Article pubs.acs.org/EF
Emission of Organically Bound Elements during the Pyrolysis and Char Oxidation of Lignites in Air and Oxyfuel Combustion Mode Fiona Low, Anthony De Girolamo, Bai-Qian Dai, and Lian Zhang* Department of Chemical Engineering, Monash University, GPO Box 36, Clayton, Victoria 3800, Australia S Supporting Information *
ABSTRACT: This study aims to clarify the abundance of individual elements, particularly those in trace concentrations in lignites, and their emission dynamics during pyrolysis and char oxidation in both air and oxyfuel combustion modes. For this laboratory-scale study, the emission dynamics was represented by element release from the coal/char particle during thermal treatment in a drop-tube furnace. The main coal sample studied is a Victorian brown coal (VBC), which was compared with a Chinese lignite. Irrespective of elemental type, the VBC is rich in organically bound elements, which partly dissociated during the initial flash pyrolysis step. This dissociation extent varied broadly with elemental type. For element release during char oxidation, As release rates in both N2 and CO2 bulk gases were slower than char surface consumption rate, because of internal diffusion limitations and scavenging of a portion of As by Ca/Al/Fe-bearing discrete minerals. In contrast, the release rates of Pb from the char surface were faster than the carbon consumption rate. Releases of the remaining elements were simply in linear proportion to the char consumption rate for the two lignites studied, despite their differences in properties with no observable (element release) difference between air and oxyfuel combustion mode.
1. INTRODUCTION
The research in this paper covers the emission rates of 15 individual elements, both major (Na, K, Ca, Mg, Fe, Al) and trace (As, Ba, Cr, Co, Cu, Mn, Ni, Pb, Sr), during air and oxyfuel combustion of two low-rank lignites. The Victorian brown coal (VBC) collected from the Latrobe Valley in Victoria, Australia is the main target here. It is the single largest source for power generation in the state of Victoria, meeting >85% of electricity needs.12 Coupled with the huge reserves for this coal (reserves-to-production ratio of ∼100 years)13 and the recent implementation of Australia’s carbon tax in July 2012,14 there is an urgent need to fully understand its element emission fundamentals, particularly, their behavior in advanced CO2 abatement processes. To date, there is limited knowledge in terms of trace elements for this coal, which possess distinct properties and burns differently from other lignites and highrank sub-bituminous coals that have been studied extensively in the literature.15 For comparison, a Chinese lignite collected from Xinjiang, China was also examined. The ash-forming constituents in the VBC are mainly organically associated, present either as cations associated with the carboxyl groups or as dissolved salts in the associated moisture.15 Similar associations are thought to exist for the Chinese lignite examined here, which is characterized by a low ash yield.16,17 For this coal, data are practically nonexistent due to the low exploration degree, although it contributes to ∼40% of the entire Chinese coal reserves.16 Apart from uncertainties regarding the mode of occurrences of trace elements in these two coals, their emission propensities at high temperatures have yet to be studied.
Oxyfuel combustion is one of the promising options for carbon capture and storage (CCS) to be adopted by coal-fired power plants in the short and medium terms.1 Instead of conventional air, a mixture of high-purity oxygen and recirculated flue gas is employed in the oxyfuel combustion mode. This delivers a CO2-rich flue gas that can be sequestered directly, provided that the minor impurities within it are removed appropriately.2 The inherent ash-forming elements in coal are a major cause of impurities in the flue gas. Their emissions, either as gas or fine/ ultrafine particulates, would negatively affect CO2 purity and the operating of its compression system, e.g., the basis of slagging/corrosion issues. It is also of environmental concern, since some of the trace elements such as As(III) and Cr(VI) are toxic and/or carcinogenic.3 As summarized in technical reviews reported by IEA,4−6 the emission of trace elements from conventional coal combustion in air, both in laboratory-scale and industrial-scale furnaces, have been investigated extensively. For oxyfuel combustion, many laboratory-scale studies and pilot-scale sampling have also been conducted, revealing the varying trace-element emission trends that are highly dependent on the type of coal.7 To date, the focus in literature has been on high-rank coals where the mineral-bound trace element species are predominant.8 Release of a mineral-bound species is limited by diffusion of the element vapors throughout the host mineral and its boundary layer.9 They have a higher affinity to remain in the ash matrix.10 In contrast, prior release of an organically bound element happens accessibly.9,10 Thermodynamically, experimental-based modeling affirms that coal of different ranks cannot be treated similarly.11 A solid view pertaining to low-rank brown coals (i.e., lignite), where organically bound elements are prevalent, is still lacking. © 2014 American Chemical Society
