Alternate Energy Sources for Mining Equipment.
As technology advances, more equipment manufacturers are looking at new power plants for haulage systems. Today, mines rely on either diesel- or electric-powered haulage, and two technologies have emerged from these areas. Diesel haulage has spawned natural- and hydrogen-gas combustion engines. Battery-powered haulage is evolving toward fuel cell technology. Both systems have been, to varying degrees, in the limelight over the last decade in the automotive and mass transit arenas.
Companies that maintain transportation or delivery fleets have converted to natural gas in many countries. Even some automotive manufacturers have offered a natural gas option on some vehicles. In North America, General Motors (GM), Chrysler, and Ford have offered various models with compressed natural gas (CNG) fuel systems. Showing some acceptance as fleet vehicles, mass consumer adoption of the technology still has some hurdles to overcome. In 1995, Volvo GM Heavy Truck Corp. worked with Cummins Engine Co. to offer natural gas engines as an option on Volvo XPEDITOR trucks for the refuse market. The alternative fuels application to the internal combustion engine is essentially understood as it employs the same basic design. In some cases, the engine can switch from CNG to conventional fuel while in operation.
Cummins Westport Inc., a joint venture of Cummins Inc. and Westport Innovations Inc., is involved with Placer Dome Technical Services Ltd., Komatsu Mining Systems Inc., the Cortez joint venture, and ENRG in a low-emissions, natural-gas-powered mine truck study. The goal is to establish the feasibility of testing a natural-gas-powered mine truck at the Cortez joint venture's gold mine in Nevada. An actual vehicle demonstration project would be subject to obtaining site permits and project funding. The ultimate goal is to develop natural-gas mining products, based on the Cummins QSK diesel-engine platform, that will meet future global low-emissions standards for mining, while retaining the performance of diesel engines.
Placer Dome Technical Services will determine the feasibility of using natural gas products in existing and new mines. Komatsu Mining Systems will evaluate the potential for natural gas in its ultra-class truck, the (st) 930E-2.
The Cortez joint venture will study the site implications of a natural-gas mine-truck demonstration at its Cortez gold mine near Elko, Nev. ENRG will judge the feasibility of providing liquefied natural gas (LNG) for the mine-truck demonstration.
Clean-burning fuel applications, such as those that use hydrogen or natural gas for their internal combustion engines, are showing some competition for the fuel cell. At a basic level, the two are fairly similar. Internal combustion of these fuels combines the hydrogen or natural gas with oxygen in a conventional explosive reaction, harnessing the system's energy output as kinetic energy. A fuel cell combines the hydrogen with oxygen in a more controlled manner, harnessing the systems energy output more directly on the atomic level in the form of electrons (electricity). Fuel cells designed to use hydrocarbons incorporate a converting step, which is referred to as reforming in the fuel cell industry, to produce a hydrogen-rich gas from the hydrocarbon. In the end, as with the choice between diesel and electric, it will be system efficiency that determines which technology finds a home in a given operation.
Researchers in the U.S have been investigating the technical and economic feasibility of using hydrogen as an alternative zero-emissions fuel under a project called the "Zero Emissions Utility Solution" (ZEUS). The object of the project was to demonstrate the technical and economic feasibility of using hydrogen as an alternate, zero-emission fuel that could reduce the exposure of underground miners to diesel exhaust. The 10,000-lb payload vehicle used in this project was an Eimco 975 utility truck, which is an articulated four-wheel drive, general purpose vehicle with a CAT 3304 engine modified to burn hydrogen. The engine was converted by equipping it with a spark ignition, lowering the compression ratio, turbo charging and adding an aftercooler and a parallel fuel induction system, and changing the 85-hp diesel engine to a 102-hp H [subscript]2 engine.
The rig has 21 hydride modules under the flatbed that hold 8.5-kg H [subscript]2 , which is enough to run at a high-power for about four hours. Two different types of metal hydrides are used, one for cold-starting and one for normal operation. The hydrogen diffuses into the crystal structure of metal powders where it exists as an interstitial chemical compound. The powdered hydrides are contained in heat exchangers that use waste heat from the engine's cooling system to break the chemical bonds, releasing hydrogen to the engine. The hydrides may be recharged by applying hydrogen pressure while cooling them.
Fuel cell technology has found wide acceptance in the space industry and military applications. The application is driven by its greater efficiency. For example, carbon dioxide emissions from a small car can be reduced by as much as 72% with a 60%-efficient fuel cell running on natural-gas-reformed hydrogen, compared to a 25% emissions reduction from a gasoline internal combustion engine.
There are a number of fuel cell technologies currently under development, including: phosphoric acid, proton exchange membrane, solid oxide, direct-methanol fuel cells, molten carbonate, alkaline, and regenerative. The basic concept and design is the same with operational differences stemming primarily from the electrolyte and, therefore, the chemistry involved in generating electricity.
