Many gasifiers are operated at such elevated temperatures, that the ash left behind after consumption of organic material in coal melts and forms a glassy material called slag. More precisely, slag from coal gasification is a mixture of a this glassy, silica-based phase (a.k.a. "frit") and residual carbon char, the proportions of which vary depending on operating conditions, gasifier, feed, etc. The two parts can be separated—they are not chemically bonded—and concentrated into carbon rich char and vitreous frit. Vitreous slag is much preferable to ash, because of its habit of encapsulating toxic species (such as heavy metals) into a stable, non-leachable material. As a rule, slag qualifies as non-hazardous material which may be cheaply disposed of, or even advantageously used in a number of applications.
NETL and its partners are investigating the use of both slag components, in addition to leach testing to ensure the safety of the material and its use on the environment. The goal is to find large volume uses for the byproduct. Slagging gasifiers produce tons per day (tpd) of slag, requiring disposal by plant operators. Finding a break-even or profitable use for the byproduct would increase the confidence in gasifier economics.
The University of Kentucky's Center for Applied Energy Research (CAER), working with NETL, describes some of the potential uses for beneficiated slag in the following discussion.
The frit, or coarse component, is high density, vitreous (glass-like), and abrasive. It is low in carbon and comes in many shapes from jagged and irregular pieces to rod and needle-like forms.
Some possible applications for frit include:
- Building material (Slag-based Lightweight Aggregate, or SLA)
- Sand blasting grit or other abrasive application
- Polishing media
- Roofing granules
- Cement kiln feed
- Surface material for athletic tracks
- Road surfacing
- Ground down for use as a cement additive
Char is the finer component of the gasifier slag, composed of unreacted carbon with various amounts of siliceous ash. It can be recycled back into the gasifier to increase carbon usage and has been used as a supplemental fuel source for pulverized coal combustion. The irregularly shaped particles have a well-defined pore structure and have excellent potential as an adsorbent and precursor to activated carbon.
In a project between the Department of Energy (DOE) and CAER, potential uses of char were investigated, specifically as adsorbers for emissions control. Carbon char has the potential to control mercury (Hg) and nitrogen oxides (NOx) emissions. To test for mercury adsorption—an initial study only—a mercury vapor generator and a fixed bed reactor were used. A gas sampling system and vapor analyzer were used to assess the char’s performance. Untreated gasifier char was found to be as effective at adsorbing mercury as a commercially available activated carbon designed for Hg adsorption. Interestingly, all attempts to increase the porosity of the gasifier char lessened its adsorption potential, which seems to be related to the char mineral content (mainly SO4-2 and Cl-). Not having to treat the char is significant, as treatment would be an additional cost.
Additionally, gasification char adsorbed significantly more NOx than all other test materials except for a specially designed activated carbon NOx adsorber. For this test a thermal analyzer and mass spectrometer were used. The char performed 30% as well as the specially designed activated carbon adsorber. After increasing the surface area of the gasifier char, it increased NOx capacity, while char that was laden with Hg adsorbed more NOx than Hg-free char. This presents the possibility of using gasifier char to adsorb Hg while also adsorbing some NOx prior to an activated carbon NOx adsorber. Naturally, the gasifier char, a "waste," is significantly less expensive than a specialty adsorber and being able to put it to good use makes plant operations more economical.
DOE and CAER Factsheet - Value-Added Carbon Products from PCC and IGCC Byproducts
Before either the coarse frit or the finer char can be used, the slag must be "beneficiated." This involves separating and processing the slag into its two components, and ensuring they are of acceptable quality for their respective end uses. One such beneficiation process was developed between Charah Environmental, Inc. and CAER. It is currently in use at Tampa Electric Company's Polk Station in Mulberry, Florida.
Polk Station. Slag stockpile in foreground. (source: CAER)
The Polk Station beneficiation plant began construction in 2001, and was fully operational by spring 2002. Designed for a feed capacity of 100 tons per hour (tph), the original operating procedure involved blending 200 tpd of generated slag with a minimum 600 tpd of stockpiled slag byproduct. By spring 2004, the 140,000 tons of stockpiled byproduct was processed.
||source: University of Kentucky, Center for Applied Energy Research (UK-CAER)M
The processing plant uses a mesh separation system to classify the slag into three distinct products, from coarsest to finest:
- Frit (+20 mesh), which means slag that was trapped by a standard 20 mesh screen
- Fuel (-20+100 mesh): slag that passes through 20 mesh, but is trapped by 100 mesh
- Fines (-100 mesh): the finest slag
Currently the fuel (-20+100) and fines (-100) are combined and recycled back to the gasifier as supplemental fuel. This reduces fuel consumption, increases carbon usage efficiency, and, perhaps most importantly, eliminates the need to landfill over 70% of generated by-product.