Received: February 11, 2014 Revised: May 12, 2014 Published: May 12, 2014 4167
dx.doi.org/10.1021/ef5003777 | Energy Fuels 2014, 28, 4167−4176
Energy & Fuels
Article
The entire coal combustion process involves multiple steps,18 the overlap of which would cause difficulties in terms of clarifying the emission of elements. Therefore, coal pyrolysis and the subsequent oxidation of char were carried out separately in a laboratory-scale drop-tube furnace (DTF). Coal pyrolysis was first conducted to assess the release of elements upon devolatilization, which is regarded as the initial step of coal combustion. Next, oxidation of the resulting char derived from the pyrolysis reaction was carried out in a variety of conditions to reveal the dynamic release rates of individual elements. These include the use of N2 versus CO2 as a bulk gas, different O2 and steam concentrations, and various particle residence times in the DTF. Char oxidation was modeled by computational fluid dynamics (CFD) to predict particle temperature and trace-element associations were examined using the Pearson correlation coefficient. All elements in the samples were quantified using inductively coupled plasma− optical emission spectroscopy (ICP-OES). An element of interest, arsenic (As), was also characterized by synchrotron Xray absorption near-edge structure spectroscopy (XANES).
2.2. Coal Pyrolysis and Char Oxidation Procedure. Coal combustion was carried out in a laboratory-scale drop-tube furnace (DTF) with the details described elsewhere.19,20 As illustrated in Figure 1 for the schematic of the DTF tested, coal was entrained by 1
2. EXPERIMENTAL SECTION 2.1. Coal Properties. An air-dried Victorian brown coal (VBC) from the Loy Yang mines in Latrobe Valley, Australia and a Chinese lignite (XJC) from the Xin Jiang province, China, were tested. Both coals were ground and sieved to 105−153 μm prior to use. As tabulated in Table 1, both coals have low ash contents, compared to bituminous coals in which the ash content is typically on the order of >10 wt %.
Figure 1. Schematic drawing of the DTF reactor facility employed in this study. L/min primary gas and fed through a water-cooled injector into the inner chamber of a quartz reactor installed within the DTF. In the meantime, the secondary gas, at 9 L/min, was preheated to the furnace temperature through the annulus between the inner and outer chamber of the reactor and mixed with the primary gas and coal particles at the tip of coal injector. The effective length of the entire reactor chamber is 1.8 m long. Variation in coal particle residence time was achieved by using the coal injector with different lengths. For instance, using an injector with a length of 1.2 m protruding into the reactor ensures an effective reaction zone as short as 0.6 m, which is equivalent to a coal particle residence time of 1.7 s. Similarly, the coal particle residence time of 3.4 s was achieved by halving the coal injector protruding length to 0.6 m, whereas the use of the coal injector with no part protruding into the reactor helped achieve the maximum residence time of 5.8 s for coal particle in the reactor. Unburnt char particles were collected through the use of a flask and a Whatman silica microfiber thimble filter (Cat. No. 2812259) installed downstream of the reactor, which were continuously quenched by dry ice to prevent any secondary reactions of the ash/ char particles. The high-purity thimble filter employed exhibits a particle retention rating efficiency of ≥98% for the fine particles down to 0.3 μm, which is recommended by the United States Environmental Protection Agency (USEPA) standards for flue gas sampling. Note that the dense particles larger than 5.0 μm (as has been confirmed by scanning electron microscopy (SEM) observation) (called the “coarse fraction”), mostly dropped into the flask, whereas those smaller than this size (called the “fine fraction”) were entrained by flue gas and deposited into the thimble filter when passing through it. These two ash fractions were mixed together for analysis. In terms of mass balance, the majority of the released elements are believed to be deposited on the quartz wall. The elements deposited come from the vaporization of the organically bound metals, because there are no contributions from mineral grains. Previous works on the test of a bituminous coal (exclusively mineral grains) in the same DTF witnessed a satisfactory mass balance for bituminous coal ash.21 These deposits are not regarded as contaminants, because the majority of the coal particles do not come into contact with the DTF walls (from modeling, 153 μm
size: 105−153 μm
size: 63−105 μm
size: 153 μm, 106−153 μm, 63−105 μm, and
