Erik Dahlquist

The major thermal biomass conversion techniques are combustion, gasification, pyrolysis, and tor — refaction. Combustion means 100% oxidation of all organic contents of the fuel using air/oxygen, while gasification means partial combustion where some 15-30% of the oxygen is added in rela­tion to what would be needed for 100% oxidation. In pyrolysis we only heat but without adding air and thereby gaseous components of the organic material are evaporated and later condensed as liquid hydrocarbons. Torrefaction is when you do partial pyrolysis but only to remove some of the gaseous components, where the purpose not is to produce liquid hydrocarbons but make a compact residue that can replace coal in coal fired power plants. Only combustion is really used on a large scale commercially today for biomass, although significant work has been done on development of the other techniques as well. The hurdle has been the cost as the fossil alternatives with natural gas, oil and coal have been “too cheap”. As different type of penalties are introduced on fossil fuels to compensate for the costs caused by environmental impact like greenhouse effects and acidification, the relative competitiveness will change. As the new technologies are improved they will also be cheaper, and with new system designs we can foresee a commercial expansion within the next coming 10 years also with respect to all other technologies apart from combustion. Several countries like the USA, Denmark, Finland, Germany, the Netherlands and Sweden all have strategies for research and demonstration of biomass for multiple uses such as for production of plastics, textile fibers, and many different chemicals (Andersson, 2012).

Already today, significant amounts of biomass are converted mostly into heat. The estimate is that approximately 13% of all global primary energy utilized is biomass. Still, most of this is converted with very low efficiency technologies like burning in an open fire. Then the efficiency from fuel to useful heat for e. g. cooking food is just around 10%. By introducing simple ovens the efficiency may then be increased several times, and by introducing very efficient cogeneration technologies the sum of heat and electric power in relation to the heating value of the biomass fuel may even be 117% in e. g. Sweden. This is actually quite common in the large-scale CHP (combined heat and power) plants in Sweden, where we have a heat demand at least most time of the year. In hot climates, the alternative is CCP, combined cooling and electric power production. The efficiency then can be quite high, although not as high as in Scandinavia, where also condensate heat can be utilized from the exhaust gas to reach the 117%.

In China, approximately 15% of the coal is gasified today and coal is used in the production of 50% of all chemicals. In 2005, China produced 232,820,000 tonnes of coke, 8,950,000 tonnes of calcium carbide, 25,000,000 tonnes of chemical fertilizer and about 3,500,000 tonnes of methyl alcohol from coal. Shenhua Baotou coal to olefin program has a production of 1.80 million tonnes/year; coal-to-carbinol is 600,000 tonnes/year. There are more than 10,000 coal gasification stoves in operation in China. Fixed-bed gasifiers are the most common. In ammonia-fertilizer industries, the number of water-coal gasifiers exceeds 4000 units; there are also more than 5000 two-phase gasifiers where e. g. Lurgi gasifiers are used for producing industrial fuel gas (Yasuyuki, 2007). Here the potential for gasification of biomass should be very high, as there are major resources of straw just being wasted today. As gasification is already common, it should be easier to get acceptance also for biomass gasification. Still, there is a demand for the right incentives like price or regulatory directives. Torrefaction also has a major potential, as the product is compact and easy to transport long distances in an economic way. The heating value may be up to 25 MJ/kg dry substance (DS), which is in the same range as coal. Another advantage is that the

2 E. Dahlquist pellets or briquettes produced from torrefied biomass can be used in normal coal mills without having to modify the grinding equipment normally used for the coal. This makes it easy to start using biomass as a complement to coal on a large scale.

