Which of the following describes an environmental problem that can result from the combustion of fossil fuels?

13.1 Introduction

Fossil fuel combustion is a major source of CO2 emissions in both Mexico and Canada. For instance, in 2010 the combustion of fossil fuels accounted for approximately 80% of Mexico’s CO2 emissions (Fifth National Communications, 2012, p. 179). As a member of the G20 and world’s 15th largest economy, Mexico depends heavily on its oil and gas resources for energy security; 30% of federal government budget revenues are secured from Pemex (Wood and Martin, 2018). Numerous scientists agree that a significant reduction in CO2 emissions is an important element in mitigating the effects of global warming and climate change. In 2015 the governments of Mexico and Canada committed to significantly reduce their national greenhouse gas (GHG) emissions (Paris Agreement, 2015). In 2016 Mexico ratified the Paris Agreement and committed to reduce its national GHG emissions by 22% by 2030 and by 50% from 2000 levels by 2050 (Altamirano et al., 2016). The Government of Mexico’s GHG emissions reduction goals are furthered under its Transition Strategy to promote the use of Cleaner Technologies and Fuels, which is one of the guiding instruments of the Mexican national energy policy (Mexico Secretariat of Energy [SENER], 2018a).

There are two types of CO2 emissions reduction projects that have been deployed and are operating in Canada, carbon capture, use and storage (CCUS) and carbon capture and storage (CCS) (Navarro et al., 2016). These types of projects are to be constructed and tested in Mexico, by the Mexican Government. Currently, there are two pilot projects under construction in the state of Veracruz, Mexico: “Campo Brillante” in Coatzacoalcos for enhanced oil recovery (EOR), and “Poza Rica” for CCS (Navarro et al., 2016). Pemex is interested in developing EOR operations due to the potential to capture 50 million of tons of CO2 annually (Navarro et al., 2016).

CCUS includes EOR in which CO2 is injected into pore spaces in depleted oil and natural gas reservoirs, that were previously filled with oil and gas that has been pumped out of the ground (Metz et al., 2005). In light of the additional revenue from EOR, these projects, which have been developed and operated in Canada for decades, are more economically attractive to oil and gas companies than CCS projects that have the sole objective of long-term CO2 storage to mitigate the effects of climate change. Due to the proximity of CO2 emitting industrial facilities to Pemex’s oil fields, EOR projects are one element of the Mexican government’s plans to reduce the level of national GHG emissions. In Mexico there is significant potential for CCUS projects at industrial installations that include both upstream and downstream processes, like coal-fired power and natural gas processing plants, such as those situated near the Gulf of Mexico (Lacy et al., 2013).

CCS refers to a variety of technologies used to permanently capture, inject, and store CO2 emissions from industrial processes and fossil fuel electrical generation facilities into subsurface geological reservoirs for centuries, to prevent additional emissions entering the atmosphere (Ingelson et al., 2010; Woodford, 2017). Globally, there are 37 major CCS projects, operating or in development, most of which have an approximate CO2 storage capacity of 37 million tons annually (Global CCS Institute, 2017a).

In the last 10 years the Government of Mexico has adopted policies and strategies to promote CCS and CCUS to reduce its CO2 emissions (SENER, 2018a). In 2008 Mexico joined the Global CCS Institute and the Carbon Sequestration Leadership Forum (SENER, 2018a). The government in 2010 adopted a National Energy Strategy, in which sustainable development is a critical element (SENER, 2018a). The Secretariat of Energy in 2011 carried out a study called “State that holds the technologies of CCS in Mexico,” which stated that in light of the economic and investment considerations, Mexico should incorporate both CCS and CCUS projects in its plans to reduce GHG emissions (SENER, 2018a). The “Mexican Atlas of CO2 Geological Storage” was published in 2012, as part of a trilateral project in the North American Carbon Sequestration Atlas. The General Law of Climate Change came into force in 2012, a significant milestone in Mexican environmental policy.

