| 摘要 | 1. Background and objective of this study
Recently, hydrogen energy is emerging as a new energy system substituting carbon-based energy system even if the substitution will not be realized in the near future. Hydrogen energy is efficient as well as clean when it is utilized in the final stage. However, major energy sources for producing hydrogen are natural gas and coal at present, and in this sense, hydrogen energy is not renewable, but depletable. When the production process is considered, it is difficult to regard hydrogen energy as clean and renewable energy.
Now the question arises as following: Between coal and natural gas as energy sources of hydrogen production, which one is better in the sense of environmental as well as economic aspects? Since huge investment on a new technology is irreversible, we need to take no-regret strategy. In order to do that, it is necessary to compare and evaluate economic validity as well as environmental impacts of hydrogen production from coal and natural gas. If one energy source is found as more economically efficient as well as more environmental-friendly than the other one, investment should be made to the former energy source rather than the latter one.
This research focuses on the estimation of production costs of hydrogen using coal and natural gas. More specifically, production costs of steam methane reforming and coal gasification are predicted until 2030. Secondly, energy efficiency and environmental impact of two production methods are compared for the two approaches using a life cycling assessment (LCA) analysis tool. Based on the cost estimation and LCA results, best choice is recommended.
2. Major Findings
Major contents of this research include reviews on the technologies for producing hydrogen, overview of steam methane reforming(SMR) and coal gasification (CG), literature review of hydrogen production cost for the SMR and CG methods, simulation of hydrogen production cost, and LCA of the SMR and the CG approach.
Prediction of SMR H2 production costs
We consider central system as well as on site system. Central system consists of large and middle size, with and without carbon capturing and sequestration (CCS). Prices of natural gas and electricity are predicted until 2030 using time series econometric methods. Technological progress on each facility in the SMR H2 system is assumed. Accordingly, the production cost declines as the technological progress saves unit input costs as well as the magnitude of unit input. In addition to the technological progress, we include carbon tax scenario in the simulation.
In the reference case without technological progress and carbon tax, natural gas input cost is the most significant factor in the estimation of production costs. In the middle and large scale, the proportion of natural gas price in the production cost is 64~83%, while the proportion is 39% in the on-site case. Central system with large scale shows the lowest production cost. The impact of CCS on the production cost is 50~60cents per kg of hydrogen. The production cost of on-site system is the highest among the different systems, $3.3~4.0 per kg of hydrogen higher compared to central system. However if we include infrastructure cost of central system, the whole cost will be considerably bigger.
In the case of technological progress, capital costs of each facility decline after 2015 when the production of hydrogen is on the track. For the central system, the diminishing effect from technological progress is overwhelmed by the increasing effect of natural gas price. By the way, for the on-site system, the effect of technological progress dominates the increasing effect of natural gas price.
Next, when carbon taxes are imposed on carbon emitted from the hydrogen production, the impact of carbon tax rate on the production cost was ignorable for the CCS case. In the case of technological progress, the effect of carbon taxes on the cost was reduced significantly.
Prediction of CG H2 production costs
In the reference case without CCS, coal price at the starting date was assumed as $0.24/H2kg, 17% of total production cost ($1.43). The production cost with the CCS increases into $1.62/H2kg. Compared to the SMR method, CG method was more stable with regard to the H2 production cost. More specifically the production cost of large central system without CCS increases by $0.6 between 2008 and 2030, while the production cost with the CCS increases by $0.7 during the same period.
With the technological progress, input ratio of coal declines by 8%, input ratio of electricity decreases by 68% without CCS, while declines by 53% with the CCS. The emission level of CO2 diminishes by 11% without CCS, 27% with the CCS according to the technological progress.
The production cost of hydrogen decreases considerably after 2015 as the technological progress is included. Also, the production cost with the CCS is higher than that without CCS in the initial stage, but after 2015, the gap of production cost diminishes.
The impact of carbon tax scenario was not significant for with the CCS as well as without CCS. We observed that the two production costs converge into the same price at 2030.
Most of previous studies on the estimation of hydrogen production costs did not consider changes in input energy prices such as natural gas, coal, and electricity. Nor the technological progress and carbon tax scenarios were included in most cases. This study is distinguished from other studies in these contexts.
LCA on the SMR-H2 and CG-H2
In the case of SMR-H2, CO2 was emitted at the level between 4.1 and 15.2kg per kg of H2 depending on different systems, while the level was 3.5~12.9kg per kg of H2 for the technological progress case. In the case of CG-H2, the emission level of CO2 was 6.4~21.5kg per kg of H2, while the level was reduced into 4.5~18.5kg for the technological progress.
The above outcomes show that hydrogen energy is not clean and carbon neutral when we consider the production process of hydrogen. In the context of economic costs, coal gasification is better than the steam methane reforming, but in the environmental aspect, the latter is better than the former. Thus, we need to consider both of economics and environmental aspects of hydrogen energy when we have to determine investment on a new technology.
3. Conclusion and Policy Implication
This research shows that the production cost of hydrogen based on natural gas and coal depends on various kinds of factors such as projection of natural gas price as well as coal price, forecast of electricity, progress of technological change, environmental policy such as carbon tax on the emission of green house gases, and etc. Overall, economically, hydrogen production using coal gasification is more stable and cost-effective than the SMR approach. However, when we consider environmental aspects, hydrogen production using natural gas is more environment-friendly than the CG approach. Therefore, policy makers should consider economics as well as environmental aspects in determining the energy mix strategy of hydrogen energy in the future. |
