Improvement of the Flash-Pyrolysis Process and Pilot Plant for Bio-Oils Upgrading

Summary

The project has the overall objective of studying different methods of upgrading fast pyrolysis liquids in order to overcome anticipated problems of utilising crude fast pyrolysis liquid derived from biomass. At the time that the project was proposed and agreed, there was considerable interest. in the production of liquid fuels and a belief that the crude biomass derived fast pyrolysis liquid (bio-oil) was too unstable to be used directly in power and thermal applications. There had already been some success in upgrading bio-oil to hydrocarbons by hydrotreating on a laboratory scale to produce a naphtha type product which could be upgraded by conventional refinery operations to diesel and light fuel oil; and also zeolite cracking to produce crude aromatics that could likewise be upgraded to gasoline by conventional refinery operations.

The project therefore aimed to develop and evaluate two chemical upgrading processes hydrotreating and zeolite cracking. An assessment of upgrading through chemicals recovery was included. During the contract negotiation stage an opportunity arose to participate in a bio-oil fired gas turbine test programme in Canada and this was included to gain first hand experience of the problems arising from not upgrading.

All of the initial objectives except the construction of a dedicated bio-oil hydrotreating pilot plant have been satisfactorily completed. Due to bankruptcy of the major project contractor, this element could not be completed and this was replaced by tests on an existing unit and detailed design studies. These have enabled the aims of providing a robust technical and economic evaluation to be completed.

During the course of the project, it became clear that the crude fast pyrolysis liquid - or bio-oil - could be satisfactorily used in gas turbines (also in engines in parallel projects), and the need for upgrading was thus obviated. It is also clear that the costs of producing transport fuels are far too high, even with major fiscal incentives. Of the two main routes studied, the zeolite cracking route which is integral with the fast pyrolysis process seems to offer substantial economic benefits, although it is less developed than hydrotreating. It is recommended that attention be focused on stabilising the crude liquid to make it more amenable to wider usage and to study the upgrading processes as a means of deriving higher value chemicals.

Introduction

When this contract was awarded in 1992, there was considerable interest in the production of liquid fuels and a belief that the crude biomass derived fast pyrolysis liquid was too unstable to be used directly in power and thermal applications. There had already been some success in upgrading bio-oil to hydrocarbons by hydrotreating on a laboratory scale and the trials on zeolite cracking for aromatics production appeared promising. This project therefore aimed to explore these upgrading technologies by utilising both the existing industrial and academic expertise in Europe and also by commissioning work with leading organisations in North America in order to provide the most efficient and effective way of progressing the technologies.

Objectives

The primary objective of the project is the design, construction, testing and evaluation of a pilot plant to hydrotreat fast pyrolysis bio-oils for the production of hydrocarbon fuels. Complementary objectives include an assessment of the performance and production cost of Robinia and sweet sorghum, evaluation of zeolites as an alternative upgrading route, gas turbine testing on crude bio-oils and an evaluation of the recovery of high value chemicals. The structure of the project showing the activities and contributions as well as the interactions and relationships is summarised in the

Figure below. This has been modified from the original plan to account for the consequences of the retirement of Fochi, but is otherwise largely unchanged from the original plan.

Progress

The problems met by the project are well documented in interim reports, but the most serious was the withdrawal of the lead contractor, Fochi, due to their liquidation. The resultant time delay and inability of finding another company to provide the necessary expertise and financial resources within the time constraints of the project meant that one of the primary objectives of the project to build a hydrotreating plant became increasingly difficult to realise. It was also appreciated that there were existing hydrotreating facilities at several centres in Europe, which, coupled to the success of other projects in using the crude bio-oil without upgrading, and the severe shortage of funds in the project, mitigated against pursuing construction of the pilot plant.

In 1996 it was agreed by all the partners and the Commission to modify the work programme and substitute construction of the hydrotreating pilot plant by tests and detailed design studies of hydrotreating pilot plants with cost estimates.

Activities

All objectives of the contract have been successfully completed apart from not building a hydrotreating pilot plant. With the present knowledge of pyrolysis liquids, this is now seen to be not necessary, -so does not detract from the from the project. The rest of the work can be summarised on a task by task basis, as follows:

Test on growing, harvesting, chipping and storing Robinia and sweet sorghum, economic analysis of the biomass production systems, market analysis of biomass availability and analysis of integrated supply systems. Tests have been completed on sweet sorghum and Robinia growing, harvesting, drying and transport. A linear programming model of biomass supply systems has been constructed. An analysis of biomass supply sources and systems has been carried out. The conclusions of this part of the work were that results for production of sorghum were valuable the cultivars were not as satisfactory as others tested elsewhere. Harvesting of Robinia was successful with the machine tested but the equipment needs strengthening. The model suggests that Cynara is the most promising biomass crop, followed by Arundo Donax. Combined crops provide year round supply which has implications for conversion plant specifications and design. Biomass supplies have been identified in the Umbria region which will provide a methodology and results for other areas

Characterisation of pyrolysis liquid and the upgraded products, evaluation of the hydrotreating plant and supply of bio-oil. As the hydrotreating plant was not built some objectives could not be met. Chemical and physical properties were measured on bio-oil supplied by Ensyn and Fenosa, and on partially upgraded bio-oil from Sassari, and on fully upgraded oil from the DMT tests. Upgrading processes were reviewed and it was concluded that fast pyrolysis of biomass is not yet a well proven technology, bio-oils from different sources exhibit significantly different characteristics, the DMT upgraded product consisted mostly of paraffinic hydrocarbons while there is a wide variation in upgraded product costs.

