Article pubs.acs.org/EF
Colloidal Petcoke-in-Water Suspensions as Fuels for Power Generation Gustavo A. Núñez,† María I. Briceño,*,† Cebers Gómez,† Takeshi Asa,† Hamid Farzan,‡ Shengteng Hu,‡ and Daniel D. Joseph§ †
Nano Dispersions Technology, Incorporated, Building 231, City of Knowledge, Clayton, Panama Babcock and Wilcox Power Generation Group, 180 van Buren Avenue, Barberton, Ohio 44203, United States § Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡
ABSTRACT: In this work, it is shown that, despite the low reactivity of petroleum coke (petcoke) and the presence of 40% water, a petcoke suspension having a large colloidal population burned with unprecedented high efficiencies (>99%) without a support fuel. This paper is an account of the main combustion test results, obtained in a 6330 MJ/h pilot-scale boiler simulator located at the Babcock and Wilcox Research Center. This pilot plant simulates a full-scale utility boiler in many key aspects. Combustion tests of a typical heavy fuel oil (HFO) were carried out to produce baseline data for comparison to the colloidal petcoke in water suspension (CPW) performance. The CPW fuel showed, besides high particle reactivity during combustion, some advantageous characteristics, such as ease of pumping, metering, and atomization at room temperature, using conventional equipment designed to handle and fire HFO.
1. INTRODUCTION Heavy fuel oil (HFO) is still used worldwide in industrial and power-generating boilers, including facilities in the east and west coasts of the U.S., where fuel oil can be conveniently shipped in oil tankers. However, with the rising price of oil and its attached fuel oil price, it has become necessary to find less costly alternatives to keep the operational costs of any plant at competitive levels. Some alternatives, such as coal and petroleum coke (petcoke), are quite obvious given their comparatively historical lower cost and wide availability. Some of the shortcomings of burning pulverized fuels in oil-firing boilers can be overcome by preparing mixtures of the solid material with water or oil. This is not a new idea, and back in the 1970s and 1980s, there was intensive research and development on coal−water slurry (CWS) or coal−oil slurry (COS).1 However, the interest in this kind of fuel came to an end in the U.S., mostly because of economic hurdles associated with low oil prices prevalent through the 1990s. Nonetheless, to the authors’ knowledge, there was no attempt to prepare petcoke in water slurries for use as the sole fuel in conventional boilers. At present, the oil price is increasing, and a definite higher price horizon is foreseen as new large consuming economies, such as China and India, become stronger buyers of fossil fuels. This situation is again becoming a driver to improve coal quality through cleaning procedures and more efficient combustion processes for the aforementioned technologies.1,2 Our research group has taken a new approach that deals with several of the limitations of CWS fuels, by means of the manufacturing of a mostly colloidal suspension of petcoke particles dispersed in water. The latter characteristic makes a great difference regarding fuel reactivity and stability. Colloidal particles have a total surface area many times higher than the particles present on conventional CWS. Chemical reactions, such as oxidation (combustion) or sulfation, occur on the © 2012 American Chemical Society
surface of solid fuels or minerals. Large increases in surface area imply higher reaction efficiencies.3−6 A second and also important aspect is the effect of particle size reduction in suspension stability. Increasing the colloidal fraction slows or prevents altogether sedimentation,3,5 one of the main bottlenecks of CWS in the past. In a previous paper,3 we presented the results of the preparation and combustion characteristics of a colloidal coalin-water suspension used as a reburn fuel. As expected, the colloidal nature of the fuel greatly improved the reburn performance, outdoing a higher rank pulverized coal and, under some operational conditions, approaching natural gas behavior in the same conditions.3 In this work, we present the combustion results of a colloidal petcoke in water suspension (CPW) used as the sole fuel in a small boiler simulator. The fuel was prepared in a similar way as the coal suspension described in our previous publication.3 Petcoke usually has an ash content lower than 1%. A fuel having less than 1% ash could be fired, in principle, in a boiler designed for HFO, as long as the fuel has combustion characteristics similar to or better than fuel oil. However, firing only petcoke is next to impossible in most conventional boilers, without co-firing with a suitable (supporting) fuel. This fact is attributable to the low reactivity of petcoke related to the low volatile content, which is inferior to most commercial coals. Nonetheless, petcoke is an attractive fuel from the point of view of its cost per heat value (BTU) and from its market availability, although its combustion characteristics have relegated it to mostly being fired in cement furnaces, fluidized beds, and other specially designed boilers or being co-fired with other fuels. Received: July 26, 2012 Revised: October 23, 2012 Published: October 24, 2012 7147
