Researching the Ultimate Fireless Steam Locomotive - Part II
Chemical Thermal Energy Storage
Over the past decade, research was undertaken in Japan into high-temperature chemical thermal energy storage using metallic oxides. This research was aimed at storing thermal energy at thermal power stations during off-peak periods, then using that stored energy to generate extra electric power during peak demand hours. A team of research scientists based at the Tokyo Institute of Technology included Dr Yukitaka Kato, Dr Yamashita and Dr Yoshizawa who undertook research into the thermal reaction of magnesium oxide with steam. Another research team that included Dr Matsuda, Dr Kyaw, Dr Masanobu and Dr Hasatoru at Nagoya University based their investigation on the thermal reaction of calcium oxide and carbon dioxide. Information pertaining to this research can be accessed online at http://www.scej.org/ronbun/JCEJe/e29p0119.html .
The injection of steam [H2O] into a container containing powdered magnesium oxide [MgO] produced magnesium hydroxide [Mg(OH)2] at a temperature of 300-degrees C to 350-degrees C (570-degrees F to 660-degrees F). The reaction released between 391-KJ/Kg and 556-KJ/kg or 168-BTU/lb to 239-BTU/lb of heat. The reverse reaction (decomposition) involved heating the magnesium hydroxide until it released the water vapour, leaving behind powdered magnesium oxide. Japanese researchers used a heat pump to transfer heat from a low-grade source and raise its temperature to enable magnesium hydroxide to decompose into the metallic oxide and water vapour.
Magnesium is one of several metallic oxides that will react with steam to form a hydroxide (giving off heat), which can be heated until it decomposes into a metallic oxide and steam. Others include calcium oxide (CaO) and nickel (NiO), the latter having a high density allowing it to occupy a compact package. A portion of the saturated steam in the accumulator of a fireless steam locomotive may be directed to the tank of metallic oxide, to generate the heat needed to maintain constant temperature and pressure levels in the accumulator as saturated water was being flashed into steam for propulsion. This transfer of energy into the accumulator would raise the locomotive's power levels and increase its operating range from the shunting yard to short-distance intercity service. A portion of the heat of formation of the metallic hydroxide may be heat pumped (using a high-pressure line of steam) to a higher temperature either into the accumulator or to a superheater.
Calcium carbonate [CaCO3] decomposes into calcium oxide [CaO] and carbon dioxide [CO2] when heated, a process that needs to be carefully controlled to prevent glazing the calcium oxide, rendering it unfit for further use. Reconstituting the glazed oxide involves grinding it into a powder then reacting it with water to produce calcium hydroxide [Ca(OH)2] and 15,300-calories of heat. The hydroxide can then be reacted with carbon dioxide to form calcium carbonate. When calcium oxide reacts with carbon dioxide, it produces calcium carbonate at temperatures between 700-degrees C and 1031-degrees C (1290-degrees F to 1885-degrees F), the higher temperature occurring at a gas pressure of 5-atms (74-psia) and releasing 237-Watts/Kg or 367-BTU/lb of heat energy.
The range of metallic oxides that would react with carbon dioxide would include beryllium, lead, barium, magnesium and manganese. Limestone (CaCO3), magnesite (MgCO3), and rhodochrosite (MnCO3) all occur quite naturally in nature and should be available at competitive costs. Manganese has the advantage of high density, allowing a thermal storage system using this metal to occupy a compact package that would make it more suitable for mobile operation. The carbon dioxide gas may either be stored in high-pressure containers or in low temperature carbonates that could be decomposed using exhaust heat rejected from an engine. Fireless steam locomotives using heat of formation of metallic carbonates for energy storage may carry most of the carbon dioxide in a low temperature carbonate in a tender unit, with a small amount being carried in an auxiliary pressure tank to enable the locomotive to start and restart.
To generate heat for propulsion, the metallic oxide would need to react with the carbon dioxide inside a thermal reaction chamber. This chamber may be located next to an accumulator of saturated water. It may alternatively occupy the location of a firebox in a firetube boiler. The firetubes may either use high-pressure steam or a liquid metal (like a sodium-potassium mixture) to transfer thermal energy from the reaction chamber to the boiler. In the latter case, leak-proof, corrosion-resistant, stainless-steel tubes would carry liquid metal at low pressure between the reaction chamber and boiler. In the reaction chamber, the heat of formation resulting from carbon dioxide reacting with the metallic oxide would occur at temperatures close to solid fuel combustion in a steam locomotive.
