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Technology

Pre-Combustion

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A Simplified Scheme of Pre-Combustion with Five Stations.

Pre-combustion carbon capture refers to the process of capturing CO₂ before the energy generations. It is often composed of five operating stations: oxygen generation, syngas generation, CO₂ separation, CO₂ compression, and power generation. Basically, the fuel will first go through a gasification process by reacting with oxygen to form a stream of CO and H₂, which is syngas. The products will then go through a water-gas shift reactor to form CO₂ and H₂. The CO₂ that is produced will then be captured, and the H₂, which is a clean source, will be used for combustion to generate energy. The process of gasification combined with syngas production is called Integrated Gasification Combined Cycle (IGCC). Normally, it would require an Air Separation Unit (ASU) to serve as the oxygen source. However, research proves that with the same flue gas, oxygen gasification has little advantage over air gasification, and they both have a similar thermal efficiency of roughly 70% using coal as the fuel source. Thus, the use of ASU is not really necessary in pre-combustion.

Biomass is considered "sulfur-free" as a fuel for the pre-combustion capture. However, there are other trace elements in biomass combustion such as K and Na that could accumulate in the system and finally cause the degradation of the mechanical parts. Thus, further developments of the separation techniques for those trace elements are needed. And also, after the gasification process, CO₂ takes up to 13% - 15.3% by mass in the syngas stream for biomass sources, while it is only 1.7% - 4.4% for coal. This limit the conversion of CO to CO₂ in the water gas shift, and the production rate for H₂ will decrease accordingly. However, the thermal efficiency of the pre-combustion capture using biomass resembles that of coal which is around 62% - 100%. Besides, some researches also proved that instead of using a biomass/water slurry fuel feed, using a dry system is more thermally efficient and also more practical for biomass.

Oxy-Combustion

See also: Oxy-fuel combustion process

Overview of oxy‐fuel combustion for carbon capture from biomass, showing the key processes and stages; some purification is also likely to be required at the dehydration stage.

Oxy‐fuel combustion has been a common process in the glass, cement and steel industries. It is also a promising technological approach for CCS. In oxy‐fuel combustion, the main difference from conventional air firing is that the fuel is burned in a mixture of O₂ and recycled flue gas. The O₂ is produced by an air separation unit (ASU), which removes the atmospheric N₂ from the oxidizer stream. By removing the N₂ upstream of the process, a flue gas rich in CO₂ and water vapor is produced, which eliminates the need for a post‐combustion capture plant. The water vapor can be removed by condensation, leaving a product stream of relatively high‐purity CO₂ which, after subsequent purification and dehydration, can be pumped to a geological storage site.

The key challenges of BECCS implementation using oxy-combustion method is associated with combustion process. For the high volatile content biomass, the mill temperature has to be maintain at low temperature to prevent the risk of fire and explosion. In addition, the flame temperature is lower. Therefore, the concentration of oxygen needs to be increased up to 27-30%.

Post-Combustion

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Overview of post combustion for carbon capture from biomass

In addition to pre-combustion and and oxy-fuel combustion technologies, post-combustion is a promising technology which can be used to extract CO₂ emission from biomass fuel resources. During the process, CO₂ is separated from the other gases in the flue gas stream after the biomass fuel is burnt and undergo separation process. Because it has the ability to be retrofitted to some existing power plants such as steam boilers or other newly built power stations, post-combustion technology is considered as a better option than pre-combustion technology. According to the fact sheets U.S. CONSUMPTION OF BIO-ENERGY WITH CARBON CAPTURE AND STORAGE released in March 2018, the efficiency of post-combustion technology is expected to be 95% while pre-combustion and oxy-combustion capture CO2 at an efficient rate of 85% and 87.5% respectively.

