Gasification of waste for energy carriers - A review
The primary scope and focus of IEA Bioenergy Task 33, “Thermal Gasification of Biomass”, is to follow the developments in the area of biomass gasification with the purpose of providing a comprehensive source of information on activities in this field in the participating countries. A dedicated web site is publicly available (http://task33.ieabioenergy.com).
Although the main focus of the task is material of biomass origin, also waste feedstocks are of interest as materials normally considered as “wastes” are to a large extent composed of biomass materials and the fuel characteristics (high volatile matter content, low fixed carbon content etc.) and product gas characteristics (e.g. tar contamination) are similar, and therefore similar gas cleaning techniques is used. For this reason, waste gasification is always to some extent included in the Task activities and from time to time, have been more in focus, e.g. as topic for a special project in the period 2016-2018, which this report represents. The methodology used in this study has mainly been to collect information from public sources. In addition, in some cases direct contacts were also taken to obtain more information. The work has also involved some contacts with mainly IEA Bioenergy Task 36, “Integrating Energy Recovery into Solid Waste Management”.
The report initially describes wastes in a broad sense, but excluding hazardous wastes, as a fuel for thermal treatment process. The report continues by giving a brief overview of the waste and waste treatment situation in the EU, Japan and the USA and also summarises the policy and regulatory framework for waste treatment in the above jurisdictions. This is done from a perspective of the impact of the policies and regulations on the thermal treatment of waste in general, and in particular on the impact on waste gasification technologies.
On the technical side, the report describes the gasification and gas cleaning technologies used for waste. The focus of the report is on waste gasification in combination with pre-combustion gas cleaning, i.e. advanced waste gasification technologies, as this combination is key to the main advantage of waste gasification technologies that motivates many of the developments in this field. Furthermore, the state-of-the-art in waste gasification is presented by descriptions of a number of projects for different applications and the associated developers. The technical scope of these applications ranges from direct use of the raw fuel gas in furnaces and boilers to advanced technologies where cleaned gas is used in more efficient steam cycle boilers, engines and gas turbines, as well as for the production of chemicals and liquid energy carriers.
Finally, the competitive position of advanced waste gasification technologies relative to the conventional technology (thermal treatment by one or two-stage incineration with heat recovery), the barriers for introducing the technology on the market, R&D needs and the results of a simplistic economic evaluation are discussed.
The information collected from public sources has also been used to compile a list, included as Appendix 3 and 4 of this report, of technology developers/suppliers and waste gasification projects in various stages ranging from historical projects, operational projects and projects in planning. However, this listing does not pretend to be complete and the data will change over time.
Waste as an energy resource
Waste treatment is of high importance to all societies as it is linked to other issues such as health, pollution of land, air and water as well as climate change and unsustainable resource utilization. Despite policy actions to curb waste generation, the waste quantity generated is still growing due to population increase and life-style changes. The global quantities of wastes that could be treated by thermal methods amounts to the order of 3 billion tonnes annually. Some of this is already processed in incinerators but still a dominant fraction is disposed of in landfills. Even so, in terms of the energy content and also the GHG emissions, waste overall contributes a small fraction (3-4 %) of the global energy usage and GHG emissions. Nevertheless, is it still a significant energy potential to valorise and the management of this quantity in itself is a challenge.
The disposal methods used of old such as of dumping waste on some marginal land or in rivers or oceans have proven to generate a number of problems (methane emissions, soil and groundwater contamination, plastic soup in the oceans etc.). Even if controlled and engineered landfills are used, both the material disposed of and its energy content are lost. In the waste hierarchy, which is a principle guiding policy in this area, the preferred order of waste management is prevention of waste generation, reuse, recycling and recovery of wastes, while disposal in landfills or by other means is at the bottom. Apart from prevention, reuse and recycling, thermal treatment by incineration with energy recovery is the main alternative to disposal, as in addition to the energy recovery, it sanitizes the waste and reduces its volume. Nevertheless, in the absence of policy interventions to reduce landfilling, improve landfill management procedures or to enforce waste recycling and treatment practices, disposal by landfilling is still a widely used waste management method in many parts of the world as the cost has been, and still is, relatively low. Under such circumstances, thermal treatment has not been feasible, but as landfill space becomes more limited, management within the legal perimeter becomes costlier and societal acceptance decreases, thermal treatment is gradually gaining a stronger position. In many places such as in Japan and Northern Europe thermal treatment is the dominating waste management method and the capacity is also being expanded very rapidly in e.g. China.