In 2013 a carbon tax on fossil fuels was approved and the National Climate Change Strategy (Vision 10–20–40) published. To facilitate CCUS test projects and coordinate research activities on CO2 capture and regulation, in 2014 the Minister of Energy (SENER) established a working group comprising representatives from the Minister of the Secretariat of Environment and Natural Resources (SENVNR), Pemex, the Federal Commission of Electricity, the Autonomous National University of Mexico, the National Technologic Polytechnic, and the Mario Molina Center (Mexico. Secretariat of Environment and Natural Resources [SEMENR], 2014). A Technological Roadmap for CCUS projects in Mexico was created in 2014 and is currently being used as a planning mechanism to assist in the development of CCUS technology (Mexico. Secretariat of Environment and Natural Resources [SEMENR], 2014). A National Emissions Registry has been established, under which all companies that generate GHG emissions higher than 25,000 tCO2e must prepare and submit an annual report on those emissions. The Special Climate Change Program 2014–18 was launched, and for the first time in Mexican history the Electricity Industry Law classified “geological capture and storage or carbon dioxide bio sequestration” as Clean Energy, and CCS to be a low-carbon source.

In 2015 three studies were undertaken in collaboration with the World Bank, aiming to facilitate deployment of CCUS in EOR projects and long-term CO2 storage (Navarro et al., 2016). In addition, the Energy Transition Law came into force. In 2016 a National Transition Strategy to promote the use of cleaner technologies and fuels was released (Mexico, 2016). and the following year an inventory of fixed emissions sources and CO2 storage sites in Mexico and the Mexican CCUS Center was created (SENER, 2018b). At the end of 2017 SENER created a Mexican CCUS Center to further support CCUS development by promoting collaboration among academics in the research community with industry representatives that support the development and completion of CO2 capture pilot plants (Global CCS Institute, 2017b). In 2018 Wood and Martin (2018) noted that privatization of the Mexican energy sector and “the Mexican government’s desire to build an energy sector that meets the need to shift to a low-carbon growth model” (p. 35). In 2018 the CCUS National Strategy and CCS-READY Strategy were under development, which are Mexican public policy documents intended to establish a strategy to implement CCUS and CCS technologies through the integration of criteria to promote, plan, and execute projects at different scales in the country (SENER, 2018a).

Abstract

An increasing fossil fuel combustion raise the environmental concerns in terms of environmental pollution and climate change. Biogas is a byproduct of microbial metabolism. It is a sustainable fuel made of organic waste such as biomass and may reduce the global demand in fossil fuels. Its methane (CH4) content usually reaches 50%–70%, while its second most abundant component is carbon dioxide (CO2). In order to increase the caloric value of biogas, the methane content needs to be increased through an upgrading process. In this chapter, a possibility of using electrochemical methods to upgrade biogas by removal of its two major problematic compounds will be discussed. This chapter will focus on electroreduction of carbon dioxide (CO2), which is the second major compound in biogas, as well as electrochemical oxidation of hydrogen sulfide (H2S), which is its most corrosive component. Basic fundamental aspects, electrocatalysts, electrolyzer, and fuel cell designs, as well as challenges for further development and industrial applications toward electrochemical biogas upgrading will be discussed.

Abstract

Heat engines based on fossil fuel combustion produce harmful pollutants and greenhouse gas emissions. Environmental concerns and sustainable development call for new technology for energy conversion and power generation, which is more efficient, environmentally friendly and compatible with alternative fuels and renewable energy sources and carriers. Fuel cells meet all these requirements, and are being developed as one of the primary energy technologies of the future. In this chapter, the thermodynamic performance of fuel cells is analyzed, energy conversion efficiency of fuel cells and heat engines is studied and compared, and misconceptions about fuel cell efficiency clarified. It is shown that both fuel cells and heat engines have the same maximum theoretical efficiency, which is equivalent to the Carnot efficiency, when operating on the same fuel and oxidant. However, fuel cells are free from the high temperature limit imposed by materials on heat engines and less irreversibilities associated with heat rejection. As a result, fuel cells can have higher practical efficiencies.