Development for current post-combustion technologies has not been entirely done due to several problems. One of the major concerns using this technology to capture carbon dioxide is the parasitic energy consumption..If the capacity of the unit is designed to be small, the heat loss to the surrounding is great enough to cause to many negative consequences. Another challenge of post-combustion carbon capture is how to deal with the the mixture’s components in the flue gases from initial biomass materials after combustion. The mixture consists a high amount of alkali metals, halogens, acidic elements , and transition metals which might have negative impacts on the efficiency of the process. Thus, the choice of specific solvents and how to manage the solvent process should be carefully designed and operated.

Biofuels in BECCS

See also: bioenergy

Biofuels used in BECCS are versatile, including solid biofuels, gaseous biofuels, and liquid biofuels. Biomass, a type of solid biofuels, is the main feedstock for BECCS currently. Biomass sources used in BECCS include agricultural residues & waste, forestry residue & waste, industrial & municipal wastes, and energy crops specifically grown for use as fuel. Current BECCS projects capture CO₂ from ethanol bio-refinery plants and municipal solid waste (MSW) recycling center.

Current Project

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IL-CCS phase 1

Up to date, there have been 23 BECCS projects around the world, with the majority in North America and Europe. Today, there are only 6 projects in operation, capturing CO₂ from ethanol bio-refinery plants and MSW recycling centers.

5 BECSS projects have been canceled due to the difficulty of obtaining the permission as well as their economic viability. The canceled projects include: the White Rose CCS Project at Selby, UK can capture about 2 MtCO₂/year from Drax power station and store CO₂ at the Bunter Sandstone. The Rufiji Cluster project at Tanzania plan to capture around 5.0-7.0 MtCO₂/year and store CO₂ at the Saline Aquifer. The Greenville project at Ohio, USA has capacity of capturing 1 MtCO₂/year. The Wallula project was planed to capture 0.75 MtCO₂/year at Washington, USA. Finally, the CO₂ Sink project at Ketzin, Germany.

At Ethanol plants

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IL-CCS Phase 2

Illinois Carbon Capture and Storage (IL-CCS) is one of the milestones, being the first industrial-scaled BECCS project, in the early 21^st century. Located in Decatur, Illinois, USA, IL-CCS captures CO₂ from Archer Daniels Midland (ADM) ethanol plant. The captured CO₂ is then injected under the deep saline formation at Mount Simon Sandstone. IL-CCS consists of 2 phases. The first being a pilot project which was implemented from 11/2011 to 11/2014. Phase 1 has a capital cost of around 84 million US dollars. Over the 3-year period, the technology successfully captured and sequestered 1million tonne of CO₂ from the ADM plant to the aquifer. No leaking of CO₂ from the injection zone was found during this period. The project is still being monitored for future reference. The success of phase 1 motivated the deployment of phase 2, bringing IL-CCS (and BECCS) to industrial scale. Phase 2 has been in operation since 11/2017 and also use the same injection zone at Mount Simon Sandstone as phase 1. The capital cost for second phase is about 208 million US dollars including 141 million US dollar fund from the Department of Energy. Phase 2 has capturing capacity about 3 time larger than the pilot project (phase 1). Annually, IL-CCS can capture mourned 1 million tonne of CO₂. With the largest of capturing capacity, IL-CCS is currently the largest BECCS project in the world.

In addition to the IL-CCS project, there are about three more projects that capture CO₂ from the ethanol plant at smaller scales. For example, Arkalon at Kansas, USA can capture 0.18-0.29 MtCO₂/yr, OCAP at Netherlands can capture about 0.1-0.3 MtCO₂/yr, and Husky Energy at Canada can capture 0.09-0.1 MtCO₂/yr.

At MSW recycling centers

Beside capturing CO₂ from the ethanol plants, currently, there are 2 models in Europe are designed to capture CO₂ from the processing of Municipal Solid Waste. The Klemetsrud Plant at Oslo, Norway use biogenic municipal solid waste to generate 175 GWh and capture 315 Ktonne of CO₂ each year. It uses absorption technology with Aker Solution Advanced Amine solvent as a CO₂ capture unit. Similarly, the ARV Duiven at Netherlands uses the same technology, but it captures less CO₂ than the previous model. ARV Duiven generates around 126 GWh and only capture 50 Ktonne of CO₂ each year.