The state-of-the-art thermal treatment technology is waste incineration with energy recovery to mainly power, i.e. a thermal power cycle composed of combustion of the waste to generate steam used to drive a steam turbine generator (often denoted waste-to-energy, WtE). There are on the order of some two thousand such installations world-wide of which maybe one hundred are using various gasification technologies. However, to avoid corrosion issues caused by the presence of contaminants in wastes, notably chlorine, the steam superheater temperatures are lower than for conventional thermal power plants, only 400-470 °C compared to from 500 °C up to almost 600 °C. This causes the conversion efficiency of waste incinerators to be significantly lower, only 20-25 %, compared to the efficiency of other thermal power plants using conventional fuels, 35-45 % for solid fuels and up to even 60 % in large gas turbine combined cycles. In the past, operational requirements and emission control of waste incinerators were also less regulated than today. This is still reflected in a low public acceptance of incinerators, despite that stringent regulatory requirements for efficient emission control monitoring have gradually been mandated.
But even if more stringent air pollution control and ash disposal methods have improved the emission footprint and more or less sophisticated energy recovery is used, the products of waste incineration are limited to power, generated at far lower efficiencies than other thermal power plants, and possibly heat. As the economy of an incinerator is based largely on revenue from receiving waste for treatment, the drivers are weak for increasing the efficiency beyond regulatory standards, e.g. to qualify as energy recovery (R1) rather than disposal (D10) in the EU.
In this perspective waste gasification has advantages. However, sometimes the term waste gasification is used for a technology where none or a very limited part of the improvement potential of gasification technologies is realized, i.e. the waste is converted into a combustible gas in a gasifier only to be directly combusted in a close-coupled furnace with heat recovery by steam generation, and the exhaust flue gas is then treated in conventional waste incinerator emission control equipment. Such gasifiers without pre-combustion gas cleaning can be designated as two-stage incinerators, (or sometimes “incinerators in disguise” by anti-incinerator NGOs), as opposed to “true gasification” in which more or less extensive gas cleaning takes place before the product gas is used. The performance of such two-stage gasification incineration technologies can, at best, be similar to a comparable incinerator as the presence of contaminants in the furnace and heat recovery section limits the steam temperature, and hence the efficiency, in the same way as for conventional incinerators.
Waste gasification technology
The focus of the report is therefore on the “true gasification” systems, i.e. where the use of gas cleaning is an enabling technology to not only achieving a higher conversion efficiency to power, but also to produce a synthesis gas that can be catalytically converted to chemicals and fuels by well-established commercial processes (waste-to-liquids, WtL). The figure below illustrates the differences between a conventional waste incinerator (left leg), gasifiers being two-stage incinerators (second left leg) and true gasification systems with partial and complete gas cleaning, respectively (the right-most legs).
There has been a wide variety of gasification and gas cleaning technologies used. This is in itself a sign that the technology is not mature and that the selection of alternative processing routes has not been narrowed down to a more limited number of varieties that have proven to be more cost-efficient and reliable than other options tried. The gasifiers are typically fixed beds or fluidized beds of similar designs as in incineration (grates, kiln, fluid beds etc.). Due to the fuel characteristics, entrained flow gasifiers are not in use other than for pumpable, liquid wastes (contaminated oils, etc.).
For gasification technologies, the presence of so-called tars (a mixture of heavy hydrocarbons formed during fuel devolatilization) in the concentrations typical of most gasifiers makes tar removal the primary target for gas cleaning, as tars interfere with heat recovery via gas cooling and also additional gas cleaning addressing other contaminants. The primary method for removal of tars in waste gasification systems is by thermal decomposition downstream of the gasifier operating at elevated temperatures relative to the gasifier temperature. Another less common way to remove tars used is by scrubbing with a suitable organic liquid to absorb tar hydrocarbons. Other gasification-specific issues are that sulphur compounds are present in a different chemical form than in an incinerator and the formation of ammonia and predominantly NH3 from fuel bound nitrogen.