dx.doi.org/10.1021/ef301249q | Energy Fuels 2012, 26, 7147−7154
Energy & Fuels
Article
evaporation. On average, the final petcoke content in the suspension was about 62% (w/w). A total of 12 tons of CPW fuel were manufactured in this fashion and later shipped to BWRC facilities in standard 300 gallon intermediate bulk containers (IBCs). To follow-up the CPW properties during storage, a couple of samples were kept in one 10 gallon container and one 55 gallon drum. As mentioned above, more details on the preparation method and a description of the wetcomminution apparatus can be found elsewhere.3 2.2. Evaluation of the Physical Properties. During manufacture, the particle size distribution of the petcoke slurry (before wet comminution) and CPW was measured using a laser diffraction apparatus made by Microtrac. The measurement procedure was as follows: A small portion of sample was dispersed in 10 mL of sodium polyphosphate (0.25%, w/w) and sodium lauryl sulfate (0.75%, w/w) solution. About a third of this suspension was poured into the Microtrac mixing tank before circulation through the measuring cell. It is worth mentioning that, while the dry petcoke and petcoke slurry were relatively easy to measure and results were quite reproducible, CPW particle size characterization was less reproducible. Wet-sieving the aforementioned solution in a 600-mesh standard sieve (20 μm opening) was used to validate the Microtrac results, and it was found consistently that sieving gave way to less mass retention than the laser diffraction apparatus, although the difference was on the order of 2 or 3 mass % below 20 μm. A gravimetric method was used to measure the water content; a weighed sample of slurry or CPW was placed in an oven and let to dry at 120 °C for at least 6 h. After cooling, the dry sample was weighed and the amount of water lost during the tests would allow us to calculate the initial moisture content. A moisture analyzer made by Arizona Instruments, Computrac model, was used to measure the water content of CPW during the combustion tests. A Brookfield DV-II+ Pro viscometer, set with a RV3 spindle rotating at 100 rpm, was also used to monitor viscosity during the tests. The viscosity reading was made 10 s after the start of the spindle’s rotation. A KinexusPro rheometer, manufactured by Malvern, was used to obtain steady-state flow curves for the CPW. The measuring geometry was a parallel plate system, 40 mm in diameter and set to a 1 mm gap. The ultimate analysis and other relevant properties for both CPW and HFO fuels are shown in Table 2.
This paper is an account of the main combustion test results, obtained in the 6330 MJ/h pilot-scale small boiler simulator II (SBS-II) located at the Babcock and Wilcox Research Center (BWRC), at Barberton, OH. The SBS-II tests simulated a fullscale utility boiler in many key aspects, such as flame stability, combustion efficiency, and furnace and convection pass heattransfer characteristics under various operating conditions. It also included assessing emission characteristics of CPW combustion, including CO and NOx, which are highly dependent upon combustion parameters, such as in-flame oxygen availability, fuel−air mixing, the nitrogen content of fuel, and other factors. For this purpose, about 3000 gallons of CPW fuel were manufactured in a pilot plant in a semi-batch fashion. Combustion tests of a typical HFO were carried out to produce baseline data for comparison to the CPW performance. The main purpose of these tests was to collect relevant and scalable data for commercial power generation.
2. EXPERIMENTAL SECTION 2.1. Fuel Preparation and Handling. The petcoke was supplied by DTE Energy Services (Vicksburg, Mississippi) and previously milled to −200 mesh. Table 1 shows some of the most relevant properties of this fuel-grade petcoke sample.
Table 1. Physical Properties of Fuel-Grade Pulverized Petcokea
a
petcoke properties
typical values
carbon (%) hydrogen (%) sulfur (%) vanadium (ppm) volatiles (%) moisture (%) high heating value (Btu/lb) particle size (mesh)
86 3.5 6.5
