Hi Luis, thank you for that post. The role of mid merit plant (what you call intermediate load) is of course central in grid dispatch and is becoming ever more so with the rate of penetration of wind and other intermittent renewables. No grid can possibly hope to be stable without it, bearing in mind that mid-merit plant also provides most of frequency response and spinning reserve which is essential to match loads on a second to second basis - again ever more challenging with greater wind penetration.
In the UK, mid-merit has tended to be provided by mainly pulverised coal plant and older gas CCGT, and it has required most mid merit plant to go much further than simply ramping up and down. With a couple of exceptions all UK coal plants run 2-shift regimes (i.e. turn on and off every day) for 6 - 9 months of the year. In fact this year several coal plants 2-shifted right through winter although that has partly to do with running restrictions under the Large Combustion Plant Directive.
In a post fossil world, large circulating fluidised bed (CFB) boilers running on biomass are very well suited to perform this role. Due to the large inert mass of sand in these boilers they are very well suited to retaining heat during a 6-12 hour shutdown and re-starting quite easily. An appropriate fuel for these boilers is wood chip as it is low in chlorine, ash and heavy metals which make certain types of biomass challenging to combust.
You lightly dismiss "biofuels" as having a low EROI and being subject to land constraints, and these are both genuine worries with regards to certain types of biofuel, especially those that rely on primary agricultural outputs such as cereals or oilseeds. However they simply do not stack up for solid wood biomass, let me take the two points individually:
EROI:
The largest scale biomass boilers being contemplated are about 300MW net capacity, at this level these plants can achieve 40-42% net conversion efficiency, depending on the cooling that's available. There is not a lot of difference between a biomass plant and a hard coal plant, except that no boiler manufacturer today will go supercritical on biomass because it is a step too far to deal with alkali ash melting and corrosion at those temperatures at the moment. However, once they have more experience on with big boilers they will get there, probably by using additives to control the chemistry. A port based 300MW biomass CFB burning imported wood chip from North America (where there is massive surplus potential) has an EROI which is roughly similar to our current coal plants (burning Russian coal at an average net efficiency of 36%) and very slightly lower than an average aged CCGT burning Qatari LNG Siberian piped gas. As a rough rule of thumb the energy required to ship the wood chip (using panamax class wood chip carriers) is about 1.2% of the biomass energy in the hold assuming the ship returns empty. The energy consumed in harvesting, chipping, transport to port, loading and discharge is about the same again.
Land constraints:
The underutilised resource from existing European and Eastern North American working (i.e. not protected) forests that could be sustainably harvested or collected and supplied to a European port is around 250m tonnes (depending on who is doing the estimating, we have seen much larger figures used, but this one is much closer to what is realistically practicable). That would comfortably supply about 100 of the large 300MW boilers that I mentioned earlier or about 30GW if they were running baseload. However if they were 2-shifting with say a 40% loadfactor (typical of mid-merit coal plant in the UK today) that would be about 60 GW of plant. A very respectable % of Europe's mid merit needs.
However the real resource potential is South America and Africa where huge areas of degraded land (typically used for a short period for agriculture and then abandoned) exist which would be used for sustainable forest plantations. Brazil alone estimates about 110 m Ha of abandoned or unutilised agricultural land that would be suitable for forest plantations (i.e. there would be enough rainfall), energy optimised plantations (as opposed to the pulp fibre optimised variety where thee genetics are pushed to produce as much cellulose as possible) can easily achieve 60 tonnes per net planted Ha today (and likely much more in the future). Assuming 50% net to gross planted area on 110m Ha of land that would equal enough fuel for almost a terrawatt of 40% LF mid-merit biomass plant. That alone would be far more than enough to cover the mid-merit needs of the whole of Europe and South America, and doesn't take into account other vast areas of potential in South America and Africa.
I stress that this is all land that would literally benefit from having trees grown, even though it may be a mono-culture and therefore not environmentally perfect it would still prevent soil erosion and economic depredation on a massive scale (which usually leads to overexploitation of soils and other natural resources such as high biodiversity forests) and preserve the value of the land for the long term. Most of this land is not suitable for arable crops, or at least would take massive energy-intensive inputs to be so. Soil preparations are mainly potassium and phosphorous and other micronutrients, i.e. not heavy nitrogen. They can mainly be recycled from biomass ash. By the way, North America could already produce enough from existing sustainable forestry to meet its mid-merit needs.
Biomass is not "the answer" to future power needs. Rather it has a role to play role along side baseload nuclear, variable wind and solar, and flexible hydro. Although due to subsidy regimes, the first generation of large dedicated biomass plant will be baseload (the grid has all the mid-merit it needs for now with coal and gas and not enough low carbon generation) the future of biomass is to fill the mid-merit niche, while providing traditional “thermal” spinning reserve and frequency response. It is a vital role if we harbour any hope at all that power grids can be kept stable at acceptable prices in the future, and does not deserve to be so lightly dismissed.
Comment: http://europe.theoildrum.com/node/6720#comment-684137
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