Chapter 9
Indirect Liquefaction Carbon Efficiency Downloaded by PENNSYLVANIA STATE UNIV on June 6, 2012 | http://pubs.acs.org Publication Date (Web): November 29, 2011 | doi: 10.1021/bk-2011-1084.ch009
Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2V4 *Tel: +1 780-248-1903. E-mail:
[email protected].
The carbon efficiency of indirect liquefaction is studied to determine factors affecting carbon loss. Carbon loss takes place during all conversion steps: feed pretreatment, feed-to-syngas conversion, syngas conditioning, syngas-to-syncrude conversion and refining. Carbon loss is associated with both energy and chemistry demands. Indirect liquefaction has a carbon efficiency around 34 % (coal) depending on the feed.
Introduction Biomass, coal, natural gas and waste can be employed as raw materials for the production of useful carbon-based commodities. The first step is to convert the aforementioned raw materials into a synthetic crude oil. The synthetic crude oil, like conventional crude oil, can then be refined to produce the carbon-based transportation fuels, lubricants and petrochemicals. The conversion of alternative carbon sources into a synthetic crude oil requires some work. Whenever work is performed, according to the Second Law of thermodynamics some energy must be irreversibly expended. Since the carbon sources are also energy carriers, some of the carbon can be sacrificed in order to perform the work that is required to transform the raw material into a synthetic crude oil. The energy is extracted through the combustion of the carbon and the carbon is sacrificed as carbon dioxide (CO2). Considering the current political sensitivity surrounding CO2 emissions, this is an unpalatable but unavoidable consequence of transforming a carbon based raw material into a different form. For example, the transformation of a carbon-based energy carrier into electricity has an efficiency of 45 % (1). That is, only 45 % of the carbon-based energy is converted into electric energy; 55 % of the carbon is rejected in order to perform the work. © 2011 American Chemical Society In Synthetic Liquids Production and Refining; de Klerk, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Depending on the ultimate application, an efficiency calculation based on the energy content of the product may be a good metric. However, in the case of lubricants and petrochemicals, the product is not an energy carrier, but a carbonbased product. In these cases the conservation of carbon during the transformation is a better metric. Carbon efficiency can also be employed for energy applications, but the chemical nature of the carbon determines its energy content and there is not a direct correlation between the carbon efficiency and the thermal efficiency. Furthermore, transportation fuels must meet specific requirements and it is not just the energy content that is important. The usefulness of the carbon-based energy carrier as a transportation fuel is measured against these requirements that include performance characteristics for the specific engine type. Carbon efficiency is a useful metric to quantify this transformation, because it speaks to the ultimate objective of indirect liquefaction, namely to produce a useful liquid fuel with specific performance characteristics.
Table I. Yield of main transportation fuels that could be obtained from conventional crude oil refining in United States oil refineries over time Transportation fuel
Yield of product (%)
a
1964
1974
1984
1994
2003
44.1
45.9
46.7
45.7
46.9
5.6
6.8
9.1
10.1
9.5
Diesel fuel
22.8
21.8
21.5
22.3
23.7
Total for transportation fuels
72.5
74.5
77.3
78.1
80.1
Overall yield b
90.1
92.3
92.9
95.4
96.8
Motor-gasoline Jet fuel
Units of yield are ambiguous in source, but it is likely based on volume, not mass. b Includes other potentially useful products, such as liquefied petroleum gas, fuel oil, coke and asphalt. a
Conventional crude oil is an efficient carbon-based raw material for the production of transportation fuels, as can been seen from Table I (2). Over time improvements in refining enabled increasingly higher yields of motor-gasoline, jet fuel and diesel fuel. These yields roughly correspond to carbon efficiency. When the decision is made to employ an alternative carbon source to conventional crude oil for the production of transportation fuels or chemicals, it is anticipated that the carbon efficiency will be lower. Of specific interest in this study is the carbon efficiency during indirect liquefaction and what the prognosis is for improving it over time. Thereby this work also addresses the CO2 footprint of indirect liquefaction technology.
