1.6.6.1.5. Solar driven pyrolysis

Concentrated solar energy can be used to drive thermochemical processes. The solar thermochemical conversion of carbonaceous materials is one of the most often suggested options. In addition, concentrated light can be also used as a very efficient laboratory device for studying basic kinetic pathways in very clean conditions, as for example the primary steps of the biomass thermal degradation. Solar energy is converted into chemical fuels which can be stored for long times and transported over long distances. In addition, solar energy can also assist in the processing of high temperature chemical commodities [5, 12].

The advantages of solar assisted processes of thermochemical conversion of biomass are as follows [13]:

1. The calorific value of the feedstock is upgraded and due to the exclusive use of the feedstock as the chemical source of the product gas.

2. The gaseous products are not contaminated by the byproducts of combustion, such as CO₂ and N₂ if air is used as the gasifying agent.

3. The discharge of pollutants to the environment is avoided.

4. No capital investment for an air separation unit arises compensating for the more complex reactor technology and solar radiation concentration installations.

So far, only theoretical analysis and laboratory-scale experimental data on solar driven conversion of fossil fuels and biomass are available in the literarure. Solar pyrolysis and gasification of coke, coal, cellulose, and other carbonaceous materialswas studied in vortex-flow, molten-salt pool, fluidized bed and packed-bed reactors, directly irradiated by solar or, more often, a substitute model radiator like in [14].

There are two possible configurations to integrated solar energy with gasification process: direct projection of solar irradiation to the gasifier (Figure 6, a) and indirect application to the gasifier (Figure 6, b). In the first configuration, solar reactor having direct projection usually features the use of a cavity type configuration. The cavity can effectively capture the incoming radiation through a small aperture window. Due to multiple internal reflections, the cavity that acts like a black body and absorbs most of the radiation by minimizing re radiation losses. However, a major drawback is to keep the aperture window transparent during the operation of reactor at high pressure, and gaseous environment which can blacken the window by tar, coke or ash formation.

By using indirect solar radiation the problem of clean optical window for reactor can be avoided. It can be done by a cavity receiver containing an opaque tubular absorber that enclose reaction chamber. The opaque absorber is exposed to direct solar radiation and the radiation from the hot cavity wall transmits the heat to reaction chamber by conduction, as shown in Figure 6, a. To collect high solar flux an arrangement of heliostat type mirror arrangement can be used. A similar type of 100 KW pilot project is installed in Masdar city, Abu Dhabi, as shown in Figure 7.

Figure 6. Solar reactors with a) direct and b) indirect irradiation

Figure 7. A 100 kW pilot solar collector at Masdar City, Abu Dhabi [15]

Figure 8 shows details of our own laboratory setup for experimental study of solar driven pyrolysis of biomass particles (solid wood cylinders, wood pellets, crashed wood pellets). A glass tubular retort containing a single particle or a batch of particles (Figure 8, left) is placed in the focal area of a dismountable parabolic solar concentrator with the diameter of 1 m (Figure 8, right). Temperature of a particle is measured vs time with a TCC thermocouple embedded in the particle centre. Yields of the pyrolysis products are found by weighting the particle (solid yield – char residue) and retort (liquid yield – deposits on the wall) before and after the test, and by difference – gas yield. Photographs of pyrolysis products processed at a heat flux around 700 W/m² are shown in Figure 9. It took about 40 minutes for a particle to reach temperature of 250—320 ^oC. Typical product yields under these conditions are reported in Table 5 and Figure 10.

Figure 8. Details of BNTU laboratory setup for experimental study of solar driven pyrolysis of biomass particles: parabolic concentrator with a grid sample holder in the focal area (left) and glass retorts filled with fresh biomass particles on the holder (right)

Figure 9. A glass retort with charcoal (left) and carbonized wood cylinders (right) after tests

Table 5. Product yields from solar pyrolysis of biomass particles, the present study

Sample No	Particle characteristics			Char	Liquid	Gas
	Material	Diameter, mm	Length, mm	wt%
1	Solid wood	12	4	72.1	4.4	23.5
2		6.5	30	85.3	1.2	13.5
3		10	30	40.5	4.9	54.6
4		8	40	84.5	1.1	14.4
5	Wood pellet	10	23	89.5	0.7	9.8

Figure 10. Product yields from solar pyrolysis of biomass particles, the present study