Hydrogen Gas and Hot Water Waste Heat
After the oil-supply disruptions and price shocks of 1970s, emphasis on energy saving has been intense on the industrial sector in Japan, e.g. by promoting the use of heat-cascading systems.
In transporting waste heat through pipelines, the supply of waste heat is usually located far from demand, which can make the cost of facilities (mainly pipelines) both prohibitive and unprofitable. One option is to utilize chemical reactions, which can lead to the transport of heat more intensively than via vapor or hot water in pipelines. As a result the methanol-gas decomposition/synthesis reaction, and hydrogen-absorbing alloys have been investigated as requisite technologies in Japan.
Waste heat is recovered by the endothermic reaction of methanol decomposition (CH3O
CO+2H2). The carbon monoxide and hydrogen gases derived are then transported through the pipeline to the demand side. These gases are synthesized to methanol at high temperature and pressure (250°C, 50 atm). This synthesis reaction is exothermic, which enables heat release. The synthesized methanol is carried back through another pipeline and this closed cycle is repeated continuously.
Hydrogen-absorbing alloys can react reversibly with hydrogen gas. Heat energy is generated through hydrogen absorbing process, and the metal hydride emits hydrogen gas when heat is added to it. These characteristics can be applied to waste-heat transporting system.
Processes for transportation are as follows:
1. Waste heat is injected to Reactor 1, and the metal hydride in it emits hydrogen gas;
2. The emitted hydrogen gas is carried to Reactor 4, and the metal in it absorbs the hydrogen gas;
3. The heat generated through the exothermic reaction can be utilized at the demand side.
To provide heat energy continuously at the demand side,the metal in Reactor 3 is required to absorb hydrogen gas during the processes quoted above 1–3. So, initially the metal hydride is placed in Reactor 2, and hydrogen gas is carried to Reactor 3 when the waste heat is injected to the metal hydride of Reactor 2.
The total costs (i.e. facility costs and energy cost for running the system) of waste-heat transportation systems with different energy carriers (vapor, hot water, methanol or hydrogen gas) are evaluated here.
• The demanded temperature and pressure are set as restrictive conditions besides the calorific balance between supply and demand;
• Heat and pressure losses incurred in the pipeline transmission are included in the total cost by evaluating the cost of energy for boosting or compressing;
• The model calculates the optimal system cost by evaluating the pipeline cost under various specifications (e.g. velocity and pressure) for the use of the pipelines.
Type of heat demand: heating, cooling or hot-water supply.
Heat requirement (temperature and pressure):
heating – hot water (above 110°C) or vapor (above 1.5 atm),
cooling – hot water (above 180°C) or vapor (above 9.0 atm),
hot-water supply – hot water or vapor (above 60°C).
Type of the hydrogen absorbing alloy: LaNi5,
Time period for hydrogen absorbing=30 m, Cost of the metal 5,000 yen/kg.
Pressure loss of the transmission pipeline:
Liquid flow – subject to the Darcy – Weisbach’s law
Gas flow – subject to the equation of heat loss for isothermal fluids.
Heat loss of the transmission pipeline:
Calculated from the knowledge of the environmental temperature, heat conduction and transmission ratios.
Cost of facilities and utilities:
Heat exchanger – 10,000 yens/Mcal.h, Thermal efficiency=0.8
Refrigerator (steam absorption type) – 30,000 yens/Mcal.h, COP=1.2
Refrigerator (hot-water absorption type) – 27,000 yens/kW, COP=0.5
Compressor – 100,000 yen/kW, power efficiency=0.75
Reactor for methanol decomposing/synthesis – 24,660 yens/t(gas) · s (including compressor), thermal efficiency=0.9
Pipeline: Cost of construction (yen/km) is linearly estimated by the caliber and the distance of plumbing
Electricity – 19.69 yen/kWh Urban gas – 5.0 yen/Mcal
About modeling of heat-transport process using hydrogen absorbing alloys
Temporal change of hydrogen gas flow and heat energy supplied corresponding to the change of the pressure condition are investigated. To make the investigation easier, the hypothetical system of hydrogen transmission system shown in is assumed.
Fig. 3 shows that energy costs for the compressor are relatively high in utilizing the methanol reaction and that reactor makes the total cost rather prohibitive in utilizing hydrogen-absorbing alloys.
Fig. 4 indicates the field of demand (regarding quantity of heat energy transmitted, and distance between the heat supplier and demand point) suitable for each transportation system.
• By utilizing methanol reaction, the heat energy can be transmitted intensively, which decreases the cost of the pipelines, and makes the system more cost-effective.
• The transmission system using vapor is relatively suitable for large heat-demands located near the supplier.
It is useful to compare the cost of the transportation system with that of the case of not introducing the transportation system and obtaining the required heat by burning fossil fuels.
Tags: hydrogen gas
