G. VAN ROSSUM and S. R.A. KERSTEN, University of Twente, The Netherlands

Abstract: This chapter describes various technologies for biomass reforming for the production of high-value gases. These gas mixtures can be used for the production of fuels and chemicals or as a product itself (like hydrogen). Both ‘wet’ and ‘dry’ biomass conversion technologies are detailed with and without intermediate processing steps. Throughout the chapter, the conversion of biomass via fast pyrolysis and subsequent reforming is highlighted.

Key words: biomass, steam reforming, reforming in hot compressed water, pyrolysis oil, gasification.

20.1 Introduction

Reforming is a technology to upgrade biomass into tuned gas mixtures. Synthesis gas (H2/CO), H2/CO2 gas and CH4/CO2 gas are possible products.

A combination of hydrogen and carbon monoxide can be used for the manufacturing of ethers, alcohols and Fischer-Tropsch products. H2/CO2-rich gas is a feedstock for alcohol production. Hydrogen is an interesting fuel as such. There is also an increasing demand for hydrogen in the current petrochemical industry and it is envisaged that hydrogen will become of paramount importance to make biomass compatible with fossil refinery streams. Methane can be used as a substitute natural gas (SNG) for the grid or in compressed form (CNG) as motor fuel. The reform reactions of biomass (here represented by C6H10O4) can be described by the following conceptual stoichiometric equations:

C6H10O4 + 2H2O ^ 6CO + 7H2 [20.1]

C6H10O4 + 2CO2 ^ 8CO + 5H2 [20.2]

Reaction [20.1] is steam reforming and reaction [20.2] represents dry (CO2)

reforming. Like for fossil feedstock, both reactions require catalysts. The water-

gas-shift [20.3] and methanation [20.4] reactions will typically reach equilibrium over reform catalysts.

CO + H2O « CO2 + H2 [20.3]

CH4 + H2O « CO + 3H2 [20.4]

The proposed operating regime for biomass reforming is very broad and ranges from 230°C to 1000°C and 1 bar to 300 bar. Without a catalyst, the reaction of biomass and H2O/CO2 will yield a typical fuel gas at temperatures below 1000°C:

C6H10O4 + aH2O ^ bCO + cH2 + dCO2 + eCH4 + fCxHy + gH2O + tars [20.5]

Next to steam and dry reforming, auto-thermal reforming is also a well-known reaction system:

C6H10O4 + H2O + 0.5O2 ^ 6CO + 6H2 [20.6]

Gasification and reforming of fossil feedstock have been two separate developments. For biomass feedstock, gasification and reforming processes cannot be distinguished that easily. In this chapter, gasification will be used to denote the non-catalytic processes converting biomass into gas, and reforming will be used for catalytic biomass-to-gas technologies. Biomass can be raw biomass from the fields or biomass-derived products such as pyrolysis oil and aqueous by-products from biological conversion processes. For coal and heavy oil, gasification systems are now in operation and for natural gas, associated gas and naphtha reforming is used. Gasification systems for fossil fuels are thermal processes1 while fossil fuel reforming uses a catalyst (except for the Exxon and Kellog catalytic coal gasification processes, but these never reached commercial implementation2). On the other hand, biomass gasification and biomass reforming have always been interconnected technologies. Reforming activity has been introduced originally inside low-temperature (<900°C) biomass gasifiers to upgrade the product gas catalytically. There have been also attempts to create direct contact between solid biomass and catalysts (e. g. by impregnation), but this is outside the scope of this chapter.3 It has been attempted to add catalytic active materials to the gasifier and to use dedicated down-stream catalytic reactors to remove hydrocarbons (tars) and to upgrade the fuel gas to synthesis gas or hydrogen. Later, reforming systems have been proposed for liquid biomass streams and for very wet biomass feedstock. To understand the developments in biomass reforming, it is necessary to have some insight in ‘reforming of fossil fuel or feedstock’ (Section 20.2.1), ‘gasification of fossil fuel or feedstock’ (Section 20.2.2) and ‘biomass gasification’ (Section 20.2.3). For this reason we start this chapter with short accounts on these technologies.

As mentioned before, the proposed operating regime for biomass reforming is rather broad. This is mainly because two essentially different chemical processes are considered:

(1) Steam or dry reforming, using pressures up to 30 bar and temperatures of 350-1000°C. In this process the reactants and products are in the gas/vapor phase.4

(2) Aqueous phase or hot compressed water reforming (hereafter called reforming in hot compressed water).5 This process uses sub — or super-critical water as reaction medium. Temperatures in the range of 230-700°C are used. The
pressure is chosen in such a way that water is either in the liquid or supercritical state (Tc = 274°C, Pc = 220.6 bar). A typical operating pressure for temperatures above the critical temperature lies around 250 bar.

Next in this chapter, the chemical thermodynamics of biomass steam reforming (Section 20.3) are introduced. Because most reforming catalysts are designed to obtain chemical equilibrium (there are some recent developments6 that aim at designing catalysts that produce hydrogen by reforming in hot compressed water under conditions favoring methane thermodynamically), this thermodynamic analysis gives insight in the product distribution that can be obtained at different conditions. The biomass feedstock for reforming (Section 20.4.1) is briefly discussed as well as those bio-refinery concepts and processing schemes that include reforming pyrolysis oil or its fractions (Section 20.4.2).

The heart of this chapter is the description of the ongoing research and status of proposed and tested technologies for reforming of biomass (see Figure 20.1), as summarized in the following sections:

20.5.2 Reforming of bio-liquids (e. g., pyrolysis oil and its fractions)

20.5.3 Reforming of gases/vapors produced by biomass gasifiers/evaporators.

20.5.4 Reforming of very wet biomass streams in hot compressed water.

These technologies will be compared and we will end with conclusions (Section 20.6) including R&D needs to mature these technologies.