Delivery of bio-oil from the Fenosa pilot plant and techno-economic assessments of the WFPP system with a fluid bed pyrolyser operating at 500°C and atmospheric pressure. This part of the project resulted in samples of oil being delivered to 9 organisations. Capital costs have been estimated for pyrolysis processes from 0.5 Mwe to 30 Mwe which range from 3650 ECU/kWe output for the pyrolysis plant alone (no engine) at 0.5 MWe to 2130 ECU/kWe at 30 MWe. Mass balances and electricity outputs have been reported for pyrolysis processes from 0.5 MWe to 30 MWe. It was concluded that the break-even for a 16% IRR is achieved at around 8 MWe based on bio-oil as the product. The water content of the biomass has a significant effect on the production costs. Small projects can only be justified if there is a social credit. Large projects may be financially viable for low cost biomass feed such as forestry residues.

Hydrotreating on a laboratory scale unit and design of pilot plant. The laboratory scale process was successfully operated on bio-oil for the first stage of processing. A detailed pilot plant design was produced. It was concluded that full de-oxygenation of bio-oil is possible by several routes. Feasible designs have been produced for hydrotreating pilot plants. Comparisons with bio-diesel are favourable with respect to hydrocarbon yield per ha and similar with respect to production cost. If bio-diesel is considered economically acceptable, then bio-naphtha is equally acceptable.

Techno-economic assessments of the upgrading systems and evaluation of opportunities for recovering chemicals. Capital cost estimates of seven processes for fast pyrolysis and upgrading have been provided on a consistent basis at 100 t/h dry wood input. Production costs of crude and upgraded products have been estimated on a consistent basis. A thorough review of chemicals recovery processes has been completed . It was found that none of the pyrolysis or upgrading processes gives a product that is economic currently. Crude bio-oil and zeolite cracking offer the best short term prospects for a competitively priced fuel. Hydrotreating is very costly in terms of investment and hydrogen requirement, even for partial upgrading. Future work should focus on improving the crude bio-oil and adapting applications to the crude bio-oil, as well as developing integrated processing such as the zeolite cracking process. Chemicals recovery is feasible but the economics will depend very much on market evaluation and development.

Exploratory research into existing and new catalysts for hydrotreating bio-oil to produce hydrocarbons and/or improved stability oil. A 2-3 kg/h laboratory unit was successfully modified to operate with a separate fixed catalytic bed and with replacement of the sand by catalyst, with four catalysts tested. The product proportions changed with increased gas and char/coke production and with reduced liquid yields. The liquid was generally in two phases and showed substantially different characteristics in particular being more distillable and with higher hydrocarbon levels. It was concluded that zeolite type catalysts can be successfully introduced into an Ensyn RTP pyrolysis system. Use of catalysts generally gives a significantly different product range. Lower temperatures gave higher liquid yields. The liquid had much higher levels of hydrocarbons of around 9% compared to 1% in bio-oil without catalyst.

Tests on a 2.5 MWe gas turbine on bio-oil with subsidiary objectives of characterising bio-oil and testing it for combustibility and effect on turbine components. The tests were successfully completed for a total duration of 10 hours. Emissions were measured and found to be below corresponding values for diesel apart from particulates. Flame tunnel testing of components was carried out with some deposition found, particularly on the first stage blade and first stage nozzle. It was concluded that there is a potential for bio-oil fired gas turbines. Some deposition was found on turbine components from flame tunnel tests but these are not necessarily representative. The emissions were entirely acceptable for CO, NOx, methane, equivalent total hydrocarbons and S02- particulates were higher than diesel.

Development of hydrotreating using conventional hydrotreating catalysts to achieve low severity upgrading. A laboratory scale unit was successfully operated in downflow mode and good mass balances achieved. A revised pilot plant specification and design with capital cost estimate was provided. It was concluded that low severity hydrotreating is an alternative to full de-oxygenation for improving the properties of crude bio-oil, but still requires many of the features of full hydrotreating, but at a lower capital cost. The use of nickel catalysts at low temperatures leads to alcohol formation giving a more stable but oxygenated product.

Conclusions

During the course of the project, it was found that the crude fast pyrolysis liquid - or bio-oil - could be satisfactorily used in gas turbines (also in engines in parallel projects), and the need for upgrading to hydrocarbons was thus obviated. The costs of producing transport fuels is too high, even with major fiscal incentives,. Of the two main routes studied, the zeolite cracking route which is integral with the fast pyrolysis process seems to offer substantial economic benefits, although it is less developed than hydrotreating. It is recommended that attention be focused on stabilising the crude liquid to make it more amenable to wider usage in different applications and to study the upgrading processes as a means of deriving higher value chemicals.