Hydrogen: Economics and its Role in Biorefining

FERDI SCHÜTH

1.1 Introduction

Hydrogen is perhaps one of the most promising energy carriers of the future. In renewable energy systems with high fractions of intermittent supply (e.g. wind power and solar thermal energy), potential surplus electricity could be converted into hydrogen through water electrolysis. This hydrogen can be used in a wide variety of applications. The most often discussed option, the reconversion of hydrogen into electricity, be it by gas turbines or by fuel cells, appears to be rather unattractive, due to the low round-trip efficiencies. Electrolysis – based on the process scale – can be estimated to have an efficiency of about 60% (if higher efficiencies are given, they are typically relative to the cell level). A recent NREL analysis, based on questionnaires given to manufacturers, indicate a mean efficiency value of 53% for the system. Considering that the fuel-cell efficiency on the systems' level and gas turbines (not available for hydrogen yet) is estimated at about 50–60%, the overall round-trip efficiency is thus reduced to slightly above 30%. It will certainly be possible to improve this figure to some extent, but substantial losses in the round trip from electricity to electricity will invariably always be present. Therefore, the use of "renewable" hydrogen – not for the reconversion into electricity, but rather as a feedstock for the chemical industry, in oil refineries, or in biorefineries – appears to be promising. For biomass upgrading, a substantial need for hydrogen undoubtedly exists due to the high oxygen content present in biogenic molecules.

Figure 1.1 plots the chemical composition of different energy carriers in an O/C vs. H/C diagram. The typical biomass constituents contain much more oxygen than potential target molecules. In addition, the hydrogen content often needs to be increased. Decarboxylation and decarbonylation pathways are one of the possibilities for the reduction of the O/C ratio, but they alone are insufficient for this purpose. Accordingly, for further oxygen removal, hydrogen is often required as a reducing agent in order to convert biogenic molecules into less -oxygenated target compounds. In order to increase the H/C ratio, hydrogen is needed directly either in the hydrogenation and hydrogenolysis pathways, or indirectly after dehydration, since the dehydration leads to unsaturated compounds that are often undesired intermediates, as they are very reactive, and thus may undergo side reactions, decreasing overall product yields. The various process options that hydrogen is used for in order to convert biomass into chemical intermediates and end products will be briefly discussed at the end of this chapter.

1.2 Conventional Routes for Hydrogen Production and Corresponding Costs

The vast majority of hydrogen is currently produced from fossil fuels (estimated at 49% from natural gas, 29% from liquid hydrocarbons, either directly from naphtha or related feedstocks, or indirectly by converting residues in refineries or as off-gases from chemical or refinery processes, 18% from coal, and 4% from electrolysis). Most of today's production is intended for further processing in the chemical and refinery industries, and is thus not traded on the market. It is estimated that only ca. 10% of the produced hydrogen is traded (i.e. merchant hydrogen); the rest is produced and directly used onsite (i.e. captive hydrogen). Due to this fact, there are various figures available, as the production levels are difficult to assess. From the estimates published for different years and projected growth, current global production is about 60 million metric tons per year, with wide margins of error.

For the purposes of this chapter, we will consider processes rendering hydrogen as the main product (i.e. hydrogen made on purpose). Thus, typical refinery processes (e.g. coking and visbreaking) are not further discussed. Also disregarded are petrochemical processes, such as steam cracking for lower olefins production, since here the olefin is the main product, although the hydrogen produced contributes to the overall profitability of the process. Some processes can be considered as borderline cases, such as cracking, in which the amount of produced hydrogen can be adjusted by the processes conditions, and can thus be tuned to the hydrogen requirements of the refineries.

1.2.1 Steam Reforming/Autothermal Reforming/Partial Oxidation of Fossil Feedstocks

There are three main processes for the production of hydrogen from carbon-containing feedstocks: catalytic steam reforming (SR), autothermal reforming (AR) and partial oxidation (PO), as well as other configurations, which contain various aspects of any of the aforementioned processes. The selection of the reforming technology depends on many factors, such as the intended use of the hydrogen, acceptable impurity level, pressure level of downstream processes, price and availability of hydrocarbon and fuel, investment and operational costs, catalyst price, and several others secondary factors. Overall, the reforming technologies are intimately connected to the chemical reaction networks that govern hydrogen formation, making them perhaps best to be discussed altogether.

The basic reaction of steam reforming is given as eqn (1.1) for the example of methane,

CH4 + H2O [??] 3 H2...