Hydrogen is being championed to play a key role in the United States’ future energy economy, raising the question of how to produce the quantities necessary for it to meet its undoubted potential.
In today’s energy landscape the biggest challenges that beset the demands of the US energy market are foreign resource dependency and addressing environmental concerns. Around 85 per cent of US energy needs are covered by fossil fuels with the remaining 15 per cent met by renewable (mainly hydroelectric) and nuclear power.
Of the three key fossil fuels, oil and natural gas have a large dependency on foreign resources. More than 55 per cent of US oil is imported, and by 2020 this number is forecast to rise to 70 per cent. With regard to natural gas, 20 per cent is imported, and this number is expected to increase significantly with the new housing market boom and with the fleet of new combined cycle power plants coming on line.
By contrast, the US meets all of its coal needs domestically (it has the world’s second largest coal reserves). Coal currently fuels more than 52 per cent of the country’s annual electricity needs. However, in spite of the progress made in combustion technology and in emission control for new boilers, permitting of new coal fired power stations is extremely difficult and unpredictable. This leaves the US with one of its most abundant natural resources underutilized.
All three fossil fuels, oil, gas and coal produce so-called greenhouse gases in the form of carbon dioxide in the course of combustion. Oil produces one third more carbon dioxide than natural gas per unit of energy, while coal produces two thirds more. Today there is strong public concern regarding the impact of man made greenhouse gases on global mean temperature (climate change). If the political momentum becomes strong enough to override further scientific work needed to best understand how natural variability (and/or an increase in anthropogenic carbon dioxide emissions) causes change in global mean temperature, then a shift in the energy production landscape is likely; with the coal industry facing the greatest compromise.
Fossil fuel fears
Fossil fuels, especially coal, also face serious problems due to the effects of environmental concerns. However, while coal carries the greater burden of the environmental issue, it does not suffer from the dependency complications of its brethren, oil and natural gas. The conundrum thus becomes: how to deal with both the environmental challenge and the foreign dependency problem without compromising either environmental solutions or the use of our own energy resources?
Part of the solution is reducing the consumption of fossil fuels and/or increasing the net efficiency of the conversion process. The hurdle to the latter has been, and remains, the need for a quick economical payback of the additional hardware costs to increase the net efficiency. This largely depends on the level of the fuel prices. An important contribution to a solution could come from renewable energies like wind and solar power. Although strongly improving, renewable energies cannot yet meet the demand to cover the supply that fossil fuels currently provide. In the US less than two per cent (8 GW) of installed electrical generating capacity is wind power. Though in Europe the number is higher (26 GW), it is still not enough to become a viable alternative to replace fossil fuels in the mid-term. As far as solar power is concerned, its first costs today are still too high to make it functional past the small niche application it inhabits.
Given that renewable energy (wind, solar power) cannot fill the gap in the mid-term to cover additional needs for energy or replace existing energy sources, hydrogen is the leading alternative with the necessary characteristics to overcome the above listed challenges that confront fossil fuels.
Hydrogen can be produced in quantities large enough to replace some of the fossil fuels. Hydrogen combustion is considered an emission free combustion (except for concerns regarding possible hydrogen leaks into the stratosphere). In addition, the use of a hydrogen fuel cell can transform chemical energy into electrical/mechanical energy with twice the efficiency that occurs during the combustion of gasoline in an internal combustion engine. The key question then becomes how to produce hydrogen in significant quantities without relying on imported fossil fuels? The answer to this question depends on the process to produce hydrogen.
There are essentially three processes to generate hydrogen on a large scale: electrolysis, reforming/shifting and thermo-chemical water splitting process. The electrolysis is inefficient because of its high electricity consumption of approximately 4.5 kWh per cubic metre of hydrogen, while the thermo-chemical water splitting process lacks industrial maturity. Therefore, the reforming/shifting process appears to be the best means to produce hydrogen in sufficient amounts.
The reforming process is an endothermic process and thus requires a heat source to provide the thermal energy at a temperature level of about 700à‚°C to start and sustain the process. Burning either coal or natural gas can provide this heat. Being that one of the objectives is to reduce our dependency on foreign fossil sources, natural gas is best ruled out.
Twenty years ago the question of how best to generate the endothermic heat necessary for large endothermic industrial processes was studied. The conclusion then and today remains the same: that High Temperature Gas Cooled Reactors (HTGR) are the most economical solution. With its maximum thermal power output of 3000 MW, and a reactor outlet temperature of 950à‚°C, it can provide the heat required for the reforming/shifting process. The remaining thermal energy can be transformed into process steam and electricity via a bottoming cycle, thus minimizing the discharge of thermal energy to the environment. The combination of low nuclear fuel costs (which are less than half the costs of bituminous coal per unit of energy) and high reactor outlet temperature make the HTGR the ideal solution for the supply of this endothermic process heat.
Although natural gas can be ruled out as a fuel to provide the endothermic process heat, the question of whether it also should be ruled out as feedstock to generate hydrogen remains open. In Table 1, a comparison of the costs to produce one tonne of hydrogen, using coal and natural gas as feedstock, suggests that coal is the most competitive feedstock, thus assuring a very promising solution to the issue of external dependency on fossil fuels.
Figure 1. Hydrogen production by steam gasification of coal using nuclear heat
Building on the existing supply infrastructure for coal, Nuclear Hydrogen and Electricity Production Units (NHEPU, see Figure 1) can replace old, inefficient coal fired power stations. Carbon dioxide generated in the course of producing hydrogen can be sequestered into depleted oil- and/or natural gas fields or it can be used to boost crude oil recovery at mature oil fields in the US. As noted in Table 1, the costs to sequester one tonne of carbon dioxide are assumed to be $20. Although it is recognized that coal as a feedstock poses technical problems, such as removal of ash and coal’s low reactive character, these are solvable technical problems. The cost estimates in Table 1 take these complications into account.