The rationale for addressing fossil fuel export as a component of climate policymaking

A sole focus on fossil fuel combustion might be justified if the world had collectively cut demand for fossil fuels, hence making a complementary focus on supply redundant. But policymakers have not managed to successfully decarbonize the global economy and are presently on track to exceed warming by 3°C or more, far exceeding the targets of the Paris Agreement (UNEP, 2020a). The failure of the “idealized” global climate regime to effectively tackle emissions has many questioning whether new policy approaches may be helpful, including policies focused at the point of production or export of coal, oil, and gas (IEA, 2019; SEI et al., 2020).

The exploration for new and ambitious forms of climate policy led to proposals for “supply-side” climate policies that restrain the supply of fossil fuels (Gaulin & Le Billon, 2020; Lazarus & van Asselt, 2018). Indeed, some countries have begun to address fossil fuel production in earnest. The governments of Costa Rica, Belize, France, Denmark, New Zealand, Spain, and Ireland, for instance, have all banned exploration or extraction of oil in part or all of their territories (IEA, 2019; SEI et al., 2020).While we are yet to see major fossil fuel exporters commit to reducing production and export of fossil fuels to meet climate goals, some key producers are beginning to lay the groundwork for a transition away from coal, oil, and gas. For instance, the United States has recently paused oil and gas leasing on public lands and in offshore waters to allow the opportunity to review leasing arrangements in light of climate imperatives (The White House, 2021).

In the context of fossil fuel export, a range of supply-side policy options exist that could be used to limit export, such as revoking permits for export facilities (Rafaty et al., 2020), taxing fossil fuel exports (Antón, 2020), or simply banning fossil fuel exports altogether. Some subnational governments, for instance, have used zoning laws to prohibit permitting of fossil fuel export facilities within geographical boundaries (Perron, 2020). Governments can also change the calculus of decision-making so that climate change is given more weight, for instance, by requiring that upstream emissions are reported as part of the environmental review process for new export facility projects (Burger & Wentz, 2020). Or, they can limit their financial support for fossil fuel projects, by cutting subsidies to exporters or finance for new infrastructure projects (Moerenhout & Irschlinger, 2020). Finally, governments can facilitate the transition by providing support to help export-dependent communities diversify to alternative industries (Green & Gambhir, 2020).

Proponents of supply-side polices often argue that these policies should be introduced alongside efforts to cut emissions directly through measures such as carbon pricing or investments in energy efficiency. This dual focus would allow governments to “cut with both arms of the scissors,” reducing both the supply of fossil fuels at the same time as demand (Green & Denniss, 2018). There are several reasons why this could help raise climate policy ambition and effectiveness more broadly.

An explicit focus on fossil fuel supply would help solve the current problem of overproduction of fossil fuels, which is hampering efforts to cut emissions. A recent report by UNEP examining plans from the world's largest fossil fuel producers and exporters found that governments are planning to produce 120% more fossil fuels by 2030 than would be consistent with limiting global warming to 1.5°C (IEA, 2019; SEI et al., 2020). Flooding the world with fossil fuels will make it harder to undertake the transition to renewable energy that is necessary to meet our climate goals.

Overinvestment in fossil fuels also creates the risk of “stranded assets”—fossil fuel infrastructure that becomes superfluous if countries switch to less polluting energy sources—which can cause huge liabilities for countries, companies, and communities that bet on a fossil-fueled future (Bos & Gupta, 2018; Leaton et al., 2013). Relatedly, by failing to tackle supply, we run the risk of exacerbating the problem of “carbon lock-in”—whereby our infrastructure, economy, politics, and culture become so enmeshed in a fossil-fueled way-of-life that it becomes challenging to envision and create alternatives (Erickson et al., 2015; Seto et al., 2016; Unruh, 2000). When communities decide to build out fossil fuel export terminals, for instance, they are making a long-term commitment to tie their economy to the fossil fuel industry. This “lock-in” can form a formidable barrier to building a low carbon future, as the relics of past decisions—such as the need to ensure a return-on-investment in infrastructure, the political power gained by incumbent industries, or the presence of a workforce trained for fossil fuel production—can inhibit the ability to shift to other, less-polluting industries. Preventing further build out of fossil fuel export facilities can limit exposure to this “lock-in,” making the enactment of climate responses easier in the future.