216 In Synthetic Liquids Production and Refining; de Klerk, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Indirect Liquefaction
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Indirect liquefaction is a collective term for the group of technologies that convert carbon based raw materials into synthetic crude oil through synthesis gas as intermediate product. Synthesis gas is a mixture of hydrogen (H2) and carbon monoxide (CO). It is a convenient intermediate that allows the removal of heteroatoms and metals from the feed, with a significant benefit for subsequent refining of the synthetic crude oil. It also decouples the production and composition of the synthetic crude oil from the nature of the raw material that is employed as feed.
Figure 1. Generic indirect liquefaction process flow diagram indicating the most common recycle loops and product streams. A generic indirect liquefaction process is shown in Figure 1. It indicates the pathways from gaseous, liquid and solid raw materials. Distinct steps can be identified, each of which requires work to be performed and potentially carbon to be sacrificed. It highlights the complexity associated with indirect liquefaction and the effort that is required to transform a carbon-based raw material into a synthetic crude oil. It should be noted that not all steps are required and that the design can be simplified. The extent of simplification depends on the raw material employed as feed and the technologies selected.
Feed Pretreatment Natural gas may contain associated natural gas liquids, which are heavier hydrocarbons that be recovered by condensation. These associated liquid products allow some carbon to bypass the indirect liquefaction process and this stream increases the overall carbon efficiency. There is of course work performed during the recovery of the associated liquid products, which requires energy to be expended. The recovered associated liquid products can be refined with the synthetic crude oil, as is industrially practiced at the PetroSA gas-to-liquids facility in Mossel Bay, South Africa (3). The feed pretreatment required for solid feed materials depends on the nature of the raw material and the requirements of the gasifier. Frequently it involves one 217 In Synthetic Liquids Production and Refining; de Klerk, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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or more of the following: mechanical size reduction, classifying (based on particle size), drying and slurrying. All of these feed pretreatment operations require work. For clarification, it is necessary to point out that some carbon may be “lost” during pretreating because it does not conform to the physical requirements of the gasification technology. This carbon is not rejected as CO2. The energy value of this carbon is still available and the chemical nature of carbon-carrier has not changed. For example, of the run of mine (ROM) coal prepared for moving bed gasification, approximately 25 % is rejected as fines (4). The fine coal can be employed in a different type of gasifier or it can be employed as an energy source. Carbon rejected based on particle size before entering the indirect liquefaction process is not considered a loss in carbon efficiency related to indirect liquefaction. Air Separation The use of air or pure oxygen (O2) as oxidant in reforming or gasification is a design decision. Air separation requires work. Dry air contains roughly 78 % nitrogen (N2), 21 % oxygen (O2) and 1 % argon (Ar) in addition to various minor compounds. The minimum ‘ideal’ work (Wideal) that is required to separate these three major compounds present in air, can be calculated as a function of the ideal gas constant (R = 8.314 J·mol-1·K-1), temperature (T) and mol fractions of the respective compounds (xj):
The thermodynamic efficiency of an air separation unit is around 33 % (5), and at standard conditions the energy requirement is around 0.6-0.7 MJ·kg-1 O2. There is a complex trade-off between air separation and the direct use of air for synthesis gas production. An air separation unit adds to the capital and operating cost, but inefficiencies caused by inert material are limited. When air is employed as oxidant the N2 and Ar must also be heated to reforming or gasification conditions. Even with proper waste heat recovery, the inert gases place a significant burden on the energy requirements of all downstream processes. One exception is steam reforming, where energy is indirectly provided and the oxidant is not mixed with the carbon-based feed. Large-scale industrial applications of indirect liquefaction that involves partial combustion by direct contact of the oxidant with the feed employ O2 as oxidant. Incurring the penalty up front in the process is preferable and the pure N2 that is produced as by-product from air separation also has value. Gas Reforming The term ‘gas reforming’ will be restricted to refer only to catalytic gas reforming. Thermal (non-catalytic) gas reforming technologies will be referred to as gasification. The chemistries in gas reforming and gasification are similar, but the use of a catalyst lowers the operating severity that is required. 