The Finnish bioeconomy strategy jointly put forward by the Ministry of Employment and the Economy, the Ministry of the Environment and the Ministry of Agriculture and Forestry stated how they wish to increase the turnover in bioeconomy from €60bn to €100bn and in the increase 100 000 new jobs in the area in Finland by 2025Ⅰ. At the same time the demand for chemicals from renewable sources has rapidly increased and there is an urgent need for alternative, preferably “green” production routes. One of the most sought after chemicals are pure aromatic hydrocarbons, such as benzene, toluene and xylene (BTX)Ⅱ.
The chemical industry expands into every aspect of our daily lives, giving us everything from plastics and fuels to medicines and paints. Traditionally crude oil has been utilized because of the diversity of chemicals that can be obtained from it, and because it is one of the few sources of aromatic available. Obtaining chemical products from biomass is an expanding area, especially the production of the traditional chemical raw materials used in everyday life, that is to make “drop-in” replacement chemicals from an alternative raw material; this raw material may include, for example, sugars, vegetable oils and woody biomass. The purity and consistent production of biobased chemical products is one major aspect which needs to be overcome before they are readily taken-up by major industry players. As a result the market for biofuels has been initially pursued as chemical mixtures can be used without complex purification. This unfortunately is not the case for the fine chemicals industry in which the typical purity requirement of a chemical product is over 98 % (figure 1).This increased value is directly reflected in the increased value of BTX and it’s pure fractions compared to fuel mixtures.
In the iBet project Thermochemical Bioeconomy – Integrated production of bioaromatics, other chemicals, fuels and energy, we have produced a feasible and highly selective concept for the manufacture of BTX from woody biomass. The developed processes will drastically alleviate the dependency on fossil raw materials in chemicals while facilitating the efficient conversion of domestic biomass to valuable products and energy. A demonstration facility with the ability to prepare high-quality bio-BTX in multi- kilogram scale is being built at the Bioruukki plant in Espoo. This plant will be use to further develop the synthetic route, to access the techno-economics of the process and take the next steps towards a commercial process.
Figure 2. Illustration of how gasification can lead from woody biomass to a vast range of potential products.
Gasification is a process in which the woody biomass is converted into what is known as syngas, consisting mainly of carbon monoxide and hydrogen, by heating to high temperatures above 800⁰C) without combustion and with a controlled amount of oxygen. In principle syngas reactions are not dependent on the raw material, though there are still relevant differences. Although gasification itself is an established technology, there is much less experience of biomass gasification and gas purification. The distinctive characteristic of biomass-fuelled plant is its scale–biomass gasification plants are principally one or two orders of magnitude smaller than plants using coal or natural gas. Catalytic Fischer-Tropsch is a process in which carbon monoxide and hydrogen gases are recombined into a mixture of hydrocarbons which can be used either as transportation fuel or as a source for further processing to chemicals. A good database on Fischer-Tropsch synthesis can be found in reference Ⅲ. Biomass gasification, gas cleaning and synthesis gas conversion to valuable products have been extensively studied at VTT. A recent report summarizes the detailed techno-economic assessment of 20 individual plant concepts based on a large scale (300 MWth) gasifier.Ⅳ
Despite the successful demonstration of the technology there are not any commercial production units in operation which is due to very high investment cost of first-of-a-kind plant as well as significant political uncertainties. In order to improve the economics and help the commercialization VTT has developed new processes suitable for smaller plant sizes. The bio-BTX process presented here is one example of such a process suitable for scales of 50–100 MWth.
Synthesis of high selectivity benzene, toluene and xylene (BTX)
Combining biomass gasification, Fischer-Tropsch-synthesis and aromatization has allowed us to prepare BTX in high selectivities (Figure 2).
A new two-stage tubular reactor system was constructed for the manufacture of BTX (figure 3). Synthesis gas (syngas) produced by biomass gasification is used as the raw material. Light aromatics formed in the gasification are separated and combined with the synthesis product. In the first synthesis step a product mixture rich in olefins and oxygenates is produced from synthesis gas over an iron catalyst promoted by silicon, copper and potassium. In the second stage, the whole product from the first step reacts further to a mixture of aromatic hydrocarbons over a ZSM-5-catalyst modified with lanthanum and zinc. The reaction is carried out at significantly lower pressure than usually employed in the Fischer-Tropsch synthesis. Low reaction pressure (<15 bar) both favours the formation of desired olefins and lowers the cost of the process. The principle of the reactor setup is depicted in Figure 4.
A hydrocarbon product with an exceptionally good selectivity to benzene, toluene, xylenes is formed. Typical product composition of the aromatic fraction is illustrated in Figure 5 with the distribution controlled by temperature and space velocity. To our knowledge this is the first example of a process capable of producing BTX-components with such a high selectivity from biomass.
Figure 3. Two-phase reactor system for the production of BTX.
Figure 4. Reactor setup.
Purification of BTX
Although in theory a potentially very straightforward purification there are no literature reports of successful purification from BTX mixtures produced from either syngas or biomass. Such purification opens a very significant door to unlocking the ability to produce pure BTX fractions from biomass. The quality of the crude material was determined by GC-MS to contain a majority of toluene (42.8%) and benzene (23.0 %) (figure 6). The highly mobile liquid was of excellent physical composition for direct handling in the lab without any need for pretreatments, for example, filtration or preheating.
The crude BTX-product was purified by atmospheric Vigreux distillation and cryogenic crystallization. Pure BTX components could be isolated in good purity (>80%) with over 85 % of the available benzene obtained with greater than 90 % purity with the impurity being toluene.
With pure benzene and toluene having been isolated in good recovery (over 49 % of the total available product) from the BTX mixture the next step is to valorize the material into a valuable bio-based compound that can be used in a specific niche. For this purpose it is excellent to illustrate the quality of the material by undertaking a complex synthetic step, for example, the manufacture of bio-paracetamol (figure 7).
Figure 5. Product distribution of the aromatic mixture. produced.
Figure 6. Composition of the supplied crude BTX.
We have been able to demonstrate that woody biomass can be successfully and selectively converted into benzene, toluene and xylene (BTX). Over 85 % of available benzene was isolated at over 90% purity along with almost 50 % of available toluene isolated at 70% purity (“contaminant” is xylene). The preliminary price for pure BTX fractions is calculated at €1,4 per litre which, although currently more expensive than the current crude-oil derived material (€~1.0 per litre), but a lot more competitive than other equivalent bio-based routes.
This process finds direct application in producing drop-in bio-based platform chemicals, however, both the benzene and toluene can be valorised into more useful niche compounds sure as paracetamol. This complex synthetic route is another demonstrator of the high quality of the aromatics produced by this method.
With a realistic and highly-efficient process in our hands the scale-up has begun at VTT’s BioRuukki facility. To demonstrate the industrial feasibility of the process VTT plans to prepare multi-kilo quantities of material on a tubular flow reactor.
Figure 7. Illustrating the quality of the benzene produced by the synthesis of paracetamol. v
Matti Reinikainen (DSc Chemical Engineering) is a Principal Scientist in the Catalysis and synfuels group. Matti obtained his doctorate from Helsinki University of Technology and has been a key player in catalysis research at VTT for more than 25 years. Matti has 28 peer reviewed publications and 7 patents. Currently he is working with biomass based chemicals as well as special gas chromatographic analysis techniques.
David Thomas (Ph.D in chemistry) is a Senior Scientist in the Chemical synthesis and polymerization technologies group. David studied for his doctorate in photochromic chemistry from the University of Leeds, England. He is experienced in industrial synthesis and process optimisation, having spent over 9 years working in industry. David has 7 peer reviewed publications and 2 patents. Currently he is involved with biomass valorization, advanced spectroscopy and purification.
i Ministry of employment and the economy, 8.5.2014
ii T. Werpy, G. Peterson, Top Value Added Chemicals from Biomass, August 2004, http://www.plasticstoday.com/articles/anellotech-successfully-produces-kilogram-scale-green-BTX-140527
iii A. Stranges (Ed.), Fischer-Tropsch-arkisto, http://www.fischer-tropsch.org, read 16.3.2015.
iv I. Hannula and E. Kurkela, Liquid transportation fuels via large-scale fluidised-bed gasification of lignocellulosic biomass; VTT Technology 91, 2013.
v WO 94/01394, 1994. World Intellectual Property Organization.