Post combustion

As was mentioned earlier, it is probably wise to separate the carbon dioxide from the flue gas and inject only a C02-rich stream. This is the so-call capture part of the carbon capture and storage. 

At first look, we should be able to achieve this using a process similar to those used for sweetening natural gas. However, there are several factors that complicate this. 

High Temperature - Since the source of the stream is a combustion process, this stream will be at high temperature. It may be necessary to cool the flue gas stream before sending it to the treating process. 

Low Pressure - The flue gas stream is produced at near atmospheric pressure. At a minimum, blower will probably have to be used to raise the pressure of the gas to a sufficient level such that it can flow through the process equipment. 

In addition, and perhaps more importantly, the absorption process is favored by higher pressure. 

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Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

A significant issue in combustors in the mid-1990s is the performance of the process in an environmentally acceptable manner through the use of either low sulfur coal or post-combustion clean-up of the flue gases. Thus there is a marked trend to more efficient methods of coal combustion and, in fact, a combustion system that is able to accept coal without the necessity of a post-combustion treatment or without emitting objectionable amounts of sulfur oxides, nitrogen oxides, and particulates is very desirable (51,52). 

In the electrothermic part of the furnace, electrical energy introduced via three carbon electrodes, keeps the bath molten and completes the lead oxide reduction. Fumes generated in the electrothermic section are oxidized in a post-combustion chamber by adding ambient air, before the vapor is cooled, dedusted, and released to the atmosphere. 

Wet-Throwaway Processes. By 1978, three wet-throwaway systems were in commercial operation lime scmbbing, limestone slurry scmbbing, and dual alkah (1). Time/limestone wet scmbbing (Fig. 11) remains the most common post-combustion control technique appHed to utiHty boilers (67). The waste product from the scmbbers can either be sent to a landfill or be upgraded by oxidation to become saleable gypsum. 

When NO destmction efficiencies approaching 90% are required, some form of post-combustion technology appHed downstream of the combustion 2one is needed to reduce the NO formed during the combustion process. Three post-combustion NO control technologies are utilized selective catalytic reduction (SCR) nonselective catalytic reduction (NSCR) and selective noncatalytic reduction (SNCR). 

Applicability/Limitations Most t qjes of solid, liquid, and gaseous organic waste or a mixture of these wastes can be treated with this technology. Explosive wastes and wastes with high inorganic salt content and/or heavy metals require special evaluation. This operation can create high particulate emissions which require post-combustion control. 

In the post-combustion chamber temperatures of 900 °C to 1200 °C are reached. The kiln can - like any rotary kihi - handle solid, fluid and gaseous waste streams. Based on the heat capacity of the waste, halogen content, and potential slag formation, an optimal mixture of wastes is determined. By choosing the feed carefully, production of high-quality HCl can be assured. Furthermore, in this way a minimum formation of dioxins and furans can be ensured. 

The flue gas from post-combustion is cooled from 1200 °C to 230 °C to 300 C. Here, energy is recovered. Steam is produced that is added to the steam network of the BSL Schkopau site. In the flue gas purification, the HCl is absorbed from the flue gas by water. Also, other impurities are removed from the gas. The raw HCl is then purified to a useful feedstock. 

Current C02 capture technology (first generation) is adapted from gas separation processes already in industrial use. There are several technologies and strategies to capture C02 from stationary sources pre-combustion, post-combustion and oxy-fuel (Figure 2). 

Post-combustion capture follows the conventional application of a specific purification unit applied for a particular pollutant removal (C02 in this case). Figure 3 illustrates a typical block diagram of the post-combustion process that offers a great feasibility and versatility in terms of operating conditions and process integration. 

Figure 3. Simplified scheme of a fossil-fuel power plant using a post-combustion capture unit [5].

The technologies currently available for post-combustion capture are classified into five main groups absorption, adsorption, cryogenics, membranes and biological separation. The most mature and closest to market technology and so, the representative of first generation of postcombustion options, is capture absorption from amines. 

Post-combustion capture using chemical absorption by aqueous alkaline amine solutions has been used for C02 and H2S removal from gas-treating plants for decades [6]. Amines react rapidly, selectively and reversibly with C02 and can be applied at low C02 partial pressure conditions. Amines are volatile, cheap and safe in handling. They show several disadvantages as they are also corrosive and require the use of resistant materials. Furthermore, amines form stable salts in the presence of O2, SOx and other impurities such as particles, HC1, HF and organic and inorganic Fig trace compounds that extremely constrain the content of those compounds in the treated gas. 

Selectivity for C02 it represents the C02 uptake ratio to the adsorption of any other gas (typically nitrogen for post-combustion capture, and methane for natural gas). It is an essential evaluation criterion, and affects the purity of the adsorbed gas, which will significantly influence the sequestration of C02. The simplest method to estimate the selectivity factor is to use single-component adsorption isotherms of C02 and nitrogen. 

The hydrophilic nature of most zeolite structures is considered a major drawback of zeolites especially for post-combustion C02 applications [49, 50]. Water competes with C02 on the... 

In order to provide further insight into the post-combustion ratio and the heat transfer efficiency, the factors that affect the PCR and HTE will be delineated. The factors that affect the PCR and HTE will be discussed separately with the understanding that a complex relationship may exist between the two parameters. The factors that affect the PCR are shown in Table 4, and Fig. 3 demonstrates the primary conditions for postcombustion. The PCR should be kept relatively high, since the fuel consumption decreases with an increase in the PCR at the same HTE (Aukrust, 1993). However, as mentioned, high PCR may lead to problems due to increases in ... 

Figure 3. Key factors to achieve efficient post-combustion in an iron-bath smelting reduction furnace. (From Takahashi et al., 1992.)...

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