In the case of sulphur present in the waste fuel, it is present predominantly as H2S. Pre-combustion gas cleaning technologies therefore rely on the adoption and adaption of technologies used in other industries e.g. chemical, oil and gas industries. This is an area where in particular research could assist in improving the outlook for waste gasification by providing suitable and cost-efficient cleaning methods suited for the scale of operation of thermal treatment of waste.
Ammonia, which is largely oxidised to NO when the gas is combusted or is undesirable when the gas is used for synthesis of other fuels, can be removed by scrubbing but then affects the water cleaning. Alternatively, if a combustion process is used, there is also the opportunity to use established post-combustion de-NOx technologies.
Other contaminants are removed by cleaning technologies that are similar to what is used in incineration processes and other industries (cyclones and filters for particulate removal, sorbents and scrubbers to remove acid gases and mercury, etc.), and therefore the adaption of these to suit gasification conditions does not constitute a technical barrier as significant as tars and sulphur cleaning.
However, in terms of environmental performance, legislation in the EU and elsewhere for incinerators implies that the regulated contaminants must be removed to an extent of 90-95 %, or even more, relative to their presence in typical waste fuels. For the use of the gas generated from waste for the purpose of chemical synthesis, the gas cleaning requirements are even higher than this. Therefore, gasification systems, where the exhaust gas is subject to the same regulations, cannot be expected to drastically reduce the emissions, compared to conventional incinerators. But as pre-combustion cleaning is performed on a smaller gas volume then post-combustion cleaning, it may result in less secondary wastes than the conventional incinerator system, giving some cost advantages.
In addition to the potentially higher efficiency, also other interactions between policy and technology have promoted waste gasification. In Japan, the lack of space for landfills made waste incineration a preferred technology as early as in the 1970’s. In 1998, one decisive policy intervention mandated that new waste incineration plants have ash melting facilities in order to reduce dioxin in fly ash and leaching from landfills. This triggered several developments making Japan the primary market for two-stage waste gasification technologies because such gasification systems could vitrify the ash as an integrated part of the process and without consuming external energy (electricity or fossil fuels), but this required also sacrificing part of the efficiency to energy exported. Although the vitrification was made less of an absolute requirement ten years later, in 2008, there are still some one hundred gasifiers in operation in Japan today. However, the technologies employed in Japan had difficulties to penetrate the market in Europe and USA. Another example is the UK, where the use of “advanced thermal treatment” of waste has been promoted for over a decade and has spurred project developments and some dozen installations using various forms of gasification, mainly in two-stage incineration configurations, but also in a number of cases with extensive gas cleaning.
Already with partial gas cleaning, i.e. removing chlorine using sorbents and the particulates in the gas, such a gas can be used in a downstream boiler at improved steam conditions and energy. Since 2012 a CHP plant at 50 MWe output has been in operation in Lahti, Finland on SRF and contaminated wood. This CHP plant has above 30 % conversion efficiency to electricity and if designed as power only, would reach 35 %.
There are also some examples of installations at a scale of 1.5 to 10 MWe in France, the UK and elsewhere (e.g. Morcenx, Tyseley, Fort Hunter Ligget, etc.) using cleaned gas in internal combustion engines (sometimes also including a bottoming steam cycle) and reaching efficiencies in the range 25-35 %. Furthermore, there are developers that are targeting the use of the gas in gas turbine combined cycles (e.g. Synova, Taylor Bioenergy, etc.) to reach even higher conversion efficiencies, even if a notable twin-plant project a few years back (Tees Valley 1 and 2) never succeeded to come into operation.