Combining the advantages of the HTGR with the simplicity of the proven reforming/shifting process (with built in carbon dioxide sequestration), and utilizing domestically sourced fuels (coal and uranium), is perhaps the best solution to address the issues surrounding dependency on foreign fuels and the growing concern related to the anthropogenic greenhouse gas emissions.
Development work has been going on for more than 20 years to come up with a nuclear reactor system, which in terms of safety, efficiency and cost is superior to today’s light water reactor designs. The new HTGR meets all those conditions. In addition, a nuclear fuel design with 100 per cent retention capability for radioactive isotopes eliminates an intermediate heat exchanger between the reactor system and the endothermic-gasification process. The reactor coolant (helium) provides the endothermic energy into the gasifier via a heat exchanger immersed into the fluidized bed of coal, using steam as fluidizing and gasifying agent. The fluidizing steam is taken from the reheat path of the water steam cycle (see Figure 1). In the gasifier, coal and steam are converted into hydrogen, carbon monoxide, methane and carbon dioxide. Using a carbon monoxide shifting process, further hydrogen is produced. Subsequently, the hydrogen is separated from carbon dioxide and water vapour by pressure swing adsorption. Two gasifiers with its helium heat exchangers together with a HTGR of 600 MW thermal output, form a hydrogen production module. Approximately, one third of the reactor thermal output at high temperature provides part of the required endothermic energy, while the remaining portion is captured in a heat recovery steam generator to generate process steam and feed to the steam turbine.
Figure 2. The US has the world’s second largest coal reserves
Using up to five of the above-mentioned modules the NHEPU attains a maximum thermal power output of 3000 MW and produces 115 t/h of hydrogen. To put this into perspective, 1 kg of hydrogen is equivalent to nearly 4l of gasoline. By comparing the efficiency of an internal combustion engine using gasoline with the efficiency of a fuel cell/electric drive concept, one can easily demonstrate that a car using hydrogen and the fuel cell/electric drive would travel twice the distance as a car using the same amount of energy in gasoline. Therefore, a NHEPU producing 115 t of hydrogen/h, assuming a capacity factor of 88 per cent will produce annually enough hydrogen to replace 52 million bbl of imported crude oil.
Beside the data listed in Table A to calculate the production costs of one tonne of hydrogen, the following additional assumptions were made. Specific costs for the hydrogen producing part of the NHEPU were assumed to be $1078 per MW thermal using coal as a feedstock and $952 per MW thermal using natural gas. In order to make the cost comparison of hydrogen production costs independent of the production costs per kWh electric, some fixed costs from the power generation plant were shifted to the hydrogen producing part of the NHEPU. This brought the ‘all in cost’ per MWh electric to $32 for both feedstock alternatives. An inherent marginal cost of less than $12 per MWh electric puts the NHEPU into a very competitive position in the merit order of dispatch.
Because of coal’s significantly lower costs per million BTU compared to natural gas and lower endothermic energy input requirements, it returns a lower cost per tonne of hydrogen, even after taking into account $20 of sequestration costs per tonne of carbon dioxide. In addition, coal has far less price volatility than natural gas. Considering hydrogen distribution costs of approximately $7/GJ and the above sequestration costs for carbon dioxide, we end up with a cost at the pump of $1.83 per kg hydrogen produced from coal. The comparative costs for gasoline (national average) to travel the same distance with a combustion engine of the same power output, but taking into account its lower efficiency, is more than $4.
To produce enough hydrogen to replace today’s crude oil imports of approx 10.5 million bbl per day; we would have to build 75 NHEPUs at an estimated total cost of $220 billion. This investment does not cover the costs for the hydrogen distribution infrastructure. The annual increase in coal supply for feedstock purposes is 227 million t. This number takes into account that 27 GW of old, inefficient coal fired power stations are being retired and replaced by the electrical output from the 75 NHEPUs.
There is an important question to address in the course of implementing this programme, namely the safety of nuclear power. The new HTGR is safe, which means it does not contain any physical process that could cause radiation-induced hazard outside the nuclear power plant boundary. A further advantage of the HTGR that could enhance its public acceptance is the tested alternative of using thorium/uranium 235 as the reactor fuel. This process guarantees that a negligible amount of plutonium is being produced in the fuel cycle. In addition, the thorium reserves are three times as abundant as those of uranium, which will help to keep nuclear fuel costs low for a long time.
Another concern is the safe intermediate storage of spent fuel. The HTGR spent fuel compared to light water reactor spent fuel is less vulnerable during the intermediate storage time since the fuel is designed to cool down with natural convection only.
Aside from the over-leveraged oil supply situation of the US, recent remarks by the chairman of the Federal Reserve Board highlighted that the natural gas supply situation could also become very problematic. Significant price increases in either one of the two commodities can have a strong dampening impact on a nation’s GDP growth rate.
The hydrogen-production concept described in this article reduces US dependency on imported oil practically to zero by utilizing national resources. It also avoids using natural gas to replace the electrical energy production from those retired coal plants. It addresses environmental concerns by reducing dramatically emissions per unit of energy consumed. The technology to implement the concept is available right now, including the technology for the thorium/uranium 235 fuel cycle. Carbon dioxide generated in the course of hydrogen production can be safely sequestered into depleted oil and/or natural gas fields or it can be used to boost domestic crude oil recovery from those many mature oil fields in the US. The low price volatility of nuclear fuel (in particular thorium) and coal translates directly into low hydrogen price volatility.
Although some challenging technical problems still have to be resolved to bring about the transition to hydrogen, looking at the very advanced status of the enabling technologies and the ample scientific resources available in the US, those problems are solvable within a short time frame.