BIOMASS AND BIOENERGY 38 (2012)68-94 69 Primary Conversion Conversion product Market Char Storage Charcoal Pyrolysis Bio-oil Storage Biofuels chemicals Fuel gas Turbine Gasification Engine Electricity CHP Combustion Heat Boiler Heat Fig.1-Products from thermal biomass conversion. residence times favour the production of charcoal.High Short hot vapour residence times of typically less than 2s to temperatures and longer residence times increase biomass minimise secondary reactions, conversion to gas,and moderate temperatures and short Rapid removal of product char to minimise cracking of vapour residence time are optimum for producing liquids. vapours, Three products are always produced,but the proportions can Rapid cooling of the pyrolysis vapours to give the bio-oil be varied over a wide range by adjustment of the process product parameters.Table 1 and Fig.2 indicate the product distribu- tion obtained from different modes of pyrolysis,showing the As fast pyrolysis for liquids occurs in a few seconds or less, considerable flexibility achievable by changing process heat and mass transfer processes and phase transition conditions.Fast pyrolysis for liquids production is currently of phenomena,as well as chemical reaction kinetics,play particular interest as the liquid can be stored and transported, important roles.The critical issue is to bring the reacting and used for energy,chemicals or as an energy carrier. biomass particles to the optimum process temperature and minimise their exposure to the lower temperatures that 2.1. Principles favour formation of charcoal.One way this objective can be achieved is by using small particles,for example in the flui- In fast pyrolysis,biomass decomposes very quickly to dised bed processes that are described later.Another possi- generate mostly vapours and aerosols and some charcoal and bility is to transfer heat very fast only to the particle surface gas.After cooling and condensation,a dark brown homoge- that contacts the heat source which is used in ablative nous mobile liquid is formed which has a heating value about processes that are described later. half that of conventional fuel oil.A high yield of liquid is The main product,bio-oil,is obtained in yields of up to obtained with most biomass feeds low in ash.The essential 75 wt.%on a dry-feed basis,together with by-product char and features of a fast pyrolysis process for producing liquids are: gas which can be used within the process to provide the process heat requirements so there are no waste streams Very high heating rates and very high heat transfer rates at other than flue gas and ash.Liquid yield depends on biomass the biomass particle reaction interface usually require type,temperature,hot vapour residence time,char separa- a finely ground biomass feed of typically less than 3 mm as tion,and biomass ash content,the last two having a catalytic biomass generally has a low thermal conductivity, effect on vapour cracking. Carefully controlled pyrolysis reaction temperature of A fast pyrolysis process includes drying the feed to typi- around 500C to maximise the liquid yield for most biomass, cally less than 10%water in order to minimise the water in the Table 1-Typical product weight yields(dry wood basis)obtained by different modes of pyrolysis of wood. Mode Conditions Liquid Solid Gas Fast ~500C,short hot vapour residence time ~1s 75% 12%char 13% Intermediate ~500C,hot vapour residence time 10-30s 50%in 2 phases 25%char 25% Carbonisation (slow) ~400C,long vapour residence hours-days 30% 35%char 35% Gasification -750-900°C 5% 10%char 85% Torrefaction(slow) ~290C,solids residence time 10-60 min 0%unless condensed,then up to 5% 80%solid 20%residence times favour the production of charcoal. High temperatures and longer residence times increase biomass conversion to gas, and moderate temperatures and short vapour residence time are optimum for producing liquids. Three products are always produced, but the proportions can be varied over a wide range by adjustment of the process parameters. Table 1 and Fig. 2 indicate the product distribu￾tion obtained from different modes of pyrolysis, showing the considerable flexibility achievable by changing process conditions. Fast pyrolysis for liquids production is currently of particular interest as the liquid can be stored and transported, and used for energy, chemicals or as an energy carrier. 2.1. Principles In fast pyrolysis, biomass decomposes very quickly to generate mostly vapours and aerosols and some charcoal and gas. After cooling and condensation, a dark brown homoge￾nous mobile liquid is formed which has a heating value about half that of conventional fuel oil. A high yield of liquid is obtained with most biomass feeds low in ash. The essential features of a fast pyrolysis process for producing liquids are: Very high heating rates and very high heat transfer rates at the biomass particle reaction interface usually require a finely ground biomass feed of typically less than 3 mm as biomass generally has a low thermal conductivity, Carefully controlled pyrolysis reaction temperature of around 500
C to maximise the liquid yield for most biomass, Short hot vapour residence times of typically less than 2 s to minimise secondary reactions, Rapid removal of product char to minimise cracking of vapours, Rapid cooling of the pyrolysis vapours to give the bio-oil product. As fast pyrolysis for liquids occurs in a few seconds or less, heat and mass transfer processes and phase transition phenomena, as well as chemical reaction kinetics, play important roles. The critical issue is to bring the reacting biomass particles to the optimum process temperature and minimise their exposure to the lower temperatures that favour formation of charcoal. One way this objective can be achieved is by using small particles, for example in the flui￾dised bed processes that are described later. Another possi￾bility is to transfer heat very fast only to the particle surface that contacts the heat source which is used in ablative processes that are described later. The main product, bio-oil, is obtained in yields of up to 75 wt.% on a dry-feed basis, together with by-product char and gas which can be used within the process to provide the process heat requirements so there are no waste streams other than flue gas and ash. Liquid yield depends on biomass type, temperature, hot vapour residence time, char separa￾tion, and biomass ash content, the last two having a catalytic effect on vapour cracking. A fast pyrolysis process includes drying the feed to typi￾cally less than 10% water in order to minimise the water in the P mir yra tcudorp noisrevnoC noisrevnoC tekraM ilP sleufoiB rahC egarotS laocrahC enibruT Ga noitacifis P o y ls r y is sagleuF lio-oiB egarotS sleufoiB & acimehc sl Ga noitacifis oitsubmoC n eH at elE c yticirt C& HP relioB enignE oitsubmoC n eH at relioB taeH Fig. 1 e Products from thermal biomass conversion. Table 1 e Typical product weight yields (dry wood basis) obtained by different modes of pyrolysis of wood. Mode Conditions Liquid Solid Gas Fast w500
C, short hot vapour residence time w 1 s 75% 12% char 13% Intermediate w500
C, hot vapour residence time w 10e30 s 50% in 2 phases 25% char 25% Carbonisation (slow) w400
C, long vapour residence hours / days 30% 35% char 35% Gasification w750e900
C 5% 10% char 85% Torrefaction (slow) w290
C, solids residence time w 10e60 min 0% unless condensed, then up to 5% 80% solid 20% biomass and bioenergy 38 (2012) 68 e9 4 69