A dedicated focus on fossil fuel supply in climate policy can also help ensure a more just and equitable transition to a climate-friendly future. If we only focus on building green industries as a climate response, we may forget about those regions, communities, and workers who currently depend on fossil fuel export for their survival. If instead, we also address high-carbon industries in climate policymaking, we better ensure that we have a well-managed transition to a low-carbon future that leaves no one behind. In the context of export, we may need to invest funds into decommissioning and cleaning up export sites, as well as retraining workers who currently work in fossil fuel exporting roles (Green & Gambhir, 2020). A “just transition” away from fossil fuels should also include support for communities that have been historical harmed by fossil fuel production and export, such as the fence line communities who have borne the brunt of pollution from neighboring fossil fuel facilities over the years (Healy et al., 2019). Low-income countries who rely heavily on fossil fuel export will also need assistance from the international community to help transition their economies (Armstrong, 2020; Muttitt & Kartha, 2020; Ross, 2019).

Addressing fossil fuel supply can help illuminate many of the blind spots that our current climate policy regime focused solely on emissions misses. Ultimately, nations of the world cannot, in aggregate, keep expanding production and export of fossil fuels, while also meeting their agreed climate goals. This requires grappling with the idea that fossil fuel exporting will be a less-prominent part of our future, and by planning for this fact now, we can ensure a more effective, timely, and just transition.

1.2.2 Post-combustion CO2 capture

Carbon dioxide produced during fossil fuel combustion for heat and electricity generation is a major contributor to the global CO2 emissions considered responsible for global warming due to its GHG effect. Fossil fuels such as coal are, however, expected to be continued to be used through the next several decades. About 2 billion tons of CO2 are estimated to be emitted per year from existing coal-fired power plants around the world (Brunetti et al., 2010). The consumption of coal and consequent CO2 emissions are expected to be even greater in the future, for example, global carbon dioxide emissions from all uses of coal are expected to increase from 13.0 billion metric tons in 2008 to 19.6 billion metric tons in 2035 (EIA, 2012). Therefore, it is necessary to develop approaches for the capture of the GHG CO2 produced for ‘clean’ energy generation. In addition to the fossil fuels, CO2 capture is also relevant to the utilization of ‘renewable’ fuels such as biogas and syngas produced from biomass.

The ‘pre-combustion’ mode of CO2 capture discussed in the previous section is relevant primarily for the future coal gasification and hydrogen plants since almost all of the existing fossil fuel-based power generation plants are based on combustion. CO2 captured in the ‘post-combustion’ mode from the flue gases is most relevant for the retrofit applications in the existing power plants and facilities. Post-combustion CO2 capture from flue gases is especially challenging due to typically low (near atmospheric) gas pressure and low CO2 concentration of 10–15% in flue gases. The concentration of CO2 in the flue gas may be increased by using pure or enriched oxygen instead of air in the combustion process in what is termed ‘oxy-combustion’. However, such an approach will require a cryogenic oxygen separation plant making such an approach prohibitively expensive (Figueroa et al., 2008). Due to temperature limitations of existing combustor equipment, there will also be a need for a cooled flue gas recycle.

Post-combustion CO2 captures from flue gases have been reviewed in several publications recently (e.g., Basile et al., 2011; Yave and Car, 2011) Conventional approaches for removing CO2 from flue gases include absorption in solutions, most typically amine solutions, adsorption on solid sorbents, and cryogenic separation (Brunetti et al., 2010). There has however been substantial recent interest in CO2 selective polymeric membranes for flue gas CO2 capture. Amine absorption of CO2 is a proven and mature technology and may be considered as the current leading technology for post-combustion CO2 capture. There are a number of amine-based demonstration projects currently underway for CO2 capture from power plant flue gases. Amine absorption technology is capable of achieving high level (> 90%) CO2 capture. However, it is also an energy-intensive process due to the need for thermal regeneration of spent sorbent solution in a separate column and associated pumping. The energy requirement of amine absorption plants is estimated to be in the range of 4–6 MJ/kg of CO2 removed reducing the plant efficiency by 30% (Brunetti et al., 2010). The resulting increase in the cost of electricity is estimated to be 50–90% and the cost of CO2 capture at $40–100/ton of CO2 captured (Figueroa et al., 2008).