It may become possible to produce corrosion-resistant, leak-proof stainless steel watertubes that operate at high temperature and pressure. Such watertubes can pass right through the thermal reaction chamber, using research undertaken by Enginion of Germany (http://www.autofieldguide.com/articles/070102.html and http://www.enginion.com) to generate ultra-critical pressure steam at high temperatures (1000-deg C) using advanced materials technology. When combined with advanced chemical thermal energy storage technology, this technology could enable a modern fireless steam locomotive to operate well in excess of 30% thermal efficiency. A unique oil-free piston steam engine designed by Viktor Gorodnyanskiy in Russia can be built out of ceramic material and deliver an estimated 35%-efficiency using superheated steam at 650-deg C/1200-deg F, while Enginion technology has shown that high-pressure steam at 1000-deg C/1800-deg F can enable a small, low-powered steam engine to deliver the engine efficiency of a diesel engine. Such technology could enable a future generation of condensing fireless steam locomotives to operate at the efficiency level of diesel engines while using lower costing fuels and incurring lower operating and maintenance costs.
A less efficient fireless steam locomotive may use magnesium oxide or nickel oxide as the basis of its chemical thermal energy storage system. The accumulator may operate at 800-psia (518-deg F/270-deg C) and a low-pressure steam engine at 300-psia. A portion of the steam from the accumulator would be used to react with the metallic hydroxide to produce the heat needed to maintain a constant accumulator temperature. Heat from the thermal reaction chamber may be heat pumped (using high-pressure water as the working fluid) to superheat steam prior to expansion in the engine. Using steam as the reaction gas reduces operation complexity and increases safety compared to using the more efficient carbonate-based thermal reaction system. A carbonate-based thermal system may also be combined with accumulator approach.
Steam from a 1,000-psia accumulator may piped through the thermal reaction chamber to be superheated, before the hot steam line re-enters the accumulator to maintain temperature levels. A choke valve in the steam line would reduce line pressure from 1,000-psia to 550-psia and temperature from 1000-deg F to 800-deg F on the first pass. On the second pass through the accumulator, a second choke valve in the line would reduce steam line pressure to near 300 psia and steam temperature by the same value as the first pass. Heat rejected from the hot steam line would be transferred into the accumulator to maintain pressure and temperature levels, allowing a greater percentage of the saturated water in the accumulator to be used for propulsion. This would also raise power output and extend locomotive operating range. Steam at 300-psia and at over 800-degrees F could be expanded in a steam piston engine at short inlet valve cut-off ratios to optimise engine efficiency during branch-line, short-line or commuter services.
In railway operation, safety and cost factors would be a major factor that would determine which chemical materials may be used for chemical thermal energy storage. Only low-volatility, non-corrosive materials that release water vapour, oxygen or carbon dioxide when heated to high temperature would qualify. Of these, low-cost materials that are easily accessible and that absorb and release large amounts of thermal energy for their weight and volume, would be suitable for mobile operation. Thermal energy storage materials generally exceed the useable life expectancy and energy storage densities of chemical-electrical battery energy storage systems. Many thermal energy technologies have an almost infinite life expectancy. Thermal storage technologies that do eventually deteriorate after prolonged use are generally more easily reconstituted at much lower cost than electrical energy storage technologies.
Hybrid Thermal Energy Storage:
When heated, some compounds melt while others decompose or dissociate while still in the solid state. Other compounds like sodium nitrate [NaNO3] will first melt then release an oxygen atom to become sodium nitrite [NaNO2], which can decompose into sodium nitride [NaNO], a process that involves 260-BTU/lb of energy. This phenomena occurs with several metallic nitrates/nitrites/nitrides and chlorates/chlorite/hypo-chlorites [eg; potassium chlorate KClO4] that release oxygen atoms as they decompose at higher temperatures. Many metallic compounds that can occur as hydroxides, hydrated oxides, sulphates, nitrates, or silicates will melt at one temperature then release water vapour at a higher temperature.
The reverse reaction would occur as the compounds/mixtures are allowed to cool, transferred from them while adding oxygen or steam. This would result in a series of successive levels of heat of fusion and heat of formation becoming available as a thermal energy supply for use short-distance intercity railway propulsion. Theoretically, a modern fireless steam locomotive using such technology could be developed to deliver over 10,000-Drawbar-Horsepower for traction purposes, using the combination of chemical thermal energy storage as well as the latent heat of fusion thermal energy storage. At the present time, research is underway in a variety of countries involving researchers who are investigating chemical and phase-changing means to store larger quantities of heat at higher temperatures, using combinations of easily and readily available materials. Future developments in fireless steam railway traction would benefit from such efforts.