Using gasification and gas cleaning to generate a synthesis gas has been less in focus until recently, even if the efficiency for producing fuels and chemicals is higher than for production of electrical energy, of the order of 50 % or more. There is one plant using plastic waste to produce ammonia that has been in operation in the Tokyo area in Japan for more than a decade. Another plant is in early operation in Edmonton, Alberta, Canada to produce methanol or ethanol from RDF. Yet another industrial scale plant is in construction in the Tahoe-Reno area of Nevada, USA, where RDF will be converted to FT fuels, and there is also a smaller demonstration installation in construction at Swindon, UK, where RDF will be converted to synthesis gas for further conversion to bio-methane. Fuels from waste has come more in focus in the recent years due to the interest for substituting fossil fuels in the transport sector, and both the US RFS2 system and the EU RED recognises in principle such fuels as biofuels, with some caveats regarding the fossil part of the waste.
The examples of plants highlighted above, and others, are described in more detail in the report.
Despite the efficiency advantages of waste gasification, there have been difficulties in introducing the technology on the market. In many locations, the economic incentives for any form of waste-to-energy (WtE) plant have not been attractive compared to landfilling. Furthermore, conservatism combined with strict emission regulations and market conditions have not favoured the introduction of innovative but less proven technologies. Within the EU, all thermal treatment of wastes, including gasification and any downstream combustion equipment consuming the gas, is defined as incineration and subjected to incinerator legislations. However, if the gas is sufficiently cleaned prior to its combustion (end-of-waste in the figure above), the gas becomes a product in its own right and downstream equipment is not a part of an incinerator. The status of a waste gasifier is less clear in the USA and subject to interpretation of federal legislation at the state level. In Japan, the emissions accepted by the client and local authorities is more determining than the nature of the conversion equipment.
Nevertheless, and as is described in the report, there have not been many plants in which waste gasification in combination with a more extensive gas cleaning have been used, and some of these have been associated with more or less severe teething problems. Problems have been associated with the heterogeneity of the feed wastes, in particular when directly gasifying MSW. There have also been issues caused by the quality of the RDF resulting from pre-treatment of MSW and also with achieving the gas cleaning intended.
This means that the accumulated experience from such installations is not sufficient to validate to what extent, and under which circumstances, the performance and environmental advantages of advanced gasification technologies can be realised. However, at present there are a number of new installations being built or in planning for the production of both power and fuels that are hopefully successful and can contribute to clarifying the position of waste gasification.
A simplistic economic evaluation, using what are considered average market conditions, gives some indicative results on the feasibility of gasification technologies. When the fuel cost is changed to become a gate fee revenue, the investment-related capital costs becomes the major cost driver.
The data indicates that for both conventional incinerators and gasification plants, the magnitude of the specific investment is around 10 000 €/kWe, i.e. significantly higher than for conventional power and CHP plants. For fuel production the specific investment relating to the output energy is lower, 4 000- 6 000 €/kW fuel, as the conversion efficiency is significantly higher than for the production of electric energy. This may seem surprising, but if the specific investment instead is related to the energy input, both applications are fairly similar in terms of specific investment.
Furthermore, and unlike other energy installations, the drivers to increase the efficiency of waste incineration installations per se are less strong. For both conventional and gasification-based systems, the gate-fee is the dominant revenue stream for power-only plants, and together with heat sales a very significant part of the revenues for CHP systems. Based on the numbers of the simplistic economic estimates, even conventional incinerators do not show good economic results if only seen as a mere power plant project that only receives the average market revenues for power and heat and using an opportunity fuel. Break-even is relying on combinations of supports and for monetarization of additional societal services in waste treatment such as e.g. investments subsidies and financing assistance, as in Japan, landfill taxation increasing the gate fees and incentive pricing for the electric power products produced.
So, even if gasification technologies are more efficient, the specific investment must also at least be comparable or lower to conventional incinerators to really be attractive. Investment costs for most installations are at this stage in the development in line with conventional incinerators, with the exception of the CHP plant in Finland referred to above that had a specific investment cost that is closer to a biomass CHP. However, because these gasification plants also represent first-of-a-kind installations, and there is less operating experience, the data is not quite comparable to technically and commercially mature incineration technology.