Because of the simplicity of operation and typically low operating and maintenance costs, low temperature CO2 selective polymeric membranes may be attractive for post-combustion flue gas CO2 capture. However, there are a number of issues that potential membranes need to address: low pressure and CO2 concentration of flue gases require compressing flue gases sufficiently incurring energy penalty; high temperature of flue gases which require cooling as needed for the membranes; the presence of particulates and chemical contaminants in the flue gases which require their removal prior to the membrane unit; and the goals of achieving > 80% recovery with > 80% purity as indicated by International Energy Agency (Bounaceur et al., 2006) which requires membranes with high CO2 selectivity (the US DOE NETL goals for the developmental project call for > 90% recovery of sequestration ready CO2). Furthermore, to reduce the cost of the membrane modules, high permeability for CO2 is also necessary.

There have been a number of modeling studies to estimate the cost of CO2 separation by membranes and compare it to other approaches (Carapellucci and Milazzo, 2003; Powell and Qiao, 2006; Favre, 2007). In general, the feasibility of membrane separation depends on the concentration of CO2 in the flue gas as expected. For separating CO2 from a 10% flue gas stream, the energy consumption was estimated to be much larger than the absorption process and a CO2 concentration greater than 20% was thought to be needed for making membrane separation attractive (Bounaceur et al., 2006). The process economics also depends on the use of vacuum on the permeate side instead of flue gas compression: a vacuum process needs less energy but significantly greater membrane area. Combination of partial vacuum and compression may provide a lower cost option and use of combustion air as sweep gas in the final stage of a multi-stage system also was predicted to lower costs (Merkel et al., 2010). Thus approaches appear to be available to make CO2 capture from flue gases by membranes cost effective.

A large number of membranes have been studied for their CO2 separation capabilities and include a variety of polymeric membranes as well as facilitated transport, mixed matrix, and carbon molecular sieve membranes as described in detail in a recent review (Brunetti et al., 2010). Notable among various polymer membrane candidates are: Polyactive® membrane by GKSS (Yave et al., 2010) with a CO2/N2 selectivity of 50 to 55 and CO2 permeance in the range of 1000–1500 GPU (1 GPU = 3.348 × 10–10 mol/ m2sPa); Polaris™ membrane (Merkel et al., 2010) with a CO2 permeance of 1000 GPU and a CO2/N2 selectivity of 50; and a cardo-polyimide type membrane from the RITE Institute in Japan, with a permeance of 1000 GPU and a CO2/N2 selectivity of 35 (Kasama et al., 2005). Recent developments at MTR with 1 ton/day scale unit testing have reported even greater permeance (> 2000 GPU) with similar selectivity for a second generation PolarisTM membrane (Amo et al., 2012). Although the cost of CO2 capture is claimed to be lower than that for the Selexol process, the increase in cost of electricity with the second generation membrane is above 50% and thus greater than the goal of 35%. Additional membrane permeance improvement is a key to achieving the cost target (Amo et al., 2012). Large modules of candidate membranes, however, also need to be demonstrated for commercial availability of membrane separation technology for CO2 capture from power plant flue gases.

III.B Lower Atmosphere Effects

The introduction of species from fossil fuel combustion (Section II.A.1) into the lower atmosphere leads to an extensive series of chemical reactions, most of which are of free-radical type. They can be divided into three categories: daylight (D) reactions, requiring solar near-UV radiation; night (N) reactions, which are important in the absence of sunlight; and 24-hr (DN) reactions.

The fundamental light-requiring process is

(D)NO2+hv(below398nm )→NO+O.

The O atom then forms ozone in the presence of a third body M:

(DN)O+O2+M→ O3+M,

and ozone reacts with NO to form NO2:

(DN)NO+O3→NO2+O2.

