INTRODUCTION
Biomass is a general term to describe all organic carbon-containing material produced by photosynthesis in plants. Biomass is available in many forms, comprising products as well as residues from agriculture, forestry, and the agro-industry. By definition, biomass is a renewable material as during growth of the plants, crops, and trees, CO2 is withdrawn from the atmosphere (the carbon source) and stored in the biomass as chemical energy. The CO2 cycle is closed again when the CO2 is released during conversion of the biomass and the use of the derived products. The renewable and CO2-neutral nature of biomass is the major motivation to use the material for energy generation (e.g. green electricity) and the production of chemicals, raw materials, and transportation fuels.
CHEMICALS AND RAW MATERIALS FROM BIOMASS
To date, all transportation fuels and most materials and chemicals are produced from crude oil or natural gas. At a certain moment in the future, the decreasing reserves of these fossil materials will give rise to increasing prices. To maintain the same production levels, an alternative carbon source is required and biomass is the only renewable carbon source available. Therefore, biomass will be the future feedstock for the production of transportation fuels and chemicals. This is not an illusion, as the annually amount of biomass available (i.e. not used as food or otherwise) equals about six times the total global energy consumption. In two ways chemicals can be produced from biomass, i.e. via biological and thermal refining. Both conversions routes are complementary as (typically) different feedstocks are used and another spectrum of chemicals is produced.
Biological refining
In the biological refining (or C5/C6 chemistry) advantage is taken from the C5 and C6 sugar structures present in the original biomass. Via fermentation these sugar structures are converted in products like ethanol and methane. Alternatively, by extraction processes the complex structures from the original biomass are maintained and fine-chemical products can be isolated. Disadvantage of the biological route is that only a limited number of biomass materials are suitable for biological conversion and, furthermore, suitable micro-organisms are required for fermentation. Typically, 30 up to 50 wt% of the feedstock is not converted in the biological process and this residue has to be treated in a thermal conversion process (e.g. combusted or gasified) for CHP production or alternatively used as feed for thermal refining process.
Thermal refining
In the thermal conversion (or C1 chemistry) gasification of the biomass and conversion into a product gas is the key step. The advantage of this route is that almost all biomass materials are suitable for gasification. The gasification gas contains CO/H2 as main products and, in addition, raw materials for chemicals production as well as a number of valuable chemicals products that can be directly separated. With the gasification conditions the amounts of raw materials and chemicals produced can be controlled. Due to the analogy with crude oil refinery, the fractioning and recovery of products after thermal conversion of biomass is called thermal refinery of biomass, or Thermal Bio-Refinery. In Figure 1 the overall concept of the Thermal Refinery of Biomass is shown. The refinery is illustrated as a circle to express the CO2 cycle that is closed when biomass is used.
THERMAL BIO-REFINERY CYCLE
Gasification
Gasification is the key-step in the Thermal Bio-refinery; biomass materials of various shapes and origins are converted into a product gas of more consistent quality. Also residues from biological processes can be used as feedstock. Typical constituents of a product gas are indicated in Figure 1. Besides the product gas, the gasification step also affords an ash fraction containing all the minerals from the biomass, which is very suitable as fertiliser.The gasification product gas is the actual ‘feed’ of the ‘refinery’. Although all products are present, the recovery of each individual or group of chemicals from the gas is a choice to be made depending on the specific plant conditions and economic aspects. For example, when biosyngas is the desired product high temperature gasification (i.e. in an entrained flow gasifier is preferred as this affords the highest biosyngas yield and almost no side products [1]. However, in that case the process is not considered as a thermal refinery. In the Thermal Biorefinery concept low temperature gasification is always assumed as key-step (i.e. below 1000°C).
Torrefaction
Optional is a low-temperature torrefaction step preceding the gasification, either on techno-economic grounds as pretreatment to homogenize the biomass or with the purpose to produce specific acids and solvents. Typical examples of important torrefaction products are furfural, formic acid, and acetic acid. The incorporation of a torrefaction step will have little effect on the
Gasification product gas composition.
OLGA tar removal
Aqueous scrubber
The gas after the OLGA unit is tar-free with only gaseous constituents. In an aqueous scrubber the inorganic compound ammonia (NH3) is removed. When significant amounts of HCN are present in the gas, this compound can be converted into NH3 in an upstream hydrolysis step. Alternatively, the HCN is removed from the gas by adsorption with filters. Typically, the NH3 is recovered from the scrubber water solution by precipitated as ammonium salt, e.g. as ammonium sulphate (NH4)2SO4 with sulphuric acid. Ammonia is an important ingredient for fertilizers and the existing production processes are very energy intensive. Most biomass materials contain only small amounts of nitrogen and the concentration of NH3 is usually very low, therefore, the NH3 production capacity in a Thermal Bio-Refinery is limited. However, when chicken manure is used feedstock, the concentration of NH3 in the gas can rise to several volume percentages.
Sulphur removal
Similar to nitrogen, most biomass materials contain only very small amounts of sulphur that after gasification will be present in the gas mainly in the form of H2S with minor amounts of COS. When desired COS can be converted into H2S in an upstream hydrolysis step. Typically, isolation of sulphur will not be economic interesting and sulphur removal is achieved as part of the gas cleaning with adsorption on filters. When large amounts are present, the H2S can be separated from the gas and converted into elementary sulphur, which has value as industrial base material.
CO2 removal
Biomass product gas generally contains 10 to 30% of CO2. For application to generate electricity and heat the presence of CO2 is no problem. However, CO2 removal is strongly recommended as gas conditioning step, before further cryogenic distillation of the gas or for biosyngas production. CO2 is a problematic compound in the cryogenic processes and is an undesired inert diluents in the synthesis processes for which the biosyngas is used. The removed CO2 is can be applied for pressurizing oil wells (Enhanced Oil Recovery), in green houses, for chemical synthesis, or sequestered.
Cryogenic distillation
The next step in the refinery is deep cooling of the product gas and separation of gaseous products by cryogenic distillation. The first products recovered are ethane, ethylene (C2H4) and acetylene (C2H2); the last two are very important raw materials in the chemical industry. Upon further cooling methane (CH4) is recovered. Methane can be supplied to the natural gas grid as Substitute Natural Gas (SNG) for stationary and mobile applications, used as fuel on site for steam generation, or used as raw material.
Biosyngas applications
After all the described refinery steps, i.e. removal of the chemicals and raw materials as products, the gas only contains CO and H2, making it a renewable Syngas or biosyngas. Syngas, a product by itself, is an important and generic raw material in the chemical industry. Within the objective to produce transportation fuels from biomass, the gas can be used to synthesizes Fischer-Tropsch diesel, methanol (or DME), or ethanol (or mixed alcohols) for use in combustion engines. The gas can also be shifted to produce pure hydrogen for use in fuel cell vehicles, which is considered as a long-term option.
Combined Cycle
The remaining unconverted Syngas and off-gases from the different processes can be used to generate electricity and heat in a Combined Cycle (CC). Typically, a chemical production plant will generate its own energy needs in this way, or even a surplus that can be delivered to the grid (as green electricity). Upon combustion in the combined cycle, CO2 and H2O are formed and released to the atmosphere. Herewith the CO2 cycle is closed.
INDICATIVE AMOUNTS
The compounds and chemicals discussed are always formed upon gasification of biomass (with oxygen/steam). However, the relative amounts depend largely on the gasification temperature. An exception is the amount of ammonia formed, which depends mainly on the amount of nitrogen present in the feed. In Table I, this is illustrated by a representative product gas composition calculated for oxygen/steam blown gasification of woody biomass at 850°C in a fluidized bed gasifier. The gas compositions are also translated into production numbers. Firstly, expressed as yield per ton of biomass and secondly, as annual yield based on a representative medium-size biomass gasifier (500 MWth).
Table I: Typical product gas composition for gasification of woody biomass at 850°C in a fluidized bed gasifier (second column), yield of the individual compounds per tonne of biomass (third column), and the annual yield based on a 500 MWth gasifier (fourth column).
a) on dry gas basis;
b) yield of gases products per ton of dry and ash-free biomass (wood);
c) based on 8000 hours operation;
d) examples of selected tar compounds;
e) in case of a preceding torrefaction plant.
ECONOMICS
The economic perspectives of a Thermal Bio-Refinery depend on the specific conditions for each location. Detailed economic assessment of the complete integrated concept is ongoing. In this paper the economic potential is illustrated focussing solely on the cryogenic distillation step to recover C2 hydrocarbons.
Cryogenic recovery of C2 hydrocarbons
The CO2-free product gas fed to the distillation contains CO, H2, CH4, ethane, ethylene, acetylene, and a small amount of nitrogen. The boiling points of these compounds at 1 bar are indicated in Table II.The separation of the C2 hydrocarbons can be compared to the recovery of ‘natural gas liquids’, whereas the separation of CH4 from H2/CO is comparable to the removal of N2 from natural gas to increase the calorific value of the gas. The C2 compound product stream needs further distillation to recover the pure products.
Product prices
The prices of the products depend on the oil and gas prices. A conservative crude oil price of 15 $/bbl is assumed. In the Netherlands the natural gas (NG) price is coupled to the crude oil price, which in that case costs 1.8 €/GJ (1$ = 1 €). Methane is valued equal to NG as the typical application will be fuel. Ethane is valued 10% higher, being a raw material for chemical industry.
Ethylene is typically produced from crude oil or NG, and is 1.8 times more expensive compared to NG [4]. The applications of acetylene are similar to those for ethylene and with a price of 12.2 €/GJ the costs of the products (e.g. vinyl
acetate and vinyl chloride) are competitive with routes using ethylene as feed [5]. Syngas is typically produced by partial oxidation of NG.Therefore, the price of the product biosyngas is taken 1.5 times the price of NG, i.e. 2.7 €/GJ [6]. 5.3 Investment and operational costs The investment costs of a cryogenic product gas distillation unit are estimated with natural gas liquids and nitrogen separation units as reference. For a large-scale thermal bio-refinery of 8000 MW input (for motivation of scale see [7]), the investment costs are estimated to be 225 M$ and the operational costs are taken equally to the annual capital costs (with 7000 h/y operation, 15 year depreciation period, and an interest rate of 8%). The electricity costs used are 0.04 €/kWh.
Economic perspectives
The purpose of the economic assessment is to illustrate that fractioning of the product gas into chemicals and raw materials in economically more attractive than direct use of the gas in a combined cycle. Therefore, itis assumed that the product gas on energy basis has the same value as NG, i.e. 1.8 €/GJ. The product value of the gas should be higher than the energetic value plus the costs of the cryogenic separation. The added value of the products, compared to the fuel value is 167 M€. The annual cost for energy, capital, and the operational costs are 86, 26, and 26 M€, respectively. Therefore, the net added value of the system is 30 M€ per year, making cryogenic separation economically a very attractive step.
CONCLUSIONS
In analogy with the refining of crude oil, biomass can be fractioned in a thermal biorefinery. Upon gasification the biomass is converted in a product gas containing a variety of interesting compounds that can be isolated by fractioning of the gas. For the cryogenic distillation step of the integrated refinery, the economic attractiveness was illustrated.
OUTLOOK
To date, the whole chemical industry is based on crude oil and optimized to use by and side products from the refining and synthesis as feedstock for other processes. The thermal bio- refinery opens new routes to synthesize products from other biomass derived raw materials. More important, the thermal bio-refinery allows the development of a sustainable chemical industry.
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