218 In Synthetic Liquids Production and Refining; de Klerk, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Gas reforming technologies can be divided into two main categories: steam reforming and adiabatic oxidative reforming. The main difference between these to categories is the way in which heat is supplied. In steam reforming the process feed that is converted into synthesis gas is kept separate from the heat source. The fuel and oxidant that supplies the energy to drive steam reforming can therefore be selected independently from the carbon source for indirect liquefaction. A further advantage is that the oxygen to carbon ratio in the feed can be manipulated independently of the energy requirements and it is only subject to catalytic requirements, thereby enabling some control over the synthesis gas composition:
In adiabatic oxidative reforming the oxidant is mixed with the process feed and energy is supplied directly to the process by partial combustion of the feed. This simplifies the process and results in a more compact and efficient gasifier design, but the fuel and oxidant are now part of the process feed and cannot be selected independently. Control over the synthesis gas composition is limited and for the most part determined by the energy requirements and the catalytic requirements of the process. In addition to the endothermic steam reforming (Eq. 2) and dry reforming (Eq. 3) reactions, energy is supplied by partial or complete combustion of the natural gas feed:
It is theoretically possible to convert all of the carbon in natural gas into synthesis gas. By combining Eq. 2 and 4 it can be shown that all of the carbon in the feed ends up in synthesis gas:
In practice some of the carbon is lost due to CO2 production and unconverted CH4 effectively bypassing the gas reformer (6). It is possible to recover this lost carbon by appropriate gas loop design. In practise this recovery and recycling costs work. This outcome may seem counterintuitive. How is it possible to retain all of the carbon if work is being performed to enable these high temperature transformations? Part of the answer lies in the loss of thermal efficiency due to partial carbon oxidation. The energy value of the carbon containing reforming product, carbon monoxide (CO), is considerably lower than that of the methane (CH4) feed. Even though the carbon was retained, energy was rejected as lost heat. In Eq.6 the entropy change is is positive (ΔS298K = +296 J·K-1·mol-1), indicating that some energy had to be expended to satisfy the Second Law of thermodynamics. 219 In Synthetic Liquids Production and Refining; de Klerk, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Although the carbon efficiency from natural gas reforming can theoretically be 100 %, the thermal efficiency is less. The second part of the answer is related to the H:C balance of the feed. Methane is sufficiently hydrogen-rich so that it is not necessary to sacrifice CO to produce H2 from water.
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Gasification The term ‘gasification’ is employed for the thermal conversion of natural gas, as well as other solid and liquid carbon sources, such as coal, biomass, waste and petroleum waste products (e.g. petroleum coke and ashphaltenes). Gasification takes places at very high temperature, usually in the range 800-1800 °C. All carbon containing compounds in the feed material is broken down to C1 fragments at gasification temperatures. There are many different types of gasifiers (Table II). Many of the gasifier technologies were originally developed for coal gasification and the literature on coal gasification is consequently more extensive than that for other feed types. The selection and operation of gasifier technology is very dependent on the feed material (7). The gasifier type and raw material employed as feed, both affect the composition of the syngas, but ultimately most of the carbon is converted to C1 fragments, mainly CO, CO2 and CH4. There are two important exceptions: (a) Some gasifiers co-produce pyrolysis products (>C1 fragments), and (b) not all carbon is necessarily converted and some solid carbon may remain as an unconverted solid carbonaceous product.
Table II. Main gasifier types and their associated carbon conversion characteristics Description Carbon conversion Pyrolysis products in gas H2:CO in raw syngas
Moving bed
Fluidized bed
Entrained flow
>98 %
99 %
Yes
Possibly
No
>2:1 to
