DIY 3D Solar Panels

Do It Yourself Solar Energy

Get Instant Access

Note that from Eqs. (5.3) and (5.4) it appears that, on an ideal basis, producing locally hydrogen from ammonia is more efficient than producing and on board using pure hydrogen.

Up to this moment the irreversibilities of the chemical reactions and the fuel production and distribution costs were not considered in the analysis. One can of course expect that due to the irreversibilities of the production process, the cost of ammonia is higher than that of the hydrogen from which is synthesized. However, if one considers the distribution and storage costs the picture may reverse. Therefore, to complete this analysis, the market prices of ammonia and hydrogen at production site and the costs of their distribution and storage were investigated to derive the total well-to-wheel cost of the two considered scenarios.

Hydrogen, as state above, is the most difficult to store in a compact form. Therefore the distribution and storage-related costs will impact mostly on the total costs. Depending on the production method the hydrogen cost varies from ~$1/kg at coal gasification to ~$/9.50 kg using solar energy for electricity generation that in turn is used for water electrolysis (see NRC-NAE, 2004).

After production, hydrogen is stored at the manufacturer location for certain period prior delivery. The hydrogen storage is costly, because the hydrogen molecule is small and leakage cannot be avoided. The best option to store hydrogen at the production facility location (and at the distribution pump) is in metal hydrides. As reviewed in Schlapbach et al. (2001), metal hydrides tanks may operate at pressures of 15-20 bar and store up to 25-30 kg H2/m3. In order to completely release the hydrogen from the tank, some amount of heat is needed.

It must be noted that metal hydrides do not appear as a feasible solution for transportation of hydrogen. For 1 kg of hydrogen, the metal hydride tank will have about 160 kg. For transportation, hydrogen must be either compressed to extremely high pressures (~300-800 bar) or cooled for liquefaction at cryogenic temperatures. According to NRC-NAE (2004), liquefaction adds at least 30% to the hydrogen price per kg, and in top of this one must add the energy consumed to keep the storage tank at cryogenic temperatures during the storage time. If the transportation takes 1-3 days, the minimum cost penalty for hydrogen storage on transport vehicle (cistern) is CN $ 0.3/kg for compressed H2 and CN$ 0.7/kg for liquefied H2, where CN$ stands for Canadian dollars.

If one assumes, for example, that the hydrogen transport is made in pressurized containers at 345 bar the transported energy content is 8 GJ/m3, i.e., four times smaller than for gasoline case (32 GJ/m3). In NRC-NAE (2004) it is shown that if a pipeline is to be developed to distribute hydrogen at such high pressure the tubes' thickness must be more than 50% thicker with respect to natural gas pipes.

At distribution points (fuel stations) the hydrogen may be stored also in metal hydrides. Additional costs are associated with leakages during hydrogen delivery to consumers. The high explosion risk of hydrogen will raise the price even more because of the safety measures. It is shown in NRC-NAE (2004) that due to these factors, the estimated minimum cost of hydrogen distribution is more than $1/kg H2. Further, one realizes that storage of hydrogen on vehicles (that is made either in compressed gas or in cryogenic liquid) implies additional costs due to leakages or continuous running of the cryogenic plant to maintain the hydrogen in liquid phase.

Thus, if one considers the production, storage, and distribution costs the minimum expected hydrogen price at delivery point should be more than CN $2.5/kg, if produced from coal, and ~CN $11/kg from electrolysis driven by solar energy. The goal of DOE (2008) for 2015 is to achieve $2-3/kg H2 delivered, un-taxed and regardless of the production method. As a matter of fact, the North American selling price of hydrogen in 2002 varied, according to IG (2008), from 7.4 to $11.3 /kg.

Ammonia is facile to store and it has a distribution network on roads, rail, ships, and pipeline already in place. The production of ammonia from fossil fuels has a common route with hydrogen production, because it involves gasification to produce syngas, gas cleaning, and CO2 removal. In addition, the following steps are necessary for ammonia synthesis: compression of the reactants, catalytic conversion, and ammonia separation through condensation.

