ABSTRACT: Rapid pyrolysis of biomass generates a liquid with properties that are particularly attractive for production of hydrocarbons that could be substituted for liquid fuels derived from petroleum. However, the high oxygen content of the biomass derived liquids presents a number of problems because of the high water content and the considerable concentration of carboxylic acids. Measurements of total acid number (TAN) of pyrolysis oil (bio-oil) samples show that values in the 90-100 range are fairly common. This level of acidity has been shown to cause corrosion problems that have to be addressed in the selection of structural materials that are used in the production, subsequent processing, storage and transport of the pyrolysis oils. Chemical analyses have been performed and laboratory corrosion studies have been conducted in order to assess the aggressiveness of the raw pyrolysis oil from several sources as well as the corrosion caused by a bio-oil that has been treated to reduce the acid and oxygen content. Components of biomass pyrolyzers have also been fabricated from various candidate alloys, and these components have been exposed for extended periods during operation of the pyrolyzers. This paper will report on results of these analyses and corrosion studies. INTRODUCTION Thermochemical processing of biomass can produce solid, liquid and/or gaseous products depending on the temperature and exposure time used for processing. The liquid product, known as pyrolysis oil or bio-oil, offers the potential of a replacement for imported petroleum, but it also introduces other problems. As produced pyrolysis oil contains a significant amount of oxygen, primarily as components of water, carboxylic acids, phenols, ketones and aldehydes. As a result of these constituents, these oils are generally quite acidic with a Total Acid Number (TAN) that can reach levels as high as 100.
INTRODUCTION ABSTRACT Ethylene is one of the principal building blocks in the petrochemical industry, and world-wide production and consumption have been steadily increasing. Production of ethylene is accomplished primarily by the pyrolytic stripping of hydrogen from ethane or a higher molecular weight hydrocarbon. This cracking process, sometimes referred to as steam cracking, is currently accomplished in metallic tubes using high temperature furnaces and has a conversion efficiency for ethane of 60-65%. Operation at significantly higher temperatures could increase the efficiency as much as 20%, but materials with better high temperature strength would be required. To help identify suitable materials, tests have been conducted to determine the behavior of selected ceramic materials in environments similar to those anticipated for a high-efficiency, advanced steam cracking system. The effects of exposure on weight change, mechanical strength, and microstructure have been determined in a series of 100 hour tests. In addition, 500 hour tests have been conducted to determine the effect of time on material behavior. From these tests, several strong candidates have been identified. The Department of Energy-Office of Industrial Technologies (OIT), in cooperation with Stone & Webster Engineering Corporation, is developing a high pressure heat exchanger system (HiPHES) for ethylene production. Conventional production of ethylene is by a process known as pyrolysis or steam cracking. During normal cracking operation, hydrocarbon feedstock is initially mixed with steam and heated to temperatures of approximately 815-900 ºC (1500- 1650ºF) and pressures of 1.7-2.4 atm (25-35 psia) while flowing through metal alloy tubes within a direct-fired furnace, During this cracking operation, an undesirable product, coke, is also produced and it accumulates on the walls of the metal tubes. Decoking of the tubes is required and is accomplished through a process in which steam and air are passed through the coil at about 870 ºC (1600 ºF) to burn the deposited coke. Steam cracking technology has evolved to permit utilization of reactors at increasingly higher temperatures and shorter residence times so as to enhance the overall yield of ethylene and other light olefins. The present technology is constrained by operating temperature limits of common commercial alloys. The tube material temperatures typically reach 1149 °C (2100 ºF) at the end of normal operation with a coked tube. Use of advanced ceramics, such as silicon carbide or a multiphase ceramic containing acme silicon carbide, would allow higher operating temperatures. Commercial success of this project depends directly upon development of a heat exchange system that utilizes monolithic ceramics ardor ceramic composite materials, The concepts of applying advanced materials to steam cracking are being studied in this project. Stone & Webster is evaluating the benefits of elevated operating temperatures on ethylene yield at their Bench Scale Unit in Houston, Texas. The effect of exposure to simulated steam cracking environments on corrosion behavior, mechanical strength, and microstructure of candidate ceramic materials is being determined in stiles being conducted at Oak Ridge National Laboratory, Oak Ridge, Tennessee. This paper will summarize the work done at Oak Ridge.
INTRODUCTION ABSTRACT Conventional steam reforming of methane to synthesis gas (CO and H2) has a conversion efficiency of about 85%. Replacement of metal tubes in the reformer with ceramic tubes offers the potential for operation at temperatures high enough to increase the efficiency to 98 to 99%. However, the two candidate ceramic materials being given strongest consideration, sintered alpha silicon carbide and silicon carbide particulate-strengthened alumina, have been shown to react with components of the reformer environment. The extent of degradation as a function of steam partial pressure and exposure time has been studied, and the results suggest limits under which these structural ceramics can be used in advanced steam-methane reformers. Studies sponsored by the U.S. Department of Energy (DOE), Office of Industrial Technologies, indicate that some high-temperature processes in chemical, petrochemical, and other industries could achieve significantly greater efficiencies if improved materials permitted operation at higher temperatures, higher pressures, and/or in more corrosive environments. Consequently, the high-pressure heat exchange system (HiPHES) program was established by DOE to demonstrate the advantages of higher temperatures and/or pressures in industrial-sized systems. Two of these projects involve the assessment of materials for heat exchangers in hazardous, industrial waste incinerators. In another HiPHES project, Stone & Webster Engineering Corporation (SWEC) of Boston, Massachusetts, is developing a high-temperature, high-pressure steam-methane reformer. SWEC proposes to use ceramic tubes in the reformer for containment of the reactant and product gases because metallic tubes would not have sufficient strength to contain the pressure differential across the wall at the proposed operating temperature. Metal tubes would, however, be used within the ceramic tubes to provide a parallel counter flow path for the product gas as shown schematically in Figure 1. The proposed commercial-sized reformer would contain more than 600 ceramic tubes that would have an outside diameter of about 8.9 cm (3.5 in.) and a length, based on design constraints, of about 9 m (30 ft). The length of the tube could likely be reduced if a ceramic-to-metal joining technique were identified that would produce a joint capable of operation at higher temperatures and pressures. Although evaluation of joining techniques is an important aspect of this study, it is not included in this paper. Rather, the paper focuses on the effects of steam partial pressure and time on the corrosion of the two prima~ ceramic materials, sintered a-SiC and SiC-strengthened alumina.