Minimum feeding rate of activated carbon to control dioxin emissions from a large-scale municipal solid waste incinerator

Abstract

To obtain a minimum feeding rate (F_min) of activated carbon (AC), a series of measurements on dioxin emission concentration were carried out in a large-scale municipal solid waste incinerator. It was found that dioxin removal efficiency (η) increased with an increase in AC feeding concentration. This had an almost linear function to F/Q when F/Q was less than 65 mg/Nm³, where F was the AC feeding rate (mg/min), and Q was the volumetric flow rate of flue gas (Nm³/min). However, it did not seem to be affected by F/Q, when F/Q was larger than 150 mg/Nm³. On the basis of the experimental data obtained in this study, the removal efficiency of dioxins by the application of AC could be correlated as η (%) = 100/[1.0 + (40.2/(F/Q)³)]. It is valid in appropriate conditions (F/Q = 10–300 mg/Nm³) suggested by the study with a statistical error of ±18%. The correlation would be applied to estimate the dioxin removal efficiency (η) using the F/Q value. For engineering applications, the (F/Q)_min could be solved using a graphic illustration method, by which the minimum feeding rate (F) was obtained if the flue gas volumetric flow rate (Q) was known.

Introduction

Incineration is an attractive alternative for municipal solid waste (MSW) treatment and has significant benefits. However, in recent years atmospheric emissions from many thermal processes have been the subject of public concern. Combustion, commonly known as incineration, constitutes a controlled oxidation process in which chemical reactions transform carbon species into CO₂. The carbon content of waste cannot be totally converted into CO₂ and minor amounts of unacceptable products of incomplete combustion (PICs) are found. The combustion of organic matter leads to toxic pollutant emissions. The presence of chlorine and metals is widely recognized as a major source of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and other toxicants into the environment [1], [2], [3].

Dioxin (abbreviation for PCDD/F in this article) is one of the most toxic chemicals known. All published reports confirm that dioxin is a cancer hazard to people. It can cause immune system damage and interfere with the regulatory hormones. As a result, stringent regulations, mainly governing stack gas emissions, have been enforced in recent years with the aim of reducing pollutant emissions into the air. Dioxin emits from incinerators in the flue gases, the fly ash, and the bottom ash or slag. Technology advances in the past 20 years has dramatically decreased the dioxin levels in the fly ash and slag. Recent studies have shown that secondary treatment of incinerator residues can reduce the dioxin levels by more than 99% [4].

In modern plants, the first stage in dioxin abatement strategies consists of applying primary combustion measures, which mainly affect the thermal process stability. However, the raw gas produced during the combustion process requires exhaustive cleaning before being released into the air, for legislative standards to be achieved. A second common step consists of an appropriate gas-cleaning system, which drastically reduces the emissions of these contaminants into the air. However, a substantial amount of dioxins still remained in the gases using these two methods. Therefore, satisfactory dioxin abatement requires additional steps, such as, the injection of activated carbon (AC) or other additives. AC sprayed into a dry/semi-dry scrubbing unit positioned after the particulate removal device, but prior to the stack, has become a standard component in gas-cleaning trains. Bag filtration coupled with AC injection adsorption as end-of-pipe treatments can play a great role in the prevention or minimization of dioxins in the final flue gas emission into the atmosphere [5], [6], [7].

Previous researches have been conducted on the basis of the performances of AC injection and dioxin reduction. For example, Milligan and Akwicker [8], [9] studied dioxin removal using flue gas treatment with pulverized AC. They indicated the significant influence of AC feeding rate on dioxin emission. Meanwhile, Chang and Lin [10] stated that dioxin removal efficiency would decrease due to the dioxin memory effect, which increased the dioxin concentration in flue gas under AC injection and caused a lowering of removal efficiency. Everaert et al. [11] assessed the dioxin adsorption concepts. They used the results obtained from operational tests in incineration plants for municipal and industrial waste, to describe the efficiency of dioxin-sorption in an entrained bed (pneumatic transport of pulverized adsorbent) and a fixed/moving bed (granular adsorbent). Variations in the dosing rate and the use of different adsorbents, as well as the operating temperature, were used to identify the selection criteria for determining the optimum process-engineering conditions for flue gas cleaning. Mori et al. [12] studied the multi-component behavior of dioxin adsorption using AC filters. They found that the transfer rate in the adsorption of dioxin isomers tends to increase with decreasing numbers of chlorine substituents, and thus affects adsorption overall efficiency of AC.

Karademir et al. [13] studied the removal of dioxins from flue gas using a fixed-bed AC filter in a hazardous waste incinerator. They pointed out that dioxin removal efficiencies decreased as the chlorination level increased, which was explained by the difference in gas/particle portioning of the compounds. However, dioxins had a concentration value, up to which, no adsorption occurred. This could be attributed to the insufficient contact time at low concentrations in the flue gas. McKay [14] reviewed dioxin characterization, formation, and minimization during MSW incineration. He pointed out that the more the AC injection, the higher the dioxin removal efficiency. However, the spent AC with dioxin content must be carefully treated because of dioxin toxicity. The AC usage amount should be reduced as low as possible considering post-treatment cost. Kim et al. [15] have studied removal characteristics of dioxins from municipal solid waste incinerator by dual bag filter (DBF) system. They pointed out that AC consumption was less in case of DBF (40 mg/Nm³) as compared to single bag filter, which discharged about 100 mg/Nm³.

Besides the effect of AC feeding rate as described as above, chemical and physical characteristics of dioxin congeners will also influence dioxin emission in flue gas. One of them is the size and molecular weight of PCDD/F congeners. In general, PCDD/F congeners are significantly larger than other vapor-phase pollutants achieved with activated carbon adsorption. Hence, these vapor-phase pollutants would inhabit the PCDD/F adsorption with the variation of different pore size (micropore, mesopore and macropore) on activated carbon, and thus decrease dioxin adsorption efficiency.

Many findings from the previous researches indicated that the AC feeding rate significant affected dioxin removal efficiency [15], [16]. However, Aikyo and Suzuki [17] indicated that there were some problems for dioxin removal from flue gas using AC injection, for instance the disposal of spent AC with absorbed dioxins. Most owners or operators of MSW incineration plants always think that the AC cost, including purchase, and disposal cost, is expensive compared with other O&M (operation and management) items. Minimizing the AC cost has become an important issue. From the view of engineering application, this study was designed to take extensive measurements to determine the AC feeding rate effects on dioxin removal efficiency. The minimum feeding rate for AC will be estimated, to reduce O&M cost and control the dioxin emitted from MSW Incinerators.