that for each case, irrespective of product, more than 50 % of the product cost price, was included in the costs for electrolysis. And as the electrolysis technologies require electricity to operate, it can be concluded that electricity is the determining factor.
can mitigate a part of these emissions, by converting carbon species to useful products. The typical route for
CCU involves: carbon capture, compression, transportation and utilization.
This thesis takes on a slightly different approach. Instead of utilization at a centralized location, the utilization of carbon species will be realized on location. This is beneficial, since no transportation of CO2 is
required. Three different high partial pressure carbon sources will be addressed: tail-gas (TG) from steam
methane reforming, blast furnace gas (BFG) from the steel industry and biogas from the bacterial degeneration of biomass. Each of these gas streams are composed of up to 50-60 mol % CO2 and or CO. By capturing
the carbon species in these gases or removing some of the inert components, different ratios of CO2, CO, H2
and inerts can be obtained. Electrolysis technologies will be used to upgrade the captured gases, with the
addition of hydrogen or by adjusting the CO/CO2 ratio. The electrolysis technologies considered are PEM
and SOEC. The advantage of electrolysis is the potential for integration with renewable energy sources such
as, wind, solar and hydro power. Depending on the electrolysis technology and carbon source, syngas with
CO/CO2 ratios varying between 0-4 can be obtained. Five different cases were defined with CO/CO2 ratios
equal to 0, 0.25, 1, 3 and 4. The upgraded syngas can then be used in the conversion towards several liquid
fuels, such as methanol and Fischer-Tropsch (FT) liquids. Both methanol and FT liquids require a stoichiometric ratio of at least 2. For these two product routes a techno-economic analysis was performed.
As a first production route, methanol synthesis was discussed. The feasibility for each of the five cases was
assessed, by comparing the cost price of the methanol with the respective market price. A reference methanol
plant from literature was implemented in Aspen Plus to obtain the energy and mass balances for each case.
The mass balances showed the amounts of hydrogen and carbon species required to produce one ton of
methanol. The total cost price of the methanol consists of the operational expenses (OPEX) and capital expenses (CAPEX). The OPEX reflects the costs required to run the process. This includes the prices for the feedstock and labour costs for example. The CAPEX consists of the capital investments. Based on the CO/CO2,
Case 3 with CO/CO2 = 4, should be the cheapest case as less hydrogen has to be added to produce one ton
of methanol. However, Case 4 with a CO/CO2 = 0.25 is the cheapest option. Case 4 originated from the TG
stream, which already had a high fraction of H2 present in the TG. Therefore, Case 4 requires less addition
of hydrogen. Capture costs were also lower in Case 4, compared to the other cases. At a market price for
methanol of €400 per ton, a maximum electricity price of €30 per MWh was allowable.
FT liquids were overall found to be an unfeasible product, when considering renewable production of hydrogen and CO from CO2 electrolysis. In FT liquids, waxes (C20+) are a requirement in the product fraction as
these can be sold at the highest price. When considering the production of gasoline and diesel, the renewable
FT route proved to be unfeasible. In general it was found that for each case, irrespective of product, more than
50 % of the product cost price, was included in the costs for electrolysis. And as the electrolysis technologies
require electricity to operate, it can be concluded that electricity is the determining factor.