Most emission of NOx by combustion is in the form of NO, and direct air oxidation of NO to NO2 is too slow to be of much significance; at typical concentrations the formation of O3 as an intermediate permits this oxidation to occur on a timescale of 1 hr or so in typical urban air. A further D process is

(D) O3+hv(below308nm)→O*+O2,

where O* denotes an excited state (the 1D state) of the oxygen atom, followed by

(DN)O*+H2O →2OH.,

whereas at night ozone reacts further with NO2:

(N)O3+NO2→O2+NO3.

A further source of the hydroxyl radical OH· is

(DN)2NO2+H2 O→HNO2+HNO3,

(D)HNO2+h v(295−410nm)→OH⋅+NO.

The nitrate radical formed at night leads to production of nitric acid, HNO3:

(N)NO3+NO2→N2O5,

(N)N2O5+H2O→2HNO3⋅

Nitric acid is also produced from OH·:

(DN)OH⋅+NO2→HNO3⋅

Thus solar irradiation of a mix of air and NO leads to a buildup of the oxidant O3, the reactive free radical OH· and the strongly acidic HNO3; after sunset, OH· production ceases but that of NO3 begins, leading through N2O5 to HNO3. The latter acidifies water droplets, causing the formation of rain, snow, mist, or fogs with low (i.e., acid) pH. Ozone and OH· undergo further reactions in the presence of hydrocarbons RH (i.e., volatile organic compounds or VOCs):

OH⋅+RH→R⋅+H2O,R⋅+RCHO→RH +RCO⋅,R⋅+O2→RO2⋅,RO 2⋅+NO→RO⋅+NO2,RCO⋅+O2→RC(O)OO⋅(peroxyacyl radical),RC(O)OO⋅+NO2→RC(O)OONO2(peroxyacyl nitrate or PAN ).

Thus the presence of organic molecules acts to speed the production of NO2. The PAN compounds formed in the last step are potent eye irritants; it is the mix of such substances with O3 and the brown NO2 that constitutes “smog.” Carcinogenic nitrosamines can be formed from secondary amines with HNO2:

R2NH+HNO2→R2NNO(nitrosamine)+H 2O.

Carcinogenic condensed-ring hydrocarbons such as benzo(a)pyrene

are emitted in the exhaust mix of unburned HC; they undergo further reactions with O3 or N2O5 to yield mutagenic epoxides or nitro compounds.

Particulates, especially the soot emitted in incomplete combustion or by diesel engines, play a role in the pollution chemistry of the lower atmosphere. They adsorb carcinogens of low volatility such as the condensed aromatics, thereby retaining them in the atmospheric reservoir available for inhalation, and also catalyze the further reaction of these species with oxidants.

Conversion of SO2 to SO3, leading to sulfuric acid formation, is slow in the gas phase but is probably accelerated when the SO2 is adsorbed on soot grains. In this way particulates contribute to acid precipitation. Extremely fine particulates also seem to play a role in the generation of smog, perhaps by adsorbing and stabilizing organic free radicals R· and RO·, which serve as intermediates in the production of ozone and other oxidant species. Hence there is increased interest in the possibility of designating very fine particulates (with diameters of a few microns or less) as a separate category of pollutant to be subjected to EPA monitoring and control.

Industry

Many industrial processes emit CO2 through fossil fuel combustion. Several processes also produce CO2 emissions through chemical reactions that do not involve combustion, for example, the production and consumption of mineral products such as cement, the production of metals such as iron and steel, and the production of chemicals. Fossil fuel combustion from various industrial processes accounted for about 14% of total US CO2 emissions and 12% of total US GHG emissions in 2012. Many industrial processes also use electricity and therefore indirectly cause the emissions from the electricity production. Figure 7.3(B) also indicates that the top four CO2 emitters in 2013 covered 58% of global emissions: China (28%), United States (14%), EU28 (10%), India (7%). China becomes the largest country to emit CO2 with annual growth rate of 4.2% (shown in Fig. 7.3(C)).

Four countries or zones account for 58% [in 2013] of global CO2 emissions, these countries/zones are P.R. China (28%), United States (14%), European Union [28 countries/states] (10%) and India (7%), summarised in (Fig. 7.3(C)). It was found that the bunkers fuel used for international transport is 3% of global emissions. Statistical differences between the global estimates and sum of national totals is 3% of global emissions [10].

