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Role of CCS for Emission Reduction from Fertiliser and Cement Industry

The IPCC’s pledge to reach net zero GHG target by 2050 to keep global warming below 1.5oC means that it is now critical to focus on some non-power high emitting industrial sectors from which CO2 emission is unavoidable either due to the inherent process chemistry producing CO2 or burning fossil fuel to provide required energy to the process. Fossil fuels are an essential input to several vital industries, not as a primary energy source to generate electricity, but to provide necessary thermal input that can’t be replaced by renewables. Currently, fossil CO2 emissions continue to grow by over 1% annually and reached a record high of 37 gigatonnes of CO2 in 2018. To reach net zero emissions, CO2 emissions have to fall by 45% by 2030 and to net zero by 2050. The necessity of net zero has been recognised by many countries around the world along with UK which has set net-zero GHG target for 2050 into law. 
 
Deep emission reductions from high emitting industrial sectors e.g. cement, steel, fertiliser, chemical is an absolute necessary. CCS is the only large-scale technology available that can enable significant decarbonisation of these industries. Energy efficiency measures must be complemented by CCS in order to achieve deep emission reduction. 
 
This paper builds on the concept developed during OGCI Net Zero Teesside project and further developed focusing on CO2 emission reduction and associated challenges from two such high emitting industries - Fertiliser and Cement. For this paper, we have created generic cases using typical plant data and generic amine performance data. Plants are assumed to be located in the UK as a design basis, but the design will be applicable to similar plants elsewhere. Both industries have been techno-economically evaluated for ~90% CO2 capture, transportation of supercritical CO2 at 110 bara to permanent geological storage. Two sensitivity cases have been developed for the fertiliser industry to assess the impact of lower emission reduction on overall economics. 
 
Typically, nitrogen fertilisers are produced by energy-intensive processes using natural gas as initial feedstock and deriving the required nitrogen from the air. The first stage in the ammonia production process is the synthesis of hydrogen from natural gas by steam methane reforming. Carbon dioxide is removed from the syngas to produce hydrogen rich gas which is then combined with nitrogen to form ammonia. There are therefore two CO2 sources from a fertiliser plant: relatively pure process CO2 stream that is captured from syngas and CO2 in the flue gas from the reformer that can be removed by CCS. It is important to note that process CO2 emissions cannot be easily reduced as they are an inherent by-product of the production process. Approximately 2/3 of the overall CO2 emission from a fertiliser plant is process CO2; rest is in the flue gas. 
 
Three cases have been developed for fertiliser plant: Case 1 (~1.2 MTPA of CO2) includes both process CO2 and ~90% CO2 capture from reformer flue gas; Case 2 is based on process CO2 only (~0.8 MTPA of CO2) with reformer flue gas assumed to be emitted to atmosphere; Case 3 is ~90% CO2 capture from reformer flue gas only excluding process CO2 stream. These cases have been developed to evaluate the impact of CO2 captured from the different sources on the project cost and Levelised Cost of Capture (LCOC). 
 
Case 2 represents the lowest LCOC case at $57/tCO2 with ~ 67% overall CO2 capture whereas highest LCOC is associated with the Case 3 at ~$134/tCO2 capturing only ~ 33% CO2. Case 1 achieves a LCOC of ~$81/tCO2 with an overall >90%. This clearly shows the advantage of the availability of process CO2 on the overall capture rate and cost of capture as well. Case 3 suffers from higher LCOC as it doesn’t benefit from having a readily available process CO2 stream that doesn’t require any capture plant. 
 
In the cement manufacturing process, more than half of the CO2 emissions are process-related (generated from decarbonation of limestone in the kiln) and rest are from fossil fuel combustion. Generally, hot flue gas from the combustion in the rotary kiln combines with additional flue gas generated by the pre-calciner burner and heats up the raw mix entering the rotary kiln. After as much useful heat as possible is recovered from the flue gas, and bag filters remove the dust before flue gas release to atmosphere through the stack. 
 
The cement carbon capture plant considered for this paper is based on post combustion capture of ~0.5 MTPA of CO2 from the flue gas. The cement process does not have any requirement for steam, therefore new boiler is needed to generate steam for the carbon capture reboiler. A natural gas fired auxiliary boiler is assumed for steam generation and flue gas from this auxiliary boiler is sent to the same capture plant. 
 
NO2 in the flue gas degrades amine-based carbon capture solvents as it reacts to produce nitrite and nitrate heat stable salts (HSS). Most of the available carbon capture licensed technologies have very stringent NO2 acceptable limit of typically <10 ppm. Accumulated HSS will reduce absorption performance and cause operational problems such as corrosion and foaming. The cement plant carbon capture configuration has been developed for a scenario with high NOx (as NO2) content in the flue gas which will be the case for many of the existing cement plants. A high dust Selective Catalytic Reduction (SCR) plant located in the hot gas path of the cement plant is assumed to reduce the NOx to an acceptable limit for the capture solvent. The advantage of locating the SCR on the hot flue gas path is that no additional flue gas reheating is needed. But the retrofit of SCR in an existing cement plant is likely to be a complex project in its own right. 
 
A high-level capital cost and operating cost has been developed for this case. The configuration achieves a LCOC of ~$154/tCO2 with an overall ~90% from the flue gas.