Commercially available, phosphoric acid fuel cells (PAFC) produce electricity at roughly a 40% efficiency and operate at about 400[degree sign]F. This type of system has been adopted primarily as backup power systems in critical operations such as hospitals, power plants, and airports.
The U.S. Department of Energy (DoE) sees proton exchange membrane fuel cell technology (PEMFC) as the primary candidate for light-duty vehicles, buildings, and potentially smaller applications such as replacements for rechargeable batteries. A proton exchange membrane allows hydrogen ions to pass through it. The membrane is coated on both sides with metal alloy particles (mostly platinum) that operate as active catalysts. Hydrogen is fed to the anode side of the fuel cell where the hydrogen atoms release electrons that generate the electric current. After traveling through the electrical circuit, the electron reaches the oxygen-rich cathode. The other half of the hydrogen atom, the proton has diffused through the membrane to the cathode where the hydrogen atom is recombined and reacted with oxygen to produce water, completing the process. These cells operate at relatively low temperatures (about 200[degree sign]F), have high-power density, and can vary their output quickly to meet shifts in power demand.
The solid-oxide fuel cell (SOFC) usually uses a ceramic electrolyte, allowing for higher operating temperatures than systems that use liquid. Some SOFC designs are close to commercialization with some demonstration cells producing as much as 220 kw.
The direct-methanol fuel cell (DMFC) is similar to the PEMFC in that it uses a polymer membrane as an electrolyte. However, a catalyst on the DMFC anode draws hydrogen from liquid methanol, eliminating the need for a fuel reformer.
Molten carbonate fuel cells (MCFC) promise high fuel-to-electricity efficiencies and operate at about 1,200[degree sign]F. Many different fuels may be used in this type of cell, including hydrogen, carbon monoxide, natural gas, propane, landfill gas, and marine diesel. Several 10- to 2,000-kw MCFCs have been tested on a variety of fuels.
The energy provided to a fuel cell system must come from hydrogen provided from an external fuel reserve. In the simplest of systems, the hydrogen would come from a pressure vessel that is refilled in the same manner as a fuel tank. If a pure pregenerated hydrogen source is not available, the hydrogen must be produced from a reformed hydrocarbon. The act of reforming refers to the process of removing the hydrogen from a suitable source prior to reaction in the fuel cell.
Pure hydrogen has the best energy-to-weight ratio of any fuel, but also has the lowest storage density of all fuels at atmospheric pressure. There are a number of ways to store hydrogen for fuel cell applications, such as a gas, liquid, metal, and/or chemical (liquid) form.
Hydrogen can be supplied through conventional compressed gas cylinders. The structural requirements of a compressed-gas storage tank make this type of system cumbersome. Hydrogen accounts for less than 10% of the total weight of a compressed-gas storage tank when full, making the energy density of the system less than ideal.
Liquid hydrogen has a very high-energy density, and its requirements for storage are similar to those for compressed gas. Because hydrogen boils at -253[degree sign]C, its storage containers also require insulation in order to reduce evaporation. The energy required to liquefy hydrogen reduces overall efficiency of the system. It amounts to 30% to 40% of the energy content of the gas.
Fuel Cell Comparison
Some metals can absorb up to 1,000 times their own weight in hydrogen when heated, forming a metal hydride fuel "storage tank" when cooled. Reheating the "tank" frees the hydrogen. A metal hydride fuel source bypasses the structural concerns of a gas or liquid system, but these type of metals tend to be very heavy and the temperature range of operation is not small.
Methanol has attracted a lot of attention as a possible hydrogen source. It is commonly produced from natural gas but can also be manufactured from crude oil, coal, and from renewable sources as well. Methanol is a liquid at room temperature and one atmosphere pressure, simplifying the fuel's storage and handling requirements. As a simple hydrocarbon void of sulfur, methanol has a good hydrogen density.
The same benefits touted for civilian applications have potential for the mining environment, especially underground mining. Replacing a diesel power plant with a fuel cell eliminates virtually all of the issues associated with a diesel operation. Fuel cells produce electricity, heat, water, and oxygen-depleted air, compared to diesel, which produced kinetic energy, heat, noise, and various carbon, nitrogen, and sulfur oxides mixed with oxygen-depleted air.
Those who attended MinExpo 2000 may recall the fuel-cell powered skid-steer loader developed at South Dakota State University with funds from NRECA's Cooperative Research Network. Also present was a fuel-cell powered 4-st locomotive, designed by the Fuelcell Propulsion Institute. The Institute's next development is to be a fuel cell-battery hybrid underground loader, weighing 26 mt, develop 300-kw peak power, and store 30-kg H2 on board as a metal hydride.
Application of the technology in a large scale would effect some mining in more indirect ways as well. Markets for metals like titanium and nickel, both used in the production of fuel cells, could see significant upswings with wide-spread adoption of the technology.