CAER Factsheet - Demonstration of a Beneficiation Technology for Texaco Gasifier Slag
Slag-Based Lightweight Aggregates (SLA)
In 1996, a DOE study investigated lightweight and ultra-lightweight aggregates (LWA, ULWA) with densities between 15 and 50 lb/standard cubic feet (scf). The project also attempted to use all size components of the slag, including fines (the smallest bits) through binding with clay.
As discussed above and on the general page on slag byproduct utilization, slag is composed of two general parts: carbon-rich char and glassy, siliceous frit. For aggregate purposes, the presence of char is detrimental, so the char is removed. Rather than wasting it, the recovered char was upgraded from 50 to 70% carbon (by ash removal), which is suitable for use in kilns or a fluidized bed fuel. Specifically, it could substitute for 50% of rotary kiln fuel or 80% of the fuel in a fluidized bed.
The use of slag as aggregate for building and other purposes was tested according to American Society for Testing and Materials (ASTM) standards.
- Three materials were tested for use as roof tiling material: expanded slag, a 50/50 slag-clay extruded pellet mix, and a commercial, lightweight-aggregate as a control. Aggregate and cement were mixed with five parts aggregate to two parts cement and then commercial accelerator and super-plasticizer were added. At 2,800 psi and over seven days, the compressive strength of the slag aggregate was 83% as strong as the control. In addition, its density of 93 lb/scf was 87% of the control and, if necessary, sand could be added to increase both strength and weight. The 50/50 slag-and-clay mix was similar to the 100% slag test specimen.
- Insulating Concrete
SLA was then tried as an insulating concrete. It was found to have a density of 51 lb/scf and seven-day strength of 175 psi, but work is still in progress. In addition, SLA as a loose fill insulating material is being investigated.
The test study concluded that SLA can be produced with densities between 20 and 50 lb/scf using two different methods: rotary kiln and fluidized bed expansion. These products meet weight requirements for almost all lightweight and some ultra-lightweight aggregate uses. SLA also has some advantages:
- No mining costs to obtain the slag (as the coal would have been mined regardless)
- Energy savings, as the temperature for SLA creation is less than that for traditional clays and shales
- Recovered char can provide between 50 and 80% of the fuel required for slag expansion depending on whether a kiln or fluidized bed is used
- Slag can be used by itself or mixed with expansible clay, allowing for the use of slag fines. This could also allow slag to be used as a substitute for clays and shales in existing lightweight aggregate plants
Rotary Kiln Expansion
A direct-fired rotary kiln was used to lower the slag density—by expansion—for slag-based lightweight aggregate applications. After separating the slag into various size fractions, the coarse fraction (1/4" x 50-mesh) was fired as discrete particles. Density was able to be varied between 30 and 50 lb/scf by temperature control and could be lowered below 20 lb/scf but particle fusion became an issue. At a temperature of 1,450-1,500°C, plus 10-mesh slag was expanded to densities of 20-30 lb/scf. These temperatures are 300-400°F lower than typical temperatures for expanding clays and shales, which translates to significant savings in energy.
The slag fines are difficult to expand as is, so experiments were performed using a clay binder. Binding with clay was successful, as the clay was able to be blended with the slag fines for expansion. Minus 50-mesh slag was mixed with a 20-50% (by weight) expandable clay binder. The mix was then extruded and pelletized, where the size of the aggregate can be controlled by the extrusion process. Using a larger proportion of slag results in lower pellet moisture, which affects overall fuel requirements; more slag means less fuel costs.
The firing temperature for representative samples (80/20 and 50/50 slag-clay blends) ranges from 1,800-1,900°F. This temperature is higher than slag by itself but lower than clay alone. In addition, there was no sign of fusion with any of the mixtures up to 2,000°F. These expanded mixtures had densities between 27 and 33 lb/scf and the temperatures required were nearly 200°F lower than that for clay, which again saves energy.
Fluidized Bed Expansion
Various slag sizes were expanded as discrete particles to produce lightweight aggregates with densities between 18 and 26 lb/scf. Again using the extruded slag/clay pellets from minus 50-mesh fines, the pellets were granulated to 20-mesh aggregates for use in block and roof tile applications. These slag/clay aggregates had a minimum density of 30 lb/scf.
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