The trends can be modeled using an assumption that consumption and production of energy are correlated, using China as example, it can be seen that during the last 15 years there has been a growth in consumer goods, services, and products requiring more natural resources (internal and imported) and more CO2 emissions (Fig. 7.3(D)).

Acidification potential

The SOx and NOx emissions from the fossil fuel combustion cause acidification potential. As indicated in Figs. 6 and 7, the gate-to-gate acidification potential for the CEBC spray process Case 1 and Case 4 is 88% and 19% of that for the MC process, respectively, and the cradle-to-gate impact of the spray process is 96% (Case 1) and 85% (Case 4) compared to the MC process. Similar to the cradle-to-gate global warming potential, the cradle-to-gate acidification potential is also due primarily to the fossil fuel-based pX and other raw materials production. These results suggest that while acetic acid usage dictates the acidification potential from on-site emissions, fossil fuel-based energy generation is a major contributor to the overall acidification potential when considering cradle-to-gate emissions.

5.1 Introduction

Anthropogenic CO2 production is primarily driven by fossil fuel combustion and the current energy situation gives no indication that fossil fuel combustion demand will change in the near future. Consequently, it is increasingly necessary to find ways to reduce CO2 emissions when fossil fuel is used. Of the various potential reduction options, CO2 capture and storage (CCS) appears to be among the most promising for large stationary power plants. All of the CCS technologies for power plants involve producing an almost pure stream of CO2 either by concentrating it in some manner from flue gases, or by using effectively pure oxygen as the combustion gas (Buhre et al., 2005). The latter option, referred to as oxy-fuel combustion, has been well studied for pulverized coal combustion (Toftegaard et al., 2010), but to date has received relatively little attention for oxy-fuel circulating fluidized bed combustion (CFBC), although the concept was examined over 20 years ago for bubbling FBC (Yaverbaum, 1977). More recently, the boiler companies Alstom and Foster Wheeler have explored the oxy-fuel CFBC concept using pilot-scale testing (Eriksson et al., 2007; Stamatelopoulos and Darling, 2008). Alstom’s work included tests in a unit of up to 3 MWt in size, but did not involve recycle of flue gas (Liljedahl et al., 2006). Foster Wheeler’s work (Eriksson et al., 2007) also involved pilot-scale testing, using a small pilot-scale (30–100 kW) CFBC unit owned and operated by VTT (Technical Research Centre of Finland) and this work along with CanmetENERGY’s work with its own 100 kW CFBC, appears to be the first in which units were operated with oxy-fuel combustion using flue gas recycle.

The advantages of CFBC are already well known in terms of its ability to burn a wide range of fuels, both individually and co-fired, to achieve relatively low NOx emissions, and accomplish SO2 removal by limestone (Grace et al., 1997). Another advantage of CFBC technology, in the context of oxy-fuel firing, is the relatively low heat flux in the furnace. This low heat flux may allow either a significant reduction of the amount of recycled flue gas or, alternatively, permit the use of a much higher oxygen concentration in the combustor. Both of these will improve the economics of oxy-fired CFBC relative to pulverized coal (PC) or stoker firing by reducing the size of the CFBC boiler island by as much as 50% (Liljedahl et al., 2006). In considering the scale-up of CFBC units above 300 MWe, both Foster Wheeler and Alstom are now offering much larger units and Foster Wheeler has in operation a 460 MWe supercritical CFBC boiler (Stamatelopoulos and Darling, 2008; Hotta et al., 2008).

More-difficult-to-quantify advantages for the technology relate to the possibility of co-firing biomass, so that with CCS, the overall combustion process may potentially result in a net reduction of anthropogenic CO2, and the potential for this technology to be used with more marginal fuels, as premium fossil fuels become in short supply. The co-firing option offers a potentially interesting advantage of CFBC technology since it is well established that CFBC can burn biomass and fossil fuels at any given ratio in a range of 0–100%, thus offering the possibility of using local and seasonally available biomass fuels in a CO2 ‘negative’ manner.