In contrary to the power and heat generation, the economy of producing renewable fuels looks very interesting, even if it requires the most extensive gas cleaning. The efficiency is high compared to incinerators that produce only power, which reduces the specific investments and generates a high output stream, and the value of this stream is higher than for power or heat on an energy basis. Furthermore, there is no other established waste value chain that competes for this type of product.
There are also some policy issues that can change the outlook for gasification systems. Policy interventions to decrease disposal of waste directly such as landfill bans, restrictions, or taxation promote recycling and treatment of residual wastes by e.g. thermal treatment in general. However, the competitive situation between well-established waste incineration technologies and the emerging gasification technology means that such measures on their own may not be sufficient to make gasification installations feasible.
Another and stronger form of policy driver, and in line with circular economy principles, is by setting ambitious efficiency performance targets for new installations and using these as a driver for technology development. Although this would be technology neutral and stimulate innovations also in the state-of-the-art commercial technologies, this would also require developments and associated costs for these established technologies, thereby reducing the distance to gasification-based technologies where the potential for higher efficiency would then be more appreciated.
Outside the conventional thermal conversion of wastes, the policies for decarbonization of the transport sector have, among other pathways, recognized the potential for utilizing wastes for the production of transport fuels, waste-to-liquids (WtL). Promotional policies and incentives can directly stimulate developments in this field. This area has already attracted some interest recently, as noted above. For this application, gasification is a key technology both for biomass and wastes and there is no other well-established conversion technology to compete with.
• Waste gasification technologies integrated with more or less extensive gas cleaning (“true gasification”) enable a higher conversion efficiency to power than in conventional incinerators at similar capacity, i.e. making better use of the energy potential of wastes. When cleaned, the gas can be used in boilers at higher steam temperatures than in incinerators, or in internal combustion engines and gas turbine combined cycles.
• Waste gasification systems not applying gas cleaning before the combustion of the gas (two stage incineration) suffers from the same corrosion-related limitations in steam superheat temperature as conventional incinerators and can, at best, achieve efficiency similar to these. This efficiency of a conventional waste incinerator is also significantly lower than in other thermal power plants.
• However, incinerator economics are more relying on the revenue generated by accepting waste for treatment, i.e. the gate fee, than from selling energy. Therefore, in the absence of regulatory interventions setting more ambitious minimum efficiency targets, the drivers to increase the efficiency, even if present, are less pronounced than for conventional thermal power plants.
• Waste gasification technologies integrated with extensive gas cleaning can also produce synthesis gas for the production of fuels that can assist in the decarbonisation of the transport sector. This is an interesting application which also appears economically attractive compared to using waste to generate electric energy, and where there is no established technology to compete with.
• Conventional incineration technologies and waste gasification technologies alike must achieve a high level of contaminant removal to meet ever-more stringent statutory limiting emission values. Therefore, gasification technologies cannot be said to deliver major environmental benefits in terms of emissions compared to conventional technologies, even if there may be some cost advantages.
• The overall status of the gasification and gas cleaning technology is that it is still in development and entails both technical and non-technical risks. This also means that data regarding performance, availability, maintenance, investment and operating costs refers to first-of-a-kind installations representing a variety of gasification and gas cleaning technologies. The data is therefore limited and more difficult to generalise in comparison to data for conventional incineration technologies.
• Despite the technical and economic challenges for waste gasification technologies, a number of first-of-a-kind installations using different power cycles and fuel synthesis pathways are in early operation, commissioning, construction or in later stages of planning that together with others yet to come can assist in providing data to fill the knowledge gap.
• Policy interventions such as landfill bans or taxes are in general promoting the use of thermal treatment technologies, including gasification technologies, by increasing the availability of waste for thermal treatment, whereas waste prevention and recycling can reduce the waste quantities available, and therefore reduce the interest for innovative technologies. Setting high policy targets for the conversion efficiency or promoting the use of biofuels in transport would favour gasification technologies, due to their inherent high conversion efficiency and the possibility to produce fuels instead of just power and heat.
Lars Waldheim, Waldheim Consulting
Published by IEA Bioenergy