Focus on allowing plants (organic matter) to maximize energy efficiency resulting in environmental sustainability. Extending harvest, delay in concentrating usable organic material (e.g. coal), harvest, transport, storage, rehydration, pulverizing, processing, and fermenting uses lots of energy, but living organisms (e.g. plants) have high moisture, yet gasification requires low moisture content and anaerobic digestion needs rehydration approximating the natural growing state of around 70-90% moisture.

Example 1: Maximize plant efficiency and transportation: About a decade ago I showed how plants can be mechanically, chemically, or biologically manipulated to maximize sugar content by preventing the plant (corn for grain) from forming kernels. It takes less energy for the plant to produce sugars than subsequently converting those sugars to starch and oils. My resulting patent proves biofuels can be converted from plant carbohydrates within 24-72 hours after harvesting fresh biomass and all harvested tonnage (after that short conversion) can be converted in or near the production field then pumped through pipelines. It is an energy efficient and sustainable method using sun and photosynthetic, plant cell deterioration, and fermenting powers. Result: no waiting for plant grains to mature; no drying, harvest nor storage of grains; no distant transport of bulky products; and no rehydration required before fermentation. See Wietgrefe, Gary W., Patent number 8,641,910 (Feb. 4, 2014) Systems and Processes for Producing Biofuels from Biomass, http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=15&f=G&l=50&co1=AND&d=PTXT&s1=Wietgrefe&s2=biomass&OS=Wietgrefe+AND+biomass&RS=Wietgrefe+AND+biomass

Example 2: Transportation of bulk organic material is inefficient. Transportation of biofuel base material and resulting biofuels consume tremendous amount of energy when the concentrated energy is not produced and consumed locally. Coal (a slowly concentrated biological product) gasification is more economical at the coal mine than sending coal over road, rail, or water to a gasification plant. Whereas, shipping resulting fuel by pipeline is extremely efficient. Like coal, sugarcane is limited geographically by restrictive production climate generally away from population centers needing fuel. Secondly, massive amounts of sugarcane biomass are squeezed (efficiently) but the remainder (i.e. lignin, an organic material) is usually burnt rather than reapplying it to production fields or fermented because transportation to and from fields is extremely energy inefficient as is converting lignin to a useable biofuel. While sugars ferment quicker than starch conversion, under current systems, grains and sugarcane are allowed to mature wasting valuable natural resources (sun and time) needed to maximize biomass production. See Dakota Gasification Company, https://www.dakotagas.com; and Antunes, Felipe A.F., et al., Biofuel Production from Sugarcane in Brazil, Chapter Biofuel Production from Sugarcane in Brazil

The answer is which ever will be available in a cheaper price in the market. Researchers should emphasize on renewable, especially waste, materials which are increasing in a very fast pace. Biodiesel, Biogas, Bioethanol are very promising. Yes, there are challenges. But we should remember that research works have already made up some of them and if we do more research, some fruitful outcome will definitely be there.

Bio-methane from biogas to use for vehicle transport

I think that the most promising technologies are those which can incorporate the renewable energy peaks and the CO2 into the digestion/gasification processes. Plasma and electrolysis have demonstrated that is possible to reform methane and other molecules using CO2, producing syngas and hydrocarbons. They can also improve hydrogenate CO2 to produce methane and methanol. With the increasing amounts of renewable energy, these technologies seem more and more promising to save that energy while avoiding CO2 emissions.

Combining several biomass conversion methods such as biochemical and thermochemical and also hybrid system including two renewable energy type such as bioenergy and wind energy could improve the fuel production

Anaerobic digestion

I think using solar energy may be the best way of having clean energy