The ultimate availability of premium coal for a period of hundreds of years has also recently been called into doubt with suggestions that coal production may peak well before the end of this century. Thus, Mohr and Evans (2009), for example, have developed a model which indicates that coal production will peak between 2010 and 2048 on a mass basis and between 2011 and 2047 on an energy basis, with a best-guess scenario of peaks in 2034 on a mass basis and 2026 on an energy basis. In the event of such solid fuel shortages, fluidized bed combustion is ideally suited to exploit the many marginal coals and hydrocarbon-based waste streams available worldwide.

Currently R&D on oxy-fired CFBC technology is being explored in numerous countries, including Canada, Finland, Poland, China and the United States among others. However, to date most test work has been done at small scale (in the < 100 kW range), and/or using bottled gases to simulate recycled flue gas to achieve the necessary gas velocity and solid circulation rate in terms of heat transfer requirement. CFBC systems can potentially operate with less flue gas recycle because the hot solids that are recycled in a circulating fluidized bed can also be used to produce steam. This cools the fluidized bed, and hence less flue gas needs to be recycled, compared with a suspension-fired boiler. However, the minimum amount of flue gas recycling is governed by maintaining sufficient fluid velocity in the fluidized bed while at the same time providing enough water/steam-cooled surface area to provide adequate internal and external solids heat transfer. Probably the most important advanced test programmes in oxy-CFBC are the development of a number of large pilot-scale units. However, first an exploration of the early work will be presented, followed by a discussion of results from the various pilot-scale tests done so far, finishing with a look at the major demonstration projects now underway.

1 INTRODUCTION

Increasing atmospheric concentration of CO2 due to fossil fuel combustion is a serious environmental problem. Recently, CO2 capture and sequestration has attracted considerable attention as one of the options to reduce CO2 emission. Various processes, such as liquid solvent absorption, membrane separation, and pressure (and/or temperature) swing adsorption (P(T)SA), have been proposed for separation and recovery of CO2 emitted by power plants, steel works, etc. [1]· However, the costs of O2 separation from flue gases are accounted for approximately 70-80 % of total cost for CO2 sequestration. Therefore, it is important to develop a new efficient and energy-saving technique for CO2 separation. In addition, it is desired to downsize a plant for CO2 separation, because enormous amounts of gases must be treated.

As for the conventional PSA or PTSA process using zeolite, a dehumidification process which consumes about 30 % of total energy is necessary, because water vapor is adsorbed more strongly than CO2 on zeolite surface. Therefore, development of a new adsorbent which is able to adsorb CO2 in the presence of water vapor is required to construct a simple and energy-saving process by elimination of the dehumidification process.

Hayashi et al. showed that hydrated potassium carbonate supported on active carbon was able to absorb CO2 from flue gas containing water vapour [2]. Solid sorbents in which amines are supported on high surface area supports are also promising as sorbents for CO2 separation [3-6]. Leal et al. applied aminosilane modified silica to CO2 adsorbent [4].

Aminosilane modified silica has been extensively studied because of its widespread applications. However, it is difficult to modify micro pore wall of silica with aminosilanes molecules due to steric hindrance [7]. Therefore, mesoporous silicas such as M41S, FSM-16, and SBA-15 are suitable supports for surface modification with aminosilane on mesoporous silica, because they have large and uniform pores. Furthermore, higher loading of aminosilane than conventional silica gel is expected due to their high surface area. Recently, Xu et al. showed that polyethylenimine modified MCM-41 is an efficient CO2 adsorbent [5,6]. However, effect of water vapour on adsorption performance of polyethylenimine modified MCM-41 has not been studied.

Among various mesoporous silicas, SBA-15 is a suitable material for applications in gases containing water vapor, due to its higher hydrothermal stability [8]. In this study, we have prepared aminosilane modified mesoporous molecular sieve SBA-15 as a “water-tolerant adsorbents”, which is possible to adsorb CO2 in the presence of water vapor, and applicability for PTSA was examined by CO2 adsorption-desorption measurement in a flow system.