Gasification can be used to produce different type of chemicals. Either methane can be produced and separated directly, or the syngas with CO + H2 is converted through catalytic processes to different chemicals using e. g. the Fischer-Tropsch process. An alternative can be to heat biomass without introducing air, and we then get pyrolysis instead of gasification. Then a more complex mixture of gaseous and liquid components is produced. This can be refined in a way similar to how crude oil is refined in refineries. This technology is now being developed both “on its own” and as part of gasification systems. For example, CORTUS has a process where biomass is first pyrolysed and the pyrolysis gas is then combusted to heat the char, which is gasified using steam (http://www. cortus. se/, 2012). Chalmers in Gothenburg is also working with a similar technology together with Metso Power. Andritz is working with the Carbona process and several companies in among others Japan are working on processes with gasification combined with combustion in two separate fluidized beds. Here the char is combusted to produce heat for the gasification and to get rid of the residual char coal. A pilot torrefaction plant in Ornskoldsvik also is using pyrolysis gas for heating and driving the torrefaction, although using lower temperatures than are normally used in pyrolysis.

Aside of these thermal conversion processes we have microbial processes as well as mechanical compaction in different ways. Concerning microbial processes, in China these can be from small batch fermenters in single households to produce gas for cooking food to large scale plants like Tianguan’s biorefinery inNanyang (Henan Tianguan Enterprise, 2012), where 150 x 106m3 gas will be produced annually.

The processes are of batch type as well as continuous and the temperature can be from room temperature over mesofilic around 35°C to thermofilic around 55°C. In all these processes the basic principles are still the same. We use different types of microorganisms to convert biomass through biochemical process routes into something that is more valuable for us as humans than only CO2.

In combined systems, we can see that it would make sense to use easily decomposable sub­stances like house hold waste for biogas production, while dry, solid waste is better to convert in the thermal conversion processes. An advantage with the microbial processes is both that all nutrients like P, N and K can be recirculated to farmland after the processing, and also there will be an organic residue that has the properties to keep moisture in the soil when distributed on farmland. As the organic content has a tendency to decrease rapidly today with a lot of cereal production and less animals, this is of high importance in many countries and should be taken into account in many more in the future, to create a sustainable agriculture. To make it possible to recycle the organic material on the other hand we need to be careful with what we put into the reactors. This is especially true in wastewater treatment plants, where many different chemical substances may come to the plant like pharmaceuticals, tensides, oil, etc. Thus, we can foresee a major demand on separation of waste and avoidance of disposing toxic chemicals into waste and wastewater in the future. The complete material handling system will be integrated with the energy system.

In reality, we will need to recycle also the inorganics from the thermal conversion plants to sustain the productivity in forestry and other areas long term. Here we have just started, and have a very long way to go until we reach sustainability.

Concerning the mechanical conversion techniques the major focus is on robustness, so that the equipment and tools will last and not need replacement too frequently. For that reason e. g. briquetting may be easier than pelletizing, as the friction surface is smaller.

For pyrolysis the major difficulty is that we get a process that gives a different composition depending on what we put in. The chemical composition is affecting the liquid phase composition a lot, and if we want to produce a very homogeneous product, we have a problem. Still, by measuring the chemical composition of the biomass we put in we can to some extent control the process in such a way that we can get more homogeneous results. This is also relevant for the

optimization of combustion, gasification, and biogas production processes. In this book, we cover these aspects especially looking at NIR (near infrared spectroscopy) and RF (radio frequency) sensor systems, which are introduced for on-line applications to determine moisture content and chemical composition of biomass. Several installations are done by e. g. Bestwood for this in Sweden.

This chapter only has the aim to give a very broad overview of the different technologies and you will read more about everything in the rest of the book. Thus only a few references have been included, as more comes in later chapters instead.

REFERENCES

Andersson, K.: Report on bioenergy based economy Bioenerginytt 2, 2012. http://www. cortus. se/ (accessed March 2012).

Henan Tianguan Enterprise Group Co., Ltd: http://www. tianguan. com. cn/english/about. asp (accessed March 2012).

Yasuyuki, A.: Report on applying coal gasification technology in China’s coal based chemical industry. UNESCO report, 2007, http://www. unescobeijing. org (accessed March 2012).

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