Challenges

Environmental considerations

"Some of the environmental considerations and other concerns about the widespread implementation of BECCS are similar to those of CCS. However, much of the critique towards CCS is that it may strengthen the dependency on depletable fossil fuels and environmentally invasive coal mining. This is not the case with BECCS, as it relies on renewable biomass. There are however other considerations which involve BECCS and these concerns are related to the possible increased use of biofuels.

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BECCS in nexus context

Biomass production is subject to a range of sustainability constraints, such as: scarcity of arable land and fresh water, loss of biodiversity, competition with food production, deforestation and scarcity of phosphorus. It is important to make sure that biomass is used in a way that maximizes both energy and climate benefits. There has been criticism to some suggested BECCS deployment scenarios, where there would be a very heavy reliance on increased biomass input.

Large areas of land would be required to operate BECCS on an industrial scale. To remove 10 billion tons of CO₂, upwards of 300 million hectares of land area (larger than India) would be required. As a result, BECCS risks using land that could be better suited to agriculture and food production, especially in developing countries.

These systems may have other negative side effects. There is however presently no need to expand the use of biofuels in energy or industry applications to allow for BECCS deployment. There is already today considerable emissions from point sources of biomass derived CO₂, which could be utilized for BECCS. Though, in possible future bio-energy system upscaling scenarios, this may be an important consideration."

Upscaling BECCS would require a sustainable supply of biomass - one that does not challenge our land, water, and food security. Using bio-energy crops as feedstock will not only cause sustainability concerns but also require the use of more fertilizer leading to soil contamination and water pollution. Moreover, crop yield is generally subjected to climate condition, i.e the supply of this bio-feedstock can be hard to control. Bioenergy sector must also expand to meet the supply level of biomass. Expanding bioenergy would require technical and economic development accordingly.

The BECCS process allows CO₂ to be collected and stored directly from the atmosphere, rather than from a fossil source. This implies that any eventual emissions from storage may be recollected and restored simply by reiterating the BECCS-process. This is not possible with CCS alone, as CO₂ emitted to the atmosphere cannot be restored by burning more fossil fuel with CCS.

Technical challenges

See also: Biomass heating system

Just as other carbon capture and storage technologies, one of the challenges of applying BECCS technology is to find suitable geographic locations to build combustion plant and to sequester captured CO₂. If biomass sources are not close by the combustion unit, transporting biomass emits CO₂ offsetting the amount of CO₂ captured by BECCS. BECCS also face technical concerns about efficiency of burning biomass. While each type of biomass has a different heating value, biomass in general is a low-quality fuel. Thermal conversion of biomass has a typical efficiency of 20-27%. Coal-fired plant has an efficiency of about 37% for comparison.

BECCS also faces a question whether the process is actually energy positive. Low energy conversion efficiency, energy-intensive biomass supply, combined with the energy required to power the CO₂ capture and storage unit impose energy penalty on the system. This might lead to a low power generation efficiency.

Potential solutions

Alternative biomass sources

Agricultural and forestry forestry residues

Globally, there are 14 Gt of forestry residue and 4.4 Gt residues from crop production (mainly barley, wheat, corn, sugarcane and rice) are generated every year. This is a significant amount of biomass which can be combusted to generate 26 EJ/year and achieve a 2.8 Gt of negative CO₂ emission through BECCS. Utilizing residues for carbon capture will provide social and economic benefits to rural communities. Using waste from crops and forestry is a way to avoid the ecological and social challenges of BECCS.

Municipal solid waste

Municipal solid waste (MSW) is one of the newly developed source of biomass. Two current BECCS plants are using MSW as feedstocks. Waste collected from daily life is recycled via incineration waste treatment process. Waste goes through high temperature thermal treatment and the heat generated from combusting organic part of waste is used to generate electricity. CO₂ emitted from this process is captured through absorption using MEA. For every 1kg of waste combusted, 0.7 kg of negative CO₂ emission is achieved. Utilizing solid waste also have other environmental benefits.

Co-firing coal with biomass

See also: Cofiring

There are current 200 cofiring plants in the world, including 40 in the US. Study showed that by mixing coal with biomass, we could reduce the amount of CO2 emitted. The concentration of CO2 in the flue gas is an important key to determine the efficiency of CO2 capture technology. The concentration of CO2 in the flue gas from the co-firing power plant is roughly the same as coal plant, about 15% .This means that we can reduce our reliance on fossil fuel.

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CO₂ Emission From Co-Firing At Different Biomass Ratio.

Even though co-firing will have some energy penalty, it still offers higher net efficiency than the biomass combustion plants. Co-firing biomass with coal will result more energy production with less input material. Currently, the modern 500 MW coal power plant can take up to 15% biomass without changing the component of the steam boiler.This promising potential allows co-firing power plant become more favorable than dedicated bio-electricity.

It is estimated that by replacing 25% of coal with biomass at existing power plant in China and the U.S, we can reduce emission by 1Gt per year. The amount of negative CO2 emitted depends on the composition of coal and biomass. 10% biomass can reduce 0.5 Gt CO2 per year and with 16% biomass can achieve zero emission. Direct-cofiring (20% biomass) give us negative emission of -26kg CO2/MWh (from 93 kg CO2/MWh).

Biomass cofiring with coal has efficiency near those of coal combustion. Cofiring can be easily applied to existing coal-fired power plant at low cost. The implementation of co-firing power plant on the global scale is still a challenge. The biomass resources have to meet strictly the sustainability criteria and the co-firing project would need the support in term of economic and policy from the governments.

Even though co-firing plant may be an immediate solution to solve the global warming and climate change issues, co-firing still has some challenges that need to consider. Due to the moisture content of biomass, it will affect the calorific value of the combustor. In addition, high volatile biomass would highly influence the reaction rate and the temperature of the reactor; especially, it may lead to the explosion of furnace.

Policy

The existing policy such as Committe on Climate Change in 2015 promotes the widespread use of BECCS achieve net-zero emission goal set by Paris Agreement. If carbon taxes were used to deploy BECCS, then the revenues from the climate policies is expected to be approximately 0.3% GDP by 2030. Also, there are some future policies that give incentives to use bioenergy such as Renewable Energy Directive (RED) and Fuel Quality Directive (FQD), which require 20% of total energy consumption to be based on biomass, bioliquids and biogas by 2020.

In February, 2018, US congress significantly increased and extended the section 45Q tax credit for sequestration of carbon oxides. This has been a top priority of carbon capture and sequestration (CCS) supporters for several years. It increased $25.70 to $50 tax credit per tonnes of CO₂ for secure geological storage and $15.30 to $35 tax credit per tonne of CO₂ used in enhanced oil recovery.

Future Outlook

In the 2014 AMPERE modeling project, based on 8 different integrated assessment models, the future deployment of BECCS is predicted to meet the future 2°C scenario in the Paris Agreement. And the overall situation seems pretty optimistic. In the middle of the 21st century, the scale of the BECCS deployment ranges from 0 Mt to 1100 Mt CO₂ per year. And by the end of the century, the deployment ranges from 720 Mt to 7500 Mt CO₂ per year, while most of the models predict the scale to be within 1000 Mt to 3000 Mt by 2100.

A research group from Stanford University has modeled the technical potential of BECCS in the US in the year 2020. According to their calculations, about one-third of the potential biomass production in total is located close enough to the geological storage site, which results in a CO₂ capturing capability of 110 Mt - 120 Mt.

Article Evaluation

Great Pacific garbage patch page.

I chose to evaluate Great Pacific Garbage Patch to evaluate. This article is neutral and the idea of this article is relevant to the topic. The information of the effect on marine life and humans is up to date. The citations is linked to the article.