This article describes woodchips as feedstock for transportation fuels and their pathway in catalytic fast pyrolysis. This pathway is based on a technology developed by CERTH (Centre for Research and Technology Hellas), Royal DSM and Neste.
Woodchips as feedstock for transportation fuels – a pathway description
Catalytic fast pyrolysis – transportation fuels
This pathway is based on a technology developed by CERTH (Centre for Research and Technology Hellas), Royal DSM and Neste. Detailed information is available here (BioBoost, 2013):
Feedstock 1: Forest residues (reference feedstock)
Forest residues are co-products of forest cultivation and wood harvest: Thinning wood occurs as whole tree or delimbed stems in the thinning of young stands. Final felling yields logs for the production of timber, wood pulp or boards; co-products are tree-tops, branches and off-spec logs (bent or rotten), which may be used for energy generation. In some countries stump excavation is allowed to prepare the ground for tree planting. Depending on the site conditions, soil fertility and eventual ash return a certain share of forest residues can be taken from the forest without threatening its productivity. This sustainable amount is collected and stored at the forest road for chipping into trucks or transport in whole for chipping at the plant. Depending on site and duration of storage, the water content of forestry residues is between 30 and 50 %. In 2015, the maximum allowable weight of forest trucks was between 40 and 76 tonnes in European countries. Optional feedstocks are other wood commodities (timber processing residues, waste wood and short rotation coppice) and other ligno-cellulosic residues.
Feedstock 2: Wood chips from hedge- tree row maintenance
The main difference between the feedstocks forest residues and hedge- and tree rows on banks is that forest residues are mainly from evergreen softwood (spruce, fir, pine) while chips from hedge- and tree rows on banks are mostly from deciduous trees and bushes. Similar to forest residues the chip quality depends on the diameter of the input material, this commands for sieving, if high quality chips are required. Chipping is either on site or after transport to a yard.
Feedstock 3: Prunings
Prunings from fruit or nut orchards and vineyards are characterised by a high share of thin material, with higher ash content compared to forest residues. Different procurement technologies exist, some of which use baling. Bales can be stacked for air-drying, which reduces feedstock degradation compared to chipping and storage of fresh material. On the other side, chipping of dry baled biomass is difficult and the bale dimension (typically 120×125 cm) requires XL-chippers or crushers.
Prunings are not the preferred feedstock biomass in the scope of greenGain, however the diffinitions are not always clearly defined between agricultural residues and biomass from landscape management as in the examples of maintenance work of abandoned olive groves and the restoration of abanoned agricultural land. As qualified feedstock potential data of these biomass types is not available for the further analysis with the BioBoost optimization tool, the pruning data from Europruning project with similar density and potential has been used. The pruning potential data is available for all EU 27 member states + Switzerland on NUTS-3 level and can be considered as having similar feedstock properties as biomass feedstock from e.g. the maintenance work of abandoned olive groves and the restoration of abanoned agricultural land.
First conversion step: Catalytic fast pyrolysis
The catalytic fast pyrolysis (CFP) starts with the drying and milling of forestry residues (e.g. thinning wood, tree-tops, branches). The biomass is pyrolysed at about 500 °C in absence of oxygen in contact to a catalytic material. The catalyst splits off a high share of the oxygen which is contained in the biomass molecules (about 45 % by weight) as carbon dioxide, carbon monoxide or water. The pyrolysis vapours are rapidly cooled. The condensed biooil contains 50 % of the liquid biomass energy, is low in oxygen content (15 to 20 %) and has a heating value of about 30 GJ/t. CFP off-gases and the catalyst coke are combusted to supply the reaction heat for pyrolysis and produce power (0.83 MWh per tonne of biooil). Another co-product is crude acetic acid of which about 50 kg are produced per tonne of energy carrier. The decentralised CFP plants are erected in areas of high feedstock availability: They are expected to have a capacity of 160,000 to 520,000 tonnes forest residues per year which relates to 28 to 92 truck loads per day. In regions of good availability, transport distances would be between 60 and 120 km. Straw, lignocellulosic energy crops (e.g. Miscanthus, Switchgrass) or waste wood are alternative feedstocks for this process. Use of these biomasses as co-feedstock would shorten the average transport distance. The decentral CFP plant produces between 45,000 and 147,000 tonnes biooil per year.
Biooil energy carrier transport
With regard to transportability, a truck load of 25 tonnes forest residue chips (14 to 17 tonnes wood dry matter, the rest is water) is converted to 4 to 5 m³ of a pumpable energy carrier. A freight train of 40 railway tank wagons with a payload of 65 tonnes each could transport the energy carrier produced from 570 truck loads forest residues. This is a very cost- and environmental efficient transport mean to bring the bioenergy from several rural areas to a central refinery for upgrading by co-processing with crude oil. The energy carrier is moderately corrosive and compatible to standard crude oil transport and storage vessels.
Upgrading to transportation fuels
The good transportability of the energy carrier enables long distance railway transport for upgrading in refineries with capacities between 200,000 and 850,000 tonnes of biooil in European countries. The energy carrier is stabilized in two hydrotreatment steps consuming about 70 kg hydrogen per tonne of transport fuel. One co-product is light gases (180 kg per tonne fuel) another might be phenol(-ics) which have a higher market value for the chemical industry than for biofuel production. Due to changes in the European refining sector, it is expected that the CP biooil may replace 2 % of fossil crude. This enables use of existing capacity for steam methane reforming and hydrotreatment for the deoxygenation of the biooil. The product is co-processed with the fossil streams and distilled to the conventional transportation fuels gasoline/kerosene/diesel according to the production slate of the refinery. All fuels purely consist of hydrocarbons which guarantee drop-in blending. The fuels are fully engine compatible and do not require changes in the distribution infrastructure, two points very important for consumer acceptance. The fuels have a GHG-avoidance potential of 81 % compared to fossil fuels.
The Figure 1 shows the catalytic fast pyrolysis reference pathway in terms of energy flows (Sankey-diagram) and logistic flows.
Numbers indicate the energy flow in MW. Transport efforts are given for reference case. Colour code: Green-biomass; blue-FP-biosyncrude; red-transport fuel; orange-power, pink-natural/combustible gas, grey-process steps (from left to right): forest residue piling, forest residue chipping, feedstock pyrolysis, biooil upgrading. (S. Kühner, SYNCOM)