A highly energy consuming component of the ammonia production process is represented by the makeup gas compression which is needed to facilitate the synthesis. This apparent drawback is compensated by a very efficient synthesis process that is possible at high pressure. Moreover, ammonia synthesis is an exothermic process and modern technologies use work and heat recovery to reduce the production costs.

The minimum cost for ammonia production per unit of energy is obtained for the case of natural gas feedstock: the technical limit is 28 GJ natural gas for production of 22.5 GJ in the form of ammonia. The maximum energetic cost is obtained with coal as feedstock: ~65 GJ coal per 22.5 GJ of ammonia product. For other methods of production except solar the cost falls in between the two extremes. The actual cost of North American coal is in average ~CN $1.5/GJ and that of natural gas is ~CN $10/GJ. These figures give an estimate of ammonia price range at the production place, which is CN $5.25-CN $20.0/GJ or about CN $0.10-0.38 per NH3 kg.

The North African price is currently the lowest $0.15/kg; other costs are $0.2/kg in Trinidad Tobago (based on $9/GJ natural gas feedstock), $0.25/kg in Ukraine, $0.3/kg in the USA; ammonia cost at Terra Industries in 2007 was $0.35/kg. Ammonia price in the USA in 2007 varied between $0.2 and $0.4 per kg.

Feedstock CN $/GJ

Fig. 5.4 Cost correlation for hydrogen obtained from ammonia at distribution points.

Feedstock CN $/GJ

Fig. 5.4 Cost correlation for hydrogen obtained from ammonia at distribution points.

We used here the data from Waitzman (1979) to correlate the ammonia production price with the feedstock price in $/GJ and eventually obtained the plot from Fig. 5.4. The cost of ammonia has been upgraded with the transportation costs. Since infrastructure development is not needed for ammonia distribution (e.g., a large pipeline network exists in the USA to transport ammonia at a cost of $0.1 hydrogen equivalent per 1,000 km) the ammonia transportation costs were assimilated to the ones of gasoline. We used the data from NRC-NAE (2004) where it was shown that gasoline distribution costs represent 10-15% of the hydrogen distribution costs per kg of hydrogen equivalent. Finally, the cost of ammonia including transportation has been multiplied with 17/3=5.67 to obtain the cost per kg of hydrogen stored in ammonia as shown in Fig. 5.4.

Figure 5.4 shows that if ammonia is produced from coal (currently at about ~$1/GJ) hydrogen from ammonia is cost competitive with hydrogen transported in pure state. Furthermore, if ammonia is produced from natural gas, the hydrogen through ammonia alternative remains economically viable; it is better than hydrogen from coal solution up to natural gas prices of $8/GJ. It should be kept in mind that as the feedstock cost increases, the hydrogen production costs does increase, too.

Since ammonia is produced from hydrogen, it is interesting to estimate and compare the amount of CO2 emission at NH3 and respectively the H2 production. We assume here the natural gas as feedstock. Modern ammonia synthesis systems that use extensive heat recovery need ~30 GJ equivalent natural gas to produce 1 tonne of NH3 (see Spath and Mann, 2001). Through stoichiometry one may deduce that ~1.32 kg of CO2 is generated in order to produce 1 kg of NH3; this is equivalent to ~8 kg CO2 generated for 1 kg H2 in the form of NH3 which is similar to the amount of CO2 released during H2 production from natural gas. This figure puts in evidence the technical, economical, and ecological values of NH3 as a hydrogen source.

Moreover, ammonia can be synthesized at any location of the oil or natural gas extraction wells and the resulting CO2 is re-injected back into the ground for sequestration. Ammonia can then be easily transported via pipelines, auto-cisterns, railway cars, and ships and delivered to consumption points where it can be used as a hydrogen source, chemical, fertilizer, fuel, working fluid, refrigerant, etc.

Was this article helpful?

0 0
Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment