2.3.1 Introduction

Thermochemical conversion technology options include gasification and pyrolysis. Although both processes show long-term promise, gasification approaches show consid­erable promise for near-term economically competitive liquid fuels production. As stated

Ethanol target price is competitive

2.50 2.00

1.50 1.00 0.50 0.00

Figure 2.8 Ethanol and gasoline price comparison.

above, the thermochemical process to liquid transportation fuels adds technology robustness to a scenario for producing a significant portion of transportation fuels from biomass. It can convert low-carbohydrate, or “non-fermentable,” biomass materials such as forest and wood residues to fuels at lower technical challenge levels than the biochemical conversion process route. This section describes the R&D needed to achieve economical competitiveness for a stand-alone biomass gasification/mixed alcohol process.

Biomass gasification converts heterogeneous feedstock supplies into a consistent gaseous intermediate that can be converted to liquid fuels. The product gas called “synthesis gas” or “syngas” has a low-to-medium energy content (depending on the gasifying agent) and consists mainly of carbon monoxide, hydrogen, carbon dioxide, water, nitrogen, and hy­drocarbons. Minor components, also referred to as contaminants, include tars, sulfur and nitrogen oxides, alkali metals, and particulates. These contaminants threaten the success of downstream syngas to liquid fuels conversion and must either be reformed or removed. Commercially available and near-commercial syngas conversion processes were evaluated on technological, environmental, and economic bases by Spath and Dayton (19). Their re­port provides the basis for identifying promising, cost-effective fuel synthesis technologies that maximize the impact of biomass gasification.

Figure 2.9 shows a representative block process flow diagram of a thermochemical process that produces ethanol from lignocellulosic biomass. The process also includes ancillary supporting operations such as feedstock interface handling and storage, product recovery, and product storage not shown in the figure. Phillips and coworkers (39) provide a detailed description of this process, which is being capable of producing economically viable ethanol from a plant processing 2000 tonnes/day of biomass. Although syngas-to-liquid processes are capable of producing a variety of transportation fuels, ethanol was selected here to provide synergy with the biochemical conversion route. A brief overview of the process developed by Phillips and coworkers is described below.

The feedstock interface addresses the main biomass properties that affect the long-term technical and economic success of a thermochemical conversion process: moisture content, fixed carbon and volatiles content, impurity concentrations, and ash content. High moisture

S, N, Cl mitigation CO2 removal H2/CO adjustment

Figure 2.9 Process flow diagram with research barriers for economical thermochemical ethanol production.

and ash content reduce the usable fraction of delivered biomass. Therefore, maximum system efficiencies are possible with dry, low-ash biomass.

Gasification is a thermochemical process that involves the thermal decomposition of biomass at temperatures that maximize syngas yield. Tar and char produced during decom­position may also react with steam, CO2, and hydrogen in the gasifier to produce additional gas. This is followed by partial oxidation of the fuel with a gasifying agent — usually air, oxy­gen, or steam — to yield raw syngas. The raw gas composition and quality are dependent on a range of factors, including feedstock composition, type of gasification reactor, gasification agents, stoichiometry, temperature, pressure, and the presence or lack of catalysts.

Gas cleanup removes contaminants from biomass gasification product gas. It generally involves an integrated, multi-step approach, which varies depending on the intended end use of the product gas. However, gas cleanup normally entails removing or reforming tars, acid gas removal, ammonia scrubbing, capturing alkali metal, and removing particulates. Gas conditioning is the final modification to gas composition to make it suitable for a fuel synthesis process. Typical gas conditioning steps include sulfur polishing (to reduce hydrogen sulfide to acceptable levels for fuel synthesis) and water-gas shift (to adjust the final hydrogen-carbon monoxide ratio for optimized fuel synthesis).

Comprehensive cleanup and conditioning of the raw biomass gasification product gas yields a “clean” syngas composed of carbon monoxide and hydrogen in a given ratio. This gas can be converted to a mixed-alcohol product. Separation of the ethanol and higher molecular weight alcohols from this product yields a methanol-rich stream that can be recycled with unconverted syngas to improve process yield. The higher-alcohol-rich stream yields byproduct chemical alcohols. The fuel synthesis step is exothermic, so heat recovery is